LINEAR MOTION DEVICES

ACTUATORS...... A257 rends in the last few the assembly to your ma- decades have led de- BEARINGS...... A268 chine. Similarly, you can T signers to rethink CONTROLS...... A271 get an that is syn- concepts of industrial mo- LINEAR MOTION DEVICES ADVERTISING ...... A272 chronous--driven for tion. Companion gains in the linear actuation, motor- control theory and hard- ized, and with integral ware allow schemes for geared speed reducer. You complete motion systems not previ- can also get electrohydraulically actu- ously possible. And motion technolo- linear part of its path, with or without ated devices that use the precision of gies from fields as diverse as national special attachments. digital control with the high force of a defense and medical diagnostics are • Plastic drive tape. hydraulic cylinder. You can get any available for exploitation in other • Sliding-action leadscrew (usually such packages with integral control fields. The primary effect is that to- acme ), with nut. hardware and software. day’s designers must consider not just •Ball-bearing leadscrew (ball- industrial “power ” along screw) with nut. The balls recirculate Screw jacks shafts and through reductions, but into and out of the load zone. also the broader concept of industrial • Planetary roller screw, in which One of the older conventional actu- motion and control. And linear mo- a nut engaging several planetary ation packages, the screw jack, comes tion is an essential part of it. threaded rollers mounts on a in a single housing containing the in- Many modern processes call for threaded shaft. The threaded rollers put rotating shaft, the output linear unattended operation, exceptional also engage the shaft. Axes of shaft, motion shaft (screw), all bearings, precision, high throughput, flexibility nut, and rollers are parallel. Threads and the lubricant. The user connects a for short runs, and total manufactur- on the nut and planetary rollers are of motor and the load attachment. There ing integration. Often in such cases, the same helix angle. are three major types. humans can’t perform well enough. • Recirculating roller screw, in Machine-screw jack. In the com- Modern sensors and controls, coupled which a nut engaging several grooved mon machine-screw jack, Figure 1, ro- with diverse and precise linear and rollers mounts on a threaded shaft. tation of the input (worm) shaft turns rotary motion devices, combine to fill The rollers also engage the shaft. Dif- the worm and drive nut, which the needs. Thus, designers must con- fers from the planetary roller screw in connects rigidly to the worm gear. sider linear motion as an integral part that rollers recirculate into and out of The leadscrew (also called the lifting of industrial motion and control. the load zone. screw or stem) is of acme or modified The major componentry of linear • Skewed rollers on a rotating cy- square-thread form. It is threaded motion systems can be categorized as: lindrical rod (traction motion on a through the drive nut and converts • . threadless rod). rotary motion of the nut to linear mo- • Support systems (bearings). • Fluid-power cylinder with direct- tion, if the screw is kept from turning •Control systems and components. driving rod, rodless cylinder, or cable with the drive nut. Many equipment manufacturers cylinder. Rolling-element bearings support supply complete systems that include • Electric solenoid. the input shaft and worm gear to mini- all major types of components. • Electric linear motor, such as lin- mize frictional loss. Thrust bearings ear induction motor or linear step support the load. The stem cover stores motor. lubricant and helps protect the stem ACTUATORS • Adapted electric rotary motor. A from damage and contamination. Common linear actuation devices common type has no shaft. The rotor Machine-screw jacks come in many for single-axis motion include, but are also serves as the mating nut on a stock sizes with load ratings from less not limited to: leadscrew with axis coincident with than 1 to more than 250 tons. A jack • Various complex linkages such rotor axis. can mount stationary so the stem re- as a walking beam or slider-crank Preceding listing order does not im- ciprocates, or an external nut can be mechanism. ply relative importance. used so the stem rotates and the nut • Gear rack and set. Many linear actuation devices (attached to the load) reciprocates. • Plate or disc-cam drive with come as complete packages. For ex- Most machine-screw jacks are self- fixed-axis follower. ample, you can get a motorized ball locking. Thus, the load remains sta- • Cylindrical-cam drive with fixed- screw powered through spur or worm tionary in the event of a power failure. axis follower. gearing, and fully self-contained so The major limitation of machine- • Chain, belt, or cable drive in the that you need only mount and wire screw jacks is low efficiency, typically

1998 PT Design A257 Screw-jack application. Figure 3 de- picts a typical lifting ar- rangement, sometimes called a T-sys- tem. It shows how one motor can power sev- eral synchro- nized lifting points by us- ing and right-an- gle gearboxes. Selection of the best ar- rangement for an application is usually based on space Figure 1—Typical machine screw jack. availability and motor accessibility. 25% or less. Sliding between drive nut Here are some guide- and screw generates heat. This heat- lines: ing restricts duty cycles of machine • Keep screw load di- to 5 to 7 1/2 min/hr at full load. rection parallel to the jack. If an application screw axis as much as calls for the advantages of a machine- possible. Figure 2 — Typical ball screw jack. screw jack, but needs a longer duty • Keep the span between drive cycle or higher efficiency, a better components as short as practical. • You can include other torque-sens- choice may be a ball screw jack, Fig- That keeps interconnecting shafting ing devices into a jack or a motor to pro- ure 2, or a roller-screw jack. See also short and limits the chance of a criti- tect the whole system from overload. discussions “Ball screws” and “Roller cal-speed problem. To select a screw jack, use a system screws” which follow. The heart of a • When necessary, use pillow- design manual having illustrations, ball screw jack is an assembly com- block supports, dynamically balanced column strength charts, power and posed of a screw and nut, separated shafting, or both, to help avoid criti- torque data, and sample calculations. by a recirculating series of balls. The cal-speed vibrations. ball screw jack works like a ball bear- • Select shaft cou- ing and has similar life predictability. plings with high Rolling friction of the ball screw, strength-to-bore ra- compared with sliding friction of the tios (such as gear cou- machine screw, generates little heat. plings) to minimize This allows higher lifting speeds in system inertia. the ball screw jack and much better • Use three and efficiency, typically 92 to 95%. The four-way miter boxes higher efficiency reduces input power whenever possible, requirement to about two thirds that to shorten intercon- of a machine-screw jack for a given necting shafts. load. Besides higher efficiency than • Use limit machine-screw jacks, ball screw jacks switches to restrict have a lower ratio of starting to run- extremes of stem ning torque. Because of low rolling travel, or connect ro- friction, many ball screw jacks can be tary switches di- back-driven. See the discussion on rectly to one unused back driving under “Ball screws.” side of the double- Roller-screw jack. You can also extending input improve on a machine-screw jack’s shaft of a jack. advantages by using a roller-screw • Use a slip cou- jack. See the discussion under “Roller pling between motor and jack input Figure 3 — Typical screw jack system screws” which follows. shaft to provide overload protection. arrangement.

