Fig. 448.—Right way to run a belt. The pull should come on the lower side bringing the slack on top.
3. When belt fasteners give way there is too much strain upon belt. The greatest amount of slack in a belt is found where it leaves the driving pulley, hence the tightener should be near the driving pulley, as it takes up the slack, prevents vibration and diminishes strain on belts and bearings. More than 100 degrees of heat is injurious to belts.
4. Double belts should always run with the splices, and not against them. Quarter turn belts should be made of two ply leather, so as to diminish the side strain.
5. Friction is greatest when the pulleys are covered with leather. Friction depends upon pressure, but adhesion depends upon surface contact; the more a belt adheres to pulley surface without straining, through too much tightening, the better the driving power. Slipping occurs on wet days because the leather absorbs dampness.
6. A leather covered pulley will produce more resistance than polished or rough iron ones. A good belt dressing makes a smooth, resisting surface, and as it contains no oils which create a slippery surface to belts, it increases belt adhesion. The friction of leather upon leather is five times greater than leather upon iron.
7. Moisture and water distend the fibres, change the properties of the tanner's grease and softening compounds. Repeated saturation and drying will soon destroy leather. Leather well filled with tanner's grease or animal oil, if allowed to hang in a warm room for several months without handling, will dry out, become harsh, and will readily crack.
8. A running belt is stretched and relaxed at different times and unless there be perfect elasticity in all its parts there will not be uniform distension.
9. There should be 25 per cent. margin allowed for adhesion before a belt begins to slip.
Figs. 449 to 451.—Method of aligning engine and dynamo. In fig. 449, a line is stretched from A to E and the dynamo shifted until the line contacts with points A, D, I, and E. In a small dynamo, the pulley may be loosened and set back on the shaft as in fig. 450, while lining up the faces, and then moved back to its original position as in fig. 451. When the pulley is not easily shifted the distances at A and D (fig. 449) may be measured.
Rules for Calculating Speed and Sizes of Pulley.—When two pulleys are working together connected by a belt, the one which communicates the motion is called the driver and the other which receives it, the driven pulley.
To Find the Size of the Driving Pulley: Multiply the diameter of the driven pulley by its required number of revolutions, and divide the product by the revolutions of the driver. The quotient will be the diameter of the driver.
To Find the Number of Revolutions of the Driven Pulley: Multiply the diameter of the driver by its number of revolutions, and divide by diameter of driven. The quotient will be the number of revolutions of the driven.
To Find the Diameter of the Driven that shall Make a Given Number of Revolutions, the Diameter and Revolutions of the Driver Being Given: Multiply the diameter of the driver by its number of revolutions, and divide the product by the number of revolutions of the driven pulley. The quotient will be the diameter of the driven pulley.
Rope Drive.—In this method of power transmission, rope is run in V-shaped grooves in the rims of the pulleys; this form of drive, in some cases, is more desirable than others.
Fig. 452.—General Electric C Q back geared motor driving Hamilton sensitive drill. When slowly moving machines are to be driven, or where, for any reason, very moderate belt speeds are required, the back geared motor is desirable. Two ratios of gear reduction have been adopted as standard; they are:—4 to 1 and 8 to 1.
Ques. What are some of the advantages of rope drive?