V-belts are components in power transmission systems that transmit power (or torque) from an input shaft component (for example, a motor) to a secondary, output shaft that is typically the working apparatus of the machinery. The belts are easily identified by their trapezoidal cross-section shape and are ideal for applications that value flexibility, efficiency, ease of installation or affordability.

Figure 1. V-belt drives offer several advantages if they are correctly specified for an application. Source: Adobe/Mulderphoto

Why pick a V-belt vs. a flat belt?

The slippage between the belt and pulley is lesser in V-belts than in flat belts. This is because the V-belt jams more firmly into the pulley's groove, providing a higher level of traction and better torque and power distribution than flat belts. V-belt drives can achieve a transmission efficiency as high as 98% - the highest of all types of belt drives.

The distance between the centers of the pulleys in a V-belt drive is usually smaller than in flat belts, meaning they typically take up less space in many applications. V-belts also offer a broader range of configuration options compared to flat-belt drives. For example, engineers can set up V-belt drives to be vertical, horizontal or inclined without worrying about a significant drop in performance. Finally, V-belt drives are typically more vibration dampening than flat-belt drives.

(Learn more about V-belt drive advantages with Engineering360)

What are some key design considerations for V-belts?

1. Length of the belt

Engineers can specify the length of a V-belt using:

• Outside length
• Effective length
• Pitch (or datum) length

The outside length is a measure of the length about the outer diameter of the belt when tensile force is not acting on it. In contrast, the effective length is a measure of the length about the effective outside diameter of a sheave at a specified tension.

Pitch length describes the length of the belt measured from the belt pitch line, as shown in Figure 2. This length does not change with the belt drive’s operation and it correlates to the location of the internal tensile cord.

Figure 2. V-belt drive with pulleys of equal diameters.

Consider a V-belt drive featuring two pulleys of equal diameters and is solely used for transmitting power, as shown in Figure 2. This pulley drive configuration is such that the angle of wrap (the angle formed by the arc of contact between the pulley and V-belt) is 180°. This means half of the pulley’s circumference is in contact with the belt, and the pitch length can simply be estimated mathematically, as shown below.

Where:

C = Distance between the two pulleys

R = Radius of the pulley

Pulleys of different diameters are generally used in a power transmission application where speed reduction or torque multiplication is required. This configuration is such that the angle of wrap formed by the smaller pulley and V-belt is less than 180° while the angle of wrap formed by the larger pulley is greater than 180°, as shown in Figure 3.

Figure 3. V-belt drive with pulleys of different diameters.

The pitch length of such configuration can be calculated using:

Where:

Lp = Pitch length

D = Diameter of the larger pulley

d = Diameter of the smaller pulley

C = Distance between the centers of the two pulleys

2. The maximum tension of the belt

Like in other belt drives, there needs to be a level of tension (Ti) present in a V-belt drive when it is not transmitting power. This helps to prevent belt slippage from the pulley.

Now consider a V-belt drive featuring two pulleys of different diameters, with the smaller pulley being the driver. Once the driver pulley starts rotating, tangential force is applied to the belt, causing it to rotate and making the tension on one side of the belt greater than the tension on the other side of the belt.

This small amount of tension can be denoted by dT, and it is equivalent to the tangential force that was applied by the driver pulley. This tension causes one side of the V-belt to be tight while the other side becomes slack.

Mathematically,

vbelt_eq4

Where:

T₁ = Tension in the tight side

T₂ = Tension in the slack side

Hence,

But in addition to this tensile force, centrifugal force acts on the belt to increase the tension on the belt at very high speeds. This extra tension is called centrifugal tension, and it represents the value by which the tensions in the slack side and tight side increase. It can be estimated using:

Where:

T_c = Centrifugal tension

M = Mass of the belt

v = Velocity of the belt

Therefore, the maximum tension in the belt can be calculated using:

What are the material options for V-belts?

The base material usually employed in making V-belts is synthetic rubber. This rubber is embedded with fibers to strengthen it. Apart from the base material, V-belts also have tension members, which are best described as load-bearing cords. The tension members are usually made of polyester and they improve the capacity of the belt to bear loads.

Oil-based lubricants should never be used with a rubber V-belt; silicone lubricants are a better option.

Conclusion

There are many additional parameters when designing for or selecting a V-belt. For example, it is advisable to check standards, such as ISO 9981, when choosing V-belts for a particular application.

For further consultation, engineers are advised to discuss their application requirements with V-belt drive manufacturers.