DEFINITION OF A KEY
A key is a piece of steel in inserted between the shaft and a hub of the pulley or the gear, mainly to that power. Key are always inserted parallet to the shaft axis. They are need as temporally fastening and are subjected to considerable cursing and shearing stresses.
A key - way in a slut or recess in the shaft and hub of a pulley to accommodate a key.
A key is a machine member employed at the interface of a pair of mating male and female circular cross-sectioned members to provenet relative angular motion between these mating members.
Fulleys, gears, sprockets, levers, coupling and similar devices are employed to transmit torque to and from shafts. They are usually redgidly attached to the shaft by shrink fits, set screws, keys, splines or cotter.
Shrink fits are suitable for permanent assemblies. Sat screws for light service and cotter for axial loads. In places where the parts must be disassembled, keys or sphines are used.
Key fit into mating grooves in the shaft and mating member called the keyway and transmits torque by shear across the key strength and rigidity of the shaft is reduced by the cutting of the keyway into it. the amount of reduction depends on the shape and size of the keyway.
TYPES OF KEYS
They are classified according to whether they are constant or variable in X - section. The constant X section keys are square, flat, round or Barth. Square key with sinks ½ in the shaft and half in the hub are commonly used.
Flat keys are used where the weakening of the shaft by the keyway is serious. The width of the flat and square keys is ¼ the shaft diameter. Keyways for round keys are drilled and reamed after assembling of the mating parts. Small round keys are used for festering cranks, handwheel and other parts that so not transmit heavy torques.
The barth key is a square key with bottom too corners beveled. The beveled corner ensures tight fitting and lessing the tendency to twist when driving in either direction. There is a small clearance to permit easy removal and assembly. Variable X - sectional keys include woodruft, round taper, gibead taper and the saddle keys. The woodruff key consists of ½ of circular disk fitting into a rectangular keyway in the female member and a smsemi circular keyway in the male member. In has the advantage of less cocakening of the shaft than the square flat keys of equal torque-capacity. Woodruff key can act as bind key i.e. The female member can utterly hide the key or keyways in the malemember.
Round taper keys have a taper in diameter of 1/8 in 1ft and a nominal dia of ¼ the shaft dia for shafts under 6in dia or a nominal dia of 1/5 the shaft dia for lager shaft. They are often used with constant dia round keyways. They can easily be driven into the keyway until they are tight because of their tapering.
Head taper keys are flat keys with varying height and with a special gip-head to easy the driving and removal of the key. Usually they are used with constant dimension flat – keyways and have a taper in height of 1/8 in per foot.
Saddle keys are flat keys used without a keyway in the shaft. Since the torque is transmitted by friction on the shaft rather than by shear of the key, top or outside of the key is tapered in height to ensure a large radial pressure on to the shaft. Saddle key is employed for light work where the relative motion between the haft and its mating hub is required for adjustment and keyway cannot be cut in both.
A feather key is one which has a tight fit into one member is used when there rust be relative axial motion between the shaft and the mating hub. The bearing loading on feather keys should not exceed 1,000 psi, if members are to slide under load, the bearing load should be under 1,000 psi.
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