What is Horsepower? | Wagga Wagga Riverina NSW

What is Horsepower?

Webster defines horsepower as a mechanical unit of power for expressing the rate of doing work. This rate of doing work 1 horsepower = 33,000 foot pounds of work in one minute was expressed by James Watt, a Scottish inventor, some 200 years ago.

Horsepower = Ft/lb Torque x RPM 

Kilowatts = Nm Torque x RPM 

By Watts observation of horses working on the London docks, he calculated that a horse could raise a 250 lb. weight 88 feet (250lb x the 88 feet distance, he determined the average horse could perform 22,000 ft. lbs. of work in one minute). Watts, being an inventor of the steam engine and a salesman of his own product, reasoned that his machine did not become tired like a horse and that his machine could do more work than a horse, so he added 50% to his finding of the average horse performing 22,000 ft. lb of work per minute. He called this rate of 33,000 ft. lbs. of work per minute = 1 horsepower. His method of calculating horsepower was not a very scientific set of values, yet a good workable comparative system. This formula for horsepower is still used today as an expression of the rate of doing work.

It is very important to understand that "work" is the product of the force of lbs. x distance of feet. In mechanical terms it is expressed as ft. lbs. with no time element. In comparing the performance of mechanical devices we must measure how many foot lbs. of work can be performed in a 1 minute period of time. Only by putting the element of time, 1 minute, into the comparison can we call it power. In other words, power is the rate of doing work and is expressed in ft. lbs. per minute.

It makes no difference whether the work or force is travelling in a straight line, a crooked line, or in a series of circles. The main consideration is the distance travelled in 1 minute and lbs. of force.

In todays world of motors and engines we express this circular force in motion as "TORQUE." By Websters definition it covers both rotary or dynamic and stationary or static torque. For our purposes torque is created by a lever or lever device, be it the opposing electrical field in an electric motor or a crankshaft in an engine, or torque wrench.

Let us take a one foot lever or a one foot torque wrench. We will apply a 1 lb. force on it at the 1 foot position from its center. This would be very similar to the crankshaft in an engine. If we were to rotate this 1 foot lever with 1 lb. of force or torque and complete 1 revolution we would have produced 6.28 ft. lbs. of work or torque.

You recall that it takes 33,000 ft. lbs. of work in 1 minute to equal 1 horsepower. By dividing 33,000 ft. lbs. by our 6.28 ft. lbs. we arrive at a constant value of 5252 which is the new horsepower unit for rotary power. In other words, 1 ft. lb. of dynamic torque would have travelled 5252 revolutions in 1 minute to equal 1 Horsepower.

Horsepower = Ft/lb Torque x RPM

Torque = HP X 5252

RPM = HP X 5252 

A dynamometer is a device which establishes a controlled torque reaction. It measures this reaction to obtain the lbs. of force involved in a specific test by applying Newtons Third Law of Motion. For every action there is an equal reaction, if a force acts to change the state of motion of a body, the body offers resistance equal and directly opposite to the force.

Through this conversion of the mechanical energy, (the power created by the motor) the dynamometer converts it into heat energy, or B.T.U., of which must be cooled properly.

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