How To Size A Hydraulic Winch Motor From Line Pull And Line Speed

To size a hydraulic winch motor, start with the rope’s required pull and speed. Then record how much rope is already on the drum. Those figures tell you how hard and how fast the drum has to turn. Use the gearbox ratio to work back to the motor, then check the flow and pressure available at its ports.

If the winch must pull the same load with a nearly empty drum and a nearly full drum, check both ends of that range. On the larger rope layer, the drum needs more torque to produce the same pull. At the same drum speed, the rope also travels faster on that larger layer. These two changes can lead to different motor requirements even though the load on the rope has stayed the same.

This guide works through a hypothetical single-line pulling duty. For a machine built around an existing winch, get its approved configuration and ratings first. A motor calculation is one part of checking the complete assembly. Keep the original winch maker’s requirements for the drum, gearing, rope, brake and controls in the equipment review.

Put the pull and speed on the same line of your worksheet

Line pull is the force in the rope at the winch. Line speed tells you how fast that rope travels. On a direct, single-line layout the requested pulling force can go in the line-pull column. If the rope runs through a block arrangement, use the force and speed at the winch end of the rope. A load’s travel speed or weight alone does not describe what the rope or drum sees. Get the rope force and rope travel for the actual arrangement from the equipment designer.

Write the operating cases before looking at a motor catalogue. Put a loaded start and a steady pull on separate lines, even if they use the same rope. A third case could involve pulling a shorter length of rope at a higher speed after the load has eased. Mark which figures are design requirements and which were measured on the existing machine. Where a value is still an estimate, show who will confirm it before the order is released.

Give the supplier a simple set of units. Newtons or kilonewtons are useful for force. Metres per second or metres per minute work for rope speed. A service document may instead use pounds-force or feet per minute. Keep the original number and its unit on the worksheet and show the conversion beside it. A figure written as “12,000 pull” can mean very different things across drawings, and the supplier needs to know which unit was intended.

Take note of the conditions that produce the largest force. The highest pull might occur for a few seconds at the start. A lower steady pull may last much longer. The maximum line speed may only be required with light load. Put each case on a separate row and include its duration. Combining the largest force with the fastest speed makes sense only if the machine actually needs both at once or the designer has specified that combined point.

For a replacement, start with the old motor code, the winch model and the gearbox drawing. A working motor’s part number can help identify a configuration that has already been reviewed. An unreadable tag need not stop a first inquiry. Include photos, equipment serial information and the required rope pull and speed. Ask the winch maker which motor and gear arrangement the machine was built around before treating a similar-looking spare as a match.

How does rope layer affect winch pull?

At the same drum torque, a hydraulic winch pulls less on a larger rope layer. To maintain the same line pull as the rope builds up, the drum needs more torque. At the same drum speed, the rope also travels faster on the larger layer.

Measure from the centre of the drum shaft to the centre of the rope where it wraps onto the drum. On the first layer, this is roughly the bare drum radius plus half the rope diameter if the rope sits directly on the drum. Each full additional layer moves the rope centre outward. Where the drum grooves, winding pattern or rope size change the position, use the dimensions or layer chart for that winch.

The flange on either side of a drum helps retain the rope. Its outside diameter is a convenient measurement, but the rope does not necessarily run at that radius. Keep the drum flange dimension in the equipment drawing and enter the actual rope radius in the calculation. If the drawing supplies diameter at the rope centre, divide it by two before you multiply by line pull.

Winch manufacturers may publish pull ratings by rope layer. For one example, see the layer table in the Ramsey H-800 owner’s manual. Those ratings apply to the H-800 under the conditions in its manual.

Next, check how the machine controls rope speed. Each revolution takes up more rope on a larger layer. If you hold motor speed constant, the outer layer takes up rope faster. If your specification holds rope speed constant instead, the motor needs to run more slowly on the outer layer. Identify which speed the machine actually controls. A system with a fixed supply flow can behave differently from one in which an operator or controller trims speed as the rope builds up.

