Friction and Belt Slippage on Belt Pulleys

In our article about the relationship between pulley diameters and rotational speeds on pulleys, belt slippage was also touched on. We’re continuing our machine elements publications with this article on friction and belt slippage on belt pulleys.

Why Does Belt Slippage Occur?

Low friction force between the pulley and the belt causes belt slippage. As a result, the speed of a point taken on the circumference of the driving pulley is always greater than the speed of another point taken on the belt’s surface where it contacts the pulley.

While the belt is in motion, a radial force is created from the pulley’s rim toward its center. Multiplying this force by the coefficient of friction gives the friction force. The coefficient of friction is affected by the pulley and belt materials, the roughness of the contacting surfaces, how oiled the belt is, and belt speed. If the belt speed is known, we can approximately find the coefficient of friction with this formula:

2- We know that one side of the belt is taut while the other is slack. There’s a pulling force on each side of the belt.

If we call the force that drives the belt toward the driving pulley (F1), and the force on the other side (F2), then always: F1 > F2. Because of this, one side of the belt is always tauter than the other.

For a belt drive to run well, this depends on the forces (F1) and (F2). On single-ply belts, the ratio of (F1) to (F2) should be three. When the forces acting on the two sides of the belt differ too much, this causes belt slippage.

The belt shouldn’t be loose. A belt that’s too tight also makes it harder to run.

Belt drive systems have arrangements for tensioning the belt.
In the figure, two arrangements for tensioning the belt are shown together. In the first, there are two vertical “T” slots on plate (E). When the bolts (D) fitted in these slots are loosened, electric motor (A) slides down under its own weight and tensions the belt.

In the second arrangement, the belt is tensioned using bolts (G) on the machine housing. If the lower bolt (G) is advanced, the plate rotates slightly around pivot point (F), and since pulley (B) lifts as a result, the belt loosens. Doing the opposite tensions the belt. To keep bolts (G) from loosening over time due to vibration, they’re locked in place with nuts (H).

Using Tensioner (Idler) Pulleys

The figure shows the use of tensioner pulleys that can apply pressure with a spring, and in the image below, with a weight.

In the figure at the side, the pressure that tensioner pulley (D) applies to the belt is adjusted by loosening or tightening spring (E) with a screw arrangement.

Reducing belt slippage with a tensioner pulley happens in two ways. First, by tensioning the belt, and second, by increasing the wrap angle (a) of the belt around the small pulley.

Friction on belt pulleys — tensioning the belt using a tensioner pulley (above). In this arrangement, weight (D) is adjusted by sliding it along the arm in the direction of the arrows; in the arrangement on the right above, it’s adjusted by adding or removing weight from (D). Weights (D) are specially made.

timing belt crankshaft camshaft — Tensioning a timing belt and its tensioner pulley

Use of a tensioner pulley to tension the timing belt, used in vehicle engines to transmit motion from the crankshaft to the camshaft and to synchronize their timing (above)

Use of tensioner pulleys in a vehicle engine (above)

Increasing the Belt’s Wrap Angle

The belt’s wrap angle on the pulley should be as large as possible. In a belt drive, the belt’s wrap angle should not fall below 150°. As we just mentioned, the belt’s wrap angle can be increased using tensioner pulleys. If a tensioner pulley won’t be used, we should pay attention to the following:

A — The ratio of the large pulley’s diameter to the small pulley’s diameter should not exceed six.

If there’s a significant difference between the pulley diameters, an (intermediate transmission) should be used. Here, an intermediate shaft (B) and intermediate pulleys (C; D) are used. This way, the electric motor shaft’s rotational speed is significantly reduced as it’s transmitted to shaft (F), and belt slippage is also significantly reduced.

B — In a straight (open) drive, the center distance (1) between the shafts should be greater than the sum of the pulley diameters — that is, 1 > D + d.

Preventing Air from Getting Trapped

Especially on wide belts (over 250 mm), air trapped between the belt and the pulley increases belt slippage. To prevent this, holes about 15 mm in diameter should be drilled into the pulley’s rim.

For the same purpose, on V-grooved pulleys, cogged rubber V-belts can be used. Because cogged rubber V-belts bend more easily than other V-belts, they work quite well on small-diameter pulleys.

Making Sure the Pulley Axes Are Parallel

The pulleys’ axes should be perfectly parallel to each other. (Except for quarter-turn and right-angle drives.)

When using leather belts, the pulley’s rim should contact the smooth, hard surface (f) of the belt — that is, the hair side of the animal hide, not the flesh side.

Lubricating the Belt

The belt slips more if it dries out. What oil should be used on belts? In this case, we should periodically clean the belt and lubricate it with tallow or pure whale oil.

Should Machine Oil Be Used on Belts?

We should never let the belt come into contact with machine oils. Because: machine oils ruin the belt’s properties and increase its slipping. Friction on belt pulleys is important.

Shallow grooves (0.1 … 0.3 mm deep) should be cut around the pulley’s rim on a lathe.