A274 1997 Power Transmission Design Before selecting any jack equip- preloaded ball screws, a common back- screw does add resisting torque. In ment, determine: lash range is 0.002 to 0.013 in., de- some applications such as numeri- • Number of lifting points. pending on screw size and ball diame- cally controlled machines, in order to • Total load per jack. ter. To avoid , one nut can be specify the screw’s motor drive com- • Load direction (tension or com- preloaded against another on the pletely, you must know preload pression). screw so there is no play. Similarly, torque in addition to load torque. Con- • Speed at which load must move. one ball circuit can be preloaded sult the ball screw manufacturer. • Total distance load must move. against another within the one nut. In Life expectancy. A ball screw is Also, consider duty cycle and envi- some assemblies, slots cut in the nut like a ball bearing in many ways, and ronmental conditions such as tempera- make it act as a preloaded spring when life expectancy is based on similar ture and contaminants. Next, using a mounted, or a clamping device can principles. For a ball screw, life expec- system design manual, make calcula- draw it up against the . tancy depends on: tions to determine input torque, speed, Mountings. Critical speed of any • Applied force, including accelera- and power requirements. Then check long shaft, including a ball screw, is a tion and friction loads. column strength and select the jack. speed at which it vibrates violently in • Number and length of load a transverse direction. It results from strokes. Ball screws rotating-system unbalance. The mem- For best life, the load should be ap- ber can have several critical speeds, plied along the same axis as that of The ball screw jack is a specific use all multiples or submultiples of one the ball screw. Side loads and loads of a more general linear actuation de- predominating “first-order” speed. A that tend to overturn the nut cause vice called a ball screw or ball-bearing machine cannot operate long at criti- uneven load distribution on the balls screw. The first ball screws appeared a cal speed. Bidirectional thrust bear- and reduce life expectancy. half-century ago, but their popularity ings at each end of a ball screw provide Life expectancy is based on a standard surged in the last few decades as greatest support for critical-speed and rating of 1 million in. of travel. Most ball needs for efficient mechanical linear stiffness considerations; bidirectional screw manufacturers offer charts to de- actuation grew rapidly. Early models bearings at one end with the other end termine life in inches of travel for various were seen as low-friction actuators in totally free, the least support. loads. For example, if the load is halved automotive steering gear and similar Stiffness. A ball screw’s diameter from rated load, expected life increases uses. Sliding friction of threads on the and the number and size of its load- by about nine times. nut and screw is replaced by rolling carrying balls determine assembly Other factors affecting life rating are friction of balls in approximately cir- stiffness. Preload and end configura- screw and nut materials and hard- cular-form threads on the nut and tion can also affect stiffness. nesses. If hardness of a stainless steel screw, instead of Acme or modified When a screw mounts with single- ball screw is reduced from, say, 56 to 45 square threads of the machine-screw direction thrust bearings at each end Rockwell C, then load capacity is re- actuator. A deflector forces the balls or thrust bearings at only one end and duced to about 30% of what it had been. out of the screw’s threads and into the freely at the other, then its spring Column load. Excessive compres- ball guide on the nut. The guide di- rate varies directly with its cross-sec- sion load on a ball screw may cause it rects them diagonally back to the op- tional area at the root diameter; di- to buckle. Column-load capacity of a posite end of the nut and rechannels rectly with operating load; and in- ball screw varies with length, type of them to the screw’s thread, so the balls versely with length. The spring rate of end mounting (fixity), and screw root circulate continuously. This self-con- the ball screw system varies inversely diameter. In general, these parame- tainment allows use of fewer balls. with the sum of the reciprocals of the ter changes increase the tendency for The ball guide can be an external tube spring rates of the screw, the nut, and the screw to buckle as a column: that mounts on the nut or, for more the bearings. Total system spring rate • Increasing axial load. compactness, an integral interior part is always less than that of the most • Increasing length-to-diameter of the nut. compliant member. (slenderness) ratio. Formed vs. machined threads. Drive torque. For rotary-to-linear • Reducing end-support fixity. Most ball screws are of steel or stain- motion, ball screw assembly drive Another problem that axial load less steel. Threads are machined and torque is: can cause, unrelated to column buck- ground for high precision, or roll- PL ling, is that load makes the ball screw T = formed for lesser precision and lower π lead change slightly. In most cases it cost. In recent years, thread-forming 2 E is insignificant. However, for heavy techniques have improved to where where: load in tension or compression in a better grade rolled-thread ball screws T = Torque input, lb-in. high-precision application, you may approach the precision of many ma- P = Operating load, lb need to account for lead variation. chined ball screws of several years ago. L = Lead, in./rev Installation. Misalignment can Backlash. The precision of a ball E = Efficiency (about 90%) shorten life of a ball screw assembly screw assembly depends strongly on Torque values from the equation do to a small fraction of the expected life. the amount of backlash in it. Backlash not account for drag or inefficiencies Avoid misalignment by making is the measured play between nut and due to mounting or to drive compo- mounting surfaces of the nut parallel screw. Ball size and ball-and-ball- nents, to wiper drag, or to torque due or perpendicular to the ball screw cen- groove conformity dictate the amount to preload. terline, or provide a gimbal housing if of backlash. For ordinary non- Preload torque. Preloading a ball appropriate.

