Keys Cotter Joints
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Rameshaydavji@gmail.com KEYS, COTTER and JOINTS Keys: - These are temporary fasteners. - Used to hold pulleys, wheels, gears etc on the rotating shafts, such that there is no relative rotary motion between the pairs. - Functions of keys: o It is an agent in transmission of power, motion, torque between the pairs. o It is also used as a safety feature. When overloading occurs, the key will fail before the actual parts. o Keys are easy to replace and costs less. Classification of keys: Depend chiefly upon the load to be transmitted: (1) Heavy duty keys a. Sunk key - A key which goes in the key seat. The key seat is a sort of groove to hold the key. Key seat is machined partly in the shaft and partly in the part supported. - Sunk key is mostly of rectangular or square key. These two can be parallel or taper keys. - Drawing proportions: o Rectangular key: W=0.25D and T=0.66W.
o Square key: T=W.
Gib head key - Key is provided with a head at one end to facilitate withdrawal of the key.
Splines - The key is actually the solid integral part of the shaft and corresponding grooves/ keyways are made on the supported parts. - There can be number of keys on the circumference of the shaft.
(2) Light duty keys: a. Flat saddle key.
It is flat on the shaft. As it is liable to slip on shaft, so it is used for light duty.
Hollow saddle keys: - The key is curved to fit on the shaft. - It increases friction.
Round or pin key - It is a circular pin. - It is fitted in a hole drilled partly in the end of a shaft and partly in the supported part.
Woodruff key - It is a segmental disc with a flat or round bottom. - The keyway is also of disc type. - It is an self adjustable key.
COTTERS - These are again the temporary fasteners. - It is a wedge shaped piece of mild steel, tapered on one or both side (1:20-50). - It is tapered so as: o Facilitate the positioning and withdrawing of the cotter. o For lateral adjustment. - Used to hold two parts where the parts are subjected to axial forces only. - Usually driven perpendicular to the axis of the connected parts.
DIFFERENCE BETWEEN KEYS AND COTTERS
- Keys are driven parallel to the axis but cotters perpendicular to axis. - Keys are used where parts are subjected to torque but cotters are used where parts are subjected to tensile or compressive forces. - Keys resist shear over a longitudinal section but cotter resist shear over two transverse sections.
COTTERED JOINTS - Joints formed by using a cotter are called cottered joints. (1) Spigot and socket cottered joint - Used for round rods.
2. Sleeve and cotter joint.
3. Gib and cotter joint - used for square rods. - Gib is a piece of mild steel having the same thickness as cotter and is used in combination of cotter.
- Uses of gib: (a) When the cotter is driven in position, it has the tendency to bend the lower part
of the fork end away from rod. It is prevented by using a gib. (b) To increase the bearing area of contact between the mating parts.
PIN JOINT OR KNUCKLE JOINT - Joint allows a small angular movement of the rods relative to each other. - Joint is capable of transmitting rotary and transverse motion.