A hydraulic motor turns oil flow and pressure into a turning shaft: pressurized oil pushes on gear teeth, vanes, a gerotor star or pistons, and that push becomes torque, while the amount of oil going through sets the speed. The main types are gear, vane, orbital (gerotor and Geroler), axial piston (bent axis and swashplate) and radial piston motors, and they split roughly into fast, light-torque motors and slow, high-torque ones.
We rebuild and supply parts for most of these families, so this page sticks to how they actually work on a machine, the math you need to size one, and what tends to go wrong. Wherever a number appears, it comes from the manufacturer's own catalog, and we say which one.
The two formulas that matter. Theoretical torque (lb-in) = displacement (in³/rev) x pressure (psi) / 6.28. Speed (rpm) = 231 x flow (gpm) / displacement (in³/rev). In metric: torque (Nm) = cm³/rev x bar / 62.8, and rpm = 1000 x L/min / cm³/rev. These are the forms Eaton prints in its motor catalog. Flow sets speed, pressure sets torque, displacement sets the ratio.
How does a hydraulic motor work?
Danfoss puts it simply in its orbital motor manual: the motor converts hydraulic energy (pressure and flow) into mechanical energy (torque and speed). For a given motor size, the oil flow decides the speed and the pressure differential decides the torque. Rexroth says the same of its bent axis piston motors: output speed depends on pump flow and motor displacement, and torque rises with the pressure difference between the high and low pressure sides.
That last part trips people up. The motor works on the difference between inlet and outlet pressure, not the gauge reading at the inlet. Run a motor into a restricted return line or a second motor in series, and every psi of back pressure comes straight off your torque.
Inside, every type does the same thing. Oil is let into chambers on one side of a rotating element, the chambers grow as the shaft turns, and on the other side they shrink and push oil out to the return. A valve (a spool, a disc, a port plate, or the gear mesh itself) keeps the pressure on the right side. Reverse the flow and the shaft reverses. Some motors are fixed displacement; others, like variable piston motors, multi-torque vane motors and two-speed orbital motors, can change how much oil they take per turn to trade speed for torque.
What is the difference between a hydraulic motor and a pump?
On paper, a motor is a pump run backwards. Rexroth describes the external gear motor as the reverse of the pump process, and Parker sells its F11 and F12 bent axis units as motor/pumps. In practice the two are built for different lives:
- A motor often has to run both ways. Rexroth's one-direction gear motors drain internal leakage to the outlet, so outlet pressure is limited by the shaft seal. Its reversible gear motors route leakage to a separate drain so they can take pressure on either port, which is what lets them run in series.
- A motor has to start under load. Starting torque is lower than running torque (more on that below), so motor catalogs rate it separately.
- A motor can be driven by its load. Rexroth notes a reversible gear motor becomes a pump when the load overruns it. That is why winch and propel circuits carry counterbalance valves and brakes.
So can you use a pump as a motor? Some units are designed for it. Many are not. Rexroth notes that on standard gear pumps and motors the shaft seal is connected to the low pressure side and only withstands about 3 bar, so pressure on the wrong port pushes the seal out. Check the catalog for that exact unit before you try it. For the pump side of the story, see what a hydraulic pump is and how a piston pump works.
What is motor displacement, and how do you calculate hydraulic motor torque and speed?
Displacement is the volume of oil the motor takes to turn the shaft one full revolution, in cubic inches (in³/rev) or cubic centimeters (cm³/rev, often written cc). Everything else follows from it.
| What you want | US units | Metric |
|---|---|---|
| Theoretical torque | lb-in = in³/rev x psi / 6.28 | Nm = cm³/rev x bar / 62.8 |
| Shaft speed | rpm = 231 x gpm / in³/rev | rpm = 1000 x L/min / cm³/rev |
| Hydraulic power into the motor | hp = psi x gpm / 1714 | kW = bar x L/min / 600 |
| Mechanical power out of the motor | hp = lb-in x rpm / 63,025 | kW = Nm x rpm / 9549 |
The 6.28 is 2 x pi (one revolution in radians), and 231 is the number of cubic inches in a US gallon. All four lines are from Eaton's low speed, high torque motor catalog (E-MOLO-MC001-E8).
