{"id":4218,"date":"2026-10-10T22:00:00","date_gmt":"2026-10-10T14:00:00","guid":{"rendered":"https:\/\/prancehydraulic.com\/blog\/hydraulic-motor-displacement-measurement\/"},"modified":"2026-10-10T22:00:00","modified_gmt":"2026-10-10T14:00:00","slug":"hydraulic-motor-displacement-measurement","status":"publish","type":"post","link":"https:\/\/prancehydraulic.com\/es\/blog\/hydraulic-motor-displacement-measurement\/","title":{"rendered":"Medici\u00f3n de la desviaci\u00f3n del motor hidr\u00e1ulico: comprobaciones en banco y limitaciones"},"content":{"rendered":"
\n

To measure hydraulic motor displacement, relate fluid volume to counted shaft revolutions under a defined test condition. Inlet flow divided by speed gives an apparent displacement, not automatically the motor's geometric displacement: internal leakage, fluid compression, displacement setting and measurement error affect the result. A useful hydraulic motor displacement measurement therefore records pressure, temperature, speed, flow location and drain routing alongside the calculation. For an unidentified motor, treat the result as evidence to compare with an exact model code and supplier documentation, rather than a sufficient basis for ordering a replacement. This guide explains the calculation and the limitations that matter in an OEM acceptance review.<\/p>\n

\"Reference-based
Reference-based A2FM exterior illustration. Not a measured displacement, internal cutaway or approved test setup.<\/figcaption><\/figure>\n

Define which displacement you are trying to establish<\/h2>\n

Geometric displacement is the nominal volume swept by the working chambers per shaft revolution at a specified configuration or setting. It is usually stated in cm\u00b3\/rev, equivalent to mL\/rev. A fixed-displacement motor has a nominal geometric value, while a variable-displacement unit has a range and a position-dependent value. Neither value is identical to every measured inlet volume per revolution in service.<\/p>\n

Apparent displacement is a ratio calculated from an observed flow and speed. In motor operation, inlet oil can include both the oil associated with useful chamber motion and leakage flow. Consequently, the apparent inlet volume per revolution can exceed the geometric value. The difference can become important at low speed, high differential pressure or a worn operating condition.<\/p>\n

Document the purpose of the measurement before choosing a method. Identifying an approximate displacement family, checking a supplier's declared nominal value, and measuring volumetric efficiency are different tasks. A field flow-to-speed ratio may support the first task. A defensible acceptance claim for the others requires a specified procedure, traceable instruments and a confirmed motor configuration.<\/p>\n

Read the complete identity before connecting a test<\/h2>\n

Record the manufacturer information, full model code, serial or batch identifier and any displacement-control code. Photograph the nameplate, shaft, mounting face, work ports and drain ports. Compare the full code against the supplier's documentation; an abbreviated number stamped on one housing component can identify that component rather than the complete assembled motor.<\/p>\n

Do not infer displacement from body size alone. Units in one housing family can have different chamber geometry or configured displacement. A similar flange, shaft and overall outline are useful mechanical clues but are not proof of the internal volume. An external A2FM-style motor photograph, such as the illustration in this guide, cannot verify a particular displacement or pressure rating.<\/p>\n

For a variable motor, establish the control position and whether it can change during the test. A spring-biased setting, pressure-dependent control or remote command can change displacement as the operating condition changes. If the control state is unknown, record that uncertainty and avoid presenting a single measured ratio as the motor's nominal displacement.<\/p>\n

The flow-to-speed calculation<\/h2>\n

When inlet flow Q is measured in L\/min and shaft speed n in rev\/min, the apparent displacement is D_app = 1000 \u00d7 Q \/ n in cm\u00b3\/rev. The factor 1000 converts liters to cubic centimeters. Both measurements must describe the same steady operating interval and the same motor branch; whole-machine pump flow is not necessarily motor inlet flow.<\/p>\n

