Fig. 97.
Example: Diameter of pulleys 12 and 36 inches and speed of L 500, what is speed of S? Place points of dividers on 12 and 36. Now, if dividers are raised and one point placed on 500 and the other above it on scale A, it will come beyond the top of the scale. Hence go to scale B, placing lower point on revolution scale at 500 and the other point above, which will fall upon 1500, the answer.
Rule 2. Pulley Diameters.—When the speed of both pulleys and the diameter of one is given, to find diameter of the other: Place points of dividers on the two speeds on scale A or revolution scale B. Then place one point of dividers on given diameter and the other above it to find diameter of L, or below it for diameter of S. The figure thus indicated is the required diameter.
Example: Speeds 180 and 450 and diameter of smaller pulley 20. What must be diameter of L?
Place points of dividers on 180 and 450 on scale A. Then place one point on 20 (the given diameter). The other point falls at 50, the required diameter of L.
If the point falls between two graduations in any problem, the result can be closely judged by the relative position.
The other and more labor-saving use for this chart is its application to belting problems. It is generally conceded that there is no subject of more general interest in practical mechanics and none on which there is a greater difference of opinion than the proper allowance to be made in the selection of belt sizes for given requirements. The general formula for the horse-power transmitted by belting is
HP = WS/C in which HP = horse-power,
W = width of belt in inches, S = speed of belt in feet per minute, and C = constant.
The proper values of this constant, or the feet per minute that each inch of width must run to transmit a horse-power, under certain conditions, is the point in question.