machine design lecture topic 2.0-Keys.pdf
- 2. ▪ Key - is a machine member employed at the interface of a pair of mating male and female circular sectioned members to prevent relative angular motions between these mating parts. The key fits into mating grooves in the shaft and mating member called keyway.
- 3. Rectangular key Square key Parallel sunk key Gib head key Feather key Woodruff key Flat key Hollow key
- 4. ▪ Sunk keys - Fits half in the keyway of the shaft and half in the keyway of the hub.
- 5. ▪ Rectangular, square & parallel key b b t t 𝒃 ≠ 𝒕 𝒃 = 𝒕 Parallel key = a taperless rectangular or square key
- 6. ▪ Feather keys - A type of key where it allows the hub to move along the shaft but prevents rotation on the shaft. - Clearance is provided between the hub and key so that hub slides freely over the key.
- 7. ▪ Gib head keys - Are square or flat and tapered. (see figure below) - This key is useful when the small end is inaccessible for tapping it out; the head permits its easy removal - The head should not be left exposed, because it might caught on workmen’s clothing that can cause possible serious injury.
- 8. ▪ Pin key - Also known as round key. It may be straight or tapered - It is usually drive fit, size is about one-fourth of the shaft diameter. - (b) it can transmit heavy power - (c) it cannot transmit nearly as much power; usually used as a shear pin
- 9. ▪ Rollpin key - Also called as spring pin - These are hollow, cylindrical tube-style press fit fasteners with chamfered ends - The chamfer end makes it easy to drive in, and the pressure it exerts holds it in place.
- 10. ▪ Woodruff keys - The woodruff key is a sunk key, almost in the form of semicircular disc of uniform thickness. - The curve portion of key sits inside slot made on shaft whereas flat portion sits inside slot made on hub - The extra depth of woodruff key prevent its tendency to slip over during heavy load condition - Extensively used in tapered shafts because it can align easily with the hub slot by slight rotations. - Disadvantage: depth of the keyway weakens the shaft.
- 11. ▪ Saddle key - Is a key which fits in key way of hub only and transmits torque between hub and shaft by means of friction. - It is used for comparatively light loads. - It is usually used as a temporary fastening in fixing and setting eccentrics, cams etc. - A flat saddle key is a taper 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. - 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.
- 12. ▪ Kennedy keys - These are tapered square keys, with or without gib heads. - Are also said to be tangential keys. - It transmit the torque only in one direction by means of compressive force alone. - The advantage of this configuration is a significant increase in capacity. - Two tangent keys separate by 90 degrees angle force hub and shaft to concentric position.
- 13. b = width of key t = thickness of key L = length of key D = shaft diameter F = crushing/shearing force acting on the key T = torque to be transmitted P = power to be transmitted n = number of revolutions per unit time 𝑇 = 1 2 𝐹𝐷 𝑜𝑟 𝐹 = 2𝑇 𝐷 From the diagram, the torque being transmitted by the pulley or gear and the crushing/shearing force developed in the key is given by equation;
- 14. 𝑆𝑠 = Τ 𝐹 𝐴𝑠 = Τ 2𝑇 𝐷 𝑏 𝐿 ; 𝑜𝑟 𝑇 = 𝑆𝑠 𝑏 𝐿 𝐷 2 𝑜𝑟 𝐿 = 2𝑇 𝑏 𝐷𝑆𝑠 Shear stress:
- 15. 𝑆𝑐 = Τ 𝐹 𝐴𝑐 = Τ 2𝑇 𝐷 ൗ 𝑡 2 𝐿 ; 𝑜𝑟 𝑇 = 𝑆𝑠 𝑡 𝐿 𝐷 4 𝑜𝑟 𝐿 = 4𝑇 𝑡 𝐷 𝑆𝑠 Compressive stress: t t
- 16. The usual material for keys is cold finished, low carbon steels (0.2% C or less) although heat-treated alloy steels are used where needed. In design, the design stress in foregoing equations are based on the yield strength of the material involved. The allowable stress in compression is take according to the weakest of the three parts involved; the shaft, the key or the hub. The suggested margins of safety based on yield strength are; 1.5 = for smooth load, 2.0 – 2.25 = for minor shock 4.5 = for severe shock and reversing load. Typical hub lengths fall between 1.25 D and 2.4 D, where D is the outside shaft diameter. If the needed key length is greater than about 2D, consider using 2 keys, 180° apart.
