How do you test the efficiency of a hydraulic motor?

Test a motor's efficiency by measuring three similar numbers, including mechanical efficiency, volumetric efficiency, and overall efficiency, under the control of bench-based conditions. The test involves operating the motor at the rated pressure and speed while taking measurements of output flow and input flows (or leakage) as well as the demand for torque input and the output torque. Then, you compare the results against manufacturers' recommended values. Any drop that is greater than 10% from the base efficiency usually indicates internal wear, usually in the rotating part of the motor.

Motors for hydraulics convert power from fluid into mechanical energy for rotation; however, no motor can convert the energy in a perfect manner. The internal clearances of the motor, wear on seals, and friction losses reduce the output. Being aware of the efficiency of a motor and observing how efficiency changes over time is among the most reliable methods to identify the problem before it leads to unexpected downtime.

Why is testing the efficiency of hydraulic motors important?

Efficiency isn't just a purely academic term. Motors operating at a lower than efficiency rating waste energy, produce heat, and frequently signal the internal parts are failing. Being able to detect this in time lets maintenance teams plan a repair or replacement rather than reacting to an unexpected failure in the line.

Efficiency testing is also important when commissioning. Motors that are new, rebuilt units, and motors coming back from repairs must be tested against OEM specifications prior to going back to service. The motor, which "runs fine" on visual examination, could be losing 20 percent of its power due to internal slip.

Three types of efficiency in hydraulic motors

Hydraulic motor performance is divided into 3 distinct efficiency levels that are usually tested to address the three in isolation before combining the three figures.

Volumetric efficiency

Volumetric efficiency determines the amount of fluid that is entering the motor and actually is a part of the shaft's rotation, versus the amount that leaks outwards past the gears, pistons, or vanes but does not do any useful work. It's calculated using:

Volumetric Efficiency = (Theoretical Flow Required for Actual Speed / Actual Flow Input) x 100

Internal leakage, also referred to as "slip" -- increases as clearances become less open with time or the temperature of the fluid decreases. A motor that has a drop in volumetric efficiency will require a greater flow of fluid to keep the same output speed.

Mechanical performance

Mechanical efficiency is the ability to measure friction losses in seals and bearings as well as sliding surfaces. It compares the amount of torque an engine could theoretically generate at a certain pressure to the actual torque that it produces on the shaft:

Mechanical Efficiency = (Actual Output Torque / Theoretical Torque) x 100

Mechanical losses usually manifest as heat or as a shortfall in torque when under stress, even though flow and speed appear normal.

Overall effectiveness

Overall efficiency is the product of volumetric as well as mechanical performance, expressed in terms of percent:

Overall Efficiency = Volumetric Efficiency x Mechanical Efficiency

The majority of industrial hydraulic motors have between 85% and 95% efficiency overall in their initial state, based on the motor's type (gear vane, piston, or gerotor). Piston motors tend to be on the upper part of the range, while gear motors are generally lower.

Set up of a hydraulic motor efficiency test

Necessary testing equipment

A valid bench test must include:

• A flow meter that is calibrated to the motor's intake to measure flow at the input
• A transducer or pressure gauge at the outlet and inlet ports.
• A loading valve or dynamometer to apply a controlled different load onto the shaft that is output
• A torque sensor and tachometer are mounted on the output shaft and are used to determine the torque delivered and also the rotational speed
• A method to measure the flow of the drain in the case, as leakage from the drain of the case is a clear indicator of wear on the internals
• A temperature sensor is required, as efficiency figures only apply at the stated temperature of the fluid and viscosity

Test conditions

Efficiency numbers have no meaning without a specific test condition. A majority of test protocols, which are in line with ISO 4409, specify testing at the motor's maximum pressure, rated speed, and a set temperature for the fluid (commonly 40-50 degrees Celsius) with a fluid that has a known viscosity. Tests that are not conducted in accordance with these parameters—for instance, using high-viscosity, cold oil—can result in artificially high efficiency readings that aren't reflective of the real performance of the motor.

Step-by-step test procedure

Step 1: Establish the no-load requirement.

It is recommended to run the motor without load at the speed it is designed to run and take note of the input flow in addition to input pressure and the flow of draining from the case. This provides a reliable reference for internal leakage, with no external torque demands.

Second step: apply load incrementally.

Utilizing the dynamometer, or loading valve, to apply load in controlled increments that are equal to the motor's recommended pressure. When each step is completed, note:

• Pressure and flow in the input
• Speed and torque of output
• Case drain flow
• Temperature of the fluid

Step 3: Calculate volumetric efficiency at each load point

Each load stage examines the theoretical flow required to generate the measured output speed to the actual flow provided. The difference, which is explained mostly by drain case leakage -- provides an efficiency of volumetric at the load point.

Fourth step: Determine the mechanical efficiency at each point of load.

Check the theoretical torque that the motor is expected to produce at the specified inlet pressure to the actual torque that is delivered to the shaft. This eliminates the friction-related losses.

Step 5: Calculate efficiency against load

Efficiency isn't a fixed number; it fluctuates depending on speed and load. The plotting of mechanical, volumetric, and overall efficiency over the entire range of pressures in a motor yields an efficiency curve that is more helpful to diagnose than a single point measurement. The majority of motors have peak efficiency in the mid-to-upper portion of their pressure range, and efficiency drops to a minimum at extremely small load (where mechanical losses are the most prevalent) and often at the top of the spectrum (where leakage from the internals increases).

Step 6: Compare with OEM baseline

Check the curve's performance with the manufacturer's performance information, or against the baseline test that was conducted at the time of its newness. A constant gap throughout the load range, especially in the volumetric efficiency, suggests worn vanes, pistons and gears, and bearing surface.

Field testing in contrast to bench testing

Testing on a bench using a dynamometer isn't necessarily feasible for motors that are still within the machine. When working in the field, technicians usually depend on a simple test for measuring drain flow at the rated pressure and then checking it against the manufacturer's maximum leakage specifications. The flow of the case at operating temperatures and pressures is among the quickest gauges of decreasing volumetric efficiency, even if there isn't the full measurement of speed and torque.

Interpreting the results

A motor that is testing at or below five percent of its rating efficiency is usually considered to be healthy. A reduction of 10 to 15% is often sufficient to allow the operation to continue but requires more monitoring and scheduling of reconstruction. Anything greater than 20 percent efficiency loss typically indicates an extensive internal wear and a high possibility of a continued decline because worn clearances tend to increase wear and tear because leakage can increase the local temperature and velocity of fluids.

What is the best performance rating of a motor that is hydraulic?

The latest industrial hydraulic motors generally range from 85 to 95% overall efficiency, with piston motors at the upper end and gear motors on the lower end.

What is the cause of the low volumetric efficiency of a hydromotor?

The low volumetric efficiency is typically caused by wear inside vanes, gears, and pistons or bearing surfaces. This permits fluid to escape the chambers of operation instead of being a contributor to shaft rotation. It can also be caused by lower fluid viscosity due to overheating.

Are you able to measure the efficiency of a hydraulic motor without the use of a dynamometer?

Yes, a simpler field test compares the case drain leakage at the rated pressure to the OEM's maximum permitted specification; however, it's only a measure of the volumetric efficiency and does not account for mechanical losses.

When should the hydraulic motor efficiency be checked?

Critical or high-duty-cycle motors are typically checked annually or at regular maintenance intervals. Motors that are showing indications of performance decline should be checked immediately.

Does the temperature of fluid affect the efficiency of tests?

Testing should be performed at a specified fluid temperature and viscosity because cold, high-viscosity liquid results in artificially inflated efficiency measurements that do not reflect actual operating conditions.