1997 Power Transmission Design A275 Get more life out of ball screws The ball screw on the right has seen many Many ball screws are hours of service, but it repairable. Common B — Chipped and is more economical to broken lands on this problems, such as loss of repair it than to ball screw are not discard it. After repeatability due to wear severe enough to can be fixed by regrind- repair, it will look like scrap it. the assembly at left. ing the ball thread grooves and then using larger balls in the ball- screw assembly. If a mi- nor crash bends the screw a bit, it can often be straightened to its original accuracy and returned to service. Surface C — Before repair, brinnelling of ball problems such as screw threads is seen spalling, brinelling, inin thethe foregroundforeground asas chipping, or feath- vertical “dashes.” ering, Figure A, Figure A — can be reground Common forms of and replated. All of wear or damage these repairs may on the threads of prove more eco- ball screws. nomical than re- are the first to wear. Thus, new, placement with a larger balls are used at every level to whole new restore preload and repeatability. ball-screw as- The secret to proper ball replace- sembly. ment: for every 0.003 in. of wear, use To deter- a 0.001 in. larger ball. The screw is mine when a also straightened because a bow as ball screw little as a thousandth of an inch can needs repair, put excess moment on the ball nut, and approxi- ceptable lash is which can later result in failure. mate how 0.002 to 0.004 in. Level 2 (seven days) adds re- D — A close-up • much repair, For diameters grinding of the ball nut to the steps view of the inside of the measure its ball nut shows spalling of the two ranging from taken in Level 1. Ball-nut thread diametral middle threads. 0.5625 to 0.6250 grooves wear faster than the screw backlash, or in., the acceptable threads because they are subject to lash. Diametral lash (not axial back- lash is 0.004 to 0.008 in. more ball travel. lash) is a measurement that can be As noted, a ball screw with 80% or • Level 3 (seven days) adds re- taken in the plant. The ball-screw more wear is likely irreparable. Four grinding of the ball screw threads, assembly is placed in V blocks. An repairs is about the maximum for and as required, rebuilding of the engineer lifts the ball nut vertically any ball screw. After that, bury the journal diameters with eutectic and with a gage, measures the play ball screw with honors. spraying and grinding them back between the ball nut and screw. For Levels of repair. When a ball to size. a preloaded assembly, a diametral screw arrives at a repair facility, it is • Level 4 (14 days) adds regrind- lash of 0.0005 in. indicates a wear inspected and evaluated for the type ing of the ball nut and ball screw. factor of 50%. Minimal, or Level 1, of needed repair. This process can This level may cost 55% of a new as- repair is needed to bring the screw take up to three or four days. sembly, but the repaired ball screw back into use. A lash of 0.0035 in. In general, there are four levels of will have a normal new-screw life. represents 80% wear and indicates cost-effective repair. While each suc- When the repair cost goes over 65%, either a Level IV repair or a dead ball cessive level adds cost, this cost is buy a new assembly. screw. Similarly for nonpreloaded still less than a new ball-screw These four levels of repair are clas- assemblies, a diametral lash of 0.009 assembly. sified by the most common repairs in. is 50% wear and needs Level 1 All repair levels involve the same and do not cover all contingencies. and 0.015 in. is 80% wear and needs four basic steps: inspect, clean, re- Level IV or replacement. ball, and straighten. Excerpted from an article by For ball-circle diameters ranging • Level 1 (three days) repairs loss Thomson Saginaw Ball Screw Co., in from 0.03934 to 0.04875 in., the ac- of repeatability due to wear. Balls the June 1996 issue of PTD.

A276 1997 Power Transmission Design A common installation procedure: duced side loads on the ball screw as- preloaded, bearings, slides or rails, Mount the ball screw in its bearing at sembly. If torque from one end to the and frame. The user aligns and bolts either end; loosely connect the nut to other is constant, there is no signifi- down the entire frame. You need not its mounting; traverse the nut from cant binding in the mounting system. align or preload any component. You one end to the other so it can seek its Several manufacturers now supply can get motorized versions, too. own center; then tighten the nut complete ball screw assemblies, in- Back driving. The ball screw as- mounting. This tends to alleviate in- cluding nut and screw properly sembly has the capability of either the

Selecting lubricants for ball screws

Lubricants maintain the low-fric- cosity is expressed in centistrokes (1 • Operating environment. tion advantage of ball-screw assem- cSt = 1 mm2/sec.) Various grades have • Load. blies by minimizing rolling resistance been selected for standardization • Speed. between balls and tracks and sliding (DIN 51512). • Judgments based on knowledge friction between adjacent balls. To determine the nominal viscosity of of the application. Proper lubrication helps keep most the oil for an application, establish the In addition: contaminants out, which reduces the mean speed of rotation of the ball screw • If the ball screw is horizontal, damage foreign matter can cause. and, from it, the limiting speed, dnm, fac- add nothing to the flow rate, Qmin, to Oil and grease provide corrosion tor. You also need the temperature at account for orientation; if vertical, protection, but lubricant choice de- which the ball nut is likely to stabilize. add 25%. pends on the advantages and disad- Mean speed of rotation accounts for • If the application is clean and vantages of each. Oil can be applied at the ball screw’s duty cycle: dry, add nothing to Qmin to account for a controlled flow rate directly to the environment; if not, add 25%. point of need. It will clean out moisture =÷+÷+ If the screw is not subject to high nnqm 11()100 nq 2() 2 100 • and other contaminants as it runs loads or speeds, add nothing to Qmin to ÷ + through the ball nut and provides cool- nq33()100 ... account for severe running conditions; ing. Oil disadvantages include: if it is, add 50%. • Possibility of excess oil contami- where: Grease lubrication. Grease is not nating the process. nm = mean speed, rpm so widely used as oil for ball-nut lubri- • Cost of a pump and metering sys- n1, 2, 3 ... = speed for time q1,2,3 ..., rpm cation, though it lubricates accept- tem to apply oil properly. q1,2,3 = time at speed n1,2,3 ..., % ably. Speeds that are high for ball Grease is less expensive than oil to of total screws are no problem for grease, so

apply and requires less frequent ap- For typical applications, nm ranges speed is no criterion for selection. plication, and it does not contaminate from 200 to 500 rpm. One problem with grease: It tends

process fluids. Grease disadvantages: The dnm factor is given by: to be fed out of the nut and onto the • It is hard to keep inside the ball ball screws, accumulating at the ex- = nut and has a tendency to build up at dnmm()( d n ) tremities of travel where it collects the ends of ball nut travel, where it ac- contaminants. It must be replenished cumulates chips and abrasive particles. where: regularly. • Incompatibility of old grease d = ball screw nominal diameter, Grease is a complex subject. with relubrication grease can create a mm Greases consist of a mineral or syn-

problem. Typical values of dnm range from thetic oil, additives, and a thickening Oil lubrication. Operating tem- 15,000 to 25,000 mm/min. Values of agent such as lithium, bentonite, alu- perature, load, and speed determine dn to 100,000, where n is the maxi- minum, and barium complexes. For the oil viscosity and application rate mum speed of rotation, are becoming most applications, use a grease with a for an installation. If the oil is too vis- more common, and in such cases the drop point above 220 C, a service tem- cous or if you use too much, heat may lower viscosity should be used if the perature range of -30 to 130 C, and a be generated. If the oil is too thin or oil selection guide indicates a grade limiting speed factor (dn) above you use too little, parts may not be midway between two adjacent viscos- 1,000,000. Such a grease is classified coated adequately; friction and wear ity curves. as HL91 Grade 2 (DIN 51818) and is may result. Ball nut operating temperature based on Mil-9-7711A. The following guidelines are appro- should be about 20 C, however, it usu- As a rule of thumb, replenish grease priate for most applications, but if ex- ally stabilizes a few degrees above at least every 800 hr. However, because tremes of temperature, load, or speed screw-shaft operating temperature. If conditions vary so widely, you should are involved, consult a lubrication you can’t measure nut temperature, confirm this interval by inspection, and specialist. assume it to be 30 C for your initial se- readjust if needed. For extreme condi- The recommended nominal viscos- lection of oil viscosity. tions, such as dn values above 50,000, ity of the oil at 40 C is based on the Required oil flow rate is a function consult a lubrication expert. mean speed of the ball screw, its di- of: Excerpted from an article by Thom- ameter, and the temperature at which • Number of ball circuits. son Industries Inc., in the February the ball nut is likely to stabilize. Vis- • Ball-screw orientation. 1995 issue of PTD.

