A conveyor turns normally after it is moving, but the motor stalls whenever product sits on the belt overnight. The inlet gauge reaches 160 bar, so a larger pressure rating looks like the obvious purchase. Yet the return port is at 35 bar during startup, the chain drive adds a changing radius, and the catalog torque was read from a running curve rather than a starting-torque line.
The calculator was not wrong. The inputs described the wrong operating moment.
Short answer
For a metric screening calculation, use the pressure difference across the motor, not pump pressure alone:
This result narrows the displacement range; it does not approve a model. Breakaway load, starting efficiency, continuous versus intermittent pressure, speed, side load, brake release, return backpressure, oil condition and the exact performance curve still control the decision.
Start at the driven load, not at the catalog
The shaft does not know the pump’s maximum pressure or the number printed on a relief valve. It only sees the torque demanded by the load and the torque that survives losses between the high-pressure and return ports. Before opening a catalog, reduce the machine to the motor-shaft requirement: force at a known radius, drum pull, conveyor tension, auger resistance, wheel tractive effort or measured drive torque. The BLINCE hydraulic orbital motor range shows the available family direction, while the guide to the three common hydraulic motor types helps decide whether an orbital, gear or piston architecture belongs in the calculation at all.
Steady running load is only one line in the duty profile. A loaded winch, compact mixer or auger can require materially more torque to start than to keep moving; a reversing drive adds acceleration and shock; a wheel drive may impose radial load through the hub. A motor that passes a theoretical running calculation can still hesitate at the worst rotor position or overload its bearing arrangement. The orbital-motor working-principle guide explains why the displacement mechanism produces low-speed torque, and the gear-motor versus orbital-motor comparison shows why the same calculated torque does not make different architectures interchangeable.
Evidence boundary: what the calculator knows
The equation is a physics relationship. Parker publishes the motor form as M = D × Δp × ηm ÷ 63, with displacement in cm³/rev and pressure differential in bar. Danfoss likewise separates theoretical torque from hydraulic-mechanical efficiency and shows that starting torque can be lower than running torque at the same pressure drop. Tokyo Keiki gives the equivalent SI relationship and treats pressure as the difference between motor inlet and outlet. These sources agree on the useful boundary: pressure differential and displacement establish theoretical torque; model data and efficiency determine what reaches the shaft.
BLINCE product numbers in this article are Published values from the current OMH/BMH page, not generic promises. The family table lists geometric displacements of 203.2, 255.9, 316.5, 406.4 and 489.2 cm³/rev; continuous pressure limits vary by size, and published torque and speed limits must take priority over an equation. Review the OMH/BMH family product data together with the 200, 250 and 500 cc selection guide before treating a rounded displacement label as an exact input.
Step 1: calculate the torque demanded at the motor shaft
For a tangential force acting at a drum, sprocket or wheel radius:
Load torque (N·m) = force (N) × effective radius (m)
If a gearbox sits between motor and load, reflect the load back to the motor shaft:
Define gear ratio here as load-shaft torque divided by motor-shaft torque for an ideal reducer. State the convention in every calculation because ratio notation differs among drawings and suppliers.
A radius measured to the bare drum is not enough when cable builds up in layers. Conveyor tension can change with material loading; wheel torque must include grade and rolling resistance; acceleration adds inertial torque. Record continuous, startup and peak cases separately. The BLINCE high-torque OMH 400 page provides a concrete product boundary, while the motor speed calculation guide prevents a torque-led selection from silently missing the target rpm.
Step 2: measure useful pressure differential
Use pressure at the motor inlet minus pressure at the motor outlet during the same operating moment:
Δp = pinlet − poutlet
Suppose the inlet is 165 bar and the outlet is 35 bar during startup. The working differential is 130 bar, not 165 bar. With a 250 cm³/rev motor, the theoretical values are 657 N·m at 165 bar and 517 N·m at 130 bar. Ignoring return pressure overstates this example by 140 N·m, or about 27% relative to the useful-differential result.
Outlet pressure is not always a harmless fixed number. A restrictive directional valve, small hose, narrow elbow, undersized quick coupler, loaded return filter or cold cooler can consume pressure that the motor needed for torque. Measure both ports with suitable instruments and follow the machine’s safe test procedure. A system-level reading is more useful when interpreted beside the hydraulic pump and motor relationship in the 200/250/500 cc guide and the BLINCE general motor-type guide, because backpressure, architecture and duty change what the same gauge value means.
Step 3: calculate theoretical and estimated running torque
Worked example — calculated, not a product promise
Assume an auger drive needs 620 N·m at the motor shaft while running and 780 N·m to break away. Measurements at the motor ports during the loaded event show 175 bar inlet and 25 bar outlet. Available differential is therefore 150 bar. For screening only, use an assumed hydraulic-mechanical efficiency of 0.88; the exact value must come from the selected model’s curve at the relevant pressure, flow, viscosity and temperature.
