Keys & Couplings | Mechanical Engineering SSC JE (Technical) PDF Download

KEYS AND COUPLINGS

• A key is a piece of mild steel inserted between the shaft and hub or boss of the pulley to connect these together in order to prevent relative motion between them.
  Keys are used as temporary fastenings and are subjected to crushing and shear stresses.
   
• Type of Keys 
  • Sunk Keys. The sunk keys are provided half in the keyway of the shaft and half in the keyway of the hub or boss of the pulley.
  • Rectangular sunk key : The usual proportions of this key are width of key (W) = d/4 and
    thickness of key (t) = 
    
    d = diameter of the shaft, The key has taper 1 in 100 on the top side only.
  • Square sunk key : The only difference between a rectangular sunk key and a square sunk key is that its width and thickness are equal i.e. W = t = d/4
  • Parallel sunk key : The parallel sunk keys may be of rectangular or square section uniform in width and thickness throughout. A parallel key is a taperless and is used where the pulley, gear or other mating piece is required to slide along the shaft.

• Gib head key : It is usually provided to facilitate the removal of key.
  • Feather key : A key attached to one member of a pair and which permits relative axial movement is known as feather key. It is special type of parallel key which transmits a turning moment and also permits axial movement. it is fastened either to the shaft or hub, the key being a sliding fit in the keyway of the moving piece.

• Woodruff key : The woodruff key is an easily adjustable key. It is a piece from  a cylindrical disc having segmental cross-section. A woodruff key is capable of tilting in a recess milled out in the shaft by a cutter having the same-curvature as the disc from which the key is made. This key is largely used in machine tool and automobile construction. It accommodates itself to any taper in the hub or boss of the mating piece. It is useful on tapering shaft ends. Its extra depth in the shaft prevents any tendency to turn over in its keyway. While the depth of keyway weakens the shaft.

• Saddle keys. A flat key which fits in a keyway in the hub and is flat on the shaft. It is likely to slip round the shaft under load. Therefore it is used for comparatively light loads. A hollow saddle key is a taper key which fits in a keyway in the hub and the bottom of the key is shaped to fit the curved surface for light loads. It is generally used as a temporary fastening in fixing and setting eccentricities, of cams etc

• Tangent keys. The tangent keys are fitted in pair at right angles. Each key is to withstand torsion in one direction only. These are used in large heavy duty shaft.

• Round keys. The round keys are circular in section and fit into holes drilled partly in the shaft and partly in the hub. They have the advantage that their keyways may be drilled and reamed after the mating parts have been assembled. Round keys are usually considered to be most appropriate for low power drives.

• Splines. Sometimes keys are made integral with the shaft which fits in the keyways broached in the hub. Such shafts are known as splined shafts. These shafts usually have four, six, ten or sixteen splines. The splined shafts are relatively-stronger having a single keyway. The splined shafts are used when the force to be transmitted is large in proportion to size of the shaft as in automobile transmission and sliding gear transmission. Here we obtain axial movement as well as positive drive.
  
   
• Strength of a Key 
  • The key may fail due to shearing or crushing. The tangential shearing force acting at the circumference of the shaft.
    F = Area resisting shearing × shear stress = l.w.fs.
    Torque Transmitted by the shaft, T = l.w.fs.d/2.
    The tangential crushing force acting at the circumference of shaft
    
    Torque transmitted by shaft;
    
• When the key material is same as that of the shaft then, l = 1.571 d. 
• The keyways cut into the shaft reduces the load carrying capacity of the shaft. This is due to the stress concentration near the corners of the keyway and reduction in the cross-sectional area of the shaft. So the torsional strength of the shaft is reduced.
   
• Shaft Coupling
  • Shaft couplings are used in machinery for several purposes, the post common of which are the following :
    • To provide for the connection of shafts of units that are maufactured separately such as a motor and generator and to provide for disconnection for repairs or alternations.
    • To provide for misalignment of the shafts or to introduce mechanical flexibility.
    • To reduce the transmission of shock loads from one shaft to another.
    • To introduce protection against overloads.
    • To alter the vibration characteristics of rotating units.
       
• Types of shaft Couplings 
  • Rigid coupling. It is used to connect two shafts which are perfectly aligned. They are :
    • Sleeve or muff coupling. v Clamp or split muff or compression coupling.
    • Flange coupling. 
  • Flexible coupling.
    • It is used to connect two shafts having both lateral and angular misalignment. They are
    • Bushed pin type coupling.
    • Universal coupling.
    • Oldham coupling.