Design of keys
- 1. 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. It is always inserted parallel to the axis of the shaft. Keys aroused as temporary fastenings and are subjected to consider-able crushing and shearing stresses
- 2. 1. Sunk keys, 2. Saddle keys, 3. Tangent keys, 4.Round keys, and 5. Spines.
- 3. 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. The sunk keys are of the following type 1 Rectangular sunk key 2. Square sunk key 3. Parallel sunk key 4.Gib-head key 5. Feather key 6. Woodruff key
- 4. A rectangular sunk key is shown in above Fig. The usual proportionsof this key are : width of key, w=d /4; thickness of key, t = 2 w / 3 =d / 6 Where d =Diameter of the shaft or diameter of the hole in the hub. The key has taper 1 in 100 on the top side only
- 5. 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
- 6. The parallel sunk keys may be of rectangular or square section uniform in width and thickness throughout. It may be noted that a parallel key is a taper less and is used where the pulley, gear or other mating piece is required to slide along the shaft
- 7. It is a rectangular sunk key with a head at one end known as gib head . It is usually provided to facilitate the removal of key. The usual proportions of the gib head key are : Width, w = d / 4 ; thickness at large end, t = 2 w / 3 = d /6
- 8. A key attached to one member of a pair and which permits relative axial movement is known as feather key . It is a 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 key way of the moving piece
- 9. The woodruff key is an easily adjustable key. It is a piece from a cylindrical disc having segmental cross-section in front view as shown in Fig. 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
- 10. The main advantages of a woodruff key 1.It accommodates itself to any taper in the hub or boss of the mating piece. 2. It is useful on tapering shaft ends. Its extra depth in the shaft prevents any tendency to turnover in its keyway. The disadvantages are : 1.The depth of the keyway weakens the shaft. 2.It can not be used as a feather
- 11. The saddle keys are of the following two types 1. Flat saddle key, and 2.Hollow saddle key.
- 12. A flat saddle key is a taper key which fits in a keyway in the hub and is flat on the shaft as shown in Fig. 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 of the shaft. Since hollow saddle keys hold on by friction. therefore these are suitable for light loads. It is usually used as a temporary fastening in fixing and setting eccentrics, cams etc
- 13. The tangent keys are fitted in pair at right angles as shown in Fig. Eachkey is to with standtorsion in one direction only. These are used inlarge heavy duty shafts.
- 14. The round keys, as shown in Fig (a), 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 drive Sometimes the tapered pin, as shown in Fig. (b), is held in place by the friction between the pin and the reamed tapered hole
- 15. Sometimes, keys are made integral with the shaft which fits in the keyways broached in the hub. Such shafts are known as splined shafts as shown in Fig. These shafts usually have four, six, ten or sixteensplines. The splined shafts are relatively stronger than shafts having a single keyway. The splined shafts are used when the force to be transmitted is large in proportion to the size of the shaft as in automobile transmission and sliding gear transmissions. By using splined shafts, we obtain axial movement as well as positive drive is obtained
- 16. When a key is used in transmitting torque from a shaft to a rotor or hub, the following two types of forces act on the key : 1.Forces (F1) due to fit of the key in its keyway, as in a tight fitting straight key or in a tapered key driven in place. These forces produce compressive stresses in the key which are difficult to determine in magnitude. 2.Forces (F) due to the torque transmitted by the shaft. These forces produce shearing and compressive (or crushing) stresses in the key. The distribution of the forces along the length of the key is not uniform because the forces are concentrated near the torque-input end. The non-uniformity of distribution is caused by the twisting of the shaft within the hub. In designing a key, forces due to fit of the key are neglected and it is assumed that the distribution of forces along the length of key is uniform.
- 17. The forces acting on a key for a clockwise torque beingtransmitted from a shaft to a hub are shown in Fig
- 18. Let T =Torque transmitted by the shaft, F =Tangential force acting at the circumference of the shaft, D =Diameter of shaft, l=Length of key, w=Width of key. T =Thickness of key, and τ =Shear stresses for the material of key σc=crushing stresses for the material of key
- 19. A little consideration will show that due to the power transmitted by the shaft, the key may fail due to shearing or crushing. Considering shearing of the key, the tangential shearing force acting at the circumference of the shaft, F =Area resisting shearing × Shear stress F= L ×w×Ï„ ∴Torque transmitted by the shaft T = F × d/2 = l ×w×Ï„ × d/2……………(i)
- 20. Considering crushing of the key, the tangential crushing force acting at the circumference of the shaft, F =Area resisting crushing × Crushing stress F=l × t/2× Ïƒc ∴Torque transmitted by the shaft, T =F × d/2 = l × t/2 × Ïƒc× d/2……………(ii)
- 21. The key is equally strong in shearing and crushing EQUATION (i) = (ii) L ×w×Ï„ × d/2= l × t/2 × Ïƒc× d/2 W/t = σc/2Ï„…………………………(iii) The permissible crushing stress for the usual key material is at least twice the permissible shearing stress. Therefore from above equation we have w = t In other words, a square key is equally strong in shearing and crushing
- 22. In order to find the length of the key to transmit full power of the shaft, the shearing strength of the key is equal to the torsional shear strength of the shaft. We know that the shearing strength of key, T =l × w × Ï„ × d/2…………………(iv) torsional shear strength of the shaft T = (Ï€/16) × d³× Ï„1………..(v) Taking Ï„1= Shear stress for the shaft material
- 23. From equations ( iv ) and (v), we have w × l× Ï„ × d/2= (Ï€/16) ×d³ × Ï„1 for sunk key width= (w)= d/4 When the key material is same as that of the shaft, then Ï„ = Ï„1 l=1.571d