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 3PARTS-HANDLINGMECHANISMSSclater Chapter 3 5/3/01 10:19 AM Page 51 52MECHANISMS THAT SORT, FEED,OR WEIGHORIENTING DEVICESHere’s a common problem; Parts arrive in either open-end or closed-end first;you need a device that will orient all the parts so they feed out facing the same way.In Fig. A. when a part comes in open-end first, it is pivoted by the swinging leverso that the open end is up. When it comes in closed-end first, the part brushes awaythe lever to flip over headfirst. Fig. B and C show a simpler arrangement with pinin place of lever.A part with its open-end facing to the right (part 1)falls on a matching projection as the indexing wheelbegins to rotate clockwise. The projection retains thepart for 230º to point A where it falls away from the pro-jection to slide down the outlet chute, open-end up. Anincoming part facing the other way (2) is not retained bythe projection, hence it slides through the indexingwheel so that it too, passes through the outlet with itsopen-end up.The important point here is that the built-in magnetcannot hold on to a part as it passes by if the part has itspointed end facing the magnet. Such a correctly orientedpart (part 1) will fall through the chute as the wheelindexes to a stop. An incorrectly oriented part (part 2) isbriefly held by the magnet until the indexing wheel con-tinues on past the magnet position. The wheel and thecore with the slot must be made from some nonmagneticmaterial.Sclater Chapter 3 5/3/01 10:19 AM Page 52 The key to this device is two pins that reciprocate one after another in the horizontaldirection. The parts come down the chute with the bottom of the “U” facing either to theright or left. All pieces first strike and rest on pin 2. Pin 1 now moves into the passageway, and if the bottom of the “U” is facing to the right, the pin would kick over the partas shown by the dotted lines. If, on the other hand, the bottom of the “U” had been to theleft, the motion of pin 1 would have no effect, and as pin 2 withdrew to the right, the partwould be allowed to pass down through the main chute.53Regardless of which end of the cone faces forward as the cones slidedown the cylindrical rods, the fact that both rods rotate in opposite direc-tions causes the cones to assume the position shown in section A-A (above).When the cones reach the thinned-down section of the rods, they fall downinto the chute, as illustrated.In the second method of orienting cone-shaped parts (left), if the partcomes down small end first, it will fit into the recess. The reciprocating rod,moving to the right, will then kick the cone over into the exit chute. But ifthe cone comes down with its large end first, it sits on top of the plate(instead of inside the recess), and the rod simply pushes it into the chutewithout turning it over.Parts rolling down the top rail to the left drop to the next rail which has acircular segment. The part, therefore, continue to roll on in the originaldirection, but their faces have now been rotated 180º. The idea of droppingone level might seem oversimplified, but it avoids the cam-based mecha-nisms more commonly used for accomplishing this job.SIMPLE FEEDING DEVICESThe oscillating sector picksup the desired number of parts,left diagram, and feeds them bypivoting the required number ofdegrees. The device for oscil-lating the sector must be able toproduce dwells at both ends ofthe stroke to allow sufficienttime for the parts to fall in andout of the sector.Sclater Chapter 3 5/3/01 10:19 AM Page 53 The circular parts feed down the chute by grav-ity, and they are separated by the reciprocating rod.The parts first roll to station 3 during the downwardstroke of the reciprocator, then to station 1 duringthe upward stroke; hence the time span betweenparts is almost equivalent to the time it takes for thereciprocator to make one complot oscillation.The device in Fig. B is similar to the one in Fig.A, except that the reciprocator is replaced by anoscillating member.54Two counter rotating wheels form a sim-ple device for alternating the feed of two dif-ferent workpieces.Each gear in this device is held up by a pivotable cam sector until the gear aheadof it moves forward. Thus, gear 3, rolling down the chute, kicks down its sectorcam but is held up by the previous cam. When gear 1 is picked off (either