A hydrostatic drive (hydrostatic transmission) is a variable displacement piston pump and a hydraulic motor connected in a closed loop: the pump's outlet feeds the motor, and the motor's return oil goes straight back into the pump instead of to a tank. Tilting the pump's swashplate sets how much oil it moves per turn, so one lever controls speed and direction, from zero to full in forward or reverse, while the engine runs at a steady speed.
Around that loop sit the parts that keep it alive: a charge pump that keeps the loop full and pressurized, check valves that feed charge oil into the low side, relief valves that guard the high side, a flushing valve that bleeds hot oil out of the loop, and case drains that carry leakage back through the cooler. This page explains each one using the Danfoss (Sauer-Danfoss, Sundstrand) Series 40, 42 and 90 technical information and Eaton's heavy-duty hydrostatic documents.
What this page covers. Industrial and mobile hydrostatic pumps and motors: Sundstrand / Sauer-Danfoss / Danfoss Series 15 through 90 and Eaton heavy-duty Models 33 through 64. Small lawn and garden transaxles work on the same idea but are built and serviced differently. If your drive is acting up, go to hydrostatic transmission troubleshooting.
What are the main parts of a hydrostatic transmission?
Danfoss's Series 90 circuit diagram and Eaton's "typical hydrostatic system" drawing show the same layout. Here is what each part does.
| Part | What it does |
|---|---|
| Variable displacement pump | Axial piston pump with a swashplate that tilts both ways from center. Angle sets flow; which side of center sets direction. |
| Displacement control and servo | Turns a lever, hydraulic or electric input into servo pressure that tilts the swashplate, with feedback so the angle follows the input. |
| Hydraulic motor | Fixed or variable displacement piston motor that turns flow into shaft speed and pressure into torque. It runs in either direction. |
| Charge pump | Small pump (a gerotor on Eaton heavy-duty units) on the back of the main pump. Makes up leakage, keeps the low side pressurized, supplies the control, and provides cooling and filtering flow. |
| Charge relief valve | Sets charge pressure. Excess charge oil goes to the pump case. |
| Charge check valves | Let charge oil into whichever loop line is on the low pressure side. |
| High pressure relief valves, pressure limiter or power limiter | Protect the high side. On a Series 90 these live in the multi-function valves with a bypass function. |
| Loop flushing valve (shuttle and relief) | Usually on the motor (Series 90 motors have it built in; Series 42 offers it on the pump). Bleeds a steady stream of hot oil out of the low side of the loop into the case. |
| Case drain lines | Return internal leakage and flushing oil from the pump and motor cases to the reservoir. |
| Heat exchanger and bypass valve | Cool the case drain oil before it reaches the reservoir. The bypass valve lets oil go around the cooler when pressure across it climbs too high, as it does with cold oil. |
| Reservoir, filter and inlet line | Supply clean oil to the charge pump. Filtration is on the charge pump inlet (suction) or downstream of the charge pump. |
How does oil flow around a hydrostatic loop?
Picture two separate oil paths: the main loop between pump and motor, which never sees the tank, and the charge and drain path, which does. Follow the oil in forward:
- The charge pump draws oil from the reservoir through the inlet filter. A vacuum gauge on this line tells you whether the filter is plugging.
- Charge oil goes two places: to the displacement control as servo supply, and through a check valve into whichever main line is at low pressure. Charge oil the loop does not need goes over the charge relief valve into the pump case.
- With the swashplate tilted, the pump pushes oil out of port A, the high pressure side, to the motor.
- The motor turns, and its outlet oil, now at charge pressure, comes back to pump port B and straight into the pistons again.
- At the motor, the loop flushing shuttle shifts toward the low pressure line and lets a set flow of low-side oil out through its relief into the motor case. Fresh, cooler charge oil replaces it through the check valve.
- Leakage and flushing oil in the motor and pump cases run out the case drain lines, through the heat exchanger, and back to the reservoir.
- Move the lever through neutral to reverse and the swashplate tips the other way. Port B becomes the high side, port A the low side, and the check valve, relief and shuttle duties swap sides with it.
