Clutches and Brakes
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CLUTCHES AND BRAKES eys to effective con- FRICTION TYPES ...............................................A129 clutching and braking func- trol and transmis- ELECTROMAGNETIC TYPES ..............................A130 tions in one unit, Figure l. K sion of drive MECHANICAL LOCKUP TYPES ..........................A131 It has a friction plate and torque, speed, and power in OIL-SHEAR TYPES.............................................A132 disc combination for each many rotating drive sys- METHODS OF ACTUATION ................................A134 function. As with many tems are clutches and CLUTCHES AND BRAKES ADVERTISING...........A136 other types of clutch or brakes. Their function is to brake, various mounting either transfer torque from arrangements are possible an input shaft to an output for clutch-brake units, and shaft (clutching) or to stop and hold a or stop a load. One surface is metallic, so are various means of actuation. load (braking). Though offered as sep- generally cast iron and shaped into a In more complex designs, clutches arate components, their functions are disc, band, or drum. The other surface and brakes use multiple discs and often combined into a single unit re- has a friction facing made from molded friction plates to increase the working ferred to as a clutch-brake. organic material, and held together by (friction) surface, Figure 2. Often the Clutches and brakes can be catego- a heat-cured resin binder — usually a discs are submersed in oil to extend rized by the technique used to engage plate, shoe, or pad. Friction facings can friction component life and increase or stop the load (friction, electromag- be made from any of several types of cooling efficiency. netic, mechanical lockup), and by the material, depending on application re- Some disc brakes use calipers in method used to actuate them (me- quirements. Often, brass or aluminum place of a friction plate, Figure 3. An chanical, electric, pneumatic, hy- chips are included to extend life and advantage to this design is that addi- draulic, self-activating) . improve heat dissipation. tional calipers can be added to in- crease braking torque. Also, brake FRICTION Disc pads are easily serviced. The most popular type of clutch or In simplest form, a disc-type clutch Drum brake uses the friction developed be- or brake has a single friction plate tween two mating surfaces to engage and disc. A clutch-brake combines Drum clutches and brakes, Figure Brake armature Brake magnetic coil Clutch magnetic coil Rotor Clutch armature Hub Output shaft Figure 1 — NEMA C-face (left) and foot-mounted (right) electric clutch-brakes. To start, current applied to clutch coil generates magnetic flux that clamps armature to rotor, causing it to rotate along with attached hub and output shaft. To stop, current is removed from clutch coil and applied to brake coil. This clamps brake armature to magnet, which is fixed to housing, thereby stopping the load. 1998 PT Design A129 Figure 2 — Typical wet-disc clutch-brake with multiple discs. sively used in modern power trans- mission systems. ELECTROMAGNETIC Three types of nonfriction electric clutches and brakes are available: magnetic particle, eddy current, and hysteresis. Used primarily in applica- tions that require variable slip, elec- tromagnetic clutches and brakes use electromagnetic attraction rather than friction to perform their function. Magnetic particle The operating principles of mag- netic particle clutches are illustrated in Figure 6. The space between the in- put and output members is filled with Figure 4 — Drum type clutch. a mixture of dry iron particles. When the coil is energized, the magnetic Figure 3 — Caliper type disc brake. flux lines span this space and line up the particles on the magnetic flux 4, have cylindrical friction surfaces slightly tapered and coaxial with the lines. This results in the magnetic with a common axis (the shaft) on shaft, and they are engaged in an ax- particle chains locking the input and which the unit is mounted. Drum ial direction. In this respect, they are output members together, causing units are either constricting or ex- a cross between drum and disc types. them to rotate as a single unit. The panding types; that is, the drum is Cone clutches have light engage- amount of particle bonding deter- contacted on either its outside or in- ment forces and high power transmit- mines the amount of torque that can side diameter to force engagement. ting capabilities, but are difficult to be transmitted and is directly propor- Drum clutches and brakes wear disengage. Cone units are not exten- tional to the current flowing to the ro- evenly and transmit high torque. The tor. Torque can, contracting type responds especially therefore, be ad- fast because centrifugal force helps justed by varying withdraw the shoes rapidly, thus the amount of cur- making them