For a nominal 315 cm³/rev candidate:
Theoretical torque = 315 × 150 ÷ 62.8 = 752 N·m
Estimated running torque = 752 × 0.88 = 662 N·m
Running reserve:
Reserve = (662 − 620) ÷ 620 × 100% = 6.8%
The candidate clears the assumed running load by only 42 N·m and does not meet the 780 N·m breakaway requirement. Raising the relief setting is not the automatic answer: the exact motor, pump, valve, hose, coupling and machine structure all have pressure limits, and a jammed load may be the actual fault. The calculation changes the next action from “order the 315” to “verify starting-torque data, load condition and available differential, then consider a different displacement, reduction ratio or mechanical correction.” The OMH/BMH exact displacement table and OMT-315 product route illustrate why a nominal number alone cannot establish equivalent output or interfaces.
Step 4: solve backward for minimum displacement
When required shaft torque is known, rearrange the equation:
Required displacement (cm³/rev) = required torque (N·m) × 62.8 ÷ (Δp in bar × ηm)
Using the 620 N·m running load, 150 bar differential and assumed 0.88 efficiency:
The second result is only a screening value because starting efficiency is not necessarily the running efficiency. Danfoss’s general orbital-motor example explicitly distinguishes minimum starting torque from running torque at the same pressure drop. Select against the manufacturer’s starting-torque and permissible-duty data, then check whether the larger displacement can still reach speed. The OMH 400 application page offers a relevant high-torque direction, but the hydraulic motor speed formula must be applied before any larger unit becomes a candidate.
The calculation does not choose the duty rating
Continuous, intermittent and peak pressure are not synonyms for three interchangeable ways to reach the same torque. Continuous duty must remain inside the continuous envelope at the specified speed, oil condition and thermal state. Intermittent limits normally apply for bounded periods and cycles defined by the data sheet; peak values cover shorter transients. A motor that produces enough calculated torque at a peak condition may be unsuitable for a conveyor that holds that load for twenty minutes. Compare the duty trace with current product data rather than converting a peak rating into a continuous promise. The OMH/BMH published table separates continuous and intermittent limits, and the MS/OMV/BMV heavy-duty motor page provides another family path where operating data must be matched to an exact configuration.
Temperature moves the result indirectly. Hotter, lower-viscosity oil can increase internal leakage and reduce speed or efficiency; very cold oil raises inlet and return losses and may delay lubrication. Contamination changes clearance wear, while an overloaded cooler or restrictive return line converts hydraulic power into heat. Torque reserve that exists in a clean, stabilized test may disappear after warm-up. This is why the motor speed and efficiency guide should be read beside the orbital-motor construction overview, not treated as a separate calculation exercise.
Decision table: what to change after the result
Observed result
Candidate action
Benefit
Cost or failure risk
Confirm before selection
Running torque passes; starting torque fails
Check breakaway load, starting-torque curve, brake release and jam risk
Targets the actual weak operating moment
A larger motor may hide a mechanical fault and reduce speed
Torque and speed both require more hydraulic power
Re-evaluate the complete power unit
Prevents shifting overload to pump or prime mover
More heat, cost and circuit changes
Pump flow, prime-mover power, cooling, valve and hose losses
The table is a routing tool, not a ranking. If a calculated displacement rises, first ask whether pressure is being wasted, whether the load estimate includes a fault, and whether a gearbox can improve the operating point. The three-motor-type comparison can reopen the architecture question, while the BLINCE orbital motor category keeps the commercial search tied to available product families.
Equipment checks after torque passes
Speed and flow
A larger displacement normally gives more theoretical torque per bar but fewer revolutions from the same flow. Use n = Q × 1000 × ηv ÷ D, then compare continuous and intermittent speed limits. Also measure flow while other functions operate; the pump’s nameplate flow is not necessarily what reaches this motor. The dedicated speed-formula article develops this check, and the 200/250/500 cc guide makes the displacement-versus-speed tradeoff visible.
Shaft, flange and external load
Torque capacity does not prove that the shaft can carry a sprocket, wheel, pulley or drum at the proposed overhung distance. Verify shaft form and engagement, flange pilot, bolt pattern, coupling alignment, radial and axial load, bearing life and allowable mounting orientation. A unit that bolts on may still overload its bearing package. Compare exact drawings from the OMH/BMH product page with a heavy-duty direction such as the MS/OMV/BMV series instead of assuming that similar torque makes the housings equivalent.
Ports, drain, brake and rotation
Port thread, seal form, port position and hose clocking must match the circuit and installation space. Confirm whether the motor requires a case drain, the permitted case pressure, and the drain route. For a braked drive, release pressure, holding torque and control sequence are separate from running torque. Confirm rotation with the proposed port connection; do not infer it from a photograph. The orbital-motor category shows multiple configurations, while the motor architecture guide explains why an integrated travel or brake assembly is not just a bare motor with extra hardware.
Thermal and hydraulic power check
Input hydraulic power is approximately Δp × Q ÷ 600 in kW when Δp is in bar and flow is in L/min. At 150 bar and 60 L/min, input power is 15 kW. If overall efficiency were 0.82 as an example, estimated shaft power would be 12.3 kW and about 2.7 kW would appear as loss at that operating point. This is not a cooler-sizing calculation, but it is enough to reject the idea that more pressure and flow are thermally free. Cross-check the high-torque OMH example with the general hydraulic motor guide before the power unit, hoses or cooler are left outside the decision.
