Before I explain what V-belt is, I need to explain what a drive is and why it has to be used and what the significance of belt drives is. The need to transmit mechanical power efficiently between different parts of a machine became an impending priority due to the advent of industrial revolution and the complexity it brought to the machines being used.

The assembly of one or more belt(s) with Pulleys that transmit mechanical power is called as belt drive.

A brief overview of belt drives is as follows:

Flat Belt Drives

The Flat Belt Drives were the sole type available for Power Transmission till a large part of 20th century.

Please have a look at the following figure – Fig. 1 which shows a flat belt drive running in an old mill.

Fig. 1

It is called a ‘friction drive’ as this type of power transmission drive transmits power between the belt and Pulleys via friction. The tension of the belt is of prime importance in this type of drive. This is because friction between the belt and Pulleys is the prerequisite of power transmission and the tension creates it.

The belt will roll out of the Pulleys if they are misaligned. In turn, the drive is rendered useless unless the issue is addressed. This is a predominant issue of Flat Belt Drives.

V-Belt Drives

John Gates (of the Gates Rubber Company) had presented first Patent for the V-Belt in 1917.

This had resolved many of the issues encountered with Flat Belts. The cross-section (shape) of the V-Belts is somewhat trapezoidal. They have tapered sides resembling the letter ‘V’, hence acquiring the name – V-Belts.

The Pulleys also have tapered sides forming a ‘V’-groove in which the belt sits. By virtue of this shape the V-Belt does not get much possibility to slide away from the Pulleys. Thus the shapes of the Pulleys are adopted to the shapes of the Belt and the drive transmits power via the wedging action. Please have a look at the following figure – Fig. 2

Fig. 2

Since V-Belts transmit power through their tapered sides, the shape of the Pulley groove and how the belt fits into this groove is important. The tapered part of the Pulley is called Sheave. The following figure – Fig. 3 shows correct position of V-Belt sitting in Pulley groove. The Belt slides into the groove until it rides on the bottom of the groove if its walls are worn out (which is generally unevenly) or if the Belt is damaged. This phenomenon reduces the friction and ultimately the belt may slip.

Fig. 3

Classical V-Belts come in five sizes identified by a first letter - A to E. For example, An A70 Belt is 70 in long with a width of ½ in and height of 5/16 in. Hence the right most number corresponds to the length of the Belt and A stands for the code of a width – ½ in and height 5/16 in. Please refer to the figure Fig. 4 in this context. Also, please notice the angle of the Belt, which is 40 degrees for classical belts, also called conventional or standard V-Belts.

Fig. 4

V-Belts are usually made of rubber with fibers embedded in it to strengthen. Besides this base material, V-Belts have ‘tension members’ (or load bearing cords) which are usually made of polyester. The tension members increase the load bearing capability of the belt. If the V-Belt is overloaded (or if it ages) tension members may break, thereby decreasing the load bearing capacity of the belt. With less number of tension members, the Belt stretches and slides into the Pulley groove which makes the Belt slip or fail altogether. Figure Fig. 5 shows a V-Belt cross-section with the tension members

Fig. 5