1997 Power Transmission Design A277 nut or screw rotating when a thrust load is applied to the other member of the assembly. However, not all ball Figure 5—Typical planetary roller screw. screws can back drive. The thread’s In this style, roller helix angle determines if the assem- screws remain in bly can back drive. Generally, a ball constant contact with the screw with a helix angle of 6 deg or threaded haft. In another more will back drive; those of 4 to 6 style, roller screw deg are marginal; those under 4 deg recirculate into and out probably will not do so. Be aware that, of the load zone. in some situations in which you would not expect back driving, continuous machine vibration with the ball screw increased significantly. Roller screws nipulators, and movement of prisms unpowered and unrestrained might are more costly to produce than ball in laser measurement machines. cause slow back driving. screws and they are applied mostly These curved linear systems con- In many situations, you would not where application requirements of sist of slides or races, and rings (360 want the ball screw to back drive. For load-carrying capacity, axial stiffness, degrees of rotation) or segments of example, should power fail on a lift, it linear speed, or acceleration and decel- rings (90 or 180 degrees of rotation). could be disastrous for the load to run eration rates are especially stringent. One type of slide uses opposing fe- back. You must assure that either the Overall, roller screws are similar to male bearings with V-shaped outer ball screw cannot back drive or, if it ball screws in preload configuration, rollers in a two-and-two arrange- can, that you provide a holding means backlash, lost motion, left-hand and ment. The bearings ride on a track such as a spring-applied, electrically right-hand thread, back driving, effi- with matching V-shaped rails. A car- released brake to prevent screw rota- ciency, torque, and power require- riage plate on top of the two-and-two tion on power loss. ments. bearings is the mounting platform, Variations. Many variations in Figure 5 shows a planetary roller Figure 6. Thus, the carriage assembly ball screws and optional equipment screw. effectively runs on eight line-contact let you adapt them to special require- points on a track with varying circum- ments. For example, hollow screws Linear slides and races ferential diameters. are available for situations where low For a fixed segment of a ring, fixed system weight is important, such as Not all linear motion applications center carriage plates are the most in actuators on aircraft. consist of straight lines. Some appli- popular. A bogie carriage, Figure 7, is cations require an oc- used around S-bends, slideways with casional curve or the differing bend radii, and curves where circular motion of looseness in the movement between pure radial move- straight and curved sections is not de- ment, such as that sirable. The bogie carriage runs on found in tool changing swivel bearings, which operate on a mechanisms, mea- principal similar to that used in train surement of turbine and tram bogies to negotiate bends in blades, rotating ma- the track. Figure 4—Bidirectional 1-piece ball screw needs no joint to connect and synchronize Fixed center carriage with left and right-hand screws. two-and-two bearing support

Options in seals, wipers, mounting arrangements, housings, preload de- vices, and similar characteristics help also. And you can get specialty sys- tems such as the bidirectional 1-piece ball screw of Figure 4. It lets two nuts move in opposite directions simulta- neously, an advantage in applications such as clamping devices and robot- Opposing female ics. Also, a trunnion-mounted nut can V-shaped bearings be helpful in some applications; a tele- scoping screw in others. Track with male V-shaped rails Roller screws Figure 6 — V-ball bearing systems use opposing female bearings with V shaped outer The first roller screws appeared in rollers in a two-and-two bearing support arrangement. A fixed center carriage uses the the early 1950s. However, only in the two-and-two arrangement to support the mounting plate. Two of the V bearings have last decade or so has their popularity eccentric studs to facilitate adjustment.

A278 1997 Power Transmission Design ment). Axial run-out nears an extreme of horizontal crank is 5 microns. throw, its velocity vector is nearly For multiple car- parallel to your line of sight. The riage track systems, point seems nearly motionless, then the greatest re- motionless, then it reverses direction. Bogie carriage peatability error is If the crank turns at constant angular in the direction of velocity, that projected action of the travel and is depen- point is simple harmonic motion. dent on the play in Though simple harmonic motion is the drive mecha- fairly easy to produce, it has high lin- nism. However, it is ear acceleration at the extremes of possible to achieve travel. High acceleration means high repeatability with force—which generally means in- 0.2 mm with a belt- creased tendency for machine-part driven system. wear or breakage, and for workpiece or With all radial product damage. Cams, bar-type link- motion, engineers ages, and similar devices can modify must consider cen- crank-generated motions into profiles trifugal force. The of lesser peak acceleration. “Cycloidal force is proportional motion” is such a profile. to the square of the Moreover, modern electric motors tangential velocity. and their controls modify displace- Doubling the car- ment, velocity, and acceleration pro- riage speed quadru- files of mechanisms so that you can ples the force. It is readily get the best profiles for a pro- also inversely pro- cess without danger to components. portional to the ra- For example, a cam-and-screw Ring segment dius. Doubling the mechanism, Figure 8, adapts a con- radius halves the stant-lead cam and a ball spline to force. Often, this provide linear motion according to Figure 7 — A bogie carriage carries loads around S bends or force will also cause predetermined programs that the mo- slideways with differing bend radii. The bogie carriage runs on a moment load about tor and its control execute. The cam swivel bearings, which operate on a principal similar to that the carriage plate. mounts rigidly on two ball-spline used in train and tram bogies to negotiate bends in the track. Excerpted from an bushings that can traverse the length article by Bishop- of the spline shaft. The bushings are V-ball bearing track systems are Wisecarver Corp., in the August 1996 preloaded against each other to best suited to light loads — direct issue of PTD. prevent backlash. loads from 120 to 3,800 N (26.98 to The spline bushing can move axi- 854.38 lb) and moment loads from 0.6 Other mechanical actuators ally on ball tracks on the spline shaft, to 220 Nm (0.53 to 1,946.90 lb-in.) in a but it cannot rotate relative to the lubricated system. Refer to manufac- As listed earlier, there are many shaft. Thus if torque turns the spline turers’ tables for precise load han- linear actuators besides screws. shaft, the bushing turns with the dling capabilities. Among the oldest—still much used in shaft, causing the cam to rotate. A mo- Rings offer stability with support specific machines—is the slider-crank tor-and-reducer package turns the as near to the load as possible. A mechanism and its many variations. spline shaft, which therefore turns the gearcut rack on the outside or inside Perhaps in its eldest form it converted bushing and cam. The cam then acts diameter of the register face of the rotary to linear motion to drive pump much like it would in a typical rotary ring serves as the drive mechanism. pistons. Later versions became popu- index drive except that the traditional Linear slideways are available in lar converting linear motion to rotary roles of cam and cam follower are re- lengths to 4 m; for longer lengths, motion on steam-engine-driven rail- versed: An axially mobile cam engages slide segments are matched and but- road locomotives and paddlewheel- a straight-line array of stationary cam ted together. propelled ships. The simplest—the followers. The cam rotates through V-ball bearing systems can run dry classical Scotch yoke—converts ro- the row of followers to achieve linear or lubricated. Lubrication, through tary to linear motion in a characteris- actuation. The cam seemingly pulls it- lubricator blocks, can prolong system tic linear-motion profile called simple self along the single-file row of stand- life by as much as 150%. Every time a harmonic motion. Imagine looking ing followers, contacting at least two ring slide rotates it is wiped with oil, down at right angles to the crank’s at any instant to prevent backlash. which is also imparted to the female V axis and following the motion of a A carriage housing mounts on and of the bearing surfaces. point on its circumference. As the travels with the cam, but does not ro- For rings or segments of rings, it is point travels across the axis its entire tate. Tapered roller bearings let the possible to achieve circular motion with velocity vector is perpendicular to cam turn inside the housing but sup- radial run-out no greater than 0.05 mm your line of sight; the point seems to port thrust load only; two linear bearing per 360 deg (pro rata over angle of seg- be moving fastest. When the point systems over the actuator’s length carry