For an illustrative test, 12.0 L\/min entering a motor at 200 rev\/min gives D_app = 1000 \u00d7 12.0 \/ 200 = 60.0 cm\u00b3\/rev. That result reports inlet volume per observed revolution. It does not establish that the motor's nominal geometric displacement is exactly 60 cm\u00b3\/rev, and it does not establish the displacement code of a visually similar replacement.<\/p>\n

If a documented geometric displacement D is already known, motor volumetric efficiency can be expressed as eta_v = D \u00d7 n \/ (1000 \u00d7 Q). Rearranging gives D = eta_v \u00d7 1000 \u00d7 Q \/ n. Efficiency must come from an applicable measurement or supplier data at the relevant conditions. Choosing an assumed efficiency to force the calculation to match a desired catalog size reverses the purpose of the test.<\/p>\n

Why inlet and outlet measurements are not interchangeable<\/h2>\n

A flowmeter at the motor inlet measures the incoming branch flow. A meter in the return line measures what reaches that point after the motor and any intermediate connections. Case-drain oil may leave through a separate path, and flushing or replenishment valves may add or remove flow. A bypass connection can also make a meter reading differ from the actual work-port flow.<\/p>\n

Draw the test connections and mark every flow path before calculating displacement. Note whether the drain returns separately, joins the work-port return downstream of the meter, or participates in an approved internal arrangement. These distinctions determine what can be inferred from the readings. They also prevent an incorrect comparison between two tests that used different meter locations.<\/p>\n

Subtracting a separate case-drain flow from inlet flow can provide useful context, but it is not a universal correction to geometric displacement. Leakage may cross internally between work ports, and drain flow can include other oil paths depending on the motor design. A correction is defensible only when the circuit and leakage paths are identified by the applicable test method.<\/p>\n

A counted-volume method and its practical boundaries<\/h2>\n

A counted-volume method measures the fluid associated with a known number of shaft revolutions. In principle, D_app = V \/ N when collected volume V is in mL and the counted turns N are full revolutions. Measuring over several turns can reduce the relative influence of a single counting error or a small volume-resolution error.<\/p>\n

For example, an illustrative 300 mL associated with ten complete turns corresponds to 30 mL\/rev before considering leakage, compressibility and setup effects. A partially completed final turn, an air pocket in the line or unmeasured oil routed through another port can materially affect such a small-volume observation. Record the start and end positions, volume definition and the direction of rotation.<\/p>\n

This equation is not an instruction to hand-spin any motor or feed it through an improvised open container. Some motors require minimum charge or lubrication conditions, and some arrangements incorporate brakes, valves or bearing lubrication paths. Use a manufacturer-approved bench method performed by qualified personnel. Do not run a dry unit or modify ports merely to obtain a volume reading.<\/p>\n

Measurement methods compared<\/h2>\n\n\n\n\n\n\n\n\n\n
Method or evidence<\/th>\nWhat it establishes<\/th>\nMain limitation<\/th>\nWhat to record<\/th>\n<\/tr>\n<\/thead>\n
Full model code and supplier drawing<\/td>\nDeclared geometric displacement and configuration<\/td>\nIdentity may be incomplete or documentation may not match assembly<\/td>\nFull code, drawing revision and confirmed ordered configuration<\/td>\n<\/tr>\n
Steady inlet flow divided by speed<\/td>\nApparent inlet volume per revolution<\/td>\nIncludes leakage and depends on test condition<\/td>\nFlow location, speed, pressure difference and fluid temperature<\/td>\n<\/tr>\n
Approved counted-volume bench method<\/td>\nVolume per counted turn under the stated method<\/td>\nSensitive to routing, air, turn count and method conditions<\/td>\nVolume definition, full turns, direction and procedure revision<\/td>\n<\/tr>\n
Inlet, outlet and separate drain measurements<\/td>\nFlow balance and leakage context<\/td>\nUnknown internal paths can prevent a unique correction<\/td>\nAll ports, valve states, meter locations and synchronized readings<\/td>\n<\/tr>\n
Calibrated multi-point test against declared displacement<\/td>\nEvidence of operating volumetric behavior<\/td>\nRequires known configuration and applicable acceptance criteria<\/td>\nRepeat points, instrument uncertainty and supplier limits<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

