Linear motion

Separating rack and

This ATLANTA drive system consists of a myths planetary reducer, ISO flanged from reality pinion, and rack.

Rack and pinion drives are John Entwistle an age-old mechanism Atlanta Drive Systems Inc. Farmingdale, N.J. widely used for converting rotary motion into linear motion. However, myths Myth: Rack and are old technology that only offers Myth: Rack and pinions have low precision. too much backlash and the only and misconceptions n fact, rack and pinions are an old technology, but that isn’t a draw- way to reduce or remove it is by abound regarding the use Iback. First, not all applications require high precision; material- pushing the pinion into the rack. handling applications, for example, can have very low accuracy require- he reality is that rack and pin- TELESCOPIC ACTUATORS and application of rack ments. Tions have a theoretical center rugged transfer mechanism works even in harsh environments Second, advances in production techniques (milling, grinding, distance at which optimal meshing and pinions in this age of and heat treatment, for example) have dramatically improved the preci- occurs. By pushing the pinion into servo-driven machinery. sion and load carrying capacity of rack and pinion drives, to the point the rack, one can reduce the backlash that they are a competitive component for any linear-axis drive applica- level — but this also degrades tooth Here, we debunk some of tion. meshing, which can cause excessive those fallacies. Even rack and pinion drives with low (or zero) backlash and low (or wear and binding. zero) pitch error are possible. The best way to reduce the back- lash of a rack and Rack and pinion drive is to pinion drives increase the quality are suitable of the teeth, as well for long travel as their mounting applications, accuracy. such as this traveling gantry To completely milling remove the back- from Zayer. lash of a rack and pinion drive, a RollBeam split-pinion or du- push-pull unit al-pinion drive can loads up to 400 tons be used, where one speed up to 6.5 ft/sec pinion drives while practically unlimited stroke the other removes Ph # 800-663-4514 · [email protected] · www.serapid.us the backlash.

14 NOVEMBER 2011 MOTION SYSTEM DESIGN • motionsystemdesign.com NOVEMBER 2011 MOTION SYSTEM DESIGN • motionsystemdesign.com 15 A split pinion can be Linear motion used to eliminate axis backlash and provide precise positioning and repeatability. During assembly, errors can be in- duced into the drive where the rack sections butt together. However, using a “companion” rack, which meshes between the two rack sec- orienting the rack teeth on their side tions, can minimize this error. Oth- or facing down can minimize this. erwise, perfect rack joints with no Linear bellows or covers can also be error can be achieved by using rack used to protect the drive. assembly kits, which allow for joints By using an automatic lubrication to be precisely measured and any er- system with a felt gear applicator, ror eliminated. any debris on the tooth flanks can be wiped away so damage is pre- Myth: Rack and pinions have vented. Likewise, for washdown or pitch errors that can accumu- wet environments, using stainless- late over a long travel length. Myth: Rack and steel rack and pinions prevents cor- or long axis drives, multiple pinions can’t rosion. pieces of rack may be mount- An automatic F lubrication system be used in dirty ed end-to-end to achieve the desired using a felt gear environments. Myth: Rack and pinions can travel length. Although each piece applicator provides ack and pinions are have excessive errors at of rack has an overall pitch error continuous Rconsidered open the joint between two rack associated with it, this error can be lubrication of the gearing because they are not sections. controlled or even eliminated so rack and pinion tooth flanks. enclosed in a housing, so foreign n reality, to achieve the desired that there is no accumulated error MSD_AAGortite_quarterpage_3.25x4.625.pdfdebris 1 can 1/21/11 find its 1:38 way PM into the tooth Itravel length, multiple pieces of over the full travel length. SERAPID Inc, ACTUATORS / LB 09-10 LINEA · TEXT + DESIGN Sonntag, 12. September 2010 16:50:53 mesh and cause damage. However, rack may be mounted end-to-end. Depending on the quality of the Farbprofil: Deaktiviert rack teeth, the pitch error can be Composite Standardbildschirm between 0.012 and 0.200 mm per meter length. It is also possible to DYNAMIC SOLUTIONS mount racks in a predefined order CUSTOM ENGINEERED to always have the total pitch error100 100 FOR YOUR APPLICATION hover around zero. A&A Manufacturing Company 95 95 is your single-source provider Myth: Split-pinions use of innovative protective cover 75 75 products for: springs to remove the mesh backlash, which can deflect s Ball screws while operating. s Precision slides ctually, split-pinions con25 - 25 s Robots sist of two pinion halves — A 5 5 s Linear rails & motors one which is fixed, and one which s Machine ways is axially spring-loaded. Before0 0 operating, the split-pinion is The widest variety of 100 100 technologies and materials first adjusted to remove the mesh are used to protect in any backlash; it is then preloaded95 to 95 environmental condition. From 0.396” to 26” Diameters compress the spring up to the op- A&A can design and deliver erating load of the application.75 75 exactly what you need to keep Stainless Steel, Inconel, your equipment in motion. Preloading the spring ensures Titanium, and Hastelloy that the spring will not deflect materials under load during operation.25 In 25 Custom End Pieces fact, when the preload is set prop- erly, the spring will not move at5 all 5 A Division of A&A Manufacturing Two-Ply Construction Available during operation. The only way www.Gortite.com 0 0 Email: [email protected] the spring could deflect further is Toll Free: 800-298-2066 if the operating load exceeds the Fax: 262-786-3280 preload setting. 1