Common mistakes and their consequences
Using pump outlet pressure as Δp looks reasonable because that gauge is easy to see. It fails when the directional valve, fittings, motor outlet and return line create meaningful backpressure; the calculated torque becomes optimistic. Measure both motor ports during the same loaded event, then compare the result with the torque discussion in the displacement guide and the chosen BLINCE motor family.
Applying a generic 90% efficiency to every motor and operating point creates false precision. Efficiency changes with motor design, displacement, pressure, speed, viscosity and wear; starting torque can be lower than running torque. Use an assumption only for screening, label it, and replace it with the exact curve or published torque data. The OMH/BMH specification page is controlling for that family, while the OMT-315 route requires its own data rather than borrowed OMH values.
Choosing the next larger displacement can solve one torque shortfall and create a speed shortfall. It may also change frame size, shaft loading, hose flow, acceleration and braking behavior. Recalculate rpm, power and interfaces before calling the change an upgrade. The speed calculator guide and gear-versus-orbital comparison make those coupled decisions explicit.
Finally, never treat a higher relief setting as a free torque adjustment. The load creates pressure; the relief limits it. Increasing the setting without reviewing every pressure-containing component, prime-mover power, heat, machine structure and safe procedure can move the failure elsewhere. If the drive was previously adequate, diagnose what changed before resizing. The orbital-motor working guide provides the component boundary, and the broader hydraulic motor overview keeps that diagnosis connected to the system.
When not to order a motor yet
Pause if the load torque is guessed, only an inlet gauge is available, the complete old model code is missing, the machine has an unresolved jam, or nobody has separated continuous, intermittent and breakaway duty. Also pause for personnel lifting, suspended loads, braking or steering functions until the machine procedure, approved architecture and qualified review define the safety requirements. A calculator cannot validate a brake circuit, load-holding function or pressure rating.
A quotation should also wait when the shaft, flange, ports, rotation, drain requirement, external load or installation envelope remain unknown. Physical similarity is not compatibility. Use the BLINCE orbital motor category to identify plausible families and the OMH/BMH detailed table to see the level of model-specific data needed before approval.
Data to send for a technically reviewable quotation
Machine and load: machine function, driven element, required force or pull, effective radius, gearbox ratio and efficiency, continuous/running torque, breakaway torque, shock or reversal.
Performance: target rpm, measured flow at the motor, inlet and outlet pressure at startup and running, continuous/intermittent/peak duration, cycle and simultaneous functions.
Fluid and temperature: oil type and viscosity grade, cold-start, stabilized and maximum oil temperature, filtration and contamination history.
Interfaces: complete old code, displacement, shaft, flange pilot and bolt pattern, ports and seal form, rotation, case drain, brake, envelope, marked photographs and drawing.
Symptom or project state: new design or replacement, when the fault appears, recent repairs, debris, noise, leakage and pressure/flow measurement method.
FAQ
What is the quickest hydraulic motor torque formula in metric units?
Use T = D × Δp ÷ 62.8 for theoretical torque in N·m, with displacement in cm³/rev and pressure differential in bar. Multiply by hydraulic-mechanical efficiency for a screening estimate of running torque, then check the exact model curve and starting torque.
Should I use pump pressure or motor inlet pressure?
Neither alone is sufficient. Use motor inlet pressure minus motor outlet pressure, measured during the same loaded operating moment. Pump pressure may include losses before the motor; inlet pressure alone ignores backpressure after it.
How much safety factor should I add?
There is no universal percentage. Margin depends on load certainty, breakaway torque, shock, duty, temperature, efficiency data, braking and the consequence of a stall. Define the operating cases first, then use the machine standard and supplier data to set an appropriate margin.
Why does the motor meet calculated running torque but fail to start?
Breakaway load may be higher than running load, and minimum starting torque may be lower than running torque at the same pressure differential. Brake-release pressure, static friction, load position, oil viscosity and rotor position can also matter.
Will a larger displacement always give more usable torque?
At the same useful pressure differential, theoretical torque rises with displacement. Real output still must stay inside the exact motor’s torque, pressure, speed and duty limits, and the larger displacement will normally run slower at the same flow.
Can the calculator approve an equivalent replacement?
No. It can screen displacement and pressure. Replacement approval also needs shaft, flange, pilot, ports, seal form, rotation, case drain, brake, external load, envelope, speed, duty and exact model data.
Engineering handoff
Send BLINCE the load calculation or measured torque, target rpm, flow at the motor, simultaneous inlet/outlet pressure readings, duty cycle, oil temperature, complete old model code, shaft/flange/port details and installation photos. BLINCE can return a preliminary displacement direction, identify missing or incompatible data, compare plausible motor families and explain when the circuit or load should be corrected before a quotation proceeds.
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.
To learn more about our complete product lineup, visit our official website: www.blince.com.
