Machinery's Handbook, 31st Edition
2530 COUPLINGS and Clutches 308 = constant (see Factors Governing Motor Selection on page 2652) t = time to required speed in seconds Example 1: If the inertia is 80 lb-ft 2 , and the speed of the driven shaft is to be increased from 0 to 1500 rpm in 3 seconds, the clutch starting torque in lb-ft is T 308 3 80 1500 130 lb-ft c # # = = The heat E , in BTU, generated in one engagement of a clutch can be calculated from the formula: . E T T T WR N N 47 10 c c 1 6 2 1 2 2 2 # # # # = − − ^ ^ h h where WR 2 = total inertia encountered by clutch in lb-ft 2 N 1 = final rpm N 2 = initial rpm T c = clutch torque in lb-ft T 1 = torque load in lb-ft Example 2: Calculate the heat generated for each engagement under the conditions cited for Example 1. . . E 130 10 47 10 130 80 1500 415BTU 6 2 # # # # = − = ^ ^ h h The preferred location for a clutch is on the high- rather than on the low-speed shaft because a smaller-capacity unit, of lower cost and with more rapid dissipation of heat, can be used. However, the heat generated may also be more because of the greater slippage at higher speeds, and the clutch may have a shorter life. For light-duty applications, such as to a machine tool, where cutting occurs after the spindle has reached operating speed, the calculated torque should be multiplied by a safety factor of 1.5 to arrive at the capacity of the clutch to be used. Heavy-duty applications such as frequent starting of a heavily loaded vibratory-finishing barrel require a safety factor of 3 or more. Positive Clutches.— When the driving and driven members of a clutch are connected by the engagement of interlocking teeth or projecting lugs, the clutch is said to be “positive” to distinguish it from the type in which the power is transmitted by frictional contact. The positive clutch is employed when a sudden starting action is not objectionable and when the inertia of the driven parts is relatively small. The various forms of positive clutches differ merely in the angle or shape of the engaging surfaces. The least positive form is one having planes of engagement which incline backward, with respect to the direction of motion. The tendency of such a clutch is to disengage under load, in which case it must be held in position by axial pressure.
e
8°
h
69°
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θ
θ /2
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A
B
C
D
E
Fig. 1. Types of Clutch Teeth This pressure may be regulated to perform normal duty, permitting the clutch to slip and disengage when over-loaded. Positive clutches, with the engaging planes parallel to the axis of rotation, are held together to obviate the tendency to jar out of engagement, but they provide no safety feature against over-load. So-called “under-cut” clutches engage more tightly the heavier the load, and are designed to be disengaged only when free from
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