Char-Lynn Orbital Motor Troubleshooting: Not Turning, Slow, Weak or Leaking

A Char-Lynn (Eaton) orbital motor that will not turn, runs slow or feels weak is usually short on flow, short on pressure, or fighting back pressure, and a motor that leaks at the shaft is usually being blown out by case pressure. Put gauges on both ports and the case before you pull it; when the motor really is the problem, it is either internal wear across the Geroler and valve (a rebuild) or a shaft seal pushed out by case pressure (a seal kit plus a plumbing fix).

Plenty of motors that land on a rebuild bench had nothing wrong with them. The pump was tired, a relief was set low, or the return line was plumbed so the shaft seal never stood a chance. Twenty minutes with gauges can save you a teardown.

Two formulas that settle most arguments. Eaton gives them in its motor catalog: speed (RPM) = 231 x GPM / displacement in cubic inches per rev, and theoretical torque (lb-in) = PSI x cubic inches per rev / 6.28. If the motor turns slower than the formula says for the flow you measured, it is leaking internally. If it stalls below the torque the formula gives for your pressure drop, look at the pressure first, then the motor.

Char-Lynn motor troubleshooting chart

The last column tells you whether you are looking at a seal kit, a rebuild, or a problem somewhere else in the machine.

Symptom Likely causes The check The fix Seal kit or rebuild?
Motor will not turn No flow reaching it (valve, hose, coupling), relief set low or stuck open, parking brake not releasing, load above stall torque, broken drive or sheared splines Gauge both ports and try to run it. Full pressure and no movement points to the load, brake or drive. No pressure means the oil is going elsewhere. Fix the supply or relief. On T Series brake motors, Eaton calls for at least 17 bar [250 PSI] release pressure to fully release the brake. Rebuild if the drive or splines are broken. Otherwise it is the system.
Turns slowly Low pump flow, flow control or relief bleeding off, internal leakage across a worn Geroler or valve, oil too hot and thin Measure flow at the motor and compare actual RPM with the formula. On a twin-motor drive, swap sides and see if the problem follows the motor. Restore flow, or rebuild the motor if the flow is there and the speed is not. Rebuild when internal leakage is confirmed.
Loses torque under load Relief set below spec, high back pressure eating the pressure drop, internal leakage that grows with pressure Gauge inlet and return at stall. Torque comes from the difference between them, so 500 PSI of back pressure is 500 PSI of torque you do not get. Set the relief to the machine maker's figure, open up the return, then reassess the motor. Rebuild if inlet and return readings are right and it still stalls early.
Runs one direction only Directional valve spool or solenoid, a cross-port or integral relief stuck open on one side, counterbalance or check valve, one restricted hose Eaton builds these motors to reverse with equal torque either way, so a one-way motor is almost always the circuit. Gauge each port both ways. Repair the valve or relief cartridge on the weak side. Rarely the motor.
Jerky or erratic at low speed Oil below minimum viscosity, air in the oil, running a gerotor (H Series) where a Geroler belongs, low-speed valving run at too little pressure, worn Geroler Check oil temperature and grade against the viscosity rules below. Look for foam in the reservoir. Correct the oil, bleed the air, or move to a Geroler motor or the low-speed valving option for work under 200 RPM. Rebuild only if the Geroler is worn.
Shaft seal leaking or blowing out Case pressure above the seal rating, no case drain where one is needed, drain line tied into a pressurized return, worn shaft at the seal track, side load or bearing wear, debris, wrong seal material for the heat Tee a gauge into the case port, or the return line on motors without one, and read it under load. Compare with the limits in the next section. Fix the pressure first: case drain line, lower back pressure, or the high pressure shaft seal option. Seal kit, plus the plumbing fix. A grooved shaft or worn bearing makes it a rebuild.
External leakage at the body bolts Cap screws that have lost torque, body seals hardened by heat, scratched or burred mating faces Clean it, run it, and find where the oil starts. A wet seam between sections is a body seal. Reseal with a complete kit and torque the cap screws to the repair manual figure in sequence. Seal kit, unless the faces are damaged.
Overheating Oil above Eaton's recommended operating range, running over the relief, worn motor turning pressure into heat, no flushing on a hot closed loop Temperature gun on the motor, the case line and the reservoir. A case line much hotter than the tank means internal leakage. Fix the relief or cooling. A case port or shuttle valve option adds flushing on hot circuits. Rebuild if the motor itself is the heat source.
Noisy Cavitation or air, an overrunning load driving the motor like a pump, worn drive splines or thrust bearing, a shuttle-valve motor run with no case line Listen with the load on and off. Check inlet conditions and for play at the shaft. Cure the air and inlet problem, or rebuild the drive train. Rebuild if the drive or bearing is worn.