That is the whole trick. The main loop recirculates the same oil at high pressure, and a small, continuous charge flow keeps it topped up, cooled and filtered.
How does the swashplate control speed and direction?
Pump flow equals displacement times speed, so with the engine at a steady speed, the swashplate angle sets motor speed. Danfoss describes the Series 40 and 42 as giving an infinitely variable speed range between zero and maximum in both forward and reverse.
On a Series 90 with a manual displacement control (MDC), the handle moves a servo valve that ports charge oil to one end of a double-acting servo piston, and the piston tilts the cradle swashplate. A feedback link from the swashplate moves the valve back as the plate reaches the commanded angle, so the angle stays proportional to handle position. Danfoss calls it a high gain control: a small handle movement opens the valve fully. Electric (EDC) and hydraulic controls work the same way with a different input signal, and the feedback also corrects drift if the plate wanders.
The servo piston is spring centered. Danfoss notes that a Series 90 pump with MDC returns to neutral when the prime mover shuts down or if charge pressure is lost. On Eaton heavy-duty pumps the same job falls to springs on the servo pistons, and Eaton's repair manual has you zero the swashplate on assembly, to within .0005 inch side to side, so those springs bring it back to neutral when the lever is centered.
| Series 90 pump frame | Displacement | Rated speed | Theoretical flow at rated speed |
|---|---|---|---|
| 042 | 42 cc (2.56 in³)/rev | 4,200 rpm | 176 L/min (46 gpm) |
| 055 | 55 cc (3.35 in³)/rev | 3,900 rpm | 215 L/min (57 gpm) |
| 075 | 75 cc (4.59 in³)/rev | 3,600 rpm | 270 L/min (71 gpm) |
| 100 | 100 cc (6.10 in³)/rev | 3,300 rpm | 330 L/min (87 gpm) |
| 130 | 130 cc (7.93 in³)/rev | 3,100 rpm | 403 L/min (106 gpm) |
| 180 | 180 cc (10.98 in³)/rev | 2,600 rpm | 468 L/min (124 gpm) |
| 250 | 250 cc (15.25 in³)/rev | 2,300 rpm | 575 L/min (160 gpm) |
The frame size is the first field of the model code. Our Series 90 model code decoder breaks down the rest, including the charge pressure setting at the end of the code.
The motor can be variable too. On a Sundstrand Series 20 variable motor, the manual motor control returns to maximum displacement when released, and the optional pressure compensator control holds the motor at minimum displacement for speed until load rises, then shifts it to maximum displacement for torque.
What does the charge pump do in a hydrostatic drive?
Danfoss lists its jobs in the Series 90 technical information: make up internal leakage, keep positive pressure in the main loop, provide flow for cooling and filtration, replace leakage from external valves or auxiliary systems, and supply flow and pressure to the control. As a starting point, Danfoss sizes the charge pump at no less than 10% of the total displacement of all units in the system, and warns that low input speeds, long loop lines, high flushing flow and multiple low speed motors can make that too small.
Charge pressure is set by the charge relief valve. On a Series 90 it is set with the pump in neutral at 1,800 rpm, and it is measured above case pressure, so you subtract case pressure from your gauge reading. Danfoss also lists a minimum low loop pressure of 10 bar (145 psi) under all conditions to avoid cavitation.
When charge pressure drops, two things go wrong at once: the loop starves and cavitates, and the control loses the servo pressure it needs to hold the swashplate on stroke. That is why low charge pressure is the first thing to check on any hydrostatic complaint.
Why is charge pressure lower in forward and reverse than in neutral?
Because of the flushing valve. Eaton's heavy-duty troubleshooting guide gives typical charge pressure of 220 psi (15.2 bar) above case in neutral and 160 psi (11.0 bar) in forward or reverse. Sundstrand's Series 20 heavy-duty manual gives 190-210 psi (13-15 bar) in neutral and 160-180 psi (11-12.5 bar) minimum in forward or reverse.
