Travel motor, drive motor, wheel motor, propel motor, track drive, final drive motor -- all these terms refer to different types of hydraulic motors used to propel mobile equipment. In this Shop Talk Blog post, we are going to review some terms related to hydraulic motors that it would be a good idea for you to know. You may know them already, but a good review is always in order.

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Absolute pressure measures pressure using a vacuum as a reference point. Absolute pressure is equal to the gauge pressure plus atmospheric pressure. See gauge pressure.

An additive is a chemical that has been added to a fluid to enhance its existing properties or impart new properties. For example, sulfur is a common additive to gear oil.

Back pressure is pressure on the downstream (or on the return side) of a hydraulic component.

A bar is a common unit of pressure and is equivalent to 14.5 psig. See gauge pressure.

Breakout pressure is the minimum pressure needed to start movement within the hydraulic components.

Bore refers to a cylindrical internal passageway and can be used to refer to the internal diameter of a pipe, line, or hose.

The term burst pressure refers to the internal pressure at which a pipe, line, or hose will burst. It is usually about four times the rated pressure.

A cartridge is the part of a fluid filter that can be replaced. It may also be referred to as a filter element.

The case drain filter is a hydraulic filter on the case drain line. The case drain line is a low pressure line that carries away excess or leaked fluid (some degree of leakage is normal in hydraulic motors).

Charge pressure refers to the pressure at which replenishing fluid is supplied to the hydraulic system or component.

A connector is used to attach to pieces of tubing together or to attach tubing to a hydraulic component. See fitting.

Contamination is any material that is foreign to the hydraulic fluid and can include solids (e.g., particulate matter), liquids (e.g., water, chemical decomposition), and gases (e.g., air). Contamination adversely affects the components and the fluid power system as a whole.

Displacement is the volume of hydraulic fluid that is displaced by a complete stroke or revolution of the hydraulic motor. A variable displacement hydraulic motor is one in which the displacement can be varied.

The word downstream refers to the passage beyond a component (such as a hydraulic motor) after its outlet.

Efficiency is the ratio of output to the equivalent input and is usually expressed as a percent. See theoretical efficiency and volumetric efficiency.

A fitting is a device for connecting a pipe or a hose to a hydraulic component. See connector.

Flow is the movement of fluid caused by differences in pressure.

A filter is used to remove contaminants from the hydraulic system by filtering them out.

A fixed displacement hydraulic motor is one in which the displacement cannot be varied.

A fluid power system is a system of components that uses pressurized fluid to transfer energy. The fluid power system transmits and controls pressurized fluid flow within a closed circuit.

A flow divider is used to divide fluid in a passage into two or more different streams.

Flow rate refers to the volume, mass, or weight of fluid passing a point per unit time.

Gauge pressure is a pressure measurement that has been correct for atmospheric pressure and is typically measured in psig. Gauge pressure is equal to the absolute pressure minus the atmospheric pressure. See absolute pressure.

The gear hub is the side of the hydraulic motor that contains the speed-reducing, torque-multiplying planetary gear set.

Gear oil is the lubricant used in the gear hub.

The word gasket refers to a special type of seal that is made in the shape of a circle and a polygon cross-section. A gasket lies between parts that do not move relative to one another.

Head is a pressure measured in terms of length. It represents the pressure at the base or other reference point of a column of fluid. Head can be converted to pressure by multiplying the height of the fluid column by the density of the fluid.

Horsepower is one of several measures of power. One horsepower is equivalent to 550 ft-lbs/min or 745.66 Watts.

A hose is a flexible line used to connect hydraulic components. The inside diameter of the hose is its nominal size.

A line can refer to a pipe, a tube, or a hose.

The life expectancy of a component is a prediction of how long it will operate at an acceptable performance level under specific conditions (e.g., standard workload, regular maintenance, normal operating conditions).

A liter is a measure of volume that is equivalent to 0.2642 gallons.

A magnetic plug is used to attract and hold iron and steel (ferromagnetic) particles. This is commonly on the drain plugs from the gear hub of a final drive motor.

A manifold is fabricated set of passageways that provides multiple connection ports.

Motor speed is related to the volume of hydraulic fluid that is being delivered to the motor and the actual motor displacement.  The maximum motor speed is the speed at a given inlet pressure that can be sustained by the motor for a limited time without damaging the motor.  The minimum motor speed is the slowest, uninterrupted rotational speed that is available from the motor’s output shaft.

An o-ring is a seal used on circular or curved mating surfaces. It may have a circular or polygonal cross-section.

Overall efficiency is the ratio of output power to input power. Keep in mind that there are different measures of efficiency for hydraulic motors.

Operating pressure refers to the pressure at which the motor normally operates. Also known as system pressure.

Pipe refers to a hydraulic line that has ends with standardized threading.

Planetary hub See gear hub.

A port is the open end of a fluid passage and typically where a fitting is attached.

A pressure differential is a drop in pressure between two points. It is sometimes referred to as a pressure drop.

Rated flow is the maximum desirable flow rate that is recommended by the manufacturer.

Rated pressure is the maximum desirable operating pressure that is recommended by the manufacturer.

A return line returns the hydraulic fluid to the reservoir.

A seal prevents fluid from escaping and contaminants from entering the hydraulic system or component. When seals begin to fail, leaks will occur.

Slip is internal leakage of hydraulic fluid.

Starvation refers to a lack of fluid in the system and is often caused by clogged filters.

The stroke of a piston is its length of travel.

Torque refers to the turning force generated by a hydraulic motor.  It is related to speed and power.

The word upstream refers to the passage ahead of a device, before it’s inlet.

A valve is used to change the state of fluid that flows through it.

The valve block is the rectangular manifold that lies on top of the hydraulic side of the final drive. It has, at minimum, a set of input and output ports. It may also have a two-speed port, brake line port, and case drain port.

Viscosity is a measure of how much a fluid resists fluid flow. The higher the viscosity, the more resistant the fluid is to flow.

Volume is a measure of how much space something takes up.

Volumetric flow rate is volume of fluid flowing past a point per unit time.

Volumetric efficiency is the ratio of actual output flow to design flow or theoretical flow, all measured in gpm.

Conclusion

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