1997 Power Transmission Design A279 and continue to seal. Magnetically coupled rodless cylin- ders make slots and dynamic seals unnecessary; the piston couples mag- netically to the external carriage. Recent versions of these rodless cylinders can now handle tipping or transverse loads. Most vendors offer fully pre-engineered, out-of-the-box, bolt-it-down, hook-it-up systems. Op- tions include position sensors, posi- tion and velocity controls, end-of- stroke bumpers, shock absorbers, and other snubbing devices, brakes, ex- ternal and guides.

Figure 8—Cam-and-screw mechanism reverses usual roles of cam and followers. As reducer output turns ball spline, constant-lead rotary cam engages cam followers standing in a row. Positive cam pitch makes cam move axially, engaging succeeding followers and disengaging receding followers. the weight of the cam, carriage, and pusher-cleat attachments. Figure 10 — payload, and resist tipping moments. Many cable drives and Rodless cylinder. Standard leads for such systems power-transmission This is a band design can be up to five times more than roller chain drives which handles high, off- center loads. those of ball screws. work similarly with Other types of mechanical linear ac- attachments. Many tuation include belt, chain, and cable positioning tables, Electric linear motors drives in their linear travel paths. For single stages, and self- example, Figure 9 shows a linear actua- contained linear actuation An electric linear motor makes con- tor specifically for conveying (linearly systems use the reinforced synchronous version of rotary to linear motion un- pushing) products. The driving belt as the linear-motion element. necessary. Thus, linear motors can elements are wire-reinforced The popularity of rodless cylinders, eliminate shafts, belts, and to polyurethane synchronous belts with Figure 10, for linear motion applica- minimize space, energy, and costs. tions is increasing, For information on rotary motors, see especially in appli- the Motors Products Department in cations with space this handbook. restrictions. It can Linear motors generate force challenge mechani- rather than torque. Force to inertia cal and electrical ratio and stiffness — depending on actuators in many the type of linear motor — can be as uses, and product high as 30:1, and to 0.9 million N/mm variety has grown or 5 million lb/in., respectively. Some over the last few versions offer smooth motion to years. within a fraction of a percent of their The band cylin- nominal velocity, which can range be- der is a popular tween 1 micron/sec to over 5 m/sec. version of the rod- Others can operate continuously, pro- less cylinder. The vide 5 g (or higher) acceleration, and Figure 9—Wire-reinforced polyurethane cylinder wall has a slot running its offer a low settling time, in some synchronous belts with attachments full length. The slot lets the piston cases, less than 50 msec. provide linear motion in straight-line connect mechanically to an external Linear motors all have the same portion of belt travel. Various cleats, carriage. A dynamic strip-type seal basic structure. Imagine a rotary ac pushers, or lugs provide contact. Many over the slot’s outer surface and an- or dc motor that has been sliced along positioning tables or stages use such a device for linear motion. With other over its inner surface seal-in its axis and opened up flat, resulting attachments, roller chain drives and cable cylinder pressure and seal-out con- in two sections. One section, the pri- drives can work similarly. (Note: Not taminants. Carriage travel opens the mary, is a set of electrical coils embed- shown is the belt-driving mechanism, seal section beneath the carriage, but ded in a core (typically of steel, epoxy, which can be various devices, such as maintains the seal. As the carriage or aluminum). The structure of the .) passes, the seals reseat on the slot second section (the secondary) de-

