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Keyed joints are non-positive connections - the back surface of the key is pressed against one part of a machine, its bottom surface against the other part of the machine. By this, the two machine parts are tightened with each other.
The tightening strength depends on the pressing-in force, on the angle of inclination of the surfaces and on the friction between the tightened surfaces.
Figure 10 - Stress acting on
keyed joints
1 tension between key and hub, 2 pressing-in force, 3 tension between key and shaft
Tightening becomes strong, if the pressing-in force is great The smaller the taper of the key used, the greater the tightening effect. If there is much friction between the surface to be tightened, a great pressing-in force must be applied.
Principle of action of the keyed joint
The effect of keyed joints is based on the principle of the inclined place.
It is known that heavy loads can be pulled up on an inclined plane with little force. Especially little force is required, if the plane has a narrow angle of inclination.
This means that keys of a small taper have a great tightening effect, although only little pressing-in force is required.
Therefore, it has to be considered with shaft and hub connections that too strong pressing into place of the key-may destroy the hub or, at least, may disturb the true running. Since, in this type of connection, only the back and bottom surfaces of the key are exposed to pressure, the side faces are normally free of pressure; they have play. However, with too strong rotary forces, the side faces of the wedge get in contact with the groove in shaft and hub.
Then, a shearing stress is generated in the cross section of the wedge on the level of the shaft diameter. As a result of this, the key might shear off.
Figure 11 - Stress acting on a
keyed joint in case of overload
1 shearing stress, 2 acting forces, 3 rotary force
What types of keyed joints are
distinguished?
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What stress is the key in the keyed joint exposed
to?
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