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# KEY.COTTER AND KNUCKLE JOINTS

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### KEY.COTTER AND KNUCKLE JOINTS

1. 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. 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.