A280 1997 Power Transmission Design pends on the type of linear motor. A Electric feedthrough Heater core typical air gap of 0.024 in. (0.6 mm) Polymer core Piston separates the two sections for non- Electric contact force transmission. wires While there are several types of lin- ear motors, many rely on the interac- tion of magnetic flux that produces forces on the moving and stationary members. For the voice-coil, dc force, and step-motor types, part of this flux comes from coil current. For the in- duction motor, ac excites a coil that produces flux. In turn, this flux inter- Figure 11 — Cross-section of a solid-state actuator. Heat expands a polymer, which pushes against a piston. acts with flux produced by induction (like a transformer) and generates a force proportional to the relative expands the polymer that pushes strengths and distribution of the in- against a piston, Figure 11. As teracting fluxes. the polymer cools, the piston re- Coil Voice-coil motor. Constructed tracts. like solenoids, these motors come in Actuation speed depends on Drive rod moving-coil and moving-magnet con- the heat sink temperature, ap-Coil figurations and produce more precise plied power, and applied load. motion than solenoids. Voice-coil mo- Polymers are available to react tors maintain high linearity between at temperatures from -50 to 625 Permanent magnet applied current and developed force; F. In some cases, the heat gener- operate efficiently; and, when built ated in the application can be Figure 12 — The actuator consists of a with high-energy magnet materials, used to activate the actuator.  develop high forces and acceleration Polymer expansion is directly pro- drive rod made of the magnetostrictive alloy Terfenol-D, copper wire coil, Alnico rates, to 100 g in some versions, and portional to the received electric power. permanent magnets, and a magnetic can oscillate at high frequencies. It can be precisely positioned anywhere return circuit housed in aluminum or However, as with a solenoid, the sim- in its range of travel by varying the stainless steel. ple mechanical structure precludes electric power from 60 to 200 W. De- long strokes. pending on heat sink temperature and Single-axis actuators. Another the weight of the load (minimum of 25 The rod responds to the application short-stroke linear device is the sin- lb), continuous power input is needed or withdrawal of current almost in- gle-axis actuator. Recent develop- to maintain a specific temperature for stantaneously. The expansion is pro- ments have focused on the actuation the piston to hold position. portional and repeatable. The driver that provides the linear mo- Cooling occurs when electrical amount of force that the actuator sup- tion. One driver is a polymer that ex- power is reduced or removed from the plies to linearly move an object de- pands under applied current. The device. The polymer is stiff, with a pends on the size of the rod. A 12-mm other driver is an alloy material that bulk modulus (hydraulic stiffness) of diam rod can exert at least 200 lb of expands when subjected to a mag- 1 million psi. force. A 75-mm diam rod can exert at netic field. The other new has a least 9,000 lb. Commercial versions The polymer-based actuator com- drive rod made of Terfenol-D, a mag- of this actuator have available dis- bines the small size of solenoids, the netostrictive metal alloy of terbium, placements in the thousandths of an forces of hydraulic cylinders (to 500 dysprosium, and iron. It uses magne- inch to over 2 in. lb), and the proportional control of tostriction, where an applied mag- Thus, these actuators are for appli- electric motors. One of its benefits is netic field causes the material to cations that need high speed and high that it lets engineers stay under the change its geometric dimensions. force, such as machining. 20-lb limit of traditional short-stroke The actuator has copper wire coil Linear induction motor. A lin- actuators without buying custom and permanent magnets housed in ear induction motor resembles a com- solenoids. These actuators can oper- aluminum or stainless steel, Figure mon rotary induction motor that has ate hydraulic valves and brakes, re- 12. The copper wire is wound around been split axially and rolled out flat. place a ball screw system, actuate the drive rod. When current of 1.4 to Its speed-thrust curve, Figure 13, re- controls in car engines, and control 3.4 A from an external power supply sembles the speed-torque curve of a robotic gripper manipulators. is applied to the coil, it creates a mag- standard ac NEMA Design B rotary Rather than magnetics, it uses a netic north-south orientation at the motor. In operation, the moving part thermally reactive polymer. Heat is molecular level. This new orientation of the motor lags its synchronous supplied by an electric heating ele- causes the drive rod to lengthen as speed to develop thrust. Also, speed ment that consists of an electrically the diameter shrinks. When current depends on power-supply frequency. and thermally conductive carbon- is removed, the rod returns to its orig- Efficiency is low except when the mo- fiber or silicon-carbide grid. The heat inal shape. tor operates near design rating.

1997 Power Transmission Design A281 as a servo system stationary; the primary, with the with a closed position moving part of the system. loop. Figure 14 shows The thrust produced in an ac linear such a servomotor motor is approximately proportional system in which the to the face area between the primary linear measuring sys- and secondary parts. A modification tem could be an en- of the single-coil primary system is a coder feeding position dual-coil system, in which a primary data back to an ac part mounts at either side of a flat vector drive. (For secondary. In effect, that doubles the more on vector drives, working face area and thus the thrust see the “AC vector” of a similarly sized single primary section in the Ad- system. In most applications, the dual justable Speed Drives primaries are fixed and the secondary Figure 13—Typical speed-thrust curves of Product Department in this hand- coil is with the moving part of the sys- linear induction, force, and step motors. book). tem. Thus, you would consider first a The linear induction motor in Fig- single-coil primary system for long- The primary is a wound structure, ure 14, sometimes called an asynchro- stroke applications; a dual-coil pri- much like a conventional motor sta- nous linear motor, is a single-coil mo- mary for shorter-stroke, higher- tor. The secondary is a metallic struc- tor — the primary part (similar to a thrust applications. ture, much like a rotor. Either struc- stator in a rotary motor) holds the Linear force motor. Like the lin- ture can be the moving part of a linear windings. The secondary part (like a ear induction motor, you can think of induction motor. rotor in a rotary ac motor) consists of a linear force motor as a conventional The motor can operate directly iron and short-circuit rods of copper dc motor that has been slit axially and from line current with a fixed speed or aluminum. The magnetic field in rolled out flat, Figure 15. It comes in thrust characteristic. It can also oper- the secondary is generated by the cur- moving-magnet, moving-winding, ate in an open loop from an ad- rent induced by the moving and alter- brush, and brushless types, and with justable-speed source for adjustable- nating magnetic field of the primary. various core materials. speed applications. And it can operate In most cases, the secondary coil is Brush-type motor is the least ex- pensive. Operating from direct cur- rent, each motor has a stationary coil assembly and moving magnets. The motor cable does not move in this con- figuration. Velocity goes to 1 m/sec; and acceleration to 0.5 g. Above these values there is excessive arcing and rapid deterioration of the brushes. These motors are excluded from clean room and vacuum applications. Brushless, aluminum-core linear mo- tor operates from three phase power, with a moving coil and cable. In applica- tions requiring short travel lengths, the coil can be stationary and the magnets moving. The core of the primary en- closes the windings in aluminum. There is no magnetic attraction between the two motor parts. Therefore, it may re- Figure 14—Typical ac linear servomotor. It could be powered by a vector drive that quire a double-sided magnet assembly receives position and speed information from the linear measuring system. to close the magnetic circuit effectively.