Supporting technical context: NPTEL: hydraulic motor flow, displacement and efficiency calculations<\/a>, NPTEL: fundamentals of oil hydraulics<\/a>. This evidence checklist does not establish a product rating or universal acceptance limit.<\/p>\n

The table compares evidence types rather than assigning universal test pressures or pass limits. Those values must come from the exact motor documentation and agreed acceptance procedure.<\/p>\n

Control temperature, pressure and speed during comparison<\/h2>\n

Hydraulic fluid viscosity changes with temperature and influences leakage. A warm test and a cold test can produce different apparent displacements even when the motor's geometric displacement has not changed. Record the fluid grade and stabilized temperature. Avoid comparing a field reading to a supplier curve without checking the curve's fluid and temperature conditions.<\/p>\n

Pressure should be measured across the work ports, not assumed from one upstream gauge. Return backpressure affects the actual motor pressure difference. Case pressure is a separate condition that can influence seal exposure and must remain within the motor's limits. A nominal system relief setting is not a substitute for measuring these boundaries during the test.<\/p>\n

Speed selection also matters. At very low speed, leakage can become a large fraction of total inlet flow, and an unstable shaft can make averaging difficult. At higher speed, inlet supply and return losses may become significant. Choose test points from the approved procedure, stabilize each point and repeat readings instead of using an arbitrary speed that happens to produce a convenient number.<\/p>\n

Quantify uncertainty before comparing nearby sizes<\/h2>\n

Both flow and speed uncertainty affect the ratio. As a conservative first screening for small relative errors, add the magnitudes of the flow and speed relative uncertainties. This is a worst-case arithmetic estimate, not a full statistical uncertainty calculation. Instrument calibration, resolution, repeatability, synchronized sampling and operating drift may require additional terms.<\/p>\n

Suppose a flow reading has an applicable uncertainty of \u00b11.0% of reading and speed has \u00b10.5% of reading at the actual test range. The ratio's conservative instrument contribution is approximately \u00b11.5%, before adding the method effects. At 60 cm\u00b3\/rev that is about \u00b10.9 cm\u00b3\/rev. These are illustrative instrument inputs, not promised performance for a flowmeter or tachometer.<\/p>\n

Check whether instrument accuracy is stated as percent of reading or percent of full scale. A full-scale specification can produce substantial relative error when flow is near the bottom of the meter's range. If the uncertainty band overlaps two possible displacement families, the observation cannot distinguish them reliably. Improve the evidence or confirm the model identity rather than rounding to the preferred size.<\/p>\n

Interpret a result that does not match the nameplate<\/h2>\n

First check units, sensor location and shaft-speed identification. A sensor on an intermediate gearbox output requires the verified ratio to obtain motor-shaft speed. An imperial flow display must be converted consistently before applying a formula written for L\/min. A bypass or flushing connection may explain a flow discrepancy without any change in motor geometry.<\/p>\n

Next review the displacement-control position and operating condition. Compare repeated points under a stable fluid temperature and pressure difference. A ratio that rises with pressure can be consistent with increasing leakage, but that observation alone is not a complete failure diagnosis. A separate drain-flow investigation and the supplier's operating data can help identify the relevant paths.<\/p>\n

If the evidence remains inconsistent, report the motor as unidentified or requiring review. Do not adjust the quoted displacement, declare counterfeit manufacture or reject a supplier solely from one field ratio. The acceptance decision should distinguish an identity discrepancy, a measurement discrepancy and an operating-performance discrepancy, because each requires a different follow-up.<\/p>\n

