Shaft Key

"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 are used as temporary fastenings and are subjected to considerable crushing and shearing stresses. A key way is a slot or recess in a shaft and hub of the pulley to accommodate a key.

Types of Key

1. Sunk keys

(a) Rectangular Key

(b) Square Key

(c) Parallel Sunk Key

(d) Gib Head Key

(e) Feather Key

(f) Woodruff Key

2. Saddle keys

3. Tangent keys

4. Round keys

5. Splines

6. Kennedy Key

1. Sunk Key

The sunk keys are provided half in the key way of the shaft and half in the key way of the hub or boss of the pulley. The sunk keys are of the following types-

(a) Rectangular Key

The usual proportions of this key are : 

Width of key  w = d / 4 T

Thickness of key t = 2w / 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.

(b) Square 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

(c) Parallel Sunk Key

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.

(d) Gib Head Key

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 = 2w / 3 = d / 6

(e) Feather Key

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. The various proportions of a feather key are same as that of rectangular sunk key and Gib head key.

(f) 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. Woodruff keys are relatively short and cannot carry the same load as longer keys, and hence are used mostly in light-duty applications and mounting taper-bored components onto tapered shaft ends.

Advantages of a woodruff key are as follows : 

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 turn over in its key way. 

Disadvantages of a woodruff key are as follows : 

1. The depth of the key way weakens the shaft. 

2. It can not be used as a feather.

2. Saddle keys

Compared to sunk keys, saddle keys are not sunk into the shaft and hub instead they are only sunk into the hub. They either sit on a flat or circumference of the shaft. Power transmission is achieved through friction between the shaft and the key The saddle keys are of the following two types : 

1. Flat saddle key

A flat saddle key is a taper key which fits in a key way 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.

2. Hollow saddle key

A hollow saddle key is a taper key which fits in a key way 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.

3. 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 shafts.

4. 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 key ways 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.

5. Splines

Sometimes, keys are made integral with the shaft which fits in the key ways 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 than shafts having a single key way. 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.

There are two other types of splines are-

1. Involute splines

2. Serrations

1. Involute splines

Involute splines are in the form of concentric external and internal gear teeth. They are stub teeth with a pressure angle of 30°. These splines are specified by module. Involute splines are more popular than straight splines due to greater strength relative to their size. Involute splines are self centering and tend to adjust to an even distribution of load. However, the cost of involute splines is more than straight-sided splines.

2. Serrations

Straight-sided serrations are used in applications where it is important to keep the overall size of the assembly as small as possible. They are used as interference joints. Serration joints are also used to obtain small angular relative adjustment between the joined members.

6. Kennedy Key

The Kennedy key consists of two square keys. In this case, the hub is bored off the center and the two keys force the hub and the shaft to a concentric position. Kennedy key is used for heavy duty applications.