- 17. A cast steel gear (SAE 0030, Sy = 43750psi) with a pitch diameter of 36in is to transmit 75 hp at 210 rpm to a rock crusher and is keyed to a 3in shaft (SAE 1045, as rolled) by means of a square key made of SAE 1020 cold drawn steel. What should be the length of the square key to be used?
- 18. A 76.2mm diameter shafting of SAE 1040 grade, cold, rolled, having a yield point of 50ksi and width a ¾ x ¾ x 5 inches key. Compute minimum yield point in the key in order to transmit the torque of the shaft. The factor of safety to use is 2 and Sys = 0.5Sy. (47.12ksi)
- 19. A key is to be design for a 12.7cm shaft which will transmit power of 150 kW at 360rpm. If the allowable shear stress for the key is 920 kg/cm2 and the allowable compressive stress is 1200 kg/cm2, determine the following: a) Cross-sectional dimension of the flat key to be used. (b = 1 ¼, t = 7/8) b) Force acting on a key (62.66kN) c) Length of key under shearing stress (2.19cm) d) Length of key under compressive stress (2.39 cm) e) Safe length of key to be used (2.39cm) f) Axial force to remove the hub from the shaft if the coefficient is 0.45. (56.39 kN)
- 20. A cast steel, SAE 080 N&T, pulley is keyed to a 65mm diameter shaft by means of a standard flat key with a length of 90mm, made of cold drawn SAE 1015. The shaft is made of cold drawn SAE 1045. If the shaft is virtually under pure tension and turns 420rpm, what safe horsepower can the assembly transmit? The load is steady.
- 21. A 1 11/16 shaft rotating at 200 rpm carries a cast iron gear keyed to it by a ¼ x ¼ in woodruff key, shaft material is cold finished SAE 1045. The power is transmitted with mild shock. What horsepower maybe safely transmitted by the key if it is made of cold drawn SAE 1118? How many keys are needed to give a capacity oh 20hp?
- 22. A 20 in lever is keyed to a 1 7/8 shaft (cold finished SAE 1141) by a radial taper pin whose mean diameter is 0.5 in. Pin material is C1095, OQT 800 F. The load on the lever is repeatedly reversed, N = 2 based on endurance strength. What is the safe lever load P? (a) for the shaft, (b) for the pin key, (c) for the assembly.
- 23. 1. A keyed gear delivers a torque of 912.4 N-m thru its shaft of 63.5 mm outside diameter. If the key has thickness of 15.875mm and width of 11.1125mm, find the length of the key. Assume permissible stress value of 61.2 MPa for shear and tension at 99.8 MPa. (42.25mm) 2. A flat key is to be design for 75mm diameter shaft which will transmit 150 kW at 400rpm. If the allowable shearing stress is 200MPa and key width is 15mm, determine the length of key. (31.83mm) 3. A rectangular key is used in a pulley connected to a line shaft at 15 kW and 600 rpm. If the shearing stress of key is 230 MPa, determine the force acting on key length if key width is one-fourth of shaft diameter and key length is 1inch. (45kN) 4. A pulley is keyed to a 2.5 inches diameter shaft by a 5/8 x 7/16 in x 3 in flat key. The shaft rotates at 50rpm. The allowable shearing stress for key is 22ksi. The allowable compressive stress for the key, hub and shaft are 66ksi, 59ksi, and 72ksi, respectively. Determine: a) The torque that can be carried due to shearing stress of key (51562.6 lb-in) b) The torque that can be carried due to compressive stress of key (108281.25 lb-in) c) The torque than can be carried by the shaft (220893.23 lb-in) d) The torque than can be carried by the hub e) Maximum torque the pulley can safely deliver. 5. A 63.5mm shaft having a yield point of 50ksi with a ¾ inch width key. Compute the length of key in order to transmit the torque on the shaft. The shearing stress of the key is 22ksi and use factor of safety of 2. (3.72in)