Types Of Mechanical Keys- Design Of Keys , Keyways

Types Of Mechanical Keys- Design Of Keys, Keyways 

Introduction To Mechanical Key:

A key is the piece inserted in an axial direction between a shaft and hub of the mounted machine element such as pulley or gear etc.,
• keys are used to prevent relative rotation but sometimes may allow sliding movement along the shaft if required.
• Keys are temporary fastening and are always made of mild steel because they are subjected to shearing and compressive stresses caused by the torque they transmit.
• A keyway is the groove cut in the shaft or hub to accommodate a key. Keyways can be milled horizontally or vertically.

Functions of Key:

a) To transmit torque from shaft to hub of the mating element such as the pulley, gear, etc.
b)To prevent relative motion between the shaft & mating machine element
Types Of keys
Types Of keys

Types Of Keys:

There is a large variety of machine keys and they may be classified under four broad headings:
  • Sunk keys, 
  • Flat keys, 
  • Saddle keys
  • Pins or round keys
Sunk keys may be further classified into the following categories:

(a) Rectangular sunk keys

Rectangular sunk keys are shown in figure-  They are the simplest form of machine keys and may be either straight or slightly tapered on one side. The parallel side is usually fitted into the shaft. 

(b) Gib head sunk keys

The gib head keys are ordinary sunk keys tapered on top with a raised head on one side so that its removal is easy.
gib headed key
gib headed key

(c) Feather keys

A feather key is used when one component slides over another. The key may be fastened either to the hub or the shaft and the keyway usually has a sliding fit.
Feather Key
Feather Key

(d) Woodruff keys

A woodruff key is a form of sunk key where the key shape is that of a truncated disc, as shown in the figure-. It is usually used for shafts less than about 60 mm diameter and the keyway is cut in the shaft using a milling cutter. It is widely used in machine tools and automobiles due to the extra advantage derived from the extra depth. Steel keys are widely used in securing machine parts such as gears and pulleys.

woodruff key design
woodruff key design
Saddles keys are sub-classified as :
  1. Hollow saddle key
  2. Flat saddle key 
A flat key, as shown in figure- is used for light load because they depend entirely on friction for the grip. The sides of these keys are parallel but the top is slightly tapered for a tight fit. Theses keys have about half the thickness of sunk keys. 
flat key design
flat key design
A saddle key, shown in figure-  is very similar to a flat key except that the bottom side is concave to fit the shaft surface. These keys also have friction grip and therefore cannot be used for heavy loads. A simple pin can be used as a key to transmit large torques. Very little stress concentration occurs in the shaft in these cases. 
Round Keys are sub-classified as,
  1.  Parallel pin
  2.  Taper pin 
Round key
Round key

Lewis keys, shown in figure- are expensive but offer excellent service.

They may be used as a single or double key. When they are used as a single key the positioning depends on the direction of rotation of the shaft.
lewis key
lewis key
Spline shaft and hub:
A spline shaft is used when the hub is required to slide along the shaft. These shafts are used mostly for sliding gear applications as in the automotive gearbox and propeller shaft of aircraft.
splined key
splined key

• By this means the power transmitted is equally divided amongst the number of keys giving
• A spline shaft in which are cut equiangular longitudinal groove, the metal between these grooves forming splines or feathers of uniform depth. great strength and security against total failure than by using a single key. 

Design Consideration of key :

1) Power to be transmitted.
2) Tightness of fit
3) Stability of connection
4) Cost
5) Crushing failure of key:
6) shearing failure of key
7) The material of key, shaft should be the same but the key should be weaker than the shaft.

Application Of Mechanical Keys :

1) Sunk Key: used for heavy-duty application
a) Rectangular sunk key: for preventing rotation of gears and pulleys on the shaft
b) Gib headed key: used where the key to be removed frequently.
c) feather key: Machine tool
2) Saddle key: for light-duty or low power transmission
3) Round key: Used for low Power drive
4) Splines: Where providing axial movement between the shaft and mounted member

Keyway Design : 

Effect of Keyway on strength of shaft:
  • The keyway is a slot machined either on the shaft or in the hub to accommodate the key. It is cut by a vertical or horizontal milling cutter.
  • A little consideration will show that the keyway cut into the shaft reduces the load-carrying capacity of the shaft.
  • This is due to the stress concentration near the corners of the keyway and reduction in the cross-sectional area of the shaft. It other words, the torsional strength of the shaft is reduced.
  • The following relation for the weakening effect of the keyway is based on the experimental results by
  • H.F. Moore. e = 1-0.2(w/d)-1.1(h/d)

where e = Shaft strength factor.
w = Width of the keyway,
d = Diameter of shaft, and
h = Depth of keyway =Thickness of key (t )/2
It is usually assumed that the strength of the keyed shaft is 75% of the solid shaft, which is somewhat higher than the value obtained by the above relation.

  • In case the keyway is too long and the key is of sliding type, then the angle of twist is increased in the ratio kθ as given by the following relation
  • Kθ = 1+0.4 ( w/d ) – 0.7 ( h/d )
where kѳ= Reduction factor for angular twist.

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Sachin Thorat

Sachin is a B-TECH graduate in Mechanical Engineering from a reputed Engineering college. Currently, he is working in the sheet metal industry as a designer. Additionally, he has interested in Product Design, Animation, and Project design. He also likes to write articles related to the mechanical engineering field and tries to motivate other mechanical engineering students by his innovative project ideas, design, models and videos.

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