A worksheet for an existing winch should say where the rope usually starts and where it finishes during the heavy part of a pull. For example, most of the heavy work may happen between two middle layers, even if the drum can hold several more. Keep the allowed range from the winch maker in the job specification. Record any separately specified pull at the fullest permitted drum so the review covers the point that the buyer will actually use.

How do you calculate motor torque and speed from line pull?

Multiply line pull by the working rope radius to find drum torque. Divide line travel per minute by the circumference at that radius to get drum rpm. Use the gearbox ratio and its efficiency to convert drum torque and speed to the required motor duty.

Worked example for one pull at two drum layers

Suppose a winch must provide 12,000 N of line pull at 0.30 m/s on either of two layers. The example uses a 75 mm working radius on the smaller layer and 115 mm on the larger one. These are assumed dimensions for the calculation. Use the machine’s actual rope radius and its approved limits for a real selection.

Multiply line pull by radius to get drum torque. On the smaller layer, 12,000 N × 0.075 m gives 900 N·m. On the larger layer, 12,000 N × 0.115 m gives 1,380 N·m. Both results describe the drum before you account for the gearbox. Note the position of the rope in the record so a vendor can reproduce the arithmetic.

To find drum speed, divide the rope travel per minute by the circumference. The example’s 0.30 m/s equals 18 m/min. Divide 18 m/min by 2π × 0.075 m to get about 38.2 rpm on the smaller layer. Divide it by 2π × 0.115 m to get about 24.9 rpm on the larger one. This is how fast the drum must turn if the rope is to travel at the same speed on both layers.

Now suppose the winch uses a 5:1 reduction. This means the motor makes five turns for each turn of the drum. Motor speed is drum speed × 5. The resulting motor speeds are about 191 rpm on the smaller layer and 125 rpm on the larger one. State the ratio as motor revolutions per drum revolution so both sides apply it in the same direction.

A gearbox also loses some torque to friction. For illustration, use a running efficiency of 0.85 at these points. Divide the drum torque by the ratio and that assumed efficiency. The smaller layer needs 900 ÷ (5 × 0.85), or about 212 N·m at the motor. The larger layer needs 1,380 ÷ (5 × 0.85), or about 325 N·m. Use the gear maker’s data for the actual drive, including its starting conditions and any separate restrictions.

The rope output in both cases is 12,000 N × 0.30 m/s = 3.6 kW at the stated pull and speed. Equal rope power is a quick arithmetic check. One layer needs a faster motor at lower torque and the other needs a slower motor at higher torque. The example does not include acceleration, brake drag or losses on the hydraulic side. A supplier can review each of those separately when the drive data are available.

A motor that clears 325 N·m at 125 rpm still has to reach about 191 rpm at the smaller layer with the oil the machine can supply. When you request the curve, give the supplier both rows. That avoids buying from a torque figure taken from one condition and a speed figure taken from a different one.

How much oil does a hydraulic winch motor need?

A fixed-displacement hydraulic winch motor needs enough inlet flow to reach the required rpm and enough pressure difference between its working ports to deliver torque. Estimate both at each rope layer, then check the motor maker’s curves for the actual configuration and duty.

Motor displacement is the theoretical oil volume per shaft turn. Use the pressure difference at the motor ports when estimating torque. If the return line has back pressure, subtract it from the motor inlet pressure for that operating point.

For a rough comparison, take a hypothetical 160 cm³/rev motor with volumetric efficiency of 0.90. To estimate the inlet flow needed for 191 rpm, multiply 160 cm³/rev by 191 rpm and divide by 1,000 and by 0.90. That is about 34 L/min. For about 125 rpm, the same method gives about 22 L/min. These figures show why the smaller drum layer may need the larger flow even though the larger layer needs more torque.

