Power Take-Offs: 10 Key Questions

PTOs are generally categorised by their mounting type. The three most common PTOs are: side mount, rear mount, and top mount. In another Parker blog post, ‘Understanding Why There are So Many Options for Mounting a PTO‘  you can read about the main mounting types available for a PTO.

The speed of the PTO output is dependent on the internal gearing of the PTO as well as the internal ratio of the transmission in relation to the PTO driver gear. For an automatic transmission, the minimum input speed must be maintained higher than torque converter lock-up for PTO operation – unless the transmission offers “live drive”, meaning the PTO is powered through the impeller. Depending on the internal gearing, PTO output speeds can be less than, greater, or equal to that of the transmission.

When specifying a PTO, we need to find the input horsepower required of the driven equipment. Horsepower is the measure of capacity for doing work per unit of time. Torque is the effort required to perform a twisting or turning motion. The horsepower is figured into the equation to find the torque requirements for the proper PTO to be used. Parker Chelsea categorises product series using torque values.

The equation is: Torque (T) = HP x 5,252 / RPM

Parker Chelsea classifies PTO series as either intermittent or continuous. It is important to note that for an application that is “continuous” duty, (i.e. the PTO is in operation more than five minutes in any given 15-minute period), intermittent torque values must be de-rated by 30% unless the PTO is already classified as continuous duty.

Refer to Parker Chelsea’s application catalogue to help find the appropriate torque requirements of your application. All series are annotated as continuous or intermittent.

Power take-off provisions include special wiring and programming along with apertures on the vehicle’s transmission that allow for the mounting of the PTO. The “PTO ready” option needs to be ordered and configured at the chassis manufacturer when building out a work truck.

There are two major types of independent PTOs: mechanical (i.e. 489 series) and hydraulic (i.e. new 210 series). A hydraulic shift PTO uses a clutch mechanism for engagement. Hydraulic shift PTOs apply to traditional (torque converter) automatic transmissions. A mechanical shift PTO physically engages by shifting one gear into another. This is done typically through a lever, cable or air pressure. Mechanical shift PTOs apply to manual and automated manual transmissions.

Spur gear: A gear whose teeth are cut straight across the face of the gear.

Helical gear: A gear whose teeth are cut on an angle diagonally across the gear either with a right or left-hand slant. For helical gears to mate, one must slant to the right and the other to the left.

Pitch line: The point on the gear tooth midway between the base of the tooth and the tip of the tooth.

Pitch line velocity: The speed of rotation in feet per minute of a gear measured at the pitch line.

Pitch (Gear): The measure of the size of the gear teeth determined by the number of teeth in a given area measured at the pitch line.  PTO gear pitch is normally classified as 5, 6, or 7-pitch.

Gear ratio: It is determined by dividing the number of teeth in the driven gear by the number of teeth in the driving gear.

It is very important to have periodic PTO maintenance for proper, safe, and trouble-free operation of the PTO. It is recommended to follow the maintenance schedule below.

Daily: Check all air, hydraulic and working mechanisms before operating the PTO. Perform maintenance as required.

Monthly: Inspect for possible leaks and tighten all air, hydraulic and mounting hardware, if necessary. Torque all bolts, nuts, etc. to Chelsea specifications. Ensure that splines are properly lubricated, if applicable. Perform maintenance as required.

A potential issue with a PTO can be premature spline fretting or wear caused by torsional vibrations. Traditionally, regular grease application between the PTO output and the PTO shafts is incorporated into the preventive maintenance schedule of the truck. This entails having the pump removed from the PTO every two to three months and having the mating shafts cleaned and re-greased. Parker Chelsea provides spline lubrication grease with every PTO that has a pump mount option. They have also developed Wet Spline technology that helps provide a constant flow of fresh oil to the PTO and pump shafts to mitigate the issue of spline fretting and the wear which leads to not needing the maintenance inspections. Check out what Parker Chelsea offers for Wet Spline Technology and the additional benefits it provides.