manually,or mechanically), its sector cam pivots clockwise because of its own weight. Thispermits gear 2 to move into place of gear 1—and frees cam 2 to pivot clockwise.Thus, all gears in the row move forward one station.SORTING DEVICESIn the simple device (A) theballs run down two inclined andslightly divergent rails. The small-est balls, therefore, will fall intothe left chamber, the medium-sizeones into the middle-size chamber,and the largest ones into the rightchamber.In the more complicatedarrangement (B), the balls comedown the hopper and must pass agate which also acts as a latch forthe trapdoor. The proper-size ballspass through without touching(actuating) the gate. Larger balls,however, brush against the gatewhich releases the catch on thebottom of the trapdoor, and fallthrough into the special trough forthe rejects.Sclater Chapter 3 5/3/01 10:19 AM Page 54 The material in the hopper is fed to a con-veyor by the vibration of the reciprocatingslider. The pulsating force of the slider is trans-mitted through the rubber wedge and on to theactuating rod. The amplitude of this force canbe varied by moving the wedge up or down.This is done automatically by making the con-veyor pivot around a central point. As the con-veyor becomes overloaded, it pivots clockwiseto raise the wedge, which reduces the ampli-tude of the force and slows the feed rate of thematerial.Further adjustments in feed rate can bemade by shifting the adjustable weight or bychanging the speed of the conveyor belt.55Workpieces of varying heights are placed on this slowly rotating cross-platform. Bars 1, 2, and 3 have been set at decreasing heights beginning withthe highest bar (bar 1), down to the lowest bar (bar 3). The workpiece istherefore knocked off the platform at either station 1, 2, or 3, depending onits height.WEIGHT-REGULATING ARRANGEMENTSThe loose material falls down the hopperand is fed to the right by the conveyor systemwhich can pivot about the center point. Theframe of the conveyor system also actuates thehopper gate so that if the amount of materialon the belt exceeds the required amount, theconveyor pivots clockwise and closes the gate.The position of the counterweight on a framedetermines the feed rate of the system.The indexing table automatically stops atthe feed station. As the material drops into thecontainer, its weight pivots the screen upwardto cut off the light beam to the photocell relay.This in turn shuts the feed gate. The reactua-tion of the indexing table can be automaticafter a time delay or by the cutoff response ofthe electric eye.Sclater Chapter 3 5/3/01 10:19 AM Page 55 By pressing down on thefoot pedal of this mechanism,the top knife and the clampwill be moved downward.However, when the clamppresses on the material, both itand link EDO will be unable tomove further. Link AC willnow begin to pivot aroundpoint B, drawing the lowerknife up to begin the cuttingaction.CUTTING MECHANISMSThese 3 four-bar cutters providea stable, strong, cutting action bycoupling two sets of links to chainfour-bar arrangements.The cutting edges of the knives in the four mechanisms moveparallel to each other, and they also remain vertical at all times tocut the material while it is in motion. The two cranks are rotatedwith constant velocity by a 1 to 1 gear system (not shown), whichalso feeds the material through the mechanism.56Sclater Chapter 3 5/3/01 10:19 AM Page 56 57The material is cut while in motion by the reciprocatingaction of the horizontal bar. As the bar with the bottom knifemoves to the right, the top knife will arc downward to per-form the cutting operation.The top knife in this arrangement remains parallel to thebottom knife at all times during cutting to provide a truescissor-like action, but friction in the sliding member canlimit the cutting force.Slicing motion is obtained from the synchronized effort oftwo eccentric disks. The two looped rings actuated by thedisks are welded together. In the position shown, the bottomeccentric disk provides the horizontal cutting movement, andthe top disk provides the up-and-down force necessary for thecutting action.This four-bar linkagewith an extended couplercan cut a web on the runat high speeds. The four-bar linkage shown isdimensioned to give theknife a velocity duringthe cutting operation thatis equal to the linearvelocity of the web.Sclater Chapter 3 5/3/01 10:19 AM