A practical acceptance report for procurement<\/h2>\n

Include motor identity, geometric displacement claimed by the supplier, fixed or variable configuration, circuit drawing and all port assignments. List fluid, temperature, inlet and outlet pressures, case pressure, shaft speed and the flow readings used in every calculation. Specify the instruments, ranges, calibration status and applicable uncertainty at each point.<\/p>\n

Show the calculated apparent displacement separately from any corrected or declared value. State the correction method and evidence if a volumetric-efficiency adjustment is used. Preserve individual repeated readings as well as the averages; averaging can conceal unstable operation that an engineer needs to review. Attach the approved procedure and agreed criteria rather than creating a pass limit after seeing the result.<\/p>\n

For a replacement enquiry, add mounting dimensions, shaft details, rotation requirements, brakes or auxiliary valves, duty cycle and allowable pressure and speed boundaries. Displacement is one part of compatibility. A replacement that produces the expected speed can still have unsuitable case-drain requirements, starting torque or mechanical interfaces.<\/p>\n

Safety boundaries before any bench or machine test<\/h2>\n

A hydraulic motor can accelerate unexpectedly or drive connected machinery. Qualified personnel must use the machine or bench's approved restraint, guarding, pressure limitation and energy-isolation procedure. Establish a safe stop condition before starting a measurement. Stored pressure, gravity loads and accumulators require attention even after electrical power is disconnected.<\/p>\n

Change connections only after isolation and verification of the safe state. Keep case drains and required lubrication paths arranged as approved by the manufacturer, and never restrict a drain to make a calculated value appear more favorable. Where model identity or port function is unresolved, obtain the correct documentation before conducting a test that could damage the unit or endanger personnel.<\/p>\n

\"Reference-based
Reference-based A2FM exterior illustration. Not a measured displacement, internal cutaway or approved test setup.<\/figcaption><\/figure>\n

Frequently asked questions<\/h2>\n

Does flow divided by speed give the exact motor displacement?<\/h3>\n

It gives apparent inlet volume per revolution when flow is measured at the motor inlet. Leakage and the operating condition can make it differ from geometric displacement. Exact identification also requires configuration and method evidence.<\/p>\n

What units should I use in the displacement formula?<\/h3>\n

For Q in L\/min and n in rev\/min, use D_app = 1000 \u00d7 Q \/ n to obtain cm\u00b3\/rev. For measured volume V in mL over N counted revolutions, V \/ N gives mL\/rev, numerically equal to cm\u00b3\/rev.<\/p>\n

Can I subtract case-drain flow to get geometric displacement?<\/h3>\n

Not universally. The correction depends on the motor's internal leakage paths and the test circuit. Cross-port leakage and other drain contributions may prevent a simple subtraction from establishing geometric displacement.<\/p>\n

Can housing dimensions identify an unknown displacement?<\/h3>\n

They can narrow a mechanical family, but several displacements may share an exterior. Confirm the full model code, supplier drawing and approved test evidence before selecting a replacement.<\/p>\n

Should I test at very low speed to make revolution counting easier?<\/h3>\n

Use the approved method's speed range. Very low speed can increase leakage's relative effect and produce unstable rotation. A convenient counting speed is not automatically a reliable condition for estimating nominal displacement.<\/p>\n

Related products and engineering guides<\/h2>\n

A2FM bent-axis motor<\/a>, hydraulic motor range<\/a>, motor efficiency calculations<\/a>, motor case-drain flow checks<\/a>, motor acceptance-test reporting<\/a>. Provide the circuit, fluid, duty and exact ordered configuration when requesting component-specific data.<\/p>\n

University lesson: 6.3 – Hydraulic Motors<\/h2>\n

This NPTEL university lecture explains hydraulic motor flow, displacement and speed principles used in the calculations. It does not authorize an improvised motor test rig.<\/p>\n