Fluid Dynamics – A discussion on hydraulic motors.

You do not have to look too hard to find a hydraulic motor performing work on a farm. Be it either a hydrostatic drive unit on a combine or sprayer, or keeping the farmstead looking good by powering a mower deck, these wonders of simplicity are capable of many tasks.

Hydraulic motors offer many advantages over other drive systems. Some of these are nearly full torque available on start-up, high power density for the physical size, a significant reduction in shock load, and minimal maintenance and service.

Is it a motor or a pump?

Hydraulic motors share many of the same design elements as a pump with one significant difference. Pumps draw in fluid and force it out, thus converting mechanical energy into fluid energy. In contrast, a hydraulic motor has fluid forced into it and then exhausted out, converting fluid energy back to mechanical energy.

A hydraulic pump and motor are connected to create a hydrostatic drive system.

In the same way that a hydraulic cylinder is referred to as an actuator, you will find a hydraulic motor identified as a rotary actuator in most literature or shop manuals.

A hydraulic motor consists of a high-pressure fluid inlet and an exhaust, where the fluid returns to the pump after the motor has performed work. Just as an internal combustion engine has an intake track and an exhaust, so does the hydraulic motor.

There are three basic styles of hydraulic motors. They are gear, vane, and piston motors. Depending on the work that is required for the motor to perform, the necessary speed, the amount of hydraulic pressure available from the pump, and the space for packaging will determine the type used.

Within these three main categories, there are application-specific types such as internal or external gear motors and axial and variable displacement designs. Regardless of the type, the basic tenet of the fluid driving the hydraulic motor still holds true.

All hydraulic motors have internal and external sealing areas so that the fluid is directed in the proper path to perform work. There are also bearings, shaft(s), and gears along with other parts. All hydraulic motors are designed with very tight machining tolerances and internal clearances.

If the motor develops an internal leak (no visible signs of leakage), then the power output will drop since the fluid is bypassing the working parts.

The main reason for weak hydraulic motor performance is usually rooted in a lack of maintenance. Neglecting fluid and filter changes along with using poor quality hydraulic fluid and filters will result in exponential internal wear of a hydraulic motor.

Keep in mind also that the motor can only work as well as the pump. If the pump is weak, then so will the output of the motor. For this reason, the proper way to diagnose any hydraulic motor issue is with the shop manual and pressure gauges, starting at the pump.

Fluid circuits

The industry identifies hydraulic motors depending on the work it will perform. The qualifier is either traction drive or non-traction drive. On occasion, you may see the term propel drive instead of a motor that moves a machine. The non-traction drive may be called work drive and is qualified as a motor operating an auger, conveyor, or another use that rotates.

A rotary drive system can be classified as either an open or closed loop.

With an open-loop design, the hydraulic fluid is returned to a reservoir before going back to the motor. The motor and pump circuit are open to the atmosphere. The drive speed of the motor may be controlled by varying the flow with a valve, changing pump input speed, or using a variable displacement pump. Typical applications for open-loop circuits are truck-mounted booms, winches, conveyors, and others.

Most, if not all, machine drive systems are considered closed-loop circuits. With this design, there is no reservoir between the inlet and outlet of the pump and the motor. The pump output goes directly to the motor inlet, and the motor outlet (exhaust) is connected directly to the pump inlet. Motor speed is usually controlled by using a variable displacement pump. 

Closed-loop systems inherently tend to lose hydraulic pressure over time. For this reason, the motor/vehicle is equipped with some mechanical parking brake. The hydraulic motor will bleed pressure to the exhaust. It cannot be relied on to hold the machine stationary, especially when shut off.

Think like a fluid!

Though hydraulic motors are usually very trouble-free, there are things to keep in mind.

Since the shaft rides in a bearing, either roller or tapered roller, the condition of the bearing will determine not only the torque output of the motor but its impact on seal life. In many applications, the bearings are meant to be greased using the manufacturer recommend lubricant.

If this is not done, then over time, the internal friction will increase; the fluid power will be used to drive the motor instead of performing work.

Simple maintenance can dramatically extend not only the life of the motor but make it more efficient in operation. Often when a bearing goes fails and the motor is kept in service, the shaft gets scored, and the entire unit will need to be replaced.

Keeping the bolts that hold the motor together tight along with those for the inlet and exhaust fittings should be part of your PM program.

Since the hydraulic motor and its companion hydraulic pump, live and die by the chemical composition of the fluid, you must service the filter and change the fluid on a scheduled basis.

I like to look at a hydraulic motor no differently than an internal combustion engine. I perform a yearly analysis of the in-service fluid as would be done on a diesel or gas engine. Also, I believe in cutting apart the hydraulic system filter and check the media for any telltale signs of something going wrong.

Reference the owner’s manual for the equipment and use the hydraulic fluid with the required specifications and additives along with either original equipment or high-quality replacement filter. These simple steps will dramatically improve the life of the hydraulic motor and pump while lessening the possibility of a costly and often labor-intensive breakdown when you need the machine.

The industry estimates that a properly cared for external gear motor will experience bearing life of between 2,000 and 5,000 hours at FULL load. An axial-piston design is expected to provide 15,000 hours of service under the same conditions.

As mentioned previously, a hydraulic motor can experience a loss from the potential energy of the fluid under pressure delivered to it and the work it can perform.

Wear is the leading parasitic energy loss along with increased bearing friction. It must be recognized that the viscosity of the fluid and its ability to resist shear are paramount in the motor’s efficiency, even if there is no internal wear.

The best way to view this being the hydraulic fluid is the fuel for the motor. If you had an internal combustion engine and provided it with poor quality fuel, you would not expect it to perform properly, yet that logic is hardly ever applied to a hydraulic system on a farm machine.

Keep the motors externally clean and serviced and with a little luck, you will never need to see what the inside looks like!