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KEY.COTTER AND KNUCKLE JOINTS
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KEY.COTTER AND KNUCKLE JOINTS

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  • 1. 85 Keys, Cotter and Knuckle JointsUNIT 5 KEY, COTTER AND KNUCKLE JOINTS Structure 5.1 Introduction Objectives 5.2 Key 5.3 Types of Key 5.4 Gib Head Key 5.5 Cotter and Cotter Joint 5.6 Sleeve Cotter Joint 5.7 Socket and Spigot Cotter Joint 5.8 Joining of Rods 5.9 Knuckle Joint 5.10 Summary 5.11 Answers to SAQs 5.1 INTRODUCTION There are many situations where two parts of machines are required to be restrained. For example two rods may be joined coaxially and when they are pulled apart they should not separate i.e. should not have relative motion and continue to transmit force. Similarly if a cylindrical part is fitted on another cylinder (the internal surface of one contacting the external surface of the other) then there should be no slip along the circle of contact. Such situations of no slip or no displacements are achieved through placing a third part or two parts at the jointing regions. Such parts create positive interference with the jointing parts and thus prevent any relative motion and thus help transmit the force. You will remember that the rivets in a riveted joint had exactly the same role as they prevented the slipping of one plate over the other (in lap joint) and moving away of one plate from there (in butt joint). The rivets provided positive interference against the relative motion of the plate. Knuckle joint is yet another to join rods to carry axial force. It is named so because of its freedom to move or rotate around the pin which joins two rods, a motion which naturally exists at finger joints or knee. A knuckle joint is understood to be a hinged joint in which projection in one part enters the recess is the other part and two are held together by passing a pin through coaxial holes in two parts. This joint can not sustain compressive force because of possible rotation about the pin. In this unit we will study other interfering parts for geometrically different jointing parts. Objectives After studying this unit, you should be able to understand • what is a key, • what are the types of key, • how to draw a key, • the parts that are joined by key, • how are the keys made, • what is a cotter,
  • 2. 86 Machine Drawing • what are the types of cotter, • how to draw cotter joint, • how to make a pin joint, • how is a knuckle joint constructed, and • how is a knuckle joint drawn. 5.2 KEY A shaft rotates in its bearings and transmits torque. A shaft always carry upon at some other part like gear or pulley. That part of the gear or pulley which sits on the shaft by surrounding the shaft on all its circumference is called the hub. The hub and the shaft are provided with a positive interfering part which is called a key. The key is a prismatic bar inserted between the shaft and the hub so that it passes through both or one of them. It may be tapered or of uniform cross section. When placed in position the shaft and mating part rotate as a single unit without any slipping. The torque then can pass from shaft to mating part and vice versa. Apparently if the key is to pass through one or both the mating parts a proper groove, called keyway must be made. 5.3 TYPES OF KEY Several of the keys used in practice are shown in Figure 5.1. In these figures 1 is shaft and 2 is surrounding hub of the mating part and 3 is the key. The length of the key is perpendicular to the plane of the paper and often is equal to the length of the hub. Shaft is much longer. Figure 5.1 : Types of Key Round key is a cylinder and requires a hole to pass. Half of the hole is in the shaft and other half in the hub. It is used when load is low and shaft diameter is small. Making of hole is not easy and costly if made separately in two halves in two parts. Since the cylindrical holes do not have sharp corners they still represent a better choice. Taper round keys produce tighter joint. The taper may be as gentle as 1 : 100. Saddle key is shown in Figure 5.1(b). It sits on the curved surface of shaft and fits in the rectangular slot of hub. No keyway in the staff is required and frictional force between the seat of key and surface of the shaft is responsible for transmission of the torque. Either for transmission of light torque or holding the mating part in position during assembly such saddle key is used. Key on Flat is similar to saddle key on three sides except at the bottom where it is flat. It will of course require a flat narrow surface machined on the shaft, while it fits into the keyway made in the hub. Such flat region machined on the surface of the shaft does not affect the strength because much material is not removed no corners are created as will happen if keyway is machined.
