Reading Hydraulic Motor Performance Curves at The Required Duty

Mark the required shaft duty on the motor chart. The intersecting flow and pressure-difference lines tell you the hydraulic demand for that duty; the operating-region boundary tells you which rating conditions to check. Begin there when reading hydraulic motor performance curves. Keep a best-efficiency label elsewhere on the map separate from this required point.

A buyer may receive a torque table, a colored map or a single line marked “efficiency.” Those formats carry different information. Before comparing two offers, establish what each one actually describes. A pressure limit, an unloaded pressure-loss curve and an efficiency contour cannot be substituted for one another.

The worked records in this guide are hypothetical. They contain no BLINCE ratings and no measurements from another manufacturer's motor. Use their arithmetic to review the evidence in an actual quotation; obtain that evidence for the exact build being offered.

Give the chart a job

Take the later worked requirement of 125 N·m at 240 rev/min as an invented mixer duty. Place the torque and speed together on the proposed motor map. The quotation needs to establish that pair together. Separate maximum-torque and maximum-rpm entries leave the mixer's loaded working point and its hydraulic supply requirement unconfirmed.

Write which shaft the requirement belongs to. With a direct drive, the mixer and motor share the stated shaft duty. A reducer, belt or chain introduces a transmission between them. Work back through its ratio and losses to obtain the motor-side requirement before marking the motor chart. Label each shaft on the worksheet so the transmission calculation remains traceable.

Use the exact motor designation and the chart's document/page identity to head the comparison. Enter the shaft torque–speed pair beneath them. Add the other duty states on separate rows, rather than letting a column of unrelated maxima take the place of the sustained working point.

BLINCE's hydraulic motor range provides a starting place for identifying the relevant family. A category page establishes a product route; the selected motor's performance evidence must still match its complete configuration.

Read the axes before the curves

Start at the axis labels. For the torque–speed format used here, rpm runs across the page and N·m up the page. Across that grid are families of flow and differential-pressure lines, along with contours for the stated efficiency. For a different layout, work from that page's own axes, columns and legend.

The selection section in Danfoss's general orbital-motor document explains this combination of torque–speed axes and flow, pressure, power and efficiency lines. Read it as a worked manufacturer explanation of that format. BLINCE performance must be established from the relevant BLINCE build information. Danfoss orbital-motor function diagrams

Copy a value with its scale label attached. On a dual-unit graph, a torque reading might come from the N·m scale or the lbf·in scale; similarly, flow might use L/min or US gal/min. If two extracted values disagree, first trace each back to the unit scale it came from. Correct the transcription before making a motor comparison.

A scan can preserve the curve while losing the displacement heading or the legend. Put such a page on hold until a legible copy is available. Borrowing a scale from a neighboring catalog page can attach another motor size to an otherwise careful calculation. Retain the complete source page with the extracted values.

Separate an operating boundary from an efficiency contour

A boundary says something about the permitted operating region. An efficiency contour connects points with the same stated efficiency. A point can lie near an attractive contour while falling outside the permitted continuous region. Conversely, a permitted point can have an efficiency lower than the map's best number.

For a continuous production requirement, start with the applicable continuous region. Read any intermittent or peak definitions in the same document and edition. The allowed duration, recovery requirements and combinations of speed and pressure are model-dependent. Do not turn a colored area into a universal percentage of operating time.

The boundary may also change with speed. A rectangular mental model—maximum torque anywhere below maximum speed—can therefore be wrong. Read the complete operating point against the actual outline. Separate maxima printed in a summary table do not necessarily occur together.

Two quotations can list similar headline values on different rating bases. One can appear stronger because it advertises an intermittent maximum while the other quotes a continuous value. Normalize those definitions before comparing capacity. If the definition is unavailable, mark the capacity comparison open rather than treating the larger number as a demonstrated advantage.

Do not mistake unloaded pressure loss for loaded output

A rising pressure-versus-flow curve may describe oil passing through a motor whose shaft is unloaded. It tells a different story from a torque–speed map under load. Applying that pressure-loss value as the full pressure requirement for a working machine omits the pressure associated with useful shaft torque.