Why does a Char-Lynn motor blow its shaft seal? Case drain and back pressure limits

Eaton builds its spool valve motors on what it calls a three-zone pressure design: inlet, return and case. The shaft seal lives in the case zone. Whatever pressure ends up in the case is pushing on that seal, and every series has a limit.

Where the case pressure comes from. On H, S and T Series motors run without a drain line, Eaton estimates case pressure at roughly 0.6 times the pressure drop across the motor, plus the back pressure. Run one at 2,000 PSI in and 500 PSI out and the case sits around 1,400 PSI. On the J Series, internal check valves hold the case at return line pressure, so back pressure in the return is what the seal sees.

Standard shaft seal limits by series

These are the case pressures Eaton lists for the standard shaft seal in its 2018 low speed high torque motor catalog.

Series Standard seal, case pressure up to
H, S, T 100 bar [1500 PSI]
2000 140 bar [2000 PSI]
4000 100 bar [1500 PSI]
6000 70 bar [1000 PSI]
10,000 20 bar [300 PSI]
VIS 40, VIS 45 20 bar [300 PSI]
J Series 76 bar [1100 PSI] return pressure without a case drain; 220 bar [3190 PSI] peak in the return with a case drain

Those are not numbers you can run at any speed. Eaton's charts for the H, S, T and 2000 Series plot allowable case pressure against shaft speed, and it is highest at low speed. On the H, S and T chart the full 103 bar [1500 PSI] holds only at the low end, and the line falls as RPM climbs. Above the line, Eaton marks it "case drain required."

When is a case drain line required?

  • When your speed and case pressure land above Eaton's chart line for the series. Run the 0.6 rule above and check.
  • Motors piped in series. The downstream motor's inlet is the upstream motor's back pressure. Eaton recommends case porting for continuous running and high back pressure work such as series circuits.
  • Shuttle valve motors, always. Eaton says a shuttle valve motor must have a case port line to tank, or the internal drive splines do not get enough lubrication.
  • Continuous duty at high pressure. Eaton prefers the standard seal with a case port for maximum life on continuous work.

Plumb the drain to tank on its own line, never into a pressurized return, and route it so the motor stays full of oil. On H, S and T motors Eaton goes a step further and calls for a restriction in the drain that holds about 3.5 bar [50 PSI] in the case, which keeps the motor full.

Low pressure vs high pressure shaft seals

Eaton offers a high pressure shaft seal option for the H, S, T and 2000 Series. Case pressure clamps its lip harder onto the shaft instead of lifting it, and Eaton rates it for case pressure up to 200 bar [2900 PSI] at 150 RPM. It is aimed at intermittent extremes (Eaton defines intermittent as 10% of every minute) and at machines with no case line available, such as skid steer attachments. Eaton lists seal kits that convert a standard-seal motor to the high pressure shaft seal:

A high pressure seal does not fix a plumbing problem. If the case is sitting above the limit all day, Eaton's own advice is the standard seal plus a case drain. The high pressure seal buys margin for spikes, not a license to run a blocked return.

Gerotor vs Geroler: what is the difference?

Both are the orbit gear set at the back of the motor: a star that orbits inside a lobed ring, typically 6 to 8 orbits for every turn of the shaft according to Eaton. Eaton's 2018 catalog says the H Series uses a gerotor, and the rest of the Char-Lynn motors in that catalog use a Geroler.

A Geroler has rollers set into the lobes of the outer ring. Those rollers act like a roller bearing between star and ring, so there is less friction, better mechanical efficiency and less wear when the oil is thin. Eaton also says the Geroler usually runs smoother at low speed.

Gerotor (H Series) Geroler (J-2, S, T, W, disc valve, VIS)
Minimum oil viscosity 100 SUS [20 cSt], or 20 x PSI / RPM in SUS, whichever is higher 70 SUS [13 cSt]
Best viscosity range 100-200 SUS [20-43 cSt] 100-200 SUS [20-43 cSt]
Cleanliness (ISO 4406) 20/18/13 20/18/13
Low speed work Eaton says to consider a Geroler motor for applications under 100 RPM

That gerotor formula is the one people miss. An H Series motor pulling 1,500 PSI at 100 RPM needs oil of at least 20 x 1500 / 100 = 300 SUS at operating temperature, three times the usual minimum. Thin oil and a slow, loaded H motor is a recipe for chatter and wear. Eaton's recommended system operating temperature across the motor line is -34 to 82°C [-30 to 180°F].

Two options change how a motor feels, and they fool people. The free running option opens the star-to-ring clearance for rapid reversals and high-flow work; Eaton notes it reduces volumetric efficiency, so a slightly slow motor may be built that way on purpose. The low speed valving option tightens the valve for smooth running under 200 RPM, but Eaton says those motors need at least 300 PSI difference between the A port and the case.