16 NOVEMBER 2011 MOTION SYSTEM DESIGN • motionsystemdesign.com NOVEMBER 2011 MOTION SYSTEM DESIGN • motionsystemdesign.com 17 Linear motion

A helical rack and pinion drive from ATLANTA is suitable for myriad linear-motion applications.

Myth: Rack and pinions can’t excessive, the roller-pinion can de- provide the same performance velop vibration and noise, and lead as ballscrews and roller-pin- to premature failure. ion drives. As mentioned, rack and pinion n fact, ballscrews often provide drives can achieve zero backlash by Iadequate performance for axis use of a split-pinion or dual-pinion drives requiring precise positioning to preload the axis. The preload is and repeatability — by having low set either mechanically (split-pin- backlash and pitch error. Where ion) or electrically (dual-pinion) and required, zero backlash can be is always present, independent of the achieved by using preloaded nuts. mounting accuracy of the rack. In Rack and pinions can also achieve addition, for high-load applications, precise positioning and repeatabil- rack and pinion drives can outper- ity. By using hardened and ground form roller-pinion drives, as the lat- tooth flanks, low backlash and pitch ter can exhibit lower load capacity error can be achieved; zero backlash and stiffness. can be achieved by using split-pin- ion or dual-pinion drives. For long and heavy axis drives, Myth: Rack and pinion uses such as those found in the machine are limited. tool and aerospace industries, rack o — rack and pinions can and pinions can actually outper- Nbe used on any machine re- form ballscrews, because the travel quiring conversion of rotary motion lengths of the latter can be limited to linear motion. From simple, low- by their buckling strength and whip. accuracy applications to the most For high-speed applications, rack demanding high-precision tasks, and pinion drives can also handle they can be selected and applied higher linear speeds (up to six me- to achieve the desired axis perfor- ters per second) than ballscrews. mance without compromises. Roller-pinions are a system that Typical applications for rack and uses rollers instead of teeth to en- pinions include, but are not limited gage a rack. These drives are touted to: CNC routers, pick-and-place for zero backlash because multiple robots, traveling gantries, mate- rollers are engaged at one time. rial handling, automation, machine However, these drives require pre- tool, aerospace, woodworking, loading of the rollers into the rack to stir friction welding, carbon fiber ensure optimal meshing. This pre- placement and seventh-axis robotic loading can fluctuate up and down slides. depending on the rack’s mounting accuracy. If the preload is lost, back- For more information, call (800) 505- lash can develop; if the preload is 1715 or visit atlantadrives.com.

18 NOVEMBER 2011 MOTION SYSTEM DESIGN • www.motionsystemdesign.com