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Sunday, August 16, 2015

Fail Of the Week Catastrophic Damage to Hydraulic Gear Pump & Power Take Off Unit.

What is a PTO? A Power take of Unit is a specially designed gearbox that bolts to an Truck's Gear Gearbox or Transmission. They are used to drive Hydraulic Gear or Piston Pumps and to Provide Fluid power to Truck Mounted Hydraulic Systems such as Compactors, Cranes, Walking Floors, Tilt trays, Tippers and Winches.
Most PTO Systems are designed to be engaged and operated whilst the truck is stationary and running at low to medium low engine speeds.
So what happens when you forget to switch off the PTO and decide to drive off to the next destination? Overspeed, Cavitation, Heat, Noise = Catastrophic Failure.

In this case the PTO was driving a pump for a tipper application the driver may have heard the additional noise and should have felt a great reluctance for the transmission to change gears.
The Picture below shows such a failure probably caused by a combination of over speed, cavitation and possibly heat. The PTO is on the Right and Gear Pump is on the left.
Cavitation is the rapid formation and violent collapse of vapor pockets in a flowing liquid caused by areas or regions of very low pressure (read vacuum). Cavitation can and will cause Structural and Errosive damage.
Cavitation damage can also be found in other types of liquid pumps as well as boat propellers.

Running the PTO at High Speed caused the pump to Cavitate, Over Heat and rapidly Wear and Errode and eventually caused a hardened tooth to break off one of the gears. Being a solid the broken tooth jammed the rest of the gear pump and caused the whole pump to almost instantly stop. Because the PTO was still engaged and driving the pump the input shaft of the pump then sheared off in the PTO's Output shaft.

A failure like this will also require that the whole hydraulic system including hoses, pipes, cylinders, tank and suction strainer be flushed cleaned and all hydraulic oil and filters be replaced.

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