In neutral there is no high side, the flushing shuttle stays centered, and the pump's charge relief sets the pressure. On stroke the shuttle opens the low side to the relief at the motor, which is set lower, so charge oil exits there and the reading drops to that setting. Danfoss's Series 90 motor flushing relief, for example, cracks at 16 bar (232 psi), below the Series 90 pump's 18 bar (261 psi) minimum charge setting. A drop from the neutral figure to the in-stroke figure is normal. A drop below the in-stroke figure is not.
Eaton stamps the setting on the end of each charge relief cartridge as a three-digit code: add a zero to get psi, so 022 is 220 psi. It also notes the factory sets these valves at 2 gpm of flow and that pressure rises about 6.5 psi for each extra gpm over the valve.
What is loop flushing, and why does a hydrostatic drive need it?
A closed loop keeps reusing the same oil, and every pass through the pump and motor heats it. Danfoss explains that a loop flushing valve removes hot fluid from the low pressure side of the loop for additional cooling and filtering. The oil it removes goes to the motor case, out the case drain, through the cooler and filter, and comes back as fresh charge oil.
Series 90 motors use an integral, non-adjustable loop flushing valve with an orificed relief, and the orifice size sets the flushing flow. Danfoss warns that too much flushing flow can leave the system unable to build pressure, and that the charge pump has to be big enough to cover flushing flow and still hold charge pressure. That is why swapping in the wrong flushing orifice, or a motor from a different machine, can make a good pump look weak.
What do the high pressure relief valves and pressure limiter do?
On a Series 90 each side of the loop has a multi-function valve in the pump end cap. The pressure limiter senses loop pressure and, at its setting, destrokes the pump to hold that pressure, which avoids the heat a relief valve would make. The high pressure relief valve is set 30 bar above the limiter (Danfoss 520L0603, 2016; the 2015 service manual 520L0818 says about 35 bar, 500 psi) and opens only for fast spikes; Danfoss gives its response as about 20 ms. Running over the relief for long periods generates heat in the loop and can damage the pump.
Series 40 pumps use check and relief valve cartridges instead: the checks let charge oil into the low side and the reliefs protect the high side, and Danfoss stresses that relief valves are for transient overpressure, not continuous pressure control. Eaton heavy-duty pumps use system relief valves or power limiter valves, stamped with the same add-a-zero code as the charge relief, so 400 means 4,000 psi. Sundstrand's Series 20 offers a pressure override on the control for the same reason the Series 90 has a limiter: hold a set pressure without blowing oil over the reliefs.
The bypass is not a tow valve. A Series 90 pump has a bypass function: turning the bypass hex on both multi-function valves three turns counterclockwise connects loop A to loop B so the machine can be moved without running the engine. Danfoss limits this to no more than 20% of maximum speed for no more than 3 minutes, says it is not intended as a tow valve, and warns that motor damage is possible. Re-seat the hex plugs to their normal operating position afterward.
What happens in neutral?
At zero swashplate angle the pump moves no oil, so the motor stops and the drive holds the machine through the closed loop. That is hydrostatic braking, and it is not a parking brake. Danfoss requires an independent braking system, separate from the hydrostatic drive, that can stop and hold the machine if hydrostatic power is lost and hold it in place with full power applied.
Neutral is a mechanical setting. Eaton's zero-angle check and the Series 90 note that overtightening the case drain fitting in port L2 (100 N·m, 74 lbf·ft maximum) can shift the swashplate's neutral position both show how small the margin is. Sundstrand's Series 20 manual adds that the control's internal centering springs are not strong enough to overcome friction in the external linkage, so a sticky cable or linkage keeps the pump slightly on stroke. That is the classic cause of creep; see why a hydrostatic creeps in neutral. Many machines also use a neutral start switch, which keeps the engine from cranking unless the control is in neutral.
When a machine runs downhill or a load overruns, the roles swap: the motor pumps and the pump turns the engine. Eaton's sizing guide calls this the reversal of pump and motor identity during dynamic braking, and Danfoss notes that maximum unit speed usually happens in downhill braking, so the engine has to be able to absorb that braking torque without overspeeding the pump.
Why does a hydrostatic pump and motor need a case drain?