A282 1997 Power Transmission Design phase wound stator Figure 16 — The stator assembly (left) assembly, Figure 16. shows the windings that are sequentially The design helps its activated to move the armature (right). The Figure 15—Typical linear force motor. speed approach that of voice coil actu- actuator, including its own weight, can accelerate at a rate of 8,600 in./sec 2 when ators and is spatially efficient to ac- driving a load of 23 lb. The stator is 9.0 in. This type of linear motor generates commodate an increase in the amount OD ϫ 4.9 in. ID ϫ5 in. long. The armature smooth motion. of magnet material and coil windings is just under 4.9 in. in diameter ϫ 8 in. in Depending on the moving load, it can in the actuator. Minimum magnetic length. Armatures can be made to almost accelerate to 4 g’s, but can also develop circuit lengths and short air gaps aid any length to accommodate the needed eddy currents at speeds over 1 m/sec. It the efficiency and force capability. travel distance. is ideal for vacuum applications. The cylindrical design aids acceler- Brushless, epoxy-core linear motor ation, which ranges from 1,000 to is also non-ferrous with its epoxy- 1,500 ft/sec2. The working gap length, duce flux. The armature seeks a posi- based core. It too, provides smooth or circumference of the armature, is tion of least reluctance and attempts motion. It has an advantage over alu- long versus the amount of material in to remain in that position. The force minum in that it does not experience the armature. A 600 lb force output needed to move the armature, with eddy currents at high speed. It has version demonstrated speeds to 90 the coils off, is the detent force, which low stiffness (typically 35,000 N/mm) in./sec. The force/mass ratio was cannot be completely eliminated in at high coil temperatures (to 125 C) 1,360 ft/sec2. Maximum velocity is these motors. It can be varied over a and it tends to give off gases in high limited by the drive voltage available range of about 5 to 15% of the maxi- vacuum environments. and the travel distance, Figure 17. mum force capability. Brushless, steel-core linear motor These motors can be synchronized Low detent motors are available. uses steel lamination in the main with the electrical sequencing of the Reducing the detent involves the body of the primary to enclose the cop- field coils. This ability provides con- same techniques used in dc motors to per windings. This motor uses a sin- trolled velocity, acceleration, and po- reduce cogging, i.e. use of smaller gle-sided magnet assembly and is air sitioning without additional sensors. pitch for the coils and magnets, a or water cooled for high duty cycle ap- They achieve high detent forces larger pole width relative to pitch, a plications. It functions well in appli- through the permanent magnet de- larger air gap, or skewing the stator cations that require such duty cycles sign and very small air gaps. Detent poles relative to the armature. Gener- and velocities up to 200 ips (5 m/sec). forces are the result of changes in the ally, however, lowering the detent But this motor is subject to cogging. It reluctance of the magnetic circuit in force increases motor size and cost. can generate strong magnetic attrac- the linear motor. Permanent mag- These techniques also tend to reduce tion between the two motor parts, nets, even with the coils off, still pro- the force capability of the motor, re- which must be accounted for in the load carrying ca- 84 in. pacity of the bearing sys- 48 in. tem. Typical applications Linear motor Roller bearing are machine tools where Out stop assembly 2X high force, 5,000 lb (23,000 In position (in stop) Armature (piston) N), may be required, or in general automation where speed of several m/sec is needed, 120 to 200 ips (3 to ¯5.0 5 m/sec). 32 in. travel Moving-magnet linear ¯9.75 ¯9.0 Hall sensors motors. This motor is a special case of a linear force motor. It uses an uncon- ventional magnetic circuit Roller bearing assembly, rotated 90° in a cylindrical armature, similar to the shape of hydraulic and Figure 17 — This cylinder shaped linear actuator can develop 1,800 lb of force with a pneumatic actuators, and a three- travel of 32 in.

1997 Power Transmission Design A283 quiring the use of a larger motor for a equivalent rotary conversion systems. method is to provide not one flat sur- given force. • Long stroke. Travel length is lim- face, but two, butted at an angle to These motors are suited to applica- ited only by platen length — and in- each other to form one V-shaped way. tions requiring high acceleration, syn- creasing length does not lessen per- A companion V-shaped way mates chronized speed or acceleration, and formance. with it. A variation of this technique high detent forces and reliability. • Multiple motion. By overlapping uses a continuous V-shaped way with Linear step motor. This motor, trajectories, more than one forcer can companion wheels or sheaves with Figure 18, provides the same incre- operate on one platen at one time. mating circumferential cross sections. mental point-to-point precision as its They roll on and are guided by the rotating counterpart. BEARINGS way. The wheels, however, make such A linear step motor has a toothed, a system a rolling-element system. magnetic pole structure on the stator As with any other power transmis- Dovetail ways are another variation (platen) and on the slider (forcer). sion system, a linear motion system of the V-shaped way. Platen and forcer tooth structure al- must be supported and guided. Gen- By replacing the flat way with a most match. For example, the platen erally, moving parts exert some force, cylinder with axis parallel to the di- may have 11 teeth in the same length and the force must be resisted for the rection of motion and making the in which the forcer has 10. By sequen- system to remain stable. That is the companion moving piece a cylindrical tially energizing two coils that oper- chief reason for any bearing: It must rod, you create a sleeve bearing. Now, ate in conjunction with a permanent bear a load. For information on bear- however, you can use short, well- magnet (oriented parallel to the axis ings for rotary systems, see the Bear- aligned sleeve bearings in series to of motion), the step motor can be ings Product Department in this support the linear-motion device. made to move in one-quarter-tooth- handbook. Such a bearing looks like the sleeve pitch increments. You can get ex- Linear-motion bearings are of bearing used to support and guide ro- tremely fine resolution (to 25,000 many types, some much like rotary tary motion. However, it supports ax- steps/in.) with microstep controls. bearings. One of the most common ial motion, and the mechanics of mo- tion may differ. A major difference: the hydrodynamic oil-film “wedge” that develops between a rotating shaft and a sleeve bearing above a certain minimum rotary speed isn’t there when the shaft moves axially. Well-lubricated linear-motion sleeve bearings can serve well at low speed. Chief among the differences in lin- ear sleeve bearings are type of bear- ing material. Common bearing bronzes are often used, and so are graphites. So, too, are metal sleeve Figure 18—Linear step motor works on bearings with solid-lubricant inserts. same principle as rotary step motor. Here, ways to classify bearings is by type of Another type is the ceramic linear four sets of teeth on forcer are spaced in bearing-to-load contact: bearing—a metal sleeve coated with a quadrature so only one set at a time lines Plain bearings. Surfaces slide on hard ceramic. Solid plastic or metal- up with any set of platen teeth. • each other or on a lubricant film be- backed plastic sleeve bearings are tween them. also in common linear-bearing use. Linear step motors are well-suited • Rolling-element bearings. Ele- Rolling-element bearings. You for positioning applications requiring ments such as balls or rollers between can gain the advantage of lower high acceleration and high-speed, moving surfaces provide the lower re- rolling friction and, thus, higher low-mass moves. Motor systems offer: sistance of rolling friction instead of speed capacity by substituting • High throughput. Linear step sliding friction. rolling-element linear bearings for motors are capable of speeds to 100 Plain bearings. The simplest lin- plain linear bearings, much like you ips, and low forcer mass allows fast ear-motion plain bearing to visualize can with rotary bearings. For exam- acceleration. is the flat way, perhaps the oldest de- ple, you can put rolling elements be- • Mechanical simplicity. vice that lets one , tween the simple flat-way plain mat- • Reliability. Few moving parts such as the bed of a planer, move eas- ing surfaces to reduce friction. With and in some models, air bearings, ily on another. However, it is also dif- proper restraining systems, the ele- make for long life and low mainte- ficult to make well, because nearly ments could be balls or rollers. Like- nance compared with rotary systems. perfect flatness is hard to maintain wise for more complex systems such • Precise open-loop operation. Lin- over a long distance. Early machine as V and dovetail ways. Ball, roller, ear systems allow open-loop unidirec- ways were hand-scraped by crafts- and crossed-roller systems are in ser- tional repeatability to 1 micron men to remove high spots. The flat vice there. Also, the flat way bearing (0.00004 in.). way must also have a means to keep can become more like a box beam with • Small work envelope. Most linear the payload from running off due to cam-follower bearings or similar step motors need less space than any transverse load. A common bearings on two or more sides to guide