Is it the motor or the system?

Run these in order before anything comes off the machine.

  1. Gauge the inlet, the return and the case under load. Most of the answers are in those three numbers.
  2. Measure flow at the motor and compare actual RPM with the speed formula.
  3. Check the oil: grade, temperature at the motor, and whether it is clean. Eaton recommends ISO 4406 20/18/13 for these motors and says more than 70% of hydraulic failures come from contamination.
  4. Swap test. On a two-motor drive, swap the motors. If the fault moves with the motor, it is the motor. If it stays put, it is the circuit.
  5. Check the shaft for side play and the coupling or wheel hub for misalignment or side load.
  6. Watch the case drain on motors that have one. Compare its flow and temperature with a known-good motor on the same machine. Much more oil, or much hotter oil, means internal wear.

Just resealed it and now it runs backwards? Eaton's repair manuals say timing determines the direction of rotation. On the H Series, the 2015 Eaton manual has you mark one drive tooth, line it up with the timing dot on the shaft, then align a star point with that tooth for standard rotation or a star valley for reverse rotation. Get it one position off and the motor turns the wrong way. Pull the end cap and retime it; nothing is broken.

Seal kit or rebuild?

A seal kit is the right job when the motor makes its speed and torque, the only fault is oil where it should not be, and you have fixed whatever pushed the seal out. A rebuild is the right job when the motor is slow, weak or noisy with good flow and pressure, the shaft is grooved at the seal track, or the drive or bearings are worn. While it is apart, do the whole kit; the body seals are the same age as the shaft seal.

OEM specials are out there. Eaton builds proprietary motors for equipment makers such as John Deere, Case and CAT, and some of those use a different seal set from the standard catalog motor. Our kits fit the standard Eaton unit for the model code. If your motor came off OEM equipment, compare the old seals with the kit photo before you install, and the equipment maker is the source for its own special kit.

Finding the right kit and parts

Start with the full product number from the tag. The Char-Lynn part number decoder and seal kit finder turns it into a series and kit, how to identify the right Char-Lynn seal kit walks through tags and design codes, and the Char-Lynn orbital motor cross reference guide covers replacements. Every kit we list is in Char-Lynn motor seal kits, and the factory service data is collected in the orbital motor parts catalog and rebuild guide.

Parts catalogs by series:

Frequently asked questions

Why is my Char-Lynn motor not turning?

Usually no flow is reaching it, the relief is low or stuck open, a parking brake is not releasing, or the load is above stall torque. Gauge both ports: full pressure with no movement points to the load, brake or a broken drive; no pressure means the oil is going elsewhere.

Why is my hydraulic motor losing power under load?

Torque comes from the pressure difference between inlet and return, so a low relief setting or high back pressure cuts it directly. If inlet and return pressures are right and the motor still stalls early or slows down, internal leakage across a worn Geroler or valve is the likely cause, and that is a rebuild.

Why does my Char-Lynn motor shaft seal keep leaking?

Case pressure above the seal rating is the usual cause. Eaton rates the standard seal at 100 bar [1500 PSI] case pressure on H, S and T motors, 140 bar [2000 PSI] on the 2000 Series and 20 bar [300 PSI] on the 10,000 Series and VIS 40/45, and the allowable pressure falls as speed rises. High back pressure, a missing case drain or a drain tied into a pressurized return will push out any new seal.

Does an Eaton Char-Lynn motor need a case drain?

It needs one when the case pressure and speed fall above Eaton's case pressure seal limitation chart for that series, on motors piped in series, on any motor with the shuttle valve option, and for maximum seal life on continuous high pressure duty. The drain should run to tank on its own line and keep the motor full of oil.

What is the difference between a gerotor and a Geroler?

A Geroler has rollers in the outer ring lobes that act like a roller bearing: less friction, less wear with thin oil, smoother low speed running. Eaton uses a gerotor in the H Series and a Geroler in the rest of its 2018 motor catalog. A gerotor needs at least 100 SUS oil or 20 x PSI / RPM, whichever is higher; a Geroler needs 70 SUS.

Why does my Char-Lynn motor run backwards after a reseal?

The Geroler or gerotor was timed one position off. Eaton's repair manuals say timing sets the direction of rotation. On the H Series you align a star point with the marked drive tooth for standard rotation and a star valley for reverse rotation. Retime it and it will run the right way.

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.

Send us the tag number and what it is doing

Give us the full product number off the tag, the symptom, and your inlet, return and case pressure readings if you have them. We will tell you whether it is a seal kit, a rebuild or a plumbing fix. In-stock parts ship within 24-72 hours and carry a 2-year warranty. Motors sent in for repair get a free teardown evaluation and a firm itemized quote before any billable work, and our repair work carries a 1-year warranty.

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