A key is the piece inserted in an axial direction between a shaft and hub of the mounted machine element such as pulley or gear etc.,
• keys are used to prevent relative rotation but sometimes may allow sliding movement along the shaft if required.
• Keys are temporary fastening and are always made of mild steel because they are subjected to shearing and compressive stresses caused by the torque they transmit.
• A keyway is the groove cut in the shaft or hub to accommodate a key. Key ways can be milled horizontally or vertically .
Types Of Keys:
There is a large variety of machine keys and they may be classified under four broad headings:
Pins or round keys
Sunk keysmay be further classified into the following categories:
(a) Rectangular sunk keys
Rectangular sunk keys are shown in figure- They are the simplest form of machine keys and may be either straight or slightly tapered on one side. The parallel side is usually fitted into the shaft.
(b) Gib head sunk keys
The gib head keys are ordinary sunk keys tapered on top with a raised head on one side so that its removal is easy.
(c) Feather keys
A feather key is used when one component slides over another. The key may be fastened either to the hub or the shaft and the keyway usually has a sliding fit.
(d) Woodruff keys
A woodruff key is a form of sunk key where the key shape is that of a truncated disc, as shown in figure-. It is usually used for shafts less than about 60 mm diameter and the keyway is cut in the shaft using a milling cutter . It is widely used in machine tools and automobiles due to the extra advantage derived from the extra depth. Steel keys are widely used in securing machine parts such as gears and pulleys.
Saddles keysare sub-classified as :
Hollow saddle key
Flat saddle key
A flat key, as shown in figure- is used for light load because they depend entirely on friction for the grip. The sides of these keys are parallel but the top is slightly tapered for a tight fit. Theses keys have about half the thickness of sunk keys.
A saddle key, shown in figure- is very similar to a flat key except that the bottom side is concave to fit the shaft surface. These keys also have friction grip and therefore cannot be used for heavy loads. A simple pin can be used as a key to transmit large torques. Very little stress concentration occurs in the shaft in these cases.
Round Keysare sub-classified as ,
Lewis keys, shown in figure- are expensive but offer excellent service.
They may be used as a single or double key. When they are used as a single key the positioning depends on the direction of rotation of the shaft.
Spline shaft and hub:
A spline shaft is used when the hub is required to slide along the shaft. These shafts are used mostly for sliding gear application as in automotive gear box and propeller shaft of aircraft.
• By this means the power transmitted is equally divided amongst the number of keys giving
• A spline shaft in which are cut equiangular longitudinal groove, the metal between these groove forming splines or feathers of uniform depth. great strength and security against total failure than by using a single key.