Power Take Off Unit

Today I’d like to cover something not all heavy-equipment has: the “power take-off” (PTO) unit. PTOs are auxiliary drives typically found in fuel trucks, drilling rigs, cement mixers, etc. Their existence eliminates the need for a secondary engine to drive components on the chassis. This tip will examine the types of power take-off units, their design, lubricants needed, and their suffering points. The goal of this tip is to bolster our troubleshooting ability in an effort to better support our customers. Remember, using our knowledge to improve the customers’ productivity is the key to successful lubricants marketing. So as usual, grab a beverage, sit back, relax and let’s learn together.

Power Take-off Unit

 The power take-off unit is a device designed to use engine power to run auxiliary equipment (such as pumps, compressors and winches) installed on a piece of mobile equipment. (See Figure 1). An example is a power take-off unit on a fuel truck, where engine power is diverted by the PTO to drive a pump which allows for the transfer of fuel. The design of a PTO is very simple, with either spur or helical gearing, support bearing and shafts. What varies among PTOs is the location. Power take-off units may be:

  • Mounted to the transmission,
  • Located behind the transmission attached to the driveshaft, or
  • Connected directly to the engine at the crankshaft.

Proper PTO operation is dependent upon the correct gear ratio. The input speed at the PTO must be sufficient to provide the rated output speed and torque for operation of the component driven. Depth of mesh is also essential for proper PTO operation and is monitored as backlash. Backlash is the spacing between the non-contacting, or drive surfaces, of meshed gears. It must be sufficient for expansion caused by the heat of operation. Typical backlash for a power take-off unit is between .006 to .012 inches.

PTOs may be engaged while the vehicle is running or when the drive transmission is in neutral. Transmission powered units are clutch dependent and are only in use when auxiliary power is required.

Maintenance, Lubricants and Troubleshooting

A power take-off unit should be serviced at the same interval as the component to which it is attached: 250 hours for an engine location and 500 hours for a transmission or differential location. The oil used in a PTO depends upon where it is located. If the PTO is engine or transmission mounted, it will typically use the oil from the host component. If it is mounted on the differential, it will require a small amount of gear oil (ISO VG 68 - 220 EP, SAE 80-90). For the shaft mounted units, the viscosity oil selected depends on the power and speed requirements of the driven component.

Troubleshooting power take-off is essential as they may wear rather quickly if ignored.

As you know, rapid wear leads to early failure and production losses. Wear in a PTO is most often caused by excessive angles at the driveshafts entering or leaving the unit. Regularly check the angularity of these shafts. Proper shaft angles are shown in Figure 2. Other troubleshooting issues include:

  • Noisy PTO - stripped gears, worn bearings, worn shaft spline. Replace.
  • Slipping out of gear. Causes include:
    • Gears only partially engaged. Adjust shift linkage.
    • Weakened poppet spring. Replace.

Summary

Thank you for participating in this tip of the week. In it we reviewed function, location and design as well as lubricants required, and troubleshooting processes. It is our hope that you will use the knowledge gained to further value and productivity at your customer locations. Also check out our additional content on our new Mobil Lubricants North America LinkedIn page. Until next time!

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