Paul E. Sandin - Robot Mechanisms and Mechanical Devices Illustrated (779750), страница 24
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The coupler simply has differentends to accept the shafts it is coupling.Solid couplers are very simple devices. They clamp onto each shafttight enough to transmit the torque from one shaft onto the other. Theshafts styles in each end of the coupler can be the same or different. Forshaped shafts, the coupler need only have the same shape and size as theshaft and bolts or other clamping system to hold the coupler to the shaft.For smooth shafts, the coupler must clamp to the shaft tight enough totransmit the torque through friction with the shaft surface. This requiresvery high clamping forces, but is a common method because it requiresno machining of the shafts.As for online sources of couplers, and more detailed informationabout torque carrying ability, check out these web sites:•••••powertransmission.comdodge-pt.comflexibleshaftcouplings.comwmberg.commcmastercarr.com109110Chapter 3Direct Power Transfer DevicesMETHODS FOR COUPLINGROTATING SHAFTSMethods for coupling rotating shafts vary from simple bolted flange assembles to complex spring and synthetic rubber assembles.
Those includingchain belts, splines, bands, and rollers are shown here.Figure 3-1Figure 3-2Figure 3-3Figure 3-4Chapter 3Figure 3-5Figure 3-6Figure 3-7Figure 3-8Direct Power Transfer Devices111112Chapter 3Direct Power Transfer DevicesFigure 3-10Figure 3-11Figure 3-9Figure 3-12Figure 3-13Chapter 3Direct Power Transfer DevicesShaft couplings that include internal and external gears, balls, pins, andnonmetallic parts to transmit torque are shown here.Figure 3-14Figure 3-15Figure 3-16Figure 3-17Figure 3-18Figure 3-17113114Chapter 3Direct Power Transfer DevicesTEN UNIVERSAL SHAFT COUPLINGSHooke’s JointsThe commonest form of a universal coupling is a Hooke’s joint. It cantransmit torque efficiently up to a maximum shaft alignment angle ofabout 36°.
At slow speeds, on hand-operated mechanisms, the permissible angle can reach 45°. The simplest arrangement for a Hooke’s joint istwo forked shaft-ends coupled by a cross-shaped piece. There are manyvariations and a few of them are included here.Figure 3-20 The Hooke’s jointcan transmit heavy loads. Antifriction bearings are a refinementoften used.Figure 3-21 A pinned sphereshaft coupling replaces a crosspiece.
The result is a more compact joint.Figure 3-22 A grooved-spherejoint is a modification of a pinnedsphere. Torques on fasteningsleeves are bent over the sphereon the assembly. Greater slidingcontact of the torques in groovesmakes simple lubrication essentialat high torques and alignmentangles.Chapter 3Direct Power Transfer Devices115Figure 3-23 A pinned-sleeveshaft-coupling is fastened to onesaft that engages the forked,spherical end on the other shaftto provide a joint which alsoallows for axial shaft movement.In this example, however, theangle between shafts must besmall.
Also, the joint is only suitable for low torques.Constant-Velocity CouplingsThe disadvantages of a single Hooke’s joint is that the velocity of thedriven shaft varies. Its maximum velocity can be found by multiplyingdriving-shaft speed by the secant of the shaft angle; for minimum speed,multiply by the cosine. An example of speed variation: a driving shaft rotates at 100 rpm; the angle between the shafts is 20°. The minimum output is 100 × 0.9397, which equals 93.9 rpm; the maximum output is1.0642 × 100, or 106.4 rpm. Thus, the difference is 12.43 rpm.
When output speed is high, output torque is low, and vice versa. This is an objectionable feature in some mechanisms. However, two universal joints connected by an intermediate shaft solve this speed-torque objection.This single constant-velocity coupling is based on the principle(Figure 3-25) that the contact point of the two members must always lieon the homokinetic plane. Their rotation speed will then always be equalbecause the radius to the contact point of each member will always beequal. Such simple couplings are ideal for toys, instruments, and otherlight-duty mechanisms.
For heavy duty, such as the front-wheel drives ofFigure 3-24 A constant-velocityjoint is made by coupling twoHooke’s joints. They must haveequal input and output angles towork correctly. Also, the forksmust be assembled so that theywill always be in the same plane.The shaft-alignment angle can bedouble that for a single joint.116Chapter 3Direct Power Transfer Devicesmilitary vehicles, a more complex coupling is shown diagrammaticallyin Figire 3-26A. It has two joints close-coupled with a sliding memberbetween them. The exploded view (Figure 3-26B) shows these members.There are other designs for heavy-duty universal couplings; one, knownas the Rzeppa, consists of a cage that keeps six balls in the homokineticplane at all times.