  • 3. 87 Keys, Cotter and Knuckle JointsFlat key or rectangular key Figure 5.1(d) and square key Figure 5.1(f) are essentially same and used universally between shaft and any mating part like gear and pulley. Very large torque or power can be transmitted by both but square key is often preferred for equal strength in shear and crushing. Splines Figure 5.1(e) can be regarded as keys integral with the shaft. The shafts are weakened by creating keyways whose depth could be as large as 1/4 of diameter of the shaft. Hence, splines are created on the shaft surface fit into the grooves made in the mating part. Splines are routinely used when mating parts are required to slide on the shaft. Examples are change gear boxed in automobile. The cross section of the splines may be rectangular, triangular or involute. A spline normally has larger width (w) and smaller height (h) as shown in Figure 5.1(e). There may be four, six or 10 splines and both w and h reduce with increasing number of splines. w and h for permanent splined connections are respectively 0.28d and 0.09d for four splines, 0.278d and 0.056d for six splines and 0.17d and 0.05d for 10 splines. For sliding the dimensions increase. The keys are normally prismatic with either rounded or flat ends. The flat key with rounded ends is shown in Figure 5.2(a). No doubt it can also have flat ends as shown in Figure 5.2(b). The keyways for flat or square keys are made with end mill, which will end in semicircular ends. The keyways can also be made with discutters which can not be used with rounded end keys. The rounded end keyways are shown in Figure 5.2(c). (a) (b) (c) Figure 5.2 The jib headed key as shown in Figure 5.1(g) is in fact a rectangular cross section prismatic bar with taper (1 : 100) along the length and having a jib head at largest cross section. It is inserted in the keyslot and head helps both in insertion and extraction of the key. The jibhead, being a projection on the shaft, presents a hazard of collecting loose garments or cottonwaste, hence should be protected. It may be pointed out here that a taper key is not preferred in precise machines because it causes varying information of the moting hub. Woodruff key as shown in Figure 5.1(h) is a segment of a disc whose rounded part enters the corresponding shape cut in the shaft. The key provides the advantage of easy assembly and disassembly but weakens the shaft due to deep groove. The key is cut from a disc of radius R = 0.4 D with w = 0.2 D. Its total depth is 95% of radius and radius is 0.4 D. Three fourths of depth is in shaft. SAQ 1 On a shaft of diameter 200 mm a flanged-hub is to be placed. The diameter of the hub is 300 mm while its length is 200 mm. The flange is 500 mm diameter with a width of 50 mm. The shaft and flanged hub are shown in Figures 5.3(a) and (b). Draw the necessary views connecting the shaft with different keys.
  • 4. 88 Machine Drawing Figure 5.3 5.4 GIB HEAD KEY For convenience of insertion and extraction one end of a sunk rectangular flat key is sometimes provided with a jib head. Such keys normally have taper in height but have uniform thickness. The taper is generally 1 : 100, only in the upper surface. The keys are provided with heavy rotating mass for which accuracy of outer surface does not matter much, like heavy pulleys. The depth of the key at the end is taken as D/6 and width as D/4. The height and the length of the gib head are respectively 0.3D and 0.25D. D in this case is the diameter of the shaft. Figure 5.4(a) shows a gib key and Figure 5.8(b) shows it fitted with the shaft. (a) (b) Figure 5.4 SAQ 2 On a 35 mm diameter shaft carries a pulley of 900 mm diameter whose hub tapers from 75 mm at the arm to 70 mm at the edge and is 80mm long. Four arms, elliptic in section taper from a1 = 26 mm to a = 20 mm and b1 = 12 mm to b = 30 mm. Show the assembly of pulley with gib headed key and with part of the shaft whose diameter increases to 45 mm from 35 mm suddenly with a radius of 5 mm at the corner. The width of the pulley is 100 mm with a crown of 3 mm. Rim thickness at edges, 8 mm.
  • 5. 89 Keys, Cotter and Knuckle Joints5.5 COTTER AND COTTER JOINT A cotter is a metallic strip of uniform thickness but tapers in width. The taper may be very small like 1 : 100 but may be as large as 1 : 30. The cotter passes through slots made in two coaxial parts and thus prevent the relative motion between them. The cotter can pass through two specially made ends of two coaxial bars which may be circular in section or rectangular or it may pass through sleeve put on the plain ends of rod (two cotters will be needed). We shall now see both types of joints. The cotter joints are used only to transmit axial pull between two rods and they are not made to rotate. 5.6 SLEEVE COTTER JOINT Two plain cylindrical ends are made to butt each other and a single sleeve covers both. Two slots are made in the sleeve, each coinciding with the slot in the rod end. The rod end may be enlarged to compensate for the slot. Figure 5.5 Figure 5.5 shows a cotter, a rod with enlarged end and a sleeve. Two cotters are need to join two rods. The internal diameter of the sleeve match with the external diameter of the rod and the slot matches with the cotter. Figure 5.6 shows two rod ends pushed in a sleeve with a slight clearance at butting ends to accommodate cotters. The two views of sleeve cotter joint are drawn in Figure 5.7. Figure 5.6 Figure 5.7 : Two Views of Sleeve Cotter Joint
