Thermodynamics of the steam-engine and other heat-engines by Peabody Cecil Hobart 1855-

Author:Peabody, Cecil Hobart, 1855-
Language: eng
Format: epub, pdf
Tags: Engines, Steam-engines, Thermodynamics
Publisher: London, Macmillan
Published: 1889-03-25T05:00:00+00:00

water that does not come in contact with the rubbing surfaces. Sometimes the wheel is cooled by a stream of water circulating through it, sometimes the band is so cooled, or both may be. A rubbing surface which is not cooled should be of non-conducting material.

To avoid the increase of friction on the brake-bearings due to the load applied at a single brake arm, two equal arms may be used with two equal and opposite forces applied at the ends to form a statical couple.

With care and good workmanship a friction brake may be made an instrument of precision sufficient for physical investigations, but with ordinary care and workmanship it will give results of sufficient accuracy for engineering work.

All forms of transmission dynamometers should be standardized, and should have their errors determined by comparison with a friction brake.

Indicators. —Our knowledge of the errors of indicators, whether of kind or degree, is very limited. Preliminary experiments seem to show that at moderate speeds, i.e., those that give little or no oscillation of the piston and pencil motion, the largest errors are due to backlash and pencil friction. The latter may be reduced by making the pencil pressure light, but there is no remedy for the former. It probably does not introduce an error of more than one or two per cent in the diagrams taken with good indicators.

It is essential that the reducing motion should be correct, and that the indicator-cord should be short. The communication between the indicator and the cylinder should be short and direct, but if a pipe must be used it should be well wrapped to avoid radiation.

Scales. —Weighing may be done with scales adapted to the load. They should be tested with standard weights.

Weirs and Orifices. —When possible, the quantities of water involved in an engine test should be weighed directly ; and by proper provision of large tanks and scales, and with large valves, large quantities of water may be thus determined. When the water cannot be weighed directly it may be meas-

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