Volumetric and mechanical efficiency
Those formulas give theoretical numbers. Real motors lose some oil past their clearances, which costs speed (volumetric efficiency), and lose some force to friction, which costs torque (mechanical efficiency). Parker's F11/F12 catalog writes it out: overall efficiency is volumetric times mechanical. Both drop as a motor wears. A tired motor is slow under load because it leaks, and it can't lift what it used to because the pressure it needs climbs.
A worked example with real catalog numbers
Take a Char-Lynn H Series motor at 159 cm³/rev [9.7 in³/rev], run at its continuous rating of 57 L/min [15 gpm] and 114 bar [1650 psi]:
| Theoretical (formula) | Eaton catalog rating | What the gap is | |
|---|---|---|---|
| Speed | 231 x 15 / 9.7 = 357 rpm | 353 rpm | Leakage, about 99% volumetric |
| Running torque | 9.7 x 1650 / 6.28 = 2,549 lb-in [289 Nm] | 2,059 lb-in [233 Nm] | Friction, about 81% mechanical |
| Starting torque | Same 2,549 lb-in | 1,650 lb-in [186 Nm] minimum | Breakaway friction, about 65% of theoretical |
That starting torque line is the one that catches winch and conveyor builders. Size off running torque and the motor stalls on the first heavy start. Radial piston motors do much better here: Parker Calzoni lists a minimum starting torque of 91% of theoretical on its MRT motors. Plug your own numbers into our hydraulic calculator.
What are the main types of hydraulic motors?
Gear motors
Two meshing gears in a housing; oil pushes the teeth around the outside. Simple, cheap and happy at speed, but not built to crawl. Rexroth's AZMF, AZMN and AZMG external gear motors list a 500 rpm minimum and top out at 2,600 to 4,000 rpm depending on size, at 180 to 250 bar continuous. Rexroth names fan drives, road rollers, pavers and harvesters as typical jobs. Gear motors come one-direction or reversible, and the difference matters for how you plumb the drain.
Vane motors
A slotted rotor carries vanes that ride on a cam ring. Pressure on the exposed vane area turns the rotor. Vane motors run smoothly in either direction and are common in industrial drives. The Vickers MHT line is the high torque end: Vickers' catalog runs it from the MHT32 at 43 Nm per 6.9 bar [32 lb-ft per 100 psi] and 400 rpm maximum, up to the MHT1000 at 1,356 Nm per 6.9 bar [1,000 lb-ft per 100 psi] and 100 rpm, rated 138 to 276 bar depending on size. Vickers lists conveyors, feeder screws, crushers, coil winders and machine tool spindles among the uses. The 25M to 50M motors share cartridge architecture with the V series pumps, and the M2 series uses the same balanced cartridge principle. Parts and service data for all of them are on our Vickers vane motor catalog page and the Vickers motors hub.
How does an orbital motor work? Gerotor vs Geroler
An orbital motor has a star-shaped inner gear running inside a lobed outer ring. The star does not just spin; it orbits. Eaton's catalog says the star orbits several times (typically 6 to 8, depending on the geometry) for every single revolution of the shaft. That built-in reduction is why an orbital motor makes big torque at low speed in a small package. Only three main parts move: the star, the drive (a dogbone shaft that follows the orbit) and the output shaft. A valve, driven in step with the star, feeds each pocket in turn. Char-Lynn uses three valve designs: spool valve, disc valve and valve-in-star (VIS).
Eaton's quick guide shows how the valve design sets the ceiling, at maximum continuous ratings: spool valve motors to 441 Nm [3,905 lb-in] and 165 bar [2,400 psi], disc valve to 2,700 Nm [24,000 lb-in] and 205 bar [3,000 psi], VIS to 4,520 Nm [40,000 lb-in] and 345 bar [5,000 psi]. Speed runs from 1,992 rpm on the smallest J Series to 74 rpm on the biggest H Series, both at continuous flow.