Page 57 58FLIPPING MECHANISMSThis mechanism can turn over a flat piece by driving twofour-bar linkages from one double crank. The two flippers areactually extensions of the fourth members of the four-bar link-ages. Link proportions are selected so that both flippers rise upat the same time to meet a line slightly off the vertical to trans-fer the piece from one flipper to the other by the momentum ofthe piece.This is a four-bar linkage(links a, b, c, d ) in which the partto be turned over is coupler c ofthe linkage. For the proportionsshown, the 180º rotation of link cis accomplished during the 90ºrotation of the input link.VIBRATING MECHANISMAs the input crank rotates, theslotted link, which is fastened to theframe with an intermediate link,oscillates to vibrate the output tableup and down.Sclater Chapter 3 5/3/01 10:19 AM Page 58 SEVEN BASIC PARTS SELECTORSA reciprocating feed for spheres or shortcyclinders is one of the simplest feedmehanisms. Either the hopper or the tubereciprocates. The hopper must be kepttopped-up with parts unless the tube can beadjusted to the parts level.A centerboard selector is similar to reciprocat-ing feed. The centerboard top can be milled tovarious section shapes to pick up moderatelycomplex parts. I works best, however, withcylinders that are too long to be led with thereciprocating hopper. The feed can be contin-uos or as required.A rotary screw-feed handlesscrews, headed pings, shoulderedshafts, and similar parts in mosthopper feeds, random selection ofchance-oriented parts calls foradditional machinery if the partsmust be fed in only one specificposition. Here, however, all screwsare fed in the same orientation)except for slot position) withoutseparate machinery.Rotary centerblades catch small U-shaped parts effectively if their legs are nottoo long. The parts must also be resilientenough to resist permanent set from dis-placement forces as the blades cutthrough a pile of parts. The feed is usualcontinuous.A paddle wheel is effective for feeding disk-shaped parts if they are stable enough. Thin,weak parts would bend and jam. Avoid thesedesigns, if possible—Especially if automaticassembly methods will be employed.A long-cylinder feeder is a variation of thefirst two hoppers. If the cylinders have simi-lar ends, the parts can be fed withoutproposition, thus assisting automaticassembly. A cylinder with differently shapedends requires extra machinery to orientatedthe part before it can be assembled.A barrel hopper is useful if parts lend to become entangled. The parts dropfree of the rotating-barrel sides. By chance selection, some of them fall onto thevibrating rack and are fed out of the barrel. The parts should be stiff enough toresist excessive bending because the tumbling action can subject them to rela-tively severe loads. The tumbling can help to remove sharp burrs.59Sclater Chapter 3 5/3/01 10:19 AM Page 59 ELEVEN PARTS-HANDLING MECHANISMS60Gravity feed for rods. Single rods of a givenlength are transferred from the hopper to thelower guide cylinder by means of an intermit-tently rotating disk with a notched circumfer-ence. The guide cylinder, moved by a lever,delivers the rod when the outlet moves free ofthe regulating plate.Feeding electronic components.Capacitors, for example, can be delivered bya pair of intermittently rotating gearlike diskswith notched circumferences. Then a pick-uparm lifts the capacitor and it is carried to therequired position by the action of a cam andfollower.Feeding headed rivets. Headed rivets, cor-rectly oriented, are supplied from a parts-feeder in a given direction. They are dropped,one by one, by the relative movement of apair of slide shutters. Then the rivet fallsthrough a guide cylinder to a clamp. Clamppairs drop two rivets into corresponding holes.Label feed. Labels are taken out of thehopper by a carrying arm with a vacuumunit to hold the label. The label is thenplaced into the required position, and thevacuum hold is released.Horizontal feed for fixed-length rods. Singlerods of a given length are brought from the hopperto the slot of a fixed plate by a moving plate. Afterbeing gauged in the notched portion of the fixedplate, each rod is moved to the chute by means ofa lever, and is removed from the chute by a vibrat-ing table.Sclater Chapter 3 5/3/01 10:19 AM Page 60 [...]