A machine runs by the power supplied to it by a prime mover such as, motor, engine, etc. The power is transmitted from the prime mover to the machine through a coupler which couples the shafts of the prime mover and the machine. The most commonly employed method to connect a shaft and a part is to drive a small piece of metal, known as key between the shaft and the hole made in the part mounted over it. The key will be driven such that it sits partly into the shaft and partly into the part mounted on it. To introduce the key, axial grooves, called key ways are cut both in the shaft and the part mounted on it. The key is fitted between the shaft and the part mounted over it. While transmitting the power, the key will be subjected to shear and crushing forces. Keys are extensively used to hold pulleys, gears, couplings, clutches, sprockets, etc., and the shafts rigidly so that they rotate together. They are also used to mount the milling cutters, grinding wheels, etc., on their spindles.
Rods of circular and square or rectangular cross sections subjected to axial tensile or compressive forces, are connected together by a cotter joint. It is a temporary method of fastening of the two rods which will have to be frequently assembled and disassembled. Its chief advantages are that the joint can be quickly assembled and disassembled and the rods occupy exactly the same relative positions after assembly. The cotter joints are used to connect the piston rod to the cross head of the steam engine, pump or compressor. Long tie bars in steel structures are sometimes built up of round bar, of short lengths and joined together by cotter joints.
A key is a square, rectangular, circular or semi-circular piece of mild steel or wrought iron, which is inserted in a recessed shaft or hub called key ways. When the key is in position, it prevents relative rotary motion between the mating parts. Sometimes, a key also prevents axial motion in the two parts.
Types of Keys
The keys can be classified as per the shape and purpose for which they are used. Following are the types of keys:
(a) Sunk keys (b) Saddle keys
(c) Tangent keys (d) Serrated shaft and Splines
(e) Round keys or Taper pins
Classification of Keys
Keys are classified into three types as follows:
(a) Taper keys (b) Parallel or Feather keys
(c) Special keys
Taper Keys. A taper Key is of rectangular cross section having uniform width and tapering thickness. The taper keys are used to transmit only the turning moment between the shaft and the hub without any relative rotational and axial motion between them. The examples of tapered keys are
(a) Taper sunk key (b) Saddle key
(c) Flat key (d) Gib head key
Parallel or Feather Keys. A parallel key or feather key is also of rectangular cross section of uniform width and thickness throughout. Parallel keys are used to transmit the turning moment between the shaft and the hub along with the provision to allow a small sliding axial motion between them wherever required. The examples of the parallel keys are:
(a) Parallel sunk key (b) Peg key
(c) Single head key (d) Double head key
(e) Spline shaft.
Special Keys. The woodruff key, cone key and pin key are the special purpose keys used for specific applications.
A key that engages a slot formed in both pulley and shaft is known as sunk keys. The sunk keys are of following types:
(a) Rectangular key (b) Square key
(c) Gib head key (d) Woodruff key
(e) Feather key
Rectangular Key. This type of key is rectangular in cross section and is very commonly used. The dimensions of this type of key are as follows:
Width of the key W = D / 4
Thickness of the key T = D / 6
Where D is the diameter of the shaft.
A taper of 1 in 100 on the thickness and parallel in width is given to the key. The taper is given on the upper surface of the key, i.e. the hub side. The recess in the hub is also given the same taper. The thickness of the key is measured at the large end.
Square Key. The cross section of the key is square and the dimension is: Side of the key = D / 4
A taper of 1 in 100 on the thickness and parallel in width is given to the key. The taper is given on the upper surface of the key, i.e. the hub side. The recess in the hub is also given the same taper. This type of key is used for mounting pulleys or gears on shafts. Square keys are used for shafts upto 17mm in diameter.
Gib Head Key. This key can be rectangular or square in cross section having a head at the large end. The head makes it easier to remove the key from the hub and shaft. The slot for gib head key must have an open end to permit assembly. For this reason it is placed at the end of a shaft. The dimensions of a gib head key are given in image below.
Woodruff Key. A woodruff key shown in image below is segmental in shape and is an easily adjustable sunk key. The key fits into a semicircular key way cut into the shaft. The top of the key fits into a plain rectangular keyway in the hub. This key has the advantage of aligning itself with the taper of the hub and will not easily turn over, because of its extra depth in the shaft. Woodruff key largely used in automobile work and machine tools. The dimensions of the woodruff key are standardized. The dimensions of a woodruff key for ød and ø22 shaft are shown in image below. A woodruff key is designated as :
Woodruff key 5 X 9 IS: 2294
Where 5mm is the width and 9mm is the height of the key. IS: 2294 is BIS code for woodruff key.
Feather Key. The feather key is attached to the shaft or the hub and permits relative axial movement. Image below shows a feather key with a double gib head, a feather key secured to the shaft with the help of two set screws, a feather key attached to the hub with the help of one set screw and a peg feather key. The peg of the key fits into the slot provided in the hub.
Saddle Keys. They are tapered keys and are of two types:
- Flat saddle key
- Hollow saddle key
Flat Saddle Key. It is a tapered key which fits in the key way of the hub and the flat surface on the shaft. It has got the tendency to slip round the shaft, that is why, it is suitable for light duty. Image below shows the proportions of a flat saddle key.
Hollow Saddle Key. This is also a tapered key fitting into the key way of the hub and the bottom of the key is curved so as to fit the curved surface of the shaft. It is suitable for light duty work. Image below shows the proportions of a hollow saddle key.
These types of keys are used for heavy duty. The key is placed tangential to the shaft. These keys can withstand torsion on one side. If it is required to be used in a reversible unit then two keys at 90 or 120 are used. The key can have two wedge shaped pieces or a single rectangular or square piece as shown in image below.
Serrated Shafts and Splines
To prevent the key working loose, to reduce the working stresses and to give a greater bearing surface, it is now a standard practice to design shafts with keys machined integrally with the main body. The splines or serrations are milled or hobbed to size and shape and then heat treated. The slots or serrations are finished by broaching. The serrated or splined shafts are generally used in automobiles. In case of splined shaft, the number of splines can be four, six or even more. Image below shows an assembly of a serrated shaft used in a scooter kick with the view of a splined shaft and hub.
Round Key or Taper Pin
A round or taper pin is commonly used for fastening a collar and pulley to shaft as shown in image below. The hole for the pin is drilled and reamed with the parts assembled. In some instances the pin is expected to shear before other parts of the assembly are damaged. So it also acts as a safety device.
A cotter is a flat, wedge shaped piece of steel, used to connect rigidly, two parts which are subjected to axial forces only. The cotters have a uniform thickness throughout the length and taper on one or both side. The taper varies from 1 in 8 to 1 in 48. The cotters having large tapers need a locking device.
Length of the cotter L = 3.5d to 4d
Breadth of the cotter B = d to 1.32d
Thickness of the cotter T = 0.15d to 0.25d
The cotter joints are subjected to axial forces such as tensile or compressive forces. The joints differ from key joints which are used to join shafts subjected to torsional stresses only.