Key Joint

key joint [′kē ‚jȯint]

(civil engineering)

A mortar joint with a concave pointing.

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Key Joint 

a coupling between a shaft and a part fitted on the shaft, effected by means of a machine key. Axial slots are required in order to seat the key on the shaft and in the part. Keys may have an interference fit (tightly seated) or a keying fit. The former group includes tangent, tapered sunk, tapered flat, and saddle types; sunk keys of uniform cross section and Woodruff keys have a keying fit (Figure 1). Keys having an interference fit transmit both peripheral and axial forces; those having a keying fit transmit only peripheral force. Interference-fit keys shift the

Figure 1. Key joints: (a) sunk key of uniform cross section, (b) Woodruff key, (c) tangent key, (d) tapered sunk key, (e) tapered flat key, (f) saddle key

center of gravity of the part on the shaft by an amount equal to the seating clearance; this usually results in a skewing of the part, which is the principal reason such keys have limited application.

Sunk keys of uniform cross section and Woodruff keys are the most widely used types; tangent keys are used in heavy machine building. Sunk keys of uniform cross section are seated in the shaft to half their height, and they have narrow faces (Figure l,a). They can be used under a part that is slipped onto the shaft, in which case the key may be attached either to the shaft or to the part. The portion of a Woodruff key (Figure 1 ,b) seated in a shaft has a semicircular section. When torques are substantial, two or more keys are positioned along the axis of the shaft or around the periphery. Tangent keys (Figure l,c) are used to transmit substantial torques that vary in value or direction. They consist of two single-taper wedges having the same taper (1 : 100) and narrow faces. Two keys are inserted, since the interference in the coupling is created in a peripheral direction. Tapered interference-fit keys are less often used; they may take the form of sunk keys (Figure l,d) or flat keys (Figure l,e); saddle keys (Figure l,f) may also be used. When flat keys are used, a segmental groove is machined on the shaft; during a sudden increase in load, a flat key slips along the shaft, thereby cushioning the shock. Saddle keys are used to transmit relatively small torques.

S. N. POZDNIAKOV