Just like an internal combustion engine uses expanding gases to push pistons and do work, a hydraulic motor uses hydraulic fluid pressure to push pistons. The motor uses the steady hydraulic fluid pressure to actuate a series of radial pistons connected to drive shafts attached to various machines. Hydraulic motors produce no sparks, fire or waste gases, as with an internal combustion or electric motor. Although mechanical problems with hydraulic motors occur, most repairs involve correcting leaks in the high-pressure hydraulic components.
Hydraulic motors are loaded with many seals to contain the hydraulic fluid pressure. Each piston (anywhere from five to 12 depending on the motor) has three seals. The hydraulic fluid ports (inlet and discharge) have seals. The spinning shaft connected to the pistons has seals where the shaft exits the motor housing.
If a leak occurs, troubleshooting repairs begin by determining if each seal is tight under pressure. It’s important to examine the seals with the motor pressurized with hydraulic fluid. If a seal is compromised, it must be replaced with a specified seal of the same size and diameter. Stretching a seal a little too small or forcing in a seal a little too big just means there will be a repeated failure very quickly. Use only manufactured-specified seals for necessary repair work.
Hydraulic motors only use two pressure fittings, one on each hydraulic line for fluid entering and exiting the motor. Although made of stainless steel alloy, these fittings may fail at the attachment point between the fitting and the hydraulic line supplying the motor. As with seals, the best repair to a damaged fitting is replacement. Also, any time a fitting is replaced also consider replacing the hydraulic line attached to the fitting. Hoses and annealed tubing are distorted by the swage pressure needed to attach the fitting securely. Repeating a swage fitting in the same location means the hydraulic line is severely weakened and is likely to fail.
Pistons must move freely within the cylinder with exacting tolerances. Any deviation means loss of hydraulic pressure and reduced work. A piston and cylinder’s worst enemy is contamination between the piston and cylinder wall causing dents and other damage. If a piston or cylinder is damaged, the best repair is replacement. Trying to rebuild a hydraulic piston or cylinder to the exacting specifications of within 10,000ths of an inch is very laborious and time consuming. Pulling the cylinder wall, piston or both and replacing each is a much quicker and efficient repair. Also, remember that if one component--the piston or cylinder--is damaged, it’s a good bet the other is also.