Module 5. Key, cotter joints and pin joints

Lesson 11

11.1 Introduction

• Keys, cotter-joints and pin-joints are employed as removable/ temporary fasteners.
• Keys are also employed to secure gears, pulleys, discs, flange and other similar parts to shafts or axles.
• Cotter-joints and pin joints are used for rods in tensile or compressive load.
• The cotter-joint does not permit any axial mis-alignment as it is the rigid-joint.
• The pin-joint permits angular mis-alignment as it is flexible-joint.

11.2 Key Joints

A key is employed to connect two pieces, like shaft and a gear, in such a way that there is no relative rotational movement between the shaft and the parts mounted on it. Key is always made of steel because it is subjected to shearing and torsional stresses. It is inserted in the axial direction between the shaft and the boss or hub of the mating piece. A keyway is the groove cut in the shaft as well as in the hub, to accommodate the key. Fig.11.1.1 shows the parts of key joints and its assembly.

Keys may be classified as follows:

(i) taper keys and (ii) parallel or feather keys.

(i)Taper keys : It is tapered (standard taper of this key is 1 in 100) in thickness but uniform in width. A taper key joint prevents relative rotational as well as axial movement between the two pieces. The types of taper keys are; (1) Sunk taper key, which is a standard form of key and may be either of square or rectangular cross-section. Half part of its nominal thickness is sunk in the shaft key way and the remaining half portion fits in the keyway inside the hub or boss of the mating piece. The depth of the keyway is tapering in the hub but uniform inside the shaft (fig. 11.1.1), (2) Saddle keys are also made in two forms i.e. hollow and flat. A hollow saddle key has its underside hollow to fit the curved surface of the shaft. A flat saddle key inserted on a flat surface provided on the shaft. (3)Round key (pin key) is of circular cross-section, usually tapered along the length and is inserted in a hole drilled partly in the shaft and partly in the hub of mating piece. (4) Gib-head is a taper key is usually provided with a head, called a gib-head, to facilitate its removal.


Fig. 11.1 Sunk taper key joint

(ii) Parallel or feather keys : When there is a requirement of sliding or axial movement of mating piece on the shaft, a feather key is used. It is a sunk key of uniform width and thickness. It may be of various cross sections like rectangular, square or dove-tail. This key may be secured to either the shaft by means of two cap-screws, having countersunk heads or the mating piece and free to slide in the keyways in the shaft. These keys are also classified as peg feather key, single headed and double headed feather key.

Woodruf key : This is a sunk key in the form of a segmental part of a circular disc having uniform thickness. It is inserted into a corresponding form of a circular keyway cut in the shaft so that the some portion projects outside. This projecting part fits in a keyway inside the mating piece of the comparatively greater depth of the keyway cut in it. Once placed in position, the key tilts align it self on the tapered shaft.

Cotter Joints :

A cotter is a flat wedge-shaped piece of rectangular cross section and made of steel. It is used to connect two rods rigidly subjected to compressive or tensile forces and is inserted at 90 0 to the axes of the rods. It is tapering in width, generally on one side only but uniform in thickness. The bearing edge of the cotter and the bearing slots are generally made semicircular. This joint is not suitable for connecting the rotating members. The most commonly used types of cotter joints are; (i) Cotter joint with Socket and spigot end (fig. 11.1.2): A socket is formed by expanding the end of one rod in which the spigot-end of the other rod is inserted. A cotter is driven tightly through the slots (in the rods) which are kept slightly mis aligned. The cotter comes in contact with the two rods on opposite sides and leaves clearances on both sides. Clearances are absolutely required for proper functioning of the joint. (ii) Sleeve cotter joint: This is the simplest form of cotter joint used for fastening the two circular rods. The ends of the two rods are inserted from opposite sides in a cylindrical sleeve or steel box. Two cotters are inserted through the slots on each rod end. The slots in the rods and sleeve are made slightly wider than the width of cotter.


Fig. 11.2 Cotter joint with socket and spigot ends

Pin-Joint or Knuckle Joint:

The joint is used for rods whose axes intersect i.e which may not be in alignment. It is employed in tension or compression. The joint permits angular movement between the rods there fore it is not rigid. One end of a rod is formed in a single-eye end and is placed within a double-eye end or forked end of the other rod (fig. 11.1.3). A cylindrical pin is inserted through holes of single and double eye. The pin is kept in position by means of a collar and a taper pin. The rods are quite free to swivel on the cylindrical pin.


Fig. 11.3 Knuckle (pin) joint

Last modified: Tuesday, 25 September 2012, 6:14 AM