A284 1997 Power Transmission Design Perhaps the plain bearing. simplest An example of a more complex rolling-ele- rolling element linear bearing is a ment linear “smart” bearing, Figure 21. The bearing is the smart designation comes from this linear ball bearing’s ability to recognize types of bearing. Fig- misalignment and to self-align ures 19 and 20 around any or all of three axes inde- show a basic pendently. It can handle load capaci- arrangement, ties without giving up the ability to with recircu- absorb torsional misalignment with lating balls no increase in stress levels. Figure 19—Principle of recirculating that travel an oblong path. A linear Each double-track bearing plate in- linear ball bearing. ball bearing has

Machine housing Bearing plate

Ring

Balls

Shafting surface

Figure 22 — Schematic axial cross section shows how bearing plate can self-align (“pitch”) about curved inner surface of Figure 20—Typical linear ball bearing. hardened ring. Retainer guides balls and keeps them from three or more ob- dropping out should bearing be removed long circuits of from shaft. balls. Each cir- Ring inner surface cuit holds balls in Load Double track & balls a carriage along the length of the one of it straight beam. All these devices are used suc- sides in rolling cessfully in various applications, de- contact between pending on precision, complexity, and the shaft and the Bearing cost constraints. bearing race. The plate load rolls along the Recirculation tracks balls in this part of the Ring side of circuit. Balls bearing plate in the rest of the circuit re- circulate Figure 23— Double-track bearing plates have ring-side curvature freely in the that lets them self-align (“roll”) to distribute load evenly on their clearance in ball tracks. the sleeve. A retainer Center of rotation for bearing plate Bearing plate guides the balls and keeps them from falling Ring out if the bearing is re- Shaft side of bearing plate moved from with double load track the shaft. Figure 21— A transparent version of the The retainer Super Smart Ball Bushing bearing shows makes the linear provisions for high capacity and for pitch, ball bearing as roll, and yaw. Steady-state speed can be easy to handle up to 3 m/sec; coefficient of friction, down Figure 24 — Each bearing plate can turn about its geometric plan and install as a to 0.001. center to prevent skewing relative to shaft surface (“yaw”).

1997 Power Transmission Design A285 25

26 dividually compensates for three bearing is forced into types of misalignment: an interference-fit • It compensates for shaft angular housing. deflection or misaligned housing bore, Some linear ball or pitch, Figure 22. bearings have “rolling • It evenly distributes load on its keyways.” Balls roll in two ball tracks, compensating for roll, a semicircular-cross- Figure 23. section axial groove to • It rotates on a radial axis to elim- prevent rotation of the inate skewing between ball tracks entire bearing unit on and shaft, or yaw, Figure 24. the shaft. This is Self-alignment minimizes friction, much like a ball which holds performance and bearing spline, but with only life and simplifies installation. one ball channel. Life expectancy and shaft hardness A variation of the influence load capacity of a linear Figure 27 — Cylindrical rollers provide bearing-and-shaft combination. Cir- higher load capacity than balls, because cumferential positioning of load-car- they are in theoretical line contact instead rying working tracks relative to ap- of point contact. Linear roller way here is plied-load direction is important. Life a rolling guide unit. Cylindrical rollers run on a track rail to get endless linear motion expectancy (travel life) is expressed while circulating in a slide unit. as total inches of linear movement be- tween bearing and shaft. Manufactur- ers’ catalogs give normal and maxi- mum rolling load ratings. Generally, linear ball bearing is the linear-and- they are based on travel life of 2 mil- rotary unit, made up of a sleeve that lion in. Table 2 and Figures 25 and 26 holds a cylindrical array of balls in show a way to compute expected life. contact between the shaft and hous- A linear ball bearing may form a ing. It permits individual or simulta- continuous cylinder around a shaft, or Figure 26 — Load correction factor for neous linear and rotary movement— it may have an axial split that lets it linear ball bearings based on shaft a combination needed in many be preloaded on the shaft when the hardness. applications. Precise, high-load, linear travel can be gained in many applications by lin- Figure 25 — ear roller bearings instead of ball Load bearings. Rollers pass over a flat correction channel, rolling in line contact be- factor for tween channel and load. At the end of linear ball the channel, rollers recirculate, run- bearings based on ning unloaded behind the channel to required return between channel and load. travel life. Roller skewing could cause insta- bility and high friction. To prevent it, precise roller end guides, central sta-

A286 1997 Power Transmission Design bilizer bands, or similar means are and resolution of the position-mea- provides position and speed informa- provided. suring equipment. For example, with tion. Criteria for high accuracy are Dynamic load capacity of linear the ac linear servomotor shown in straightness and evenness of rack and roller bearings is generally quoted in Figure 14, it is possible to position pinion. terms of L10 life for a given travel life, within Ϯ1 increment of the encoder Interferential optical encoder sys- such as 10 million in. system. tems are for applications demanding The linear measuring system very high accuracy and resolution. CONTROLS shown in Figure 14 is generic. It could Laser interferometer measuring sys- serve many kinds of linear motion tems can offer resolution of 0.01 ␮m. Some sensing devices, such as prox- systems, and it could be: Common applications are inspection imity switches, work equally well for • An optical linear encoder. Here a machines, extreme-precision machine rotary or linear systems. Others, such scanning unit consists of a light tools, and wafer-slicing machines for as linear variable differential trans- source, photovoltaic cells, condensing semiconductor manufacturing. These formers, optical linear encoders, and lens, and grating reticule. On a linear devices require special consideration of force transducers, are especially for motor, it usually mounts on the pri- the lasers’ environment, such as air linear application. mary unit. It moves relative to a lin- temperature, pressure, and cleanliness. Much control technology applicable ear scale with line grating, producing A growing tendency among linear- to rotary motion systems applies as sinusoidal signals. The controller motion-device suppliers is to offer well to linear systems. Digital micro- counts the resulting signals to estab- complete systems as well as compo- processing allows it. For example, lish position and speed. nents for linear motion. You can get programmable controls and computer • An encoder system using cable some systems that include the actua- software simplify motion control and and pulley. With an industrial rotary tor, its bearings, framework, drive permit simultaneously controlled mo- encoder having a pulley on its shaft and control, and sensors for control tion in many degrees of freedom. A and a tension-controlled cable, you and safety limits. You need only bolt case in point is a multiaxis position- can sense position and speed. the system down and wire it up. Little ing system that includes rotary as • Rack-and-pinion encoder system. or no adjustment or tuning may be well as linear movement. Here, a gear rack transmits linear needed. In many complex linear motion sys- movement. A pinion rotates as its For more about control compo- tems today, quality of movement de- meshing rack moves. A rotary en- nents, see the Controls and Sensors pends essentially on system stiffness coder couples to the pinion shaft and Department in this handbook. ■

1997 Power Transmission Design A287