Basics
2.1.2 Engagement with Sprockets Although chains are sometimes pushed and pulled at either end by cylin- ders, chains are usually driven by wrapping them on sprockets. In the follow- ing section, we explain the relation between sprockets and chains when power is transmitted by sprockets. 1. Back tension First, let us explain the relationship between flat belts and pulleys. Figure 2.5 shows a rendition of a flat belt drive. The circle at the top is a pulley, and the belt hangs down from each side. When the pulley is fixed and the left side of the belt is loaded with tension ( T 0 ), the force needed to pull the belt down to the right side will be:
µ u
T 1 = T 0 3 e
For example, T 0 = 100 N: the coefficient of friction between the belt and pulley, µ = 0.3; the wrap angle u = π (180˚).
T 1 = T 0 3 2.566 = 256.6 N
In brief, when you use a flat belt in this situation, you can get 256.6 N of drive power only when there is 100 N of back tension. For elements without teeth such as flat belts or ropes, the way to get more drive power is to increase the coefficient of friction or wrapping angle. If a substance, like grease or oil, which decreases the coefficient of friction, gets onto the contact surface, the belt cannot deliver the required tension. In the chain’s case, sprocket teeth hold the chain roller. If the sprocket tooth configuration is square, as in Figure 2.6, the direction of the tooth’s reactive force is opposite the chain’s tension, and only one tooth will receive all the chain’s tension. Therefore, the chain will work without back tension.
Roller
Tooth Force
Chain Roller
T 0
T 1
Figure 2.5 Flat Belt Drive
Figure 2.6 Simplified Roller/Tooth Forces
12
Made with FlippingBook Digital Proposal Creator