Piston units leak on purpose. Oil escapes across the valve plate, piston bores and slippers to lubricate and cool them, and it collects in the case. The case drain line returns that leakage, and the loop flushing flow, to the reservoir. Danfoss says to use the higher of the case ports so the case stays full, keep the pump housing at least half full of oil, and route case drain oil through the heat exchanger because it is typically the hottest oil in the system. The hottest point, normally the motor case drain, is where the temperature limits apply: 104°C (220°F) rated and 115°C (240°F) maximum intermittent on a Series 90.
Case pressure has limits. The Series 90 is rated at 3.0 bar (44 psi) with a 5.0 bar (73 psi) maximum, and Danfoss points out that charge and system pressure add to case pressure, so a restricted drain can damage seals and housings. Eaton says heavy-duty case pressure should not exceed 40 psi for an extended period. Never tee a hydrostatic case drain into a return line with back pressure. Measuring drain flow is the best single wear test; see the case drain flow test.
Hydrostatic pump specifications compared
System pressure limits are differential pressures, high side minus low side. Charge and case figures are from the same documents.
| Unit | System pressure | Charge pressure | Case pressure | Cleanliness |
|---|---|---|---|---|
| Danfoss Series 40 (M25, M35, M44, M46) | 210 bar (3,000 psi) continuous, 345 bar (5,000 psi) max | M25-M44: standard 14.0 bar (200 psi), field adjustable 7.5-14 bar (110-200 psi) with shims. M46: standard 19.5 bar (285 psi), adjustable 19.5-26.2 bar (285-380 psi) | 1.7 bar (25 psi) continuous, 5.2 bar (75 psi) max | ISO 18/13 or better |
| Danfoss Series 42 (28, 32, 41, 51 cc) | 400 bar (5,800 psi) max working on 28/41 cc, 350 bar (5,075 psi) on 32/51 cc | Standard 14 bar (203 psi) on 28/32 cc, 20 bar (290 psi) on 41/51 cc | 3.4 bar (50 psi) rated, 10.5 bar (150 psi) cold start (service manual 520L0638, 2015). The older 2013 technical information 11022637 gives 3 bar (44 psi) and 5 bar (73 psi) | ISO 22/18/13 |
| Danfoss Series 90 (042-250) | 450 bar (6,525 psi) max working, 480 bar (6,960 psi) max | 18-34 bar (261-493 psi) per the 2016 technical information 520L0603. The 2015 service manual 520L0818 gives 10-35 bar (150-508 psi) | 3.0 bar (44 psi) rated, 5.0 bar (73 psi) max | ISO 22/18/13 |
| Eaton heavy-duty Models 33-64 | Motor torque rated at 414 bar (6,000 psi) | Typical 220 psi neutral, 160 psi forward or reverse | 40 psi max for extended periods | ISO 18/13 (SAE J1165) |
Your machine's settings come from its model code and the machine manual, not from the maximums above. Eaton notes higher charge settings may be set at the factory for a particular application, and Sundstrand says the same for the Series 20.
Hydrostatic pump vs pressure compensated pump: what is the difference?
Both are usually swashplate piston pumps, but they do opposite jobs. A hydrostatic pump sits in a closed loop with a motor, swings over center to reverse flow, and is controlled by displacement: the operator sets the angle and the motor speed follows. A pressure compensated pump sits in an open loop, draws from a tank, pumps one way only, and is controlled by pressure: it stays at full flow until outlet pressure reaches its setting, then destrokes. Our gear vs vane vs piston pump comparison covers open and closed loop pumps side by side.
Where are hydrostatic drives used?
- Propel drives: wheeled and tracked machines such as loaders and skid steers, where smooth, stepless speed in both directions matters. Danfoss notes that on propel drives the pressure and swashplate angle change constantly.
- Transit concrete mixer drums: Eaton wrote a dedicated troubleshooting guide for its heavy-duty units on ready-mix trucks.
- Winches, conveyors and elevating scrapers: Eaton's sizing guide covers winch drives and recommends a pressure override or power limiter on drives left unattended for long periods, such as conveyor drives and the elevator drives on self-loading scrapers.
