Đang tải... (xem toàn văn)
KEY EQUATIONS AND CHARTS FOR DESIGNING MECHANISMS FOUR-BAR LINKAGES AND TYPICAL INDUSTRIAL APPLICATIONS All mechanisms can be broken down into equivalent four-bar linkages. They can be considered to be the basic mechanism and are useful in many mechanical
CHAPTER 12FASTENING, LATCHING,CLAMPING, ANDCHUCKING DEVICESSclater Chapter 12 5/3/01 1:24 PM Page 405 406REMOTELY CONTROLLED LATCHThis simple mechanism engages and disengages parallel plates carrying couplings and connectors.A new latch mates two parallel plates inone continuous motion (see Fig. 1). Onthe Space Shuttle, the latch connects (anddisconnects) plates carrying 20 fluid cou-plings and electrical connectors. (Thecoupling/connector receptacles are oneplate, and mating plugs are on the otherplate). Designed to lock items in placefor handling, storage, or processingunder remote control, the mechanismalso has a fail-safe feature: It does notallow the plates to separate completelyunless both are supported. Thus, platescannot fall apart and injure people ordamage equipment.The mechanism employs fourcam/gear assemblies, one at each cornerof the lower plate. The gears on each sideof the plate face inward to balance theloading and help align the plates. Wormgears on the cam-gear assemblies areconnected to a common drive motor.Figure 1 illustrates the sequence ofmovements as a pair of plates is latchedand unlatched. Initially, the hook isextended and tilted out. The two platesare brought together, and when they are4.7 in. (11.9 cm) apart, the drive motor isstarted (a). The worm gear rotates thehook until it closes on a pin on the oppo-Fig. 1 The latch operation sequence is shown for locking in steps (a) through (c) and forunlocking in steps (d) through (f).Sclater Chapter 12 5/3/01 1:24 PM Page 406 site plate (b). Further rotation of theworm gear shortens the hook extensionand raises the lower plate (c). At thatpoint, the couplings and connectors onthe two plates are fully engaged andlocked.To disconnect the plates, the wormgear is turned in the opposite direction.This motion lowers the bottom plate andpulls the couplings apart (d). However, ifthe bottom plate is unsupported, the latchsafety feature operates. The hook cannotclear the pin if the lower plate hangsfreely (e). If the bottom plate is sup-ported, the hook extension lifts the hookclear of the pin (f) so that the plates arecompletely separated.This work was done by Clifford J.Barnett, Paul Castiglione, and Leo R.Coda of Rockwell International Corp. forJohnson Space Center.407TOGGLE FASTENER INSERTS,LOCKS, AND RELEASES EASILYA pin-type toggle fastener, invented byC.C. Kubokawa at NASA’s AmesResearch Center, can be used to fastenplates together, fasten things to walls ordecks, or fasten units with surfaces ofdifferent curvatured, such as a concaveshape to a convex surface.With actuator pin. The cylindricalbody of the fastener has a tapered end foreasy entry into the hole; the head isthreaded to receive a winged locknutand, if desired, a ring for pulling the fas-tener out again after release. Slots in thebody hold two or more toggle wings thatrespond to an actuator pin. These wingsare extended except when the spring-loaded pin is depressed.For installation, the actuator pin isdepressed, retracting the toggle wings.When the fastener is in place, the pin isreleased, and the unit is then tightened byscrewing the locknut down firmly. Thisexerts a compressive force on the now-expanded toggle wings. For removal, thelocknut is loosened and the pin is againdepressed to retract the toggle wings.Meanwhile, the threaded outer end of thecylindrical body functions as a stud towhich a suitable pull ring can be screwedto facilitate removal of the fastener.This invention has been patented byNASA (U.S. Patent No. 3,534,650).A fastener with controllable toggles can beinserted and locked from only one side.GRAPPLE FREES LOADSAUTOMATICALLYA simple grapple mechanism, designedat Argonne National Laboratory inIllinois, engages and releases loads fromoverhead cranes automatically. This self-releasing mechanism was developed toremove fuel rods from nuclear reactors.It can perform tasks where human inter-vention is hazardous or inefficient, suchas lowering and releasing loads from hel-icopters.The mechanism (see drawing) con-sists of two pieces: a lift knob secured tothe load and a grapple member attachedto the crane. The sliding latch-releasecollar under the lift knob is the design’skey feature.Spring magic. The grapple housing,which has a cylindrical inner surface,contains a machined groove fitted with agarter spring and three metal latches.When the grapple is lowered over the liftknob, these latches recede into the grooveas their edges come into contact with theknob. After passing the knob, they springforward again, locking the grapple to theknob. Now the load can be lifted.When the load is lowered to theground again, gravity pull or pressurefrom above