Look at the TMTHW document as an example of two different pages doing different jobs. Its unloaded pressure-drop plot carries a viscosity note; its function diagrams describe motor output. Identify the page used for each extracted value. The conditions and values on either page belong to that documented series. Danfoss TMTHW technical information

Write a short description beside every extracted number: “unloaded pressure drop at stated flow,” “loaded shaft torque,” or “continuous pressure boundary.” Retain the annotation when forwarding a value without the graph. A column headed only “pressure” often loses this distinction during procurement.

For a hydraulic gear motor efficiency curve, apply the same discipline. A gear-motor catalog may provide tabulated input flow and shaft output rather than a contour map. Retain the table's operating conditions and derive a comparison only from compatible entries. The absence of a colored map is not evidence of poor efficiency, just as a colorful map is not proof of a better motor.

Use pressure across the motor

A performance chart's pressure difference belongs across the motor's working ports under its stated definition. A pump-mounted gauge does not automatically provide that number. Supply-path losses and pressure at the motor's outlet affect the available difference.

Use both working-port readings in the comparison. For this invented steady observation:

A gauge at the pump has another location on the circuit. Account for the path from that location to the motor before using it to infer a motor-port condition.

This subtraction does not establish the acceptable outlet or housing pressure. Those have their own limits and measuring definitions. A motor can have the intended working-port differential while an excessive outlet or case condition remains unresolved. Check the outlet and case limits separately from the efficiency contour.

For actual measurements, qualified personnel must use the approved procedure and instruments suited to the circuit. Before changing a connection, secure the load and isolate/release stored hydraulic energy. Keep pressurized lines closed and preserve protective settings. Reaching a plotted point does not justify an improvised connection or protection change.

Check one complete point with power arithmetic

The following hypothetical point uses a nominal displacement of 100 cm³/rev. Inlet flow is assumed to be 30 L/min, shaft speed 240 rev/min, working-port differential 100 bar, and shaft torque 125 N·m. All five quantities describe the same steady point. No allowance for a separate flushing supply is included in the example's hydraulic input boundary.

Write “volumetric” beside 80.0% and “total” beside 62.8%. The quantities in the table determine those labels. Applying the volumetric figure to input power gives 5 × 0.8 = 4 kW; check that against the 3.14 kW obtained from the hypothetical shaft torque and speed. The power discrepancy is the reason those two percentages must remain separate.

Now check the torque basis using that same record.

All three rows retain the earlier point and the simple nominal-displacement assumptions.

The agreement confirms that these assumed quantities are arithmetically consistent. A manufacturer's performance method may use a different displacement basis, corrections or measurement boundary. Use its definitions for an actual acceptance comparison. The worked point supplies no universal minimum efficiency or product pass limit.

Keep interpolation modest and visible

Locate the entries bracketing the required condition before interpolating. Record them with the estimate so another reviewer can reproduce it. Use that estimate only for the preliminary purpose supported by the data and the manufacturer's guidance. A required point beyond the printed region needs further evidence rather than an extension of the lines.

Here is an invented two-row table at 100 bar differential. Both rows assume the same fluid and temperature conditions. None of the entries was taken from a manufacturer's graph; they are deliberately simple numbers for checking the interpolation method.

At the midpoint, retain all the inputs in the power check: input = 100 × 30.5 / 600 ≈ 5.08 kW; output = 2π × 250 × 125 / 60,000 ≈ 3.27 kW. Dividing that output by that input gives approximately 64.4%. A nearby contour label cannot replace this defined estimate. Keep the unrounded working values in the calculation record.

The 250 rev/min midpoint calculation is separate from the earlier 240 rev/min example. Their outputs are deliberately not interchangeable. A comparison sheet should identify the point in every row; otherwise an attractive torque from one row can drift into the power calculation for another.

A thick line close to an operating boundary may leave the required point unresolved. Identify that uncertainty in the request for a confirmed point or test evidence. Approval needs support for the actual point; extrapolating a curve outside the printed area supplies only a calculation beyond the available data.

