Design of Keys

Design of Keys
Pulleys, gears, sprockets, levers, couplings and similar devices are employed to transmit torque to or from shafts and usually are rigidly attached to the shaft by shrink fits, setscrews, keys, splines or cotters. Shrink fits are suitable for permanent assemblies, setscrews for light service and cotters for axial loads. When the parts must be disassembled, keys or splines generally are used. A key is a machine member employed at the interface of a pair of mating male and female circular cross-sectioned members to prevent relative angular motion between these mating members. The key fits into mating grooves in the shaft and mating member called the keyway and transmit torque by shear across the key. The cutting of the keyway into the shaft reduces its strength and rigidity by an amount depending upon the shape and size of the keyway

Types of Keys
Keys maybe classified according to whether they are constant or variable in cross section. The constant cross-sectioned keys are square, flat, round or Barth keys. The Square key, with the key sunk half in the shaft and half in the hub, is the type most commonly used. The Flat keys are used where the weakening of the shaft by the keyway is serious. The key is sunk half in each of the two mating members. Although there is no universal standard, the width of square and flat keys usually is approximately one-fourth the shaft diameter. Keyways for round keys may be drilled and reamed after assembly of the mating parts. Small round keys are used for fastening cranks, handwheels and other parts that do not transmit heavy torques. A few manufacturers employ round keys for heavy duty shafts over 6 inches in diameter because the absence of the sharp corers reduces the stress concentration below that which would exist had a square or flat key been used. The Barth key is a square key with bottom two corners beveled. This double beveling ensures that the key will fit tightly against the top of the keyway when the drive is in either direction and lessens the tendency to twist. This key does not require a tight fit and small clearance permits easy assembly and removal.

Variable cross-sectioned keys include the woodruff, round taper, gib-head taper and the saddle keys. The Woodruff key consists of one-half of a circular disk fitting into a rectangular keyway in the female member and a semi circular keyway in the male member. It has the advantage that it weakens the shaft less than does a square or flat key of equal torque capacity. Round tapered keys usually have a taper in diameter of 1/8 inches per foot and a nominal diameter of 1/4 the shaft diameter for shafts under 6 inches in diameter or a nominal diameter of 1/5 the shaft diameter for large shafts.  Gib-head taper keys are flat keys with varying height and with a special gib-head to facilitate easy driving and removal of key. Usually they are used with constant dimension flat keyway and have a taper in height of 1/8 inches per foot. Saddle keys are flat keys used without a keyway in the shaft. They are employed for light work or in cases where relative motion between the shaft and its mating hub is required for adjustment and a keyway cannot cut into both.  A feather key is one which has a tight fit into one member and a loose sliding fit in the other mating member, It is used when there must be relative axial motion between the shaft and the mating hub. The bearing leading on feather keys should not exceed 1,000 psi; if the members are to slide under load, the bearing loading should under 1,000 psi.

Number of Keys Used
Usually only one key is employed between the two mating members. For heavy duty shafts, two keys angularly spaced 90 degrees apart or three keys 120 degrees apart are used. When multiple keys are employed, the keys usually square or flat and often tapered.

Splines are permanent keys made integral with the shaft and fitting into keyways broaches into the mating hub. They are used where maximum torque capacity is desired or where axial motion of the mating parts under load is expected.


Ashok Khot, Pune, India said...

please add key tolerances in the article unless it is incomplete. e.g. you had given key fitting tolerances. if a key used is minus 0.08mm will it give the mentioned fitting? obviously not!

charlie puth said...

thanks for comment

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