forces the grapple housingdown until the latches come into contactwith a double cone-shaped release collar.The latches move back into the groove asthey pass over the upper cone’s surfaceand move forward again when they slideover the lower cone.The grapple is then lifted so that therelease collar moves up the cylindricalrod until it is housed in a recess in the liftknob. Because the collar can move nofarther, the latches are forced by theupward pull to recede again into thegroove—allowing the grapple to belifted free.A sliding release collar is a key feature ofthis automatic grapple.Sclater Chapter 12 5/3/01 1:24 PM Page 407 408QUICK-RELEASE LOCK PIN HAS A BALL DETENTA novel quick-release locking pin hasbeen developed that can be withdrawn toseparate the linked members only whenstresses on the joint are negligible.The pin may be the answer to theincreasing demand for locking pins andfasteners that will pull out quickly andeasily when desired, yet will staysecurely in place without chance of unin-tentional release.The key to this foolproof pin is agroup of detent balls and a matchinggrooved. The ball must be in the groovewhenever the pin is either installed orpulled out of the assembly. This is easy todo during installation, but duringremoval the load must be off the pin toget the balls to drop into the groove.How it works. The locking pin wasdeveloped by T.E. Othman, E.P. Nelson,and L.J. Zmuda under contract toNASA’s Marshall Space Flight Center. Itconsists of a forward-pointing sleevewith a spring-loaded sliding handle as itsrear end, housing a sliding plunger that ispushed backward (to its locking position)by a spring within the handle.To some extend the plunger can slideforward against the plunger spring, andthe handle can slide backward against thehandle spring. A groove near the frontend of the plunger accommodates thedetent balls when the plunger is pushedforward by the compression of its spring.When the plunger is released backward,the balls are forced outward into holes inthe sleeve, preventing withdrawal of thepin.To install the pin, the plunger ispressed forward so that the balls fall intotheir groove and the pin is pushed intothe hole. When the plunger is released,the balls lock the sleeve against acciden-tal withdrawal.To withdraw the pin, the plunger ispressed forward to accommodate thelocking balls, and at the same time thehandle is pulled backward. If the loadingon the pin is negligible, the pin is with-drawn from the joint; if it is considerable,the handle spring is compressed and theplunger is forced backward by the handleso the balls will return to their lockingposition.The allowable amount of stress on thejoint that will permit its removal can bevaried by adjusting the pressure requiredfor compressing the handle spring. If thestresses on the joint are too great or the pinto be withdrawn in the normal manner,hammering on the forward end of theplunger simply ensures that the plungerremains in its rearward position, with thelocking balls preventing the withdrawal ofthe pin. A stop on its forward end preventsthe plunger from being driven backward.A foolproof locking pin releases quicklywhen the stress on the joint is negligible.AUTOMATIC BRAKE LOCKS HOIST WHEN DRIVINGTORQUE CEASESA brake mechanism attached to a chainhoist is helping engineers lift and alignequipment accurately by automaticallylocking it in position when the drivingtorque is removed from the hoist.When torque is removed, the cam is forced into the tapered surface for brake action.According to the designer, JosephPizzo, the brake could also be used onwheeled equipment operating on slopes,to act as an auxiliary brake system.How it works. When torque is appliedto the driveshaft (as shown in the figure),four steel balls try to move up theinclined surfaces of the cam. Althoughcalled a cam by the designer, it is really aconcentric collar with a cam-like surfaceon one of its end faces. Because the ballsare contained by four cups in the hub, thecam is forced to move forward axially tothe left. Because the cam moves awayfrom the tapered surface, the cam and thedriveshaft that is keyed to it are now freeto rotate.If the torque is removed, a spring rest-ing against the cam and the driveshaftgear forces the cam back into the taperedsurface of the threaded socket for instantbraking.Although this brake mechanism(which can rotate in either direction) wasdesigned for manual operation, the prin-ciple can be applied to powered systems.Sclater Chapter 12 5/3/01 1:24 PM Page 408 409LIFT-TONG MECHANISM FIRMLYGRIPS OBJECTSTwin four-bar linkages are the key com-ponents in this long mechanism that cangrip with a constant weight-to-grip forceratio any object that fits within its griprange. The long mechanism relies on across-tie between the two sets of linkagesto produce equal and opposite linkagemovement. The vertical links have exten-sions with grip pads mounted at theirends, while the horizontal links are