You can do a parallel estimate for pressure difference using mechanical efficiency. For a running point with 0.85 mechanical efficiency, the pressure difference in bar is approximately 62.83 × torque in N·m ÷ displacement in cm³/rev ÷ efficiency. For the assumed 160 cm³/rev motor, 212 N·m needs about 98 bar. At 325 N·m, the same assumptions give about 150 bar. The factor 62.83 converts the metric units and accounts for one full turn.

The flow and pressure calculations use separate efficiency terms for separate effects. Leakage changes how much inlet oil it takes to reach a shaft speed. Mechanical losses change how much of the pressure difference becomes shaft torque. Both effects vary with the operating point and motor design. Treat the example as a way to prepare the inquiry, then compare actual curves. The explanation of flow, pressure difference, displacement and starting torque in Danfoss general orbital motor technical information shows how a manufacturer presents those relationships.

Suppose the supply can deliver 35 L/min at low load, but available flow falls when the circuit reaches the larger layer’s pressure. Record the flow at that pressure if the pump or control changes delivery. If another function uses the same pump at the same time, note its demand and priority. The winch inlet flow can differ from the figure printed on the pump data sheet, especially in a machine with shared circuits.

Treat the maximum working pressure of each component as a separate check. Motor torque depends on the difference between its working ports. Port, return and case limits depend on the pressure at the connections specified by the manufacturer. The torque calculation alone leaves the pressure at the individual connections open. Put both port readings and any relevant drain conditions on the inquiry sheet when the circuit calls for them.

A motor running at a high pressure drop also needs a heat check over the duty cycle. Start with the required run time and the rest time between pulls. Give the supplier the expected oil temperature range and which parts of the cycle run at the higher pressure or flow. If the work repeats for much of the shift, request a review of sustained operation. A brief successful pull does not establish that the same motor can repeat the duty all day.

Decide what to check at a loaded start

Running torque describes a motor that is already turning. Starting torque is what it can deliver as it begins moving under load. A winch that usually runs steadily may still start with a tight rope after an interruption. If loaded starts are part of the job, give the supplier the rope layer, required pull and available pressure for that case. Ask for the starting torque for the offered configuration.

A loaded start can also bring the brake and control circuit into the review. Follow the original equipment instructions for the approved release and holding arrangement. On the inquiry, identify the brake and the valve group separately from the motor. Describe whether the buyer is sourcing a bare motor or wants the supplier to review an assembly. The existing BLINCE guide to winch motor runaway and brake valve logic covers those control questions in more detail.

There may be an operating limit at low speed as well as high speed. On the outer layer, the winch can demand a slow motor speed while the pull is high. Give that point to the vendor alongside the normal production speed. Ask whether the curve or data sheet supports it for the requested time and oil conditions. If the job requires slow positioning, describe the acceptable speed range and the load while positioning.

Check how long the catalogue allows the motor to run at each point. A machine that pulls every few minutes needs a different duty review from one used for occasional recovery. Give the supplier the pull duration, number of repeats and any loaded starts.

When there is a real motor test record, use it to confirm the duty assumptions. A trace that shows line speed and pressure can reveal which point occurs first and how long it lasts. If the machine has only a pump outlet reading, write down where it was measured and what was operating. Ask the equipment or motor supplier what additional data are needed to identify the pressure at the motor. Put the gauge location, oil temperature and rope layer in the heading of the record.

Compare a motor with the gearbox and the rest of the winch

A higher reduction ratio generally lowers the motor torque required for a given drum torque and raises the motor speed required for a given drum speed. A lower ratio generally has the opposite effect. For a replacement on an approved assembly, use its existing ratio unless the equipment designer approves a change. An alternate ratio changes how the motor fits the hydraulic supply and how the drum reaches its line speed. Put both calculations on the sheet if a new design allows a choice.

