ABSTRACT
Figure 3.1a presents an elementary chain drive that consists of sprockets 1 and 2 and chain 3. Because of general thermal movements that can be considerable at large distances and the polygonal effect (see Chapter 27 and Figure 27.6), the chain should be slackened a little bit. The recommended sag S ≈ (0.015 … 0.025)C mm, where C = center distance. Usually C ≈ (30 … 50)P, where P = chain pitch. Smaller center distances (Figure 3.1b) are less appropriate because the shorter chain makes more cycles of turn in the hinges (inversely proportional to its length) and wears quicker. In addition, the angle of wrap of the small sprocket may be too small in this case. (It is recommended to have six teeth at least engaged with the chain at any moment.) On the other hand, greater center distance, if not dictated by the location of the shafts, is also undesirable because it leads to unreasonable increase in dimensions and weight of the housing and requires additional parts to support the weight of the chain (see Figure 3.5i).