soproportioned that their pads move in aninclined straight-line path. The weight ofthe load being lifted, therefore, wedgesthe pads against the load with a force thatis proportional to the object’s weight andindependent of its size.PERPENDICULAR-FORCE LATCHThe installation and removal of equipment modules are simplified.A latching mechanism simultaneously applies force in twoperpendicular directions to install or remove electronic-equipment modules. The mechanism (see Fig. 1) requires onlythe simple motion of a handle to push or pull an avionic mod-ule to insert or withdraw connectors on its rear face into orfrom spring-loaded mating connectors on a panel and to forcethe box downward onto or release the box from a mating coldplate that is part of the panel assembly. The concept is alsoadaptable to hydraulic, pneumatic, and mechanical systems.Mechanisms of this type can simplify the manual installationand removal of modular equipment where a technician’smovement is restricted by protective clothing, as in hazardousenvironments, or where the installation and removal are to beperformed by robots or remote manipulators.Figure 2 sows an installation sequence. In step 1, the han-Fig. 1 An avionics box mates with electrical connectors in the rearand is locked in position on the cold plate when it is installed with thelatching mechanism.Fig. 2 This installation sequence shows the positions of the han-dle and retention cams as the box is moved rearward and downward.Sclater Chapter 12 5/3/01 1:24 PM Page 409 dle has been installed on the handle cam and turned downward.In step 2, the technician or robot pushes the box rearward asslides attached to the rails enter grooves near the bottom of thebox. In step 3, as the box continues to move to the rear, the han-dle cam automatically aligns with the slot in the rail and engagesthe rail roller.In step 4, the handle is rotated upward 75º, forcing the box410rearward to mate with the electrical connectors. In step 5, thehandle is pushed upward an additional 15º, locking the handlecam and the slide. In step 6, the handle is rotated an additional30º, forcing the box and the mating spring-loaded electrical con-nectors downward so that the box engages the locking pin andbecomes clamped to the cold plate. The sequence for removal isidentical except that the motions are reversed.Perpendicular-Force Latch (continued )QUICK-RELEASE MECHANISMSQUICK-RELEASE MECHANISMQuick release mechanisms have many appli-cations. Although the design shown here operatesas a tripping device for a quick-release hook, themechanical principles involved have many otherapplications. Fundamentally, it is a toggle-typemechanism with the characteristic that thegreater the load the more effective the toggle.The hook is suspended from the shackle, andthe load or work is supported by the latch, whichis machined to fit the fingers C. The fingers C arepivoted about a pin. Assembled to the fingers arethe arms E, pinned at one end and joined at theother by the sliding pin G. Enclosing the entireunit are the side plates H, containing the slot J forguiding the pin G in a vertical movement whenthe hook is released. The helical spring returnsthe arms to the bottom position after they havebeen released.To trip the hook, the tripping lever is pulledby the cable M until the arms E pass their hori-zontal center-line. The toggle effect is then bro-ken, releasing the load.A simple quick-release toggle mechanism was designed for tripping a lifting hook.This quick-release mechanism is shownlocking a vehicle and plate.POSITIVE LOCKING AND QUICK-RELEASE MECHANISMThe object here was to design a simple device that wouldhold two objects together securely and quickly release themon demand.One object, such as a plate, is held to another object, suchas a vehicle, by a spring-loaded slotted bolt, which is lockedin position by two retainer arm. The retainer arms are con-strained from movement by a locking cylinder. To releasethe plate, a detent is actuated to lift the locking cylinder androtate the retainer arms free from contact with the slottedbolt head. As a result of this action, the spring-loaded bolt isejected, and the plate is released from the vehicle.The actuation of the slidable detent can be initiated by asquib, a fluid-pressure device, or a solenoid. The principleof this mechanism can be applied wherever a positiveengagement that can be quickly released on demand isrequired. Some suggested applications for this mechanismare in coupling devices for load-carrying carts or trucks,hooks or pick-up attachments for cranes, and quick-releasemechanisms for remotely controlled manipulators.Sclater Chapter 12 5/3/01 1:24 PM Page 410 411RING SPRINGS CLAMP PLATFORMELEVATOR INTO POSITIONA simple yet effective technique keeps aplatform elevator locked safely in posi-tion without an external clamping force.The platform (see drawing) contains spe-cial ring assemblies that grip the fourcolumn-shafts