Gerotor or Geroler? A gerotor has plain lobes on the outer ring, so the star slides against them. A Geroler (Eaton's name) puts rollers in those lobes. Eaton says the rollers act like a roller bearing: less friction, better mechanical efficiency, less wear with thin oil and smoother running at low speed. The Char-Lynn H Series is a gerotor; the rest of the line is Geroler. Eaton's viscosity rule shows the difference: a gerotor needs at least 100 SUS (or 20 x psi / rpm), a Geroler 70 SUS, and below 100 rpm Eaton suggests the Geroler.
Other makers use the same split under different names. Danfoss builds its OML, OMM, OMP and DH motors with plain teeth and its OMR, OMH, DS, OMS, OMT and OMV with rollers, recommending the roller sets for thin oil and constantly reversing loads. Parker calls the rollers in its Torqmotor TF, TG, TH and TL series roller vanes. The cross between brands is on our orbital motor cross reference. Parts: Char-Lynn orbital motors, Parker Torqmotor, and the Eaton, Char-Lynn and Danfoss motor guide.
Axial piston motors: bent axis and swashplate
A ring of pistons sits in a cylinder barrel parallel to the shaft. Pressure pushes the pistons out, and because they bear on an angled surface, that push becomes rotation. Change the angle and you change the displacement, which is how variable motors work.
Bent axis: the whole barrel sits at an angle to the shaft. Parker's F11/F12 run a 40 degree angle with spherical pistons and laminated piston rings, operating to 480 bar intermittent depending on frame size. Parker rates them from a 50 rpm continuous minimum up to 10,800 rpm continuous on the smallest frame and 2,400 rpm on the largest. Rexroth's A2FM fixed bent axis motor (Series 70, sizes 56 to 107) is rated 300 bar nominal and 3,000 to 3,750 rpm, with no lower speed limit but 50 rpm as the floor where smooth motion matters. The A6VM is Rexroth's variable bent axis motor. See our Parker F11 bent axis page.
Swashplate (in-line): the barrel is in line with the shaft and the pistons' slippers ride on a tilted plate. This is the classic hydrostatic transmission motor. Danfoss describes its Series 40 motors (the old Sundstrand line) as parallel axial piston and slipper units on a fixed or tiltable swashplate, rated 3,300 to 4,000 rpm and 345 to 380 bar maximum working pressure. Parts for Sundstrand Series 15 through 90 are on our Sundstrand catalog page.
What is a radial piston motor?
The pistons are arranged like spokes around the shaft. Because many pistons work at once on a big radius, radial piston motors make huge torque at very low, very smooth speeds. They are the motor you find in wheel drives, winches and heavy industrial drives.
Poclain's MS motors work their pistons against a multi-lobe cam, so each piston strokes several times per shaft revolution. The MS18 datasheet runs from 1,091 to 2,812 cm³/rev, 1,735 to 4,471 Nm theoretical at 100 bar, 75 to 170 rpm maximum and up to 450 bar. Two-displacement versions switch to a second, smaller displacement for faster travel, and the datasheet offers cams with equal or unequal lobes to set that ratio. Parker Calzoni's MRT motors do it differently, pushing on the shaft through a telescoping column of oil instead of connecting rods. They run from 7,104 to 23,034 cm³/rev with speed ranges starting at 0.5 rpm. See Poclain parts and Parker Calzoni radial piston motors.
