Machinery's Handbook, 31st Edition
2536
Friction Brakes
The rotation is clockwise. Find force F required.
F a Pb =
e
µθ
c
m
1
e
µθ
−
. 02 418 . #
24 100 4 #
. 271828
a
k
=
. 271828
1
. . 02 418 #
−
. 271828 1 271828 . . 0836 . 0836 −
24 400
. 1666 231 1 231 294 . . # − =
=
=
.
#
If a hand-held calculator is not used, determining the value of e μ θ is rather tedious, and the table on page 2534 will save calculations. Coefficient of Friction in Brakes.— The coefficients of friction that may be assumed for friction brake calculations are as follows: Iron on iron, 0.25 to 0.3; leather on iron, 0.3; cork on iron, 0.35. Values somewhat lower than these should be assumed when the veloci- ties exceed 400 feet per minute at the beginning of the braking operation. For brakes where wooden brake blocks are used on iron drums, poplar has proved the best brake block material. The best material for the brake drum is wrought iron. Poplar gives a high coefficient of friction, and is little affected by oil. The average coefficient of friction for poplar brake blocks and wrought-iron drums is 0.6; for poplar on cast iron, 0.35; for oak on wrought iron, 0.5; for oak on cast iron, 0.3; for beech on wrought iron, 0.5; for beech on cast iron, 0.3; for elm on wrought iron, 0.6; and for elm on cast iron, 0.35. The objection to elm is that the friction decreases rapidly if the friction surfaces are oily. The coefficient of friction for elm and wrought iron, if oily, is less than 0.4. Calculating Horsepower from Dynamometer Tests.— When a dynamometer is ar- ranged for measuring the horsepower transmitted by a shaft, as indicated by the diagram matic view in Fig. 5, the horsepower may be obtained by the formula: HP LPN 33000 2 π = in which HP = horsepower transmitted; N = number of revolutions per minute; L = dis tance (as shown in illustration) from center of pulley to point of action of weight P , in feet; P = weight hung on brake arm or read on scale. L
P
Fig. 5. By adopting a length of brake arm equal to 5 feet 3 inches, the formula may be reduced to the simple form: HP NP = 1000 If a length of brake arm equal to 2 feet 7 1 ⁄ 2 inches is adopted as a standard, the formula takes the form: HP NP = 2000 The transmission type of dynamometer measures the power by transmitting it through the mechanism of the dynamometer from the apparatus in which it is generated, or to the
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