If the grain side of a belt is the outside when on the pulleys, and a crack should consequently start, the destruction of the belt proceeds rapidly, because the line of crack is the weakest part of the belt, and the belt has less elasticity as a continuous body, and more at the line of crack. Cracking may, to some extent, be provided against by oiling the belt, and for this purpose nothing is better than castor oil. In the manufacture of belts, extra pliability is induced by an application of fish oil and tallow, applied when the belt (after having been wetted), is in a certain stage of progress toward drying. The oil and tallow are supposed to enter the pores of the leather and supply the place of the evaporated water.

Length of Belts.—Since the stretch of a belt is variable in different belts of the same length, no rule can be given for the amount to which a belt should be cut shorter than the measured length around the pulleys, and it follows, therefore, that the length of a belt cannot be obtained precisely by calculation. In practice the necessary length for a belt to pass around pulleys already in their places upon the shaft is usually obtained by passing a tape line or cord around the pulleys, the stretch of the tape line being allowed as that necessary for the belt. Then when the belt is placed around the pulleys it is shortened if it should appear to require more tension. If, however, the belt length for pulleys not in position is required, it may be obtained as follows, the error being so slight as to be within the margin of difference of stretch in different belts, and therefore of no practical moment:—

Fig. 2663.

For open belts let the distance between the shaft centres, as a b in [Fig. 2663], be the base of a right angle triangle, and the difference between the semi-diameters, as b c, the perpendicular. Square the base and the perpendicular, and the square root of the sum of the two will give the hypothenuse, and this multiplied by 2 and added to one-half the circumference of each pulley is the required length for the belt. This will give a belt too long to the amount to be cut out of the belt to give it the necessary tension when on the pulleys.

Example.—Let the distance between centres in [Fig. 2663] be 48 inches; diameter of large pulley 24 inches; diameter of small pulley 4 inches—

Here distance between centres48
„„„48
384
192
2304
Square of perpendicular100
2404Square root of 2404 = 49.03
Multiply by 2 2
98.06
Half circumference of large pulley 37.699
135.759
Half circumference of small pulley 6.283
Length of belt 142.042

A simpler rule which gives results sufficiently accurate for practical purposes is as follows:—

Rule.—Add the diameter of the two pulleys together, divide the result by 2, and multiply the quotient by 31⁄4, then add this product to twice the distance between the centres of the shafts, and you have the length required.