Hydraulic motor types compared
| Type | Speed (factory examples) | Torque and pressure (factory examples) | Efficiency | Relative cost | Brands we support |
|---|---|---|---|---|---|
| Gear | 500 rpm min, 2,600-4,000 rpm max (Rexroth AZMF/AZMN/AZMG) | 180-250 bar continuous (same range) | Fine at speed, poor at crawl speeds | Lowest | Quote by model number |
| Vane | 100-400 rpm max across the MHT range | 43-1,356 Nm per 6.9 bar [32-1,000 lb-ft per 100 psi], 138-276 bar rated (Vickers MHT) | Smooth in both directions | Moderate | Vickers M2, MHT, 25M-50M |
| Orbital (gerotor, Geroler) | 74-1,992 rpm across the Char-Lynn J and H ranges | To 441 Nm spool, 2,700 Nm disc, 4,520 Nm VIS, continuous (Eaton) | Moderate; Geroler better than gerotor | Low | Char-Lynn, Parker Torqmotor, Danfoss OMx by cross |
| Axial piston, bent axis | 50 rpm min, 2,400-10,800 rpm max continuous (Parker F11/F12) | 350-420 bar continuous, 420-480 bar intermittent (F11/F12) | High | High | Parker F11/F12; Rexroth A2F/A6V by quote |
| Axial piston, swashplate | 3,300-4,000 rpm rated (Danfoss Series 40) | 345-380 bar max working (Series 40) | High | High | Sundstrand / Sauer-Danfoss |
| Radial piston | 75-170 rpm max (Poclain MS18); from 0.5 rpm (Calzoni MRT) | 1,735-4,471 Nm at 100 bar, to 450 bar (MS18); 91% starting torque (MRT) | High, best starting torque | Highest | Poclain, Parker Calzoni |
Speed and pressure figures are the manufacturers' published ratings for the models named. Efficiency and cost columns are our shop view, not catalog numbers.
Where are hydraulic motors used?
Eaton splits motor jobs into two groups. Traction (propel) drives move a wheeled or tracked machine, usually in a closed loop with a variable pump: skid steer and turf wheel motors, compact track drives, road rollers. Non-traction (work) drives do everything else: winches, augers, conveyors, grapples, and the rotate function on booms and cranes. Rough rules from our bench:
- Wheel and track drives: orbital motors on light machines, radial piston or bent axis motors with gearboxes on heavier ones.
- Winches and swing drives: orbital or radial piston, always with a brake or counterbalance, because the load will try to drive the motor.
- Augers, mixers and conveyors: orbital motors, or vane motors in a plant.
- Fans and fast spindles: gear or axial piston motors.
Does a hydraulic motor need a case drain?
It depends entirely on the type, and getting it wrong is one of the most common reasons we see blown shaft seals on motors.
| Type | Case drain | Factory limit |
|---|---|---|
| Axial piston (Danfoss Series 40) | Yes | Case pressure 1.7 bar [25 psi] maximum working, 5.2 bar [75 psi] maximum |
| Axial piston (Rexroth A2FM) | Yes | Mean 2 bar above ambient, peaks to 10 bar for under 0.1 second |
| Radial piston (Parker Calzoni MRT) | Yes | 5 bar maximum housing pressure, 15 bar with the optional shaft seal |
| Vane (Vickers MHT) | Yes | Case port 0-1.7 bar [0-25 psi] |
| Gear, reversible (Rexroth AZMF) | External drain | Leakage line under 3 bar |
| Orbital (Char-Lynn J Series) | Optional | Up to 76 bar [1,100 psi] return line pressure with no drain; with a drain, 220 bar [3,190 psi] peak in the return line |
Orbital motors are the odd ones out because their high pressure shaft seals are built to take back pressure. Eaton says its R Series seal lets the motor take high back pressure without an external drain and run in series or parallel, and Parker says the same of the Torqmotor TG, TH and TL seal. A drain still helps: Eaton points out that drain flow flushes contamination, cools the motor and lowers case pressure, which extends seal life. If you do run one, Eaton's R Series catalog recommends a restriction that holds about 3.5 bar [50 psi] in the case so the motor stays full of oil.
Series or parallel? Eaton notes several motors can run off one pump in either arrangement. In series, the first motor's outlet is the second motor's inlet, so the first motor's seal sees the second motor's working pressure. That is fine on an orbital motor built for it, and it blows the seal on one that isn't. In parallel, the flow splits, and it goes where the resistance is lowest, so a wheel that loses traction takes the flow from the others.
How do you pick a replacement hydraulic motor?
- Read the tag. A product or model number identifies the motor exactly. Our guides for Parker, Poclain MS and the Char-Lynn decoder walk through them.
- Match displacement first. A different displacement changes speed and torque by the formulas above, so the machine will not behave the same.