- Vibratory, conveyor and fan drives: Danfoss calls these out as non-propel duty, where speed, pressure and swashplate angle stay nearly constant, so bearing life needs its own review.
Parts and rebuild information for the units we supply: Sundstrand Series 90 pump parts, Series 42 pump parts, Sundstrand seal kits, Eaton 54/64 hydrostatic parts and Eaton 33/39/46 hydrostatic parts. Step-by-step rebuild detail is in our Sundstrand Series 90 pump rebuild guide.
Frequently asked questions
What is the difference between a hydrostatic transmission and a regular hydraulic system?
A regular (open loop) hydraulic system pumps oil from a tank through valves to cylinders or motors and back to the tank. A hydrostatic transmission is a closed loop: the pump feeds a motor and the motor's return goes straight back to the pump, with a small charge pump making up leakage. Speed and direction come from tilting the pump's swashplate rather than from a valve.
What does the charge pump do in a hydrostatic transmission?
It keeps the loop full and pressurized. Danfoss lists its jobs as making up internal leakage, keeping positive pressure in the main loop, providing cooling and filtering flow, and supplying the control. If charge pressure drops, the loop cavitates and the control cannot hold the swashplate on stroke.
Why does my hydrostatic drive creep in neutral?
Usually the swashplate is not quite at zero. Sticky external linkage is a common cause, because the control's centering springs are not strong enough to overcome linkage friction. A leaking multi-function valve or a control out of adjustment can also do it. See our hydrostatic transmission troubleshooting guide.
Can you tow a machine with a hydrostatic drive?
Only a short distance, slowly, with the bypass open. On a Danfoss Series 90 the bypass hex on both multi-function valves is turned three turns counterclockwise, and Danfoss limits movement to 20% of maximum speed for no more than 3 minutes. It is not a tow valve, and longer moves can damage the motor.
What is a loop flushing valve?
A shuttle and relief valve, usually on the motor, that bleeds a steady flow of hot oil out of the low pressure side of the loop into the case so it goes through the cooler and filter. Charge oil replaces it. Too much flushing flow can keep the system from building pressure.
Does a hydrostatic pump need a case drain?
Yes, both the pump and the motor. The case drain returns internal leakage and flushing oil to the reservoir and keeps case pressure inside its limit, which is 3.0 bar (44 psi) rated on a Danfoss Series 90 and no more than 40 psi for extended periods on Eaton heavy-duty units. Run the drain to the cooler and tank on its own line.
What pressure does a hydrostatic transmission run at?
System pressure depends on the load, up to the relief or pressure limiter setting for the machine. Factory maximums include 345 bar (5,000 psi) on the Danfoss Series 40, 450 bar (6,525 psi) maximum working on the Series 90, and Eaton rates its heavy-duty motors at 6,000 psi. Charge pressure is much lower, typically 160-220 psi on Eaton heavy-duty units and 18-34 bar on a Series 90 per Danfoss 520L0603 (2016), where the 2015 service manual gives 10-35 bar.
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.
Hydrostatic pump or motor giving trouble?
Send us the model code off the pump and motor tags, plus your charge, case and system pressure readings if you have them. We supply parts for Sundstrand, Sauer-Danfoss and Eaton heavy-duty units and reply with price, availability and lead time. In-stock parts ship within 24-72 hours and in-stock parts carry a 2-year warranty. Rebuilds are done in our shop or through the RestoPower repair network; work done in our shop carries a 1-year repair warranty, and we arrange getting the unit to the shop doing the work.
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Related reading
- Hydrostatic transmission troubleshooting
- Sauer-Danfoss Series 90 model code decoder
- Sundstrand Series 90 pump rebuild guide
- Sundstrand / Sauer-Danfoss parts and catalogs
- Eaton heavy-duty hydrostatic pumps
- Sundstrand and Sauer-Danfoss pump identification
- Sundstrand document reference index
- Hydraulic pump case drain flow test
- Hydraulic fluid cleanliness: ISO codes and flushing
- How hydraulic motors work: types explained
- Gear vs vane vs piston hydraulic pumps