Initial tension is the key to the functioning of the flat belt as intended. There are
ways of controlling initial tension. One way is to place the motor and drive pulley on a
pivoted mounting plate so that the weight of the motor, pulley, and mounting plate and
a share of the belt weight induces the correct initial tension and maintains it. A second
way is use of a spring-loaded idler pulley, adjusted to the same task. Both of these
methods accommodate to temporary or permanent belt stretch. See Fig. 17–11.
Because flat belts were used for long center-to-center distances, the weight of the
belt itself can provide the initial tension. The static belt deflects to an approximate catenary
curve, and the dip from a straight belt can be measured against a stretched music
wire. This provides a way of measuring and adjusting the dip. From catenary theory the
dip is related to the initial tension by