  • 6. 90 Machine Drawing 5.7 SOCKET AND SPIGOT COTTER JOINT One end of a rod carries a socket while other end of another rod carries a spigot. The socket is a hollow and spigot a solid cylinder with a collar. The socket also has a collar. The spigot the socket and the cotter are shown in Figure 5.8. Figure 5.8 : Cotter, Socket and Spigot Figure 5.9 shows the spigot inserted into socket with their slots for receiving the cotter aligned. Figure 5.9 : Socket and Spigot Assembled SAQ 3 Draw the elevation and side view of cotter joint from three parts shown in Figure 5.8. SAQ 4 A rectangular fork ahead of a square section bar carries slot for a cotter and a gib as shown in Figure 5.10. A square bar carries a slot at its end similar to that in the fork and also shown in the above Figure. Assemble the four parts and draw elevation, plan and side view of the assembly. Figure 5.10
  • 7. 91 Keys, Cotter and Knuckle Joints5.8 JOINING OF RODS If a problem is put before us to create a joint between two round bars to carry axial load and use a pin to join them then a number of solution may come up. Some are shown in Figure here. Figure 5.11 shows how two rods can be joined with the help of a pin which passes through holes. The ends are finished flat through half the diameter to match to form a perfect cylinder when flats are placed in contact. Draw this joint in two views by taking diameter of rod as 25 mm and diameter of pin as 10 mm. Figure 5.11 : A Pin-Joint between Two Circular Section Rods Figure 5.12 : A Pin Joint between Two Plates Figure 5.13 : Another Pin-Joint between Two Plates Figure 5.14 : A Knuckle Joint Joining Two Rods
  • 8. 92 Machine Drawing Figures 5.12 and 5.13. show the joints between the plates. Note how the changes are introduced from Figures 5.12 to 5.13. It is also suggested that the plate ends can be cut along broken lines. Example 5.1 Draw the joint shown in Figure 5.13 for plate 10 mm thick and two parts 1 (a) and (b) each is 10 mm thick in plate1. The width of the plate is 25 mm and length can be any thing. Redraw the two views of above drawing by cutting along broken lines producing plates 15 mm wide. The pin diameter is 10 mm. Figure 5.15 : Two View of the Joint of Figure 5.13 Figure 5.16 : Two Views of Modified Joint of Figure 5.13 5.9 KNUCLE JOINT In earlier figures we developed a knuckle joint. That is a joint which connects two rods. The parts that create the joint are made integral with the rods, i.e. they become the rod ends. One is called fork which provides the recess and other is called eye which fits into the recess. The ends are shaped properly to avoid sharp corners or sharp changes in the radii. You must have noticed that in Figures 5.15 and 5.16 there is nothing to stop the pin from sliding. Some restrictions like head in the pin and a stopper at the other end must be provided. These can be seen in Figure 5.14 in which pin is marked 3. A collar with a hole through which a taper pin or a split pin is pressed is used as a stopper. The collar is marked 4 and taper pin as 5. Altogether there are five parts in a knuckle joint. These five parts are shown separately in Figure 5.17.
  • 9. 93 Keys, Cotter and Knuckle Joints Figure 5.17 : Five Parts of a Knuckle Joint It is interesting to note that all dimension in a knuckle joint are related to diameter ‘d’ of the rod. These rotations are shown in Figure 5.18 in which the five parts are assembled to form the join. You must draw the top view and side view. Parts of Figure 5.17 are assembled to make a knuckle joint in Figure 5.18. Figure 5.18 : A Knuckle Joint All Dimensions in Terms of Rod Diameter d Example 5.2 For two rods of diameter 25 mm draw elevation and plan of a knuckle joint. Show partial section of elevation. For inside and outside surfaces of fork take respective radii of 14 and 32 mm. Figure 5.19 : Elevation and Plan of a Knuckle Joint
  • 10. 94 Machine Drawing 5.10 SUMMARY Various types of keys are used in practice out of which the square key is most common for gears and pulleys. By drawings it has been shown how do the keys fit in the assembly. The cotter is another element that produces temporary joints. Different types of cotter joints and their elements have been shown in drawing. The reader should reproduce each drawing. 5.11 ANSWERS TO SAQs SAQ 1 Assume : Square key – w = 50 mm, h = 50 mm Saddle key – w = 60 mm, h = 25 mm Key on flat – w = 60 mm, h = 25 mm Splines – 4 rectangular, w = 56 mm, h = 18 mm Woodruff – w = 40 mm, Radius of key = 80 mm, Keyway depth = 57 mm, Depth of the key = 76 mm Round key – diameter = 50 mm All the keys except woodruff will be equal to hub in length, i.e. 250 mm. Figure 5.20 : Three Views of Shaft and Hub Assembly with a Square Key Figure 5.21 : Shaft and Hub Assembly with a Woodruff Key
  • 11. 95 Keys, Cotter and Knuckle Joints Figure 5.22 : Shaft and Hub Assembly with Round Key Figure 5.23 : Splined Shaft and Hub Assembly The shaft and hub assembly has been drawn for following four cases : Figure 5.20 : Shaft and hub assembly with square key. Figure 5.21 : Shaft and hub assembly with woodruff key. Figure 5.22 : Shaft and hub assembly with round key. Figure 5.23 : Shaft and hub assembly for splines. You are advised to draw similar assemblies for saddle key and key on flat. SAQ 2 Gib key dimensions 35 6 mm, 0.6 0.6 35 21 mm 6 6 D h H D= = = = = × = 35 35 9 mm, 12 mm 4 4 3 3 D D w B= = = = = = (a) A Rectangular Key with Gib Head (b) Rectangular Key with Gib Head Fitted between a Shaft and a Pulley (Third Angle Projection) Figure 5.24
  • 12. 96 Machine Drawing SAQ 3 The two views are drawn in Figure 5.25. Figure 5.25 : Spigot Socket Cotter Joint Assembly Square SAQ 4 Figure 5.26 shows three views of assembled gib and cotter joint. Figure 5.26 : Gib and Cotter Joint with Fork End