There exists a basic formula, which should be used with every PTO selection, counsels King. That formula is: PTO Torque = (HP x 5,252)/RPM
By way of example, suppose a PTO will run a pump requiring 25 hp at an input speed of 1,000 rpm.
Multiplying the horsepower figure of 25, times 5,252, nets 131,300. Dividing this by the rpm figure of 1,000, the result is 131.3 foot pounds.
ROTATION AND ENGINE SPEED
Two remaining factors complete the PTO selection process, says King: rotation and percent of engine speed.
“Pumps, as well as other driven equipment, operate in one of two directions: engine or opposite engine. The direction of the device being driven determines which PTO to choose, as they, too, operate in the same two directions.”
Next, and last, consider the PTO’s percent engine speed, advises King. “This speed can increase, decrease or directly transfer the speed needed to move the equipment to be driven by the PTO. As such, the relationship is slated as a percentage.”
Assuming the same pump used in the preceding example again requires 1,000 rpm to operate. However, an engine operating speed of 1,500 rpm is desired. The PTO needed to complete this job will need to be rated at approximately two-thirds (67 percent) of engine speed.
“On the other side of the coin, if another pump requires 1,500 rpm and the desired engine operating speed is only 1,000 rpm, the PTO will have to be rated at approximately 150% of engine speed,” he says.
On new applications, the pump’s gpm capacity may often dictate which PTO to select, adds King. Distributors and manufacturers usually maintain up-to-date catalogs that cross reference both pump and PTO requirements to satisfy most any application need.
RELIABILITY IS THE RESULT
“Done step by step, the power take-off selection process is not all that complicated,” concludes King. “As a matter of fact, it may be one of the easier jobs a maintenance professional will ever have to face.”
By following the few basic dos outlined within this article, and avoiding the don’ts, a vehicle’s PTO should easily end up being just what it was intended to be: a most reliable add-on component, he says.
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