keys cotter joints

Rameshaydavji@gmail.comKEYS, 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: W=0.25D and T=W.

(b) Gib head key - Key is provided with a head at one end to facilitate withdrawal of the key.

(c) 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.

b. Hollow saddle keys:

- The key is curved to fit on the shaft. - It increases friction.

c. 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.

d. 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.