- Match the mount, shaft, ports and case drain port. Then the rotation: on orbital motors, timing sets which way the shaft turns for a given port.
- Check the duty, not just the pressure. Intermittent ratings are short: Eaton defines intermittent as 10% of every minute, Parker as 6 seconds in any minute. Size for continuous.
- Don't forget the options: brake, two-speed, speed sensor, shuttle or flushing valve.
- Watch for OEM specials. Motors built for Deere, Case or CAT can differ from the catalog unit, so a catalog match is not a guaranteed fit.
If the old motor is worth saving, a rebuild is often the cheaper route. Our Char-Lynn rebuild guide covers timing and bolt torque by series, and the troubleshooting guide helps decide whether the motor is the problem at all.
Frequently asked questions
How does a hydraulic motor work?
Pressurized oil fills chambers on one side of a rotating element (gears, vanes, a gerotor star or pistons) and pushes it around, and the oil leaves through the other side. The flow rate sets the speed, the pressure difference across the motor sets the torque, and the displacement sets the ratio between them.
What is the difference between a hydraulic motor and a hydraulic pump?
A pump turns shaft power into flow; a motor turns flow back into shaft power. Motors are built to run in both directions, start under load and take pressure on either port, which is why many pumps cannot simply be used as motors.
What is the difference between a gerotor and a Geroler motor?
A gerotor has plain lobes on the outer ring. A Geroler, Eaton's name, adds rollers in those lobes, which Eaton says reduces friction and wear, improves mechanical efficiency and gives smoother low speed running. Eaton calls for at least 100 SUS oil on a gerotor and 70 SUS on a Geroler.
What is the formula for hydraulic motor torque?
Theoretical torque in lb-in equals displacement in cubic inches per revolution times pressure in psi, divided by 6.28. In metric, torque in Nm equals cm³/rev times bar divided by 62.8. Actual torque is lower by the motor's mechanical efficiency.
Why are orbital motors low speed and high torque?
The star in an orbital motor orbits several times for each turn of the shaft, typically 6 to 8 times according to Eaton, which gives a built-in reduction. That lets a compact motor turn slowly with high torque and no gearbox.
What is a radial piston motor used for?
Very high torque at very low speed: wheel drives, winches, and heavy industrial drives. Poclain's MS18 makes up to 4,471 Nm theoretical at 100 bar, and Parker Calzoni lists 91% starting torque on its MRT motors, which is why they start heavy loads cleanly.
Can you run hydraulic motors in series?
Yes, if the motors are built for it. In series, each motor's outlet carries the next motor's working pressure, so the seal has to take that back pressure. Eaton rates motors such as the R Series for series or parallel use; most piston motors need a free-flowing case drain and cannot take it.
Does a hydraulic motor need a case drain?
Piston and vane motors do, with low case pressure limits (Danfoss Series 40 motors 1.7 bar [25 psi] maximum working, for example). Many orbital motors do not: Eaton rates the Char-Lynn J Series for 76 bar [1,100 psi] return line pressure with no drain.
About the technical information on this page. Figures, procedures and part numbers here come from manufacturer service literature and are provided for reference only. They are only as accurate as the factory data, and the revision we have on file may not be the latest for your unit. Always confirm against the current manufacturer documentation for your exact model and follow safe work practices. RestoPower is not responsible for damage, injury or loss resulting from the use or misuse of this information, or from incorrect assembly, adjustment, installation or operation of any unit.
Tell us what is on the tag and what the motor drives
Send the model or product number, and what the motor does on the machine. We will match the parts or the seal kit, or quote a rebuild. Motors sent in get a free teardown evaluation and a firm itemized quote before any billable work.
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Related reading
- Hydraulic calculator: torque, speed, flow and horsepower
- What is a hydraulic pump?
- What is a hydraulic piston pump and how does it work?
- Char-Lynn orbital motor rebuild guide
- Poclain MS motor guide
- Sundstrand pump and motor guide
- Hydraulic Motor Repair & Rebuild
- Hydraulic Pump Rotation: How to Tell It and How to Change It