Another constant-velocity joint, the Bendix-Weiss,also incorporates balls.Figure 3-25Figure 3-26Figure 3-27 This flexible shaft permits any shaft angle. Theseshafts, if long, should be supported to prevent backlash andcoiling.Figure 3-28 This pump-type coupling has the reciprocatingaction of sliding rods that can drive pistons in cylinders.Figure 3-29 This light-duty coupling is ideal for many simple, low-cost mechanisms. The sliding swivel-rod must bekept well lubricated at all times.Chapter 3Direct Power Transfer Devices117COUPLING OF PARALLEL SHAFTSFigure 3-30 One method of coupling shaftsmakes use of gears that can replace chains,pulleys, and friction drives. Its major limitationis the need for adequate center distance.However, an idler can be used for close centers, as shown.
This can be a plain pinion oran internal gear. Transmission is at a constantvelocity and there is axial freedom.Figure 3-31 This coupling consists of twouniversal joints and a short shaft. Velocitytransmission is constant between the inputand output shafts if the shafts remain paralleland if the end yokes are arranged symmetrically. The velocity of the central shaft fluctuates during rotation, but high speed and wideangles can cause vibration.
The shaft offsetcan be varied, but axial freedom requires thatone shaft be spline mounted.Figure 3-32 This crossed-axis yoke couplingis a variation of the mechanism shown in Fig.2. Each shaft has a yoke connected so that itcan slide along the arms of a rigid cross member. Transmission is at a constant velocity, butthe shafts must remain parallel, although theoffset can vary. There is no axial freedom. Thecentral cross member describes a circle and isthus subjected to centrifugal loads.Figure 3-33 This Oldham coupling providesmotion at a constant velocity as its centralmember describes a circle. The shaft offsetcan vary, but the shafts must remain parallel.A small amount of axial freedom is possible.A tilt in the central member can occurbecause of the offset of the slots.
This can beeliminated by enlarging its diameter andmilling the slots in the same transverse plane.118Chapter 3Direct Power Transfer DevicesTEN DIFFERENT SPLINED CONNECTIONSCylindrical SplinesFigure 3-34 Sqrare Splines make simpleconnections. They are used mainly for transmitting light loads, where accurate positioning is not critical. This spline is commonlyused on machine tools; a cap screw isrequired to hold the enveloping member.Figure 3-35 Serrations of small size areused mostly for transmitting light loads.
Thisshaft forced into a hole of softer materialmakes an inexpensive connection. Originallystraight-sided and limited to small pitches,45º serrations have been standardized (SAE)with large pitches up to 10 in. dia. For tightfits, the serrations are tapered.Figure 3-36 Straight-Sided splines havebeen widely used in the automotive field.Such splines are often used for sliding members. The sharp corner at the root limits thetorque capacity to pressures of approximately 1,000 psi on the spline projectedarea.
For different applications, tooth heightis altered, as shown in the table above.Chapter 3Direct Power Transfer Devices119Figure 3-37 Machine-Tool splines havewide gaps between splines to permit accurate cylindrical grinding of the lands—for precise positioning. Internal parts can be groundreadily so that they will fit closely with thelands of the external member.Figure 3-38 Involute-Form splines are used where high loads are to be transmitted.Tooth proportions are based on a 30º stub tooth form. (A) Splined members can be positioned either by close fitting major or minor diameters.
(B) Use of the tooth width or sidepositioning has the advantage of a full fillet radius at the roots. Splines can be parallel orhelical. Contact stresses of 4,000 psi are used for accurate, hardened splines. The diametral pitch shown is the ratio of teeth to the pitch diameter.Figure 3-39 Special Involute splines are made by usinggear tooth proportions.
With full depth teeth, greater contact area is possible. A compound pinion is shown made bycropping the smaller pinion teeth and internally splining thelarger pinion.Figure 3-40 Taper-Root splines are for drivers that requirepositive positioning. This method holds mating partssecurely. With a 30º involute stub tooth, this type is strongerthan parallel root splines and can be hobbed with a range oftapers.120Chapter 3Direct Power Transfer DevicesFace SplinesFigure 3-41 Milled Slots in hubsor shafts make inexpensive connections.
This spline is limited tomoderate loads and requires alocking device to maintain positive engagement. A pin andsleeve method is used for lighttorques and where accurate positioning is not required.Figure 3-42 Radical Serrationsmade by milling or shaping theteeth form simple connections.(A) Tooth proportions decreaseradially. (B) Teeth can be straightsided (castellated) or inclined; a90º angle is common.Figure 3-43 Curvic Coupling teeth are machined by a face-mill cutter. When hardenedparts are used that require accurate positioning, the teeth can be ground. (A) Thisprocess produces teeth with uniform depth.
They can be cut at any pressure angle,although 30º is most common. (B) Due to the cutting action, the shape of the teeth willbe concave (hour-glass) on one member and convex on the other—the member withwhich it will be assembled.Chapter 3Direct Power Transfer Devices121TORQUE LIMITERSRobots powered by electric motors can frequently stop effectively without brakes. This is done by turning the drive motor into a generator, andthen placing a load across the motor’s terminals. Whenever the wheelsturn the motor faster than the speed controller tries to turn the motor, themotor generates electrical power. To make the motor brake the robot, theelectrical power is fed through large load resistors, which absorb thepower, slowing down the motor. Just like normal brakes, the load resistors get very hot. The energy required to stop the robot is given off in thisheat.
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