Motion Transmission with Belt Pulleys

In the subject of motion transmission with belt pulleys, machine elements vary depending on whether the shafts the pulleys are mounted on are parallel or perpendicular to each other. Different methods are used to transmit motion between perpendicular shafts than between parallel shafts.

Methods of Motion Transmission with Belt Pulleys

Belts transmit motion from one shaft to another with the help of pulleys. The pulley on the shaft the motion is taken from is called the (driving pulley), and the pulley on the shaft the motion will be transmitted to is called the (driven pulley).

In belt drives, there are two types of shaft arrangement;

• Shafts parallel to each other,
• Shafts perpendicular to each other.

Motion transmission with a belt between parallel shafts:

In all such drive arrangements, the belt wraps around the driving and driven pulleys in an endless loop. While the belt is stationary, the tension in sides (C) and (D) is equal. Once motion is imparted to the driving pulley (A), due to the friction force created between this pulley and the belt, side (C) of the belt becomes taut. Of the two sides between the pulleys, one becomes taut and the other becomes slightly slack. (Figure below)

If the pulleys are meant to rotate in the same direction, the belt must wrap around the pulleys as shown in the figure. This type of arrangement is called an (open belt drive).

 Stacked (Compound) Drive

This is used to transmit motion from a single pulley wide enough for the belt, simultaneously, to two separate pulleys.

In locations with excessive belt slippage (where the difference between diameters is large and, additionally, the shafts are close together), a stacked drive can be used if widening the intermediate pulley is undesirable.

Motion Transmission with a Belt Between Perpendicular Shafts:

The axes of perpendicular shafts can lie in two separate planes, or within the same plane. Motion can be transmitted with a belt in both cases; let’s look at each separately.

Motion transmission with a belt between shafts whose axes lie in two separate perpendicular planes

In this type of arrangement, the shafts (or their extensions) do not intersect, and if one is projected onto the other, they form a (90°) angle. The arrangement shown in (Figure: 45) is also called a (quarter-turn drive).
In a quarter-turn drive, motion is only obtained in one direction. If the pulley is rotated in the opposite direction (relative to Figure: 45), the belt will come off.

In this arrangement, the belt shifts back and forth on the driven pulley (B), along the direction of the pulley’s axis. For this reason, the driven pulley’s face width must be wider than a normal pulley face.
In a quarter-turn drive, the pulley position is found by the following methods.

The theoretical method:

The centerline (xy) of driving pulley (A) should be a distance (e₁) from the face of driven pulley (B), where (e₁ = 0.1 … 0.2 b). The centerline (x’y’) of the driven pulley, in turn, is a distance (e₂) from the face of the driving pulley (e₂ = 0.5 … 0.6 b).
To establish the distances (e₁) and (e₂) between the two pulleys relative to each other, a plumb line is dropped from the center point of driving pulley (A)’s face. Driven pulley (B) is adjusted according to the plumb line’s cord.

The practical method:
The theoretical method helps us find the approximate position of the driven pulley relative to the driving pulley. If the formulas given for the theoretical method’s (e₁) and (e₂) values are examined, they turn out not to be precise. This is because the values of (e₁) and (e₂) are affected not only by the belt width, but also by the distance between the shafts, the belt thickness, and even the belt’s flexibility. For this reason, the position of the pulleys relative to each other is determined by running the belt.

As mentioned earlier, while the belt is running, it moves back and forth on the driven pulley along the direction of that pulley’s axis. The belt is set in motion, and the driven pulley is adjusted by sliding it so that the midpoint of the belt’s travel on the driven pulley aligns with that same pulley’s (x’y’) centerline.

In a quarter-turn drive, if the distance between the shafts is too small, the belt will be forced to bend excessively, which is incorrect. For this reason, the minimum required distance between shafts in a quarter-turn drive is found using the formula below:

In the formula:
l = the minimum distance between the shafts,
b = the width of the belt,
D = the diameter of the larger pulley used in the arrangement.

(Figure: 47) shows motion being transmitted from shaft (A) to shaft (B) using guide pulleys (E) and (E’). This figure shows motion transmission, with the help of guide pulleys, between pulley (C) and pulley (D), which are in a quarter-turn position relative to each other.
Because the belt operates under unfavorable conditions, the power transmission capacity of a quarter-turn drive is (25%) less than that of the open drive shown in (Figure: 41).

Motion transmission with a belt between shafts that are perpendicular to each other and lie within the same plane:

In this type of arrangement, since the shaft axes lie in the same plane, the extensions of the shafts intersect each other. This kind of arrangement is called a (right-angle drive) (Figure: 48).

In a right-angle drive, a guide pulley such as (C) is required for motion to be transmitted between pulley (A) and pulley (B). The diameters of pulleys (A) and (B) depend on the motion transmission ratio, and their face widths depend on the belt width. The diameter (D) of guide pulley (C) used in this arrangement must be (2 … 2.5) times the largest of the other pulleys. The guide pulley’s face width, in turn, must be no less than (1.5) times the width of the belt used.

In a right-angle drive, the belt’s direction of travel can be whatever is desired. In other words, there is no issue with changing the belt’s direction of travel while the pulleys’ positions remain unchanged.