Retain the conditions printed below the graph

Read the test-condition note before carrying a plotted point into a quotation comparison. Compare fluid temperature, viscosity information, measuring definitions and any notes about the representative nature of the data. A chart drawn for one condition cannot establish identical delivery or torque at every condition the machine will encounter.

The practical purchasing question is often narrower than “what efficiency does this motor have?” It may be whether a proposed motor can start in the site's cold condition, sustain the normal warm condition and remain within limits during the hottest expected duty. Those are several operating questions, not a single correction factor applied to a catalog percentage.

Match a chart's viscosity note to viscosity at the relevant condition. The grade on the fluid container does not supply that value at every temperature. Where the chart basis and machine condition differ materially, obtain the appropriate data instead of inventing a numerical adjustment.

For offers supported by different test conditions, leave an explicit comparison gap. Request corresponding points on a common agreed basis. That request is more useful than adding a generic safety margin to an efficiency estimate whose underlying conditions have never been reconciled.

Plot the duty, including the slow portion

A machine that spends much of its cycle creeping may spend little time near a motor's best-efficiency contour. Put the slow loaded point on the map even if the normal production point looks comfortable. Add loaded starts, reversals and any overrun state as separate review items where they matter.

Put starting, creeping and braking in the review as their own operating states. Check the supplied documentation for each one. If it covers only running at finite speed, a line drawn back to zero adds no starting measurement. The missing state needs applicable information from the motor supplier.

List the time spent at each point along with its speed and torque. A simple duty table can expose a quotation that was selected around the occasional fast movement while the prolonged slow movement remains unverified. It also helps distinguish which points require continuous operation and which belong to a properly defined intermittent condition.

The existing BLINCE motor speed-formula guide explains the relationship between flow, displacement and rpm. Use the performance map to add the corresponding loaded torque and hydraulic demand, rather than repeating a no-load speed calculation as the full selection.

Compare energy over the duty without averaging percentages

An arithmetic average of efficiency percentages can mislead when the points transmit different powers or last for different periods. Calculate energy input and useful shaft energy for the defined intervals. Their ratio gives an efficiency for that limited duty boundary. It still excludes other machine losses unless they are explicitly included.

Here is a second invented exercise. Assume two candidate motors meet the same shaft-output duty. Supplier confirmation of their actual capability is outside this fictional comparison. Candidate A and B are labels for arithmetic, not BLINCE models or competing brands.

The energy account below uses each interval's duration in hours.

For the slow interval alone, B has the lower assumed input. Across the listed durations, A has the lower total. Recalculate the account if the duty changes. The choice in this fictional comparison depends on those durations; it makes no prediction for an installed product or a site's bill.

The listed energy terms cover the motor boundary. A whole-machine assessment would add the relevant pump, valve, cooler and prime-mover terms on a defined basis. Keep the fictional 0.05 kWh difference in this worksheet; a fuel or electricity cost claim needs the actual operating records and applicable prices.

Complete the installation review separately

Performance evidence does not replace interface and installation checks. The motor's shaft, mounting, ports, rotation definition, drain arrangement and applicable outlet/case limits still have to match the proposed installation. An efficient point on an unsuitable shaft configuration cannot complete a replacement decision.

Write external shaft forces in their own column. A pulley or sprocket brings a radial-load question; axial loading needs its own assessment where present. Give the supplier the force and its position relative to the motor mounting. The applicable shaft and bearing information must support that arrangement in addition to the torque–speed point.

Likewise, a chart showing motoring operation does not by itself establish the requirements for an overrunning load or a holding function. Those questions need the relevant circuit and protection information. If the machine can move dangerously after supply is removed, secure that load through the approved procedure; a selected efficiency contour provides no holding assurance.

Attach an installation list to the candidate build and performance worksheet. Put an evidence reference against each resolved item and name the missing input against each open item. Purchasing then has a record of what has been established for the operating points and what remains before installation approval.

Normalize the evidence in two offers

Check the build and definitions even when the nominal displacement matches in two documents. Confirm the exact series and configuration in each offer. If a performance page is shared across variants, ask which conclusions apply to the proposed shaft, port, bearing or integrated-valve option and which require additional checks.