A larger-displacement hydraulic motor can develop more torque at a given pressure difference, subject to its efficiency and ratings. At the same speed it also needs more inlet flow. Try the two load points against the available supply before committing to that change. If the pump can meet pressure but has little extra flow, increasing displacement may cost rope speed. If the original problem is a loaded start, get the starting torque curve before drawing a conclusion from displacement alone.

For an existing winch, the gearbox may sit between the motor and the drum. Confirm the exact shaft connection and mounting before comparing quotations. Check flange pilot, bolt pattern, shaft form and usable engagement, plus which way the motor must turn viewed from the same end on both drawings. Include motor body and hose clearances in the machine. A brochure view can identify a family. The order drawing identifies the unit that has to fit.

Check who supplies the mating pieces. A motor might come with a shaft suited to a separate coupling that the buyer already has. Another quote could include a different hub or adapter. Ask both suppliers to mark what stays and what changes. Record which drawing revision covers the assembled motor and gearbox, and ask who confirms the fit if the quote adds parts the machine did not previously use.

If the new motor drives a separate gear through an external shaft connection, show that layout on the drawing. Include any shaft load that comes from the connection. The motor and gear manufacturer can then identify which bearings carry those forces. The line-pull calculation establishes the required drum torque. The physical drawing tells the supplier how that torque and any additional load reach the motor.

The BLINCE hydraulic motor range can help identify the broad family for an inquiry. If the original unit is an orbital motor, compare its approved operating points and connection drawing with the BLINCE orbital hydraulic motor range. A motor family page is a place to start the comparison. Ask for data for the exact product and code offered for the winch.

Read the two quotes against the same rope duty

Put the same table of rope layers and duty points into both quote requests. Ask each vendor to return motor speed, torque, flow and pressure difference for those points and the documents behind the figures. Record any assumed gearbox efficiency and any starting torque condition. A vendor can then identify whether a gap comes from the oil supply, the motor rating or the gearbox ratio.

Keep test evidence beside the quote when the buyer has it. Note whether the test was run on a bare motor or on a complete winch, whether it used the same gearbox, and at which rope layer the reading was made. A motor bench test can support a motor performance point. A line-pull test on an assembly describes the unit and setup that were tested. If the test used a different layer or ratio, write that next to the result before comparing it with your own requirement.

If both quotes can meet the required duty, compare the installation drawing, the allowed run time and the documentation the vendor can supply. Ask for a revised offer if the configuration or mounting changes during discussion. For a spare, record the original model and the reviewed replacement drawing together. A later buyer can see what was approved without reconstructing the conversation from a part number alone.

If a vendor quotes only the motor’s peak torque, send back the normal running point and the start case as separate lines. Request the duration for which the peak figure applies and the speed range covered by the quote. Give the supplier a chance to identify which part of the request is still uncertain. A short answer with a named open item is useful when the team is waiting on a machine drawing or a measured pressure.

What to send with a hydraulic winch motor inquiry

Begin with the winch model, gearbox ratio and old motor code if available. Attach a drawing showing the mounting face, shaft and port layout. Give the working rope radius at each important layer, required pull and speed, loaded starts and run time. State which functions share the oil supply. Where you have pressure readings, include both working ports and their measurement points so the vendor can estimate the actual pressure difference.

If you only have the first measurements, send the two critical load points and the equipment details you know. Mark what could still change the choice, such as an unverified gearbox ratio or the radius of the fullest working layer.

If the worked example were a real inquiry, one line on the duty sheet might read “12 kN at 0.30 m/s, 115 mm working radius, 5 motor turns per drum turn.” The next line would give the smaller radius and the same pull and speed. Add the gearbox maker’s efficiency data when available, and label a starting pull separately. The supplier can see which speed and torque go together before it looks for a motor.

Send the duty sheet and drawing through BLINCE’s technical inquiry page. Ask for the proposed order code, the motor curve with your operating points marked, and the matching installation drawing. Record anything the supplier still needs before it can confirm the fit or the duty.

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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.

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