The break horsepower of comparatively small engine is determined by means of a Prony Break.
The break consist of a flexible band or strap carrying a number of wooden blocks, which encircle the engine flywheel or a break pulley mounted upon the engine shaft. The wooden blocks are in contact with the surface of the wheel and the strap or band may be tightened or loosened upon the break wheel by means of a hand adjustment.
The horizontal distance from C to the center of the engine shaft is the length (r) of the break arm. The Prony device is a friction device and the power developed at the rim of the break wheel is dissipated as heat.
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Case:
Determine the length of a Prony Break arm so that bhp= 0.001 Nw.
bhp = 2 (phi) r Nw / 33,000
r = (0.001 x 33,000) / 2 (phi)
r = 5.25 ft