... motion between processing operations These conditions are illustrated in the accompanying drawings of feed mechanisms Sclater Chapter 3 5 /3/ 01 10:20 AM Page 63 63 Sclater Chapter 3 64 5 /3/ 01 10:20 AM Page 64 Sclater Chapter 3 5 /3/ 01 10:20 AM Page 65 SEVEN LINKAGES FOR TRANSPORT MECHANISMS Transport mechanisms generally move material The motion, although unidirectional, gives an intermittent advancement... the explorer foot of a wire-stitching machine raises a vertical plunger which releases a latch lever A rotary cam then raises a lever that retains the clutch-operating plunger 83 Sclater Chapter 3 5 /3/ 01 10:22 AM Page 84 Automatic Stopping Mechanisms (continued ) 84 Sclater Chapter 3 5 /3/ 01 10:22 AM Page 85 85 Sclater Chapter 3 5 /3/ 01 10:22 AM Page 86 Automatic Stopping Mechanisms (continued ) Fig... varying heights on the containers The unit’s cut -and- stacked label capacity is 4,500 An electric eye is provided for cutting labels in web-roll form 75 Sclater Chapter 3 5 /3/ 01 10:21 AM Page 76 HIGH-SPEED MACHINES FOR ADHESIVE APPLICATIONS Viscous liquid adhesives are used to glue fabrics and paper, apply paper labels, make cardboard and wooden boxes and shoes, and bind books Specially designed machines... Sclater Chapter 3 5 /3/ 01 10:20 AM Page 73 TRAVERSING MECHANISMS FOR WINDING MACHINES The seven mechanisms shown are parts of different yarn- and coil-winding machines Their fundamentals, however, might be applicable to other machines that require similar changes of motion Except for the leadscrews found on lathes, these seven represent the operating principles of all well-known, mechanical traversing devices. .. 80 Sclater Chapter 3 5 /3/ 01 10:22 AM Page 81 Fig 19 A paste belt applicator passes around the pulley in a pastepot and slides over the label stack Fig 20 A dauber assembly is moved horizontally between a glue pot and work by an eccentric pin on a gear Vertical movements are produced by a crank-operated bar over the dauber shaft 81 Sclater Chapter 3 5 /3/ 01 10:22 AM Page 82 AUTOMATIC STOPPING MECHANISMS. .. by one, in a given direction, and are then moved individually into the corresponding indents on transfer platforms Vertical feed for rods Rods supplied from the parts-feeder are fed vertically by a direction drum and a pushing bar The rod is then drawn away by a chucking lever 61 Sclater Chapter 3 5 /3/ 01 10:20 AM Page 62 SEVEN AUTOMATIC-FEED MECHANISMS The design of feed mechanisms for automatic or semiautomatic... holding clips attached 70 Sclater Chapter 3 5 /3/ 01 10:20 AM Page 71 The wedging action of the side belts shapes paper sacks for wrapping an packing A reciprocating pusher plate is activated by an eccentric disk and two cams on a drive shaft A pusher-type conveyor can have a drive on either side 71 Sclater Chapter 3 5 /3/ 01 10:20 AM Page 72 Synchronous chains with side arms grasp and move packages A rotary... and are supported in the frame of the machine Bearings E are also supported by the frame of the machine and the rail A-A is fixed 65 Sclater Chapter 3 5 /3/ 01 10:20 AM Page 66 Fig 3 In another type of action, the forward and return strokes are accomplished by a suitable mechanism, while the raising and lowering is imparted by a friction slide Thus it can be seen that as the transport supporting slide... stationary guides for presealing heat treatment Hooks on a cable-driven conveyor and an automatic cradle for removing coils A double belt sandwiches shoe soles during their cycle around a spiral system and then separates to discharge the soles A matchbook carrier links with holding clips that are moved intermittently by sprockets 69 Sclater Chapter 3 5 /3/ 01 10:20 AM Page 70 One of several possible kinds of... The methods and machines shown here have been adapted to the application of adhesives in mass production They might also work well for the application of liquid finishes such as primers, paint, and lacquer Fig 1 A gravity spreader has an open bottom and a grooved lip Fig 2 Spiral conveyors feed the applicator roller by the force or gravity 76 Sclater Chapter 3 5 /3/ 01 10:21 AM Page 77 Fig 3 An applicator . drawingsof feed mechanisms. Sclater Chapter 3 5 /3/ 01 10:20 AM Page 62 63Sclater Chapter 3 5 /3/ 01 10:20 AM Page 63 64Sclater Chapter 3 5 /3/ 01 10:20 AM. CHAPTER 3PARTS-HANDLINGMECHANISMSSclater Chapter 3 5 /3/ 01 10:19 AM Page 51 5 2MECHANISMS THAT SORT, FEED,OR WEIGHORIENTING DEVICESHere’s