Compare either total efficiencies on matching boundaries or compatible input/output quantities from which a defined comparison can be calculated. A volumetric efficiency from one offer cannot be ranked against total efficiency from another. Use the buyer's working point in both offers, keeping any separate best-point claim clearly labeled.

Where the line thickness prevents a reliable reading, record the unresolved interval or leave the value unestablished. Reporting 82.37% from a thick contour line usually gives the comparison more digits than the evidence supports. Request data that resolves the required condition.

File the document title, revision and page with the extracted values. If a revised offer changes the motor, compare it against its own applicable page. An unidentified email screenshot needs clarification before its numbers can be treated as established evidence for the selected build.

Turn the chart review into a useful request

Send a marked operating-point table rather than a request for the “highest-efficiency motor.” Include the torque and speed pairs that matter, their durations, available supply conditions at the motor, fluid condition and the exact candidate configuration. Add the interface or circuit information only where the current comparison needs it.

Ask the supplier to identify the applicable performance page and mark or tabulate the corresponding points. Keep the origin of each entry visible: calculation, interpolation or measurement. For a gap, give the supplier the particular torque–speed and fluid conditions that the reply must establish. This gives purchasing a concrete comparison to approve or return for clarification.

The BLINCE contact page is the appropriate route for an inquiry about a proposed BLINCE motor. Request build-specific performance and installation information for the duty under discussion. Published examples from Danfoss explain how to read a chart; they do not establish BLINCE ratings, availability, interchangeability or a supplied acceptance test.

Keep the marked table with the final quotation. If the selected displacement, control arrangement or mechanical build changes, reopen the affected checks before approving the revised offer. A model substitution should carry its own applicable evidence rather than inherit an earlier model's plotted point.

Questions that come up during a curve review

What if the machine seldom uses the best-efficiency region?

Map the actual duty first, then review the time and power associated with its points. The installation and operating limits remain part of selection even where an efficiency contour looks attractive. A best point outside the machine's normal duty supplies little basis for choosing between offers.

Can a gear-motor data table replace an efficiency map?

A compatible table of flow, pressure difference, shaft speed and torque can support a defined power comparison. Preserve its conditions and model identity. Missing quantities remain missing; do not fill them from another series simply to complete the calculation.

Is the printed curve a guarantee for the individual delivered unit?

Read the document's statement about typical or measured data and the agreed supply terms. A published family curve and a serial-number test record are different evidence. Any offered acceptance criterion needs its own stated point, method and scope.

Should a motor outside the chart be rejected automatically?

The supplied chart has not established that point. Leave the point unapproved while requesting its applicable data. The missing information also prevents a definite failure claim. Continue with a supported alternative if the condition cannot be established; retain the gap rather than converting an extrapolated spreadsheet value into an approved rating.

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Website: https://blince.com

Disclaimer

This article is a general engineering guide. Final component selection should be based on machine drawings, measured hydraulic data, working conditions, safety requirements, and confirmation from a qualified hydraulic engineer or supplier.

Blince Hydraulic Team

Blince Hydraulic is an industry-leading company dedicated to precision-engineered fluid power manufacturing and custom hydraulic solutions. Backed by decades of deep field expertise in industrial machinery and thousands of successful global deployments, our engineering team focuses entirely on high-performance hydraulic component manufacturing, including specialized orbital motors, high-pressure travel drives motor, and robust directional control valves. Our production infrastructure utilizes state-of-the-art multi-axis CNC machining systems and is fully ISO 9001 certified to guarantee repeatable volumetric accuracy across every single manufacturing run.

We deliver fast, highly dependable, and cost-efficient hydraulic solutions to heavy industry distributors, machinery OEMs, and maintenance crews across more than 150 countries. Whether your active project calls for a small-volume batch of customized shaft profiles or a large-scale production run of severe-duty cast iron gear pump, we configure our flexible production schedules to meet your target lead times with total pricing predictability. Partnering with Blince means securing maximum system efficiency, elite material quality, and uncompromised fluid power professionalism.

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