with a strong force by thesimple physical interaction of twotapered rings.Thus, unlike conventional platformelevators, no outside power supply isrequired to hold the platform in position.Conventional jacking power isemployed, however, in raising the plat-form from one position to another.How the rings work. The ring assem-blies are larger versions of the ringsprings sometimes installed for shockabsorption. In this version, the assemblyis made up of an inner nonmetallic ringtapering upward and an outer steel ringtapered downward (see drawing).The outside ring is linked to the plat-form, and the inside ring is positionedagainst the circumference of the columnshaft. When the platform is raised to thedesigned height, the jack force isremoved, and the full weight of the plat-form bears downward on the outside ringwith a force that, through a wedgingaction, is transferred into a horizontalinward force of the inside ring.Thus, the column shaft is grippedtightly by the inside ring; the heavier theplatform the larger the gripping forceproduced.The advantage of the technique is thatthe shafts do not need notches or threads,and cost is reduced. Moreover, the shaftscan be made of reinforced concrete.Ring springs unclamp the column as theplatform is raised (upper). As soon as thejack power is removed (lower), the columnis gripped by the inner ring.CAMMED JAWS IN HYDRAULICCYLINDER GRIP SHEETSA single, double-acting hydraulic cylin-der in each work holder clamps andunclamps the work and retracts oradvances the jaws as required. With thepiston rod fully withdrawn into thehydraulic cylinder (A), the jaws of theholder are retracted and open. When thecontrol valve atop the work holder isactuated, the piston rod moves forward atotal of 12 in. The first 10 in. of move-ment (B) brings the sheet-locaterbumper into contact with the work. Thecammed surface on the rod extensionstarts to move the trip block upward, andthe locking pin starts to drop into posi-tion. The next 3⁄4in. of piston-rod travel(C) fully engages the work-holder lock-ing pin and brings the lower jaw of theclamp up to the bottom of the work. Thework holder slide is now locked betweenthe forward stop and the locking pin.The last 11⁄4in. of piston travel (D)clamps the workpiece between the jawswith a pressure of 2500 lbs. No adjust-ment for work thickness is necessary. Ajaws-open limit switch clamps the workholder in position (C) for loading andunloading operations.Sclater Chapter 12 5/3/01 1:25 PM Page 411 412QUICK-ACTING CLAMPS FOR MACHINES AND FIXTURES(A) An eccentric clamp. (B) A spindle-clamping bolt. (C) A method forclamping a hollow column to a structure. It permits quick rotaryadjustment of the column. (D) (a) A cam catch for clamping a rod orrope. (b) A method for fastening a small cylindrical member to astructure with a thumb nut and clamp jaws. It permits quick longitudi-nal adjustment of a shaft in the structure. (E) A cam catch can lock awheel or spindle. (F) A spring handle. Movement of the handle in thevertical or horizontal position provides movement at a. (G) A rollerand inclined slot for locking a rod or rope. (H) A method for clampinga light member to a structure. The serrated edge on the structure per-mits the rapid accommodation of members with different thicknesses.(I) A spring taper holder with a sliding ring. (J) A special clamp forholding member a. (K) The cone, nut, and levers grip member a. Thegrip can have two or more jaws. With only two jaws, the deviceserves as a small vise. (L) Two different kinds of cam clamps. (M) Acam cover catch. Movement of the handle downward locks the covertightly. (N) The sliding member is clamped to the slotted structurewith a wedge bolt. This permits the rapid adjustment of a member onthe structure.Sclater Chapter 12 5/3/01 1:25 PM Page 412 413From Handbook of Fastening and Joining of Metal Parts,McGraw-Hill, Inc.(A) A method for fastening capacitor plates to a structure with a circu-lar wedge. Rotation of the plates in a clockwise direction locks theplates to the structure. (B) A method for clamping member a with aspecial clamp. Detail b pivots on pin c. (C) A method for clamping twomovable parts so that they can be held in any angular position with aclamping screw. (D) A cam clamp for clamping member a. (E) Twomethods for clamping a cylindrical member. (F) Two methods forclamping member a with a special clamp. (G) A special clampingdevice that permits the parallel clamping of five parts by the tighten-ing of one bolt. (H) A method for securing a structure with a bolt and amovable detail that provides a quick method for fastening the cover.