FIGURE 14 Variation of the ratio of the weight to belt tension, W/T, with the ratio ξ=b/a. Upper pair: top curve, θ=20°, =15°; bottom curve, θ=30°, =15°. Lower pair: top curve, θ=20°, =20°; bottom curve, θ=30°, =20°.

Return to equation (3-15), with center distance c now replaced by cs, the center distance when the belt is stretched, to calculate the increase in angle ζ which is equal to the decrease in angle θ since θ+α+ζ is a constant. Thus

(1-

in which ζ represents its value when the belt is new, i.e., as given by equation (3-16). Call upon equation (1-1) to calculate T/W for the case where =θ= 20° and ξ=0.2, that

is, for a=44.5 cm, to find T/W=1.095. Thus, the tension provided by the weight of the motor is only 199.73 N. Consequently, an additional mass of 9.21 kg must be added to the support to achieve the tension necessary to drive the load. Likewise for =20° and θ=30°, the ratio T/W=0.434, which means that weight of the motor alone can induce a tension of only 79.20 N. Thus an additional mass of 51.50 kg must be added. For simplicity of the following calculations, it will be assumed that the weight may be added such that the center of gravity remains along the centerline of the motor shaft.