Machine Design 3(2+1)

LESSON 15 DESIGN OF KEYS

Keys are made of ductile materials. Commonly used materials for a key are hardened and tempered steel of grades C30, C35, C40, C50 and 55Mn75 etc. Brass and stainless keys are used in corrosive environment. Factor of safety of 3 to 4 is generally taken on yield strength.

 

15.2        Types of Keys

Common types of keys are:

1. Sunk keys    2. Saddle keys    3. Tangent keys    4. Round keys    5. Splines

15.2.1    Sunk Keys

A sunk key is a key in which half of the thickness of key fits into the keyway in the shaft and half in the keyway of the hub. The sunk keys are of the following types:

Rectangular sunk key: It is the simplest type of key and has a rectangular cross-section. A taper of about 1 in 100 is provided on its top side. Rectangular sunk key is shown in Figure 15.1.

Figure 15.1                Rectangular Sunk Key

Square sunk key: Rectangular sunk key having equal width and thickness is called square sunk key.

Parallel sunk key: If no taper is provided on the rectangular or square sunk key, it is called parallel sunk key i.e. it is uniform in width and thickness throughout. It is used where the pulley, gear or other mating piece is required to slide along the shaft.

Gib-head key: It is a rectangular sunk key with a head at one end known as gib head, which is provided to facilitate the removal of key. Gib Head key is shown in Figure 15.2.

 Figure 15.2                Gib Head Key

Feather key: Feather key is a parallel key made as an integral part of the shaft with the help of machining or using set-screws. It permits axial movement and has a sliding fit in the key way of the moving piece. Feather keys are shown in Figure 15.3.

Figure 15.3                Feather Key

Woodruff key: Woodruff key is a sunk key in the form of a semicircular disc of uniform thickness. Lower portion of the key fits into the circular keyway of the shaft. It can be used with tapered shafts as it can tilt and align itself on the shaft. But the extra depth of keyway in the shaft increases stress concentration and reduces strength of the shaft. Woodruff key is shown in Figure 15.4.

Figure 15.4                Woodruff Key

 

15.2.2    Saddle Keys

Slot for this type of is provided only in the hub as shown in Figure 15.5. Torque is transmitted by friction only and cannot therefore transmit high torque and is used only for light applications. The saddle keys are of two types: Flat Saddle Key and Hollow Saddle Key. In flat saddle key, the bottom surface touching the shaft is flat and it sits on the flat surface machined on the shaft. Hollow saddle key has a concave surface at the bottom to match the circular surface of the shaft. Chances of slip in case of the flat saddle key are relatively lesser and can transmit more power than the hollow saddle key.

Figure 15.5                Saddle Keys Figure 15.6                Tangent Keys

15.2.3    Tangent Keys

Tangent keys are shown in Figure 15.6. These are used to transmit high torque. They may be used as a single key or a pair at right angles. Single tangent key can transmit torque only in one direction.

15.2.4    Round Keys

The round keys have a circular cross-section and fit into holes drilled partly in the shaft and partly in the hub. Slot is drilled after the assembly so the shafts can be properly aligned. These are used for low torque transmission. Round keys are shown in Figure 15.7.

                              

Figure 15.7                Round Key                Figure 15.8                Splines

15.2.5    Splines

A number of keys made as an integral part of the shaft are called splines. Keyways are provided in the hub. These are used for high torque transmission e.g. in automobile transmission. Splines also permit the axial movement. Splines are shown in Figure 15.8.

15.3          Design of Sunk Keys

Figure 15.9 shows the forces acting on a rectangular key having width w and height h. Let l be the length of the key. Torque is transmitted from the shaft to the hub through key. Shaft applies a force P on the key and the key applies an equal force on the hub. Therefore the key is acted upon by two equal forces of magnitude P, one applied by the shaft (on the lower portion) and the other because of the reaction of hub (on the upper portion).

As these two forces are not in same plane, they constitute a couple which tries to tilt the key. Therefore equal and opposite forces P’ also act on the key, which provide a resisting couple that keeps the key in position.

As the exact location of force P is not known, to simplify the analysis it is assumed that the force P acts tangential to the shaft. If T is the torque transmitted,

where, d          = diameter of the shaft

                                                  

 Figure 15.9          Forces Acting on KeyFigure                                    15.10         Failure of Key a. Shear Failure b. Crushing Falure

In the design of key two types of failures are considered, shear failure and crushing failure.

Area resisting shear failure = w l

 Shear stress,

       Crushing Area =  l h/2

 Crushing stress,

Tables are available which give standard cross-sections for square and rectangular keys corresponding to different shaft diameters. But in the absence of such data, following relations are generally used:

For Rectangular Key:  w = d / 4 and  h = d / 6

For Square Key:         w = h = d /4

For a known diameter of shaft, w and h can be calculated using these relations and then using the above strength equations required length of the key is calculated for given values of allowable stresses. Length is calculated both for shear and crushing and then maximum value out of the two is considered.

 

References:

1. Design of Machine Elements by VB Bhandari
2. Analysis and Design of Machine Elements by VK Jadon
3. A Text Book of Machine Design by RS Khurmi