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HP to kW and back
Convert band sawmills' power sources back to front? Easy!
Electricity? Petrol? Diesel? The power is yours! You can convert the power source and you can switch back should circumstances indicate altering how a Wood-Mizer sawmill is driven. The calculations below show how comparative power sources may be calculated using Wood-Mizer's calculator.
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'Horsepower' (HP) is the name which covers several units of measurement of power. In our calculator we use metric horsepower of 75 kgf-m per second, which is roughly equivalent to 735.499 watts.
Generally, HP is equivalent to 735.5-750 watts, usually stated as approximately 746 watts. In the early 18th century, during Great Britain's industrial revolution, horsepower needed to be defined to compare the output of steam engines with the power of the shire horses which powered and pulled machinery -- 33,000 foot-pounds of work per minute, would you believe?
Subsequently, horsepower became the comparative measurement of the output of piston engines, turbines, electric motors and other machinery.
It was taken up worldwide and inevitably, the definition of the unit varied between geographical regions. Most countries now use the SI unit Watt (or kW) for measurement of power. However, forever seeking standardisation, the European Union ruled in Directive 80/181/EEC on January 1, 2010 that the use of horsepower within the EU is only permitted as a supplementary unit. One can almost hear the gasp of the ghost of Boxer, the self-sacrificing horse in George Orwell's book, Animal Farm, 1945.
Where horsepower came from...
The development of the steam engine led to a comparison of the output of horses with that of the engines. In 1702, the inventor Thomas Savery wrote (in what is today garbled English) in The Miner’s Friend: "So that an engine which will raise as much water as two horses, working together at one time in such a work, can do, and for which there must be constantly kept ten or twelve horses for doing the same. Then I say, such an engine may be made large enough to do the work required in employing eight, ten, fifteen, or twenty horses to be constantly maintained and kept for doing such a work…"
The idea was later used by the Scotsman, James Watt to help market his improved steam engine. Watt calculated that a horse could turn a mill wheel 144 revolutions an hour (2.4 each minute). The wheel was 12 feet in radius so the poor horse travelled 2.4 × 2π × 12 feet in one minute. Watt discovered that the creature could pull with a force of 180 pounds and he rounded it to an even 33,000 ft·lbf/min.
He also worked out that a pony could lift an average 220 lbf (0.98 kN) 100 ft (30 m) per minute in a four-hour shift. Next he adjudged a horse was 50% more powerful than a pony and thus arrived at the 33,000 ft·lbf/min figure.
At first, the English civil engineer, John Smeaton worked out that a horse could produce 22,916 foot-pounds per minute but John Desaguliers increased that to 27,500 foot-pounds/min. In turn, James Watt, in 1782 found that a brewery horse could produce 32,400 foot-pounds per minute. In 1783 James Watt and his fellow engineer Matthew Boulton standardized it to 33,000.
Even so, horsey people who understand their animals' capabilities claim that Watt was either a optimistic or wanted to under-promise and over-deliver. Certainly, few horses can maintain the effort for long.
Nevertheless, the comparison with a horse became the yardstick.
Fit human beings can deliver approximately 1.2 hp briefly and keep up 0.1 hp indefinitely; athletes manage up to about 2.5 hp briefly and 0.3 hp sustainably.
Metric horsepower
In the 19th century metric horsepower emerged in Germany and was adopted in Europe and Asia. Units that indicate this definition (PS, CV, hk, pk, and ch) translate as horse power so it is common to see these values referred to as horsepower or hp in the press releases or media coverage of the German, French, Italian, and Japanese automotive companies. British manufacturers often intermix metric horsepower and mechanical horsepower depending on the origin of the engine in question. Sometimes the metric horsepower rating of an engine is conservative enough so that the same figure can be used for both 80/1269/EEC with metric hp and SAE J1349 with imperial hp.
Metric horsepower, as a rule, is defined as 0.73549875 kW, or roughly 98.6% of mechanical horsepower. Obviously, when measurement systems varied considerably and engines delivered less thrust than today this was not at the top of technicians' agendas but today it is very important. European 'Trophy' vehicles such as the McLaren F1 and Bugatti Vevron are often quoted using incorrect definitions, their output being sometimes converted as much as twice through confusion over whether the horsepower figure from the start was metric or mechanical.
Horsepower definitions!
Defining horsepower is awkward because it varies between applications:
- Mechanical horsepower AKA imperial horsepower of exactly 550 foot-pounds per second is roughly equivalent to 745.7 watts.
- Metric horsepower, 75 kgf-m per second is equivalent to about 735.499 watts.
- Boiler horsepower, for rating steam boilers translates to 34.5 pounds (≈15.65 kilograms) of water evaporated hourly at 212 degrees Fahrenheit (=100 degrees Celcius, =373.15 Kelvin), or 9,809.5 watts.
- An imperial horsepower for rating electric motors is the same as 746 watts.
- Outside the UK, on the European continent electric motors formerly had dual ratings, based on 0.735 kW for 1 HP
- The Pferdestärke PS (that's German for horsepower) denotes a group of similar power measurements in Germany in the late 19th century. Each was around one metric horsepower in size.
- Finally, The Royal Automobile Club (RAC) horsepower or GB-tax horsepower is an overall estimate of several engine dimensions.
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