79. Work

The overcoming of resistance of any kind through space is called work. It implies a change of position. The fundamental formula for work, therefore is

W(Work) = F(Force) x s(Space).

80. Energy

The capacity of a body for doing work is called energy. It is measured by the amount of work that can be done by that body,. The unit of work is also the unit of energy. Energy is manifested in many forms, but in our work on machines we refer to mechanical energy.

81. Power

The rate at which any agent, such as steam, electricity, animal force, etc., does or has the capacity to do work is called power, and is measured by the amount of work that it does or has the capacity to do in a given unit of time. The formula by which power is determined is represented thus: -

P(Power)= w(Work)/t(Time).

The unit in which power is estimated is called horse power. A horse power (H.P.) is the power to do 33,000 foot pounds of work per minute, or 550 foot pounds per second of time, - i.e. the power equivalent to raising 33,000 pounds 1 foot in 1 minute against the force of gravity. It was established by James Watt, the inventor of the steam engine, who considered that a horse could do this amount of work per minute and introduced this term as the unit of measure. It is probable, however, that the average horse has not that amount of power.

In manual training work steam, gas, or electric power is used to run the woodworking machines. Electricity is preferred on account of its quiet, ease of manipulation, economy of space, and cleanliness. Gas is used extensively, but is not as desirable as electric power. Many manual training plants are also furnished with steam power.

A manual training shop should be equipped with the following power machines: 1 band saw (2 to 3 H.P.) 1 scroll saw (2 H.P.), 1 swing saw (1 to 2 H.P.), 1 combination saw bench (2 to 4 H.P.), 1 single surfacer (4 H.P.), 1 jointer (2 H.P.), 1 grindstone (J H.P.), 20 10" swing wood turning lathes, and 1 pattern makers' lathe (2 H.P.). For running this amount of machinery the writer would recommend two lines of shafting, one for the lathes and one for the other machines. Let the lathe shaft be driven by a 16 H.P. electric motor and the shaft which runs the other machines by an 18 H.P. motor.

Many schools are now equipped with an electric light and power plant. A combined engine and dynamo, like Fig. 123, is desirable for this work. To furnish power sufficient to run 1000 16-candle-power lights, 2 arc lights for the stereopticon lanterns, 1 18H.P. motor for the lathes, and 1 18-H.P. motor for the other machines, would require a 200-H.P. engine and a 125-kilowatt generator. This estimate is based upon the assumption that the generating set should be large enough to furnish all of the power at one time, but it would seldom be necessary to use all of the power at one time, so that possibly a 100 kilowatts generator would develop sufficient power. To furnish sufficient power to operate the light and power plant indicated above, The Ridgway Dynamo and Engine Company, Ridgway,.Pa., recommend a McEwen simple direct connected engine and a Thompson-Ryan direct connected generator with the following specifications: -

Fig. 123. Engine and dynamo.

Fig. 124. Electric motor.

Specification For Engine

The engine to be capable of developing 200 indicated horse power when cutting off at 1/4 stroke, with an initial steam pressure of 90 pounds in the steam chest.

Shipping weight of engine complete, including sub-base, outboard bearing, and extended shaft, 21,200 pounds.

Specification For Generator

The generator to have a rated capacity of 125 kilowatts when running at its normal speed.

The generator will run at the above rated capacity for 24 hours, with a rise in temperature not exceeding 75° F. above the surrounding atmosphere.

The commercial efficiency (the ratio of the energy delivered by the generator to that applied to it), under the conditions of speed and load given below, should not be less than the following: -

Speed 250 revolutions per minute. Load 62 1/2 K.W. Efficiency, 90.3 %. Load 93 3/4 K.W. Efficiency, 91.5%. Load 125 K.W. Efficiency, 92 %.

The generator is able to deliver 25% more than its full rated load for two hours, and 75 % more than its full rated load momentarily, without injuriously heating or sparking and without requiring a change in the position of the brushes.

Other large manufacturers make similar recommendations. If it is not advisable to put in a power plant, power can be secured from the local plant to operate motors in the manual training shop.