Machinery's Handbook, 31st Edition
Friction Brakes 2537 apparatus in which it is to be utilized. Dynamometers known as indicators operate by simultaneously measuring the pressure and volume of a confined fluid. This type may be used for the measurement of the power generated by steam or gas engines or absorbed by refrigerating machinery, air compressors, or pumps. An electrical dynamometer is for measuring the power of an electric current, based on the mutual action of currents flowing in two coils. It consists principally of one fixed and one movable coil, which, in the normal position, are at right angles to each other. Both coils are connected in series, and, when a current traverses the coils, the fields produced are at right angles; hence, the coils tend to take up a parallel position. The movable coil with an attached pointer will be deflected, the deflection measuring directly the electric current. Formulas for Block Brakes F = force in pounds at end of brake handle; P = tangential force in pounds at rim of brake wheel; μ = coefficient of friction between the brake block and brake wheel. F
Block brake. For rotation in either direction: F P a b b # = +
1 µ
a b Pb = + a k 1 µ
b
a
Fig. 1.
Block brake. For clockwise rotation:
Pb Pc
F
F a b = + − µ
b c 1 µ
a b Pb
= + − a k
c
For counter clockwise rotation: F a b Pb Pc µ = + +
b
a
a b Pb b c 1 µ = + + a k
Fig. 2.
Block brake. For clockwise rotation:
Pb Pc
F a b = + + µ
b c 1 µ
a b Pb
F
= + + a k
c
For counter clockwise rotation: F a b Pb Pc µ = + −
b
a
a b Pb b c 1 µ = + − a k
Fig. 3.
The brake wheel and friction block of the block brake are often grooved as shown in Fig. 4. In this case, substitute for μ in the above equations the value sin cos α µ α µ + where α is one-half the angle included by the faces of the grooves.
Fig. 4.
Friction Wheels for Power Transmission When a rotating member is driven intermittently and the rate of driving does not need to be positive, friction wheels are frequently used, especially when the amount of power to be transmitted is comparatively small. The driven wheels in a pair of friction disks should always be made of a harder material than the driving wheels, so that if the driven
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