well suited for high rent flowing to the cyclic operations. coil. Magnetic par- Cone Cone clutches and brakes, Figure 5, Figure 5 — have friction surfaces that are Cone type clutch. A136 1997 Power Transmission Design Hysteresis Figure 9 — Square jaw clutch. These are con- stant torque devices consists of square teeth that lock into that can be used to mating recesses in facing members. It Figure 6 — Magnetic particle clutch. provide any amount of slip, as long as provides positive lockup, but because heat dissipation capacity of the unit is it cannot slip, running engagement is ticle clutches and brakes are useful in not exceeded. Hysteresis losses trans- limited to speeds under 10 rpm. Sizes tensioning and positioning applica- mit torque in this type of clutch. A coil accommodating from 1 to 260 hp per tions where continuous changes of on the input rotor generates a mag- 100 rpm are available. speed are required. netic field in the rotor and drag cup, Figure 8. The hysteresis losses in the Spiral jaw Eddy current drag cup cause the flux to change more slowly through the cup than the Spiral jaw clutches use sloping en- Primarily used in variable speed rotor; thus torque is transmitted gagement surfaces to overcome the devices, these clutches and brakes through the drag cup. Hysteresis limited engagement speeds of square cannot be operated at zero slip. They brakes provide constant torque for a jaw types. Though the smooth, sloped consist primarily of an input drum, given control current. Torque is inde- engagement surfaces allow for en- stationary field coil, and a coupling pendent of speed up to high speeds. gagement speeds of up to 150 rpm, pole assembly that acts as an output Used primarily in fractional horse- they can operate in only one direction, rotor, Figure 7. When the field coil is power applications, these brakes ex- and have a tendency to freewheel. energized, magnetic flux links the in- hibit virtually no wear and almost un- put drum with the coupling pole as- limited life. Multi-tooth sembly. Eddy currents, developed when the input drum rotates, create a MECHANICAL LOCKUP These clutches offer the advan- new magnetic field that interacts tages of mechanical lockup clutches with the field in the pole assembly, A direct mechanical connection be- combined with the advantages of elec- creating coupling torque proportional tween input and output components is tric, pneumatic, or hydraulic actua- to coil current. used by mechanical lockup clutches to tion. Running engagement speeds of At zero slip, the eddy current brake transmit torque. Many use centrifugal up to 300 rpm are possible. They are has no torque, thus it cannot be used force, a wedging action, or wrapping ac- available in capacities to 300 hp per where holding a load is required. They tion to lock the input component to the 100 rpm, Figure 10. are useful for providing drag loads output component, and are often re- needed in applications such as ten- ferred to as self-activating types. Speed, Sprag sioning. difference in speed between input and output members, or The typical sprag type clutch has direction of rotation cylindrical inner and outer races, are used to engage with sprags filling the space in be- the torque transmit- tween, Figure 11. The sprags are ting components. An- sized, shaped, and mounted in a man- other type of mechan- ner that assures they will wedge be- ical lockup clutch, the tween the two races when rotation oc- multi-tooth design, curs in the correct direction. uses electric, pneu- matic, or hydraulic Wrap spring actuation. In wrap spring clutches, the input Square jaw shaft and output shaft are connected by a coiled spring whose inside diame- Figure 7 — Eddy current Figure 8 — Hysteresis clutch. The square jaw ter is smaller than the outside diame- clutch. clutch, Figure 9, ter of the two shaft hubs, Figure 12. 1997 Power Transmission Design A137 sleeve to the intermit- ment. Wear is greatly reduced by the tently driven hub. When oil film, which lubricates while trans- the clutch is disengaged, mitting most of the dynamic torque of the roll cage forces the engagement. rolls down the ramps The heat of engagement is gener- away from the sleeve. ated in the oil film rather than on Roll-cage position is con- working surfaces of the rotating ele- trolled by an external ments, so the heat can be readily re- trip cam and trip lever, moved and the drive’s thermal capac- which may be actuated ity is high. Also, the oil absorbs the manually or by electric, shock of engagement as the film is hydraulic, or pneumatic squeezed between discs and plates, solenoid. thus reducing drive-train stress. Small-diameter, multiple-disc designs OIL-SHEAR produce high torque-to-inertia ratios. These characteristics suit oil-shear CLUTCHES AND drives to applications ranging from BRAKES high inertia to high cycle. With ordinary control logic, drives handle 50 to 100 cy- Figure 10 — Multi-tooth clutch.