(I) A method for quickly securing, adjusting, or releasing the centermember. (J) A method for securing a bushing in a structure with aclamp screw and thumb nut. (K) A method for securing an attachmentto a structure with a bolt and hand lever used as a nut. (L) A methodfor fastening a member to a structure with a wedge. (M) Two meth-ods for fastening two members to a structure with a spring and onescrew. The members can be removed without loosening the screw.Sclater Chapter 12 5/3/01 1:25 PM Page 413 414FRICTION CLAMPING DEVICESMany different devices for gaining mechanical advantage havebeen used in the design of friction clamps. These clamps can gripmoderately large loads with comparatively small smooth sur-faces, and the loads can be tightened or released with simple con-trols. The clamps illustrated here can be tightened or releasedwith screws, levers, toggles, wedges, and combinations of them.Sclater Chapter 12 5/3/01 1:25 PM Page 414 [...]... on a common trailer-hitch ball) and a socket The socket contains all the moving parts and is the important part of this invention The socket also has a base, which contains a large central cylindrical bore ending in a spherical cup This work was done by Bruce Weddendorf and Richard A Cloyd of the Marshall Space Flight Center 427 Sclater Chapter 12 5/3/01 1:25 PM Page 428 PROBE -AND- SOCKET FASTENERS FOR... is tightened, and its outside diameter is decreased After the work is slid over the spring, the bolt handle is released The spring then presses against the work, holding it tight This spring clamp has a cam -and- tension spring that applies a clamping force A tension spring activates the cam through a steel band When the handle is released, the cam clamps the work against the Vbar Two stop-pins limit travel... righthanded threads, and • A flanged collar with left-handed external threads to mate with the shank of the firstmentioned eye, and right-handed internal threads to mate with the shank of the second-mentioned eye The flange would be knurled or hexagonal so that it could be turned by hand or wrench to adjust the overall length of the turnbuckle Sclater Chapter 12 5/3/01 1:25 PM Page 425 For fine adjustments... 425 For fine adjustments of length, the collar could be made with only righthanded threads and different pitches inside and out (Of course, the threads on the mating shanks of the eyes would be made to match the threads on the collar.) For example, with a right-handed external thread of 28 per in (pitch ≈ 0.91 mm) and a right-handed internal thread of 32 per in (pitch ≈ 0.79 mm) , one turn of the collar...Sclater Chapter 12 5/3/01 1:25 PM Page 415 415 Sclater Chapter 12 5/3/01 1:25 PM Page 416 DETENTS FOR STOPPING MECHANICAL MOVEMENTS Some of the more robust and practical devices for stopping mechanical movements are illustrated here Fixed holding power is constant in both directions A domed plunger... turnbuckles and could, therefore, fit in shorter spaces Its ends would be coaxial The design is unlike that of other short turnbuckles whose ends and the axes that pass through them are laterally offset The turnbuckle would consist of the following parts (see figure): • An eye on a shank with internal left-handed threads, • An eye on a shank with external righthanded threads, and • A flanged collar with left-handed... by John Nozzi and Cuyler H Richards of Rockwell International Corp for Marshall Space Flight Center This mechanical actuator applies an axial load to a test specimen inside or under its stand 425 Sclater Chapter 12 5/3/01 1:25 PM Page 426 GRIPPING SYSTEM FOR MECHANICAL TESTING OF COMPOSITES Specimens can be held without slippage, even at high temperatures Lewis Research Center, Cleveland, Ohio An improved... splines are self-centering and stronger Non-sliding gears can be pinned to the shaft if it is provided with a hub 420 Sclater Chapter 12 5/3/01 1:25 PM Page 421 Fig 5 A retaining ring allows quick gear removal in light-load applications A shoulder on the shaft is necessary A shear pin can secure the gear to the shaft if protection against an excessive load is required Fig 6 A stamped gear and formed wire... with a separate hub clamp Manufacturers list correctly dimensioned hubs and clamps so that they can be efficiently fastened to a precision-ground shaft 421 Sclater Chapter 12 5/3/01 1:25 PM Page 422 CLAMPING DEVICES FOR ACCURATELY ALIGNING ADJUSTABLE PARTS Methods for clamping parts that must be readily movable are as numerous and as varied as the requirements In many instances, a clamp of any design... FIG 12 As the screw is turned, it causes the movable side, which forms one side of the dovetail groove, to move until it clamps tightly on the movable member The movable side should be as narrow as possible, because there is a tendency for this part to ride up on the angular surface of the clamped part 423 Sclater Chapter 12 5/3/01 1:25 PM Page 424 SPRING-LOADED CHUCKS AND HOLDING FIXTURES Spring-loaded . left-handed externalthreads to mate with the shank of the first-mentioned eye, and right-handed internalthreads to mate with the shank of the sec-ond-mentioned. the han-dle and retention cams as the box is moved rearward and downward.Sclater Chapter 12 5/3/01 1:24 PM Page 409 dle has been installed on the handle