Sauer-Danfoss Series 51 and 51-1 Bent Axis Motor Rebuild Guide: Shaft Seal, Seal Kits, Torques and Start-Up

To rebuild a Sauer-Danfoss Series 51 or 51-1 bent axis variable motor, replace the shaft seal at the flange or cartridge carrier, service the loop flushing shuttle, charge relief, displacement limiters and control from the outside, then remove the end cap for the rotating group, fit the overhaul seal kit for your frame size, and torque everything to the Danfoss values. Set the motor charge relief 2 to 4 bar below the pump's, and confirm minimum displacement on a test stand before the motor goes back in service.

Danfoss splits Series 51 service into modules. The basic motor service manual covers minor repairs that do not require removing the end cap and do not affect warranty. Major repairs require end cap removal, and Danfoss says only its Authorized Service Centers or OEMs it has trained should do them; that procedure is not in the manual.

Series 51 or 51-1: which motor do you have?

Both are bent axis variable motors. The ball end of each piston runs in a bushing pressed into the drive shaft, a synchronizing shaft keeps the cylinder block turning with the output shaft, and a servo piston swings the rotating group to change displacement. The 51-1 has a simplified end cap and control, a 2 to 1 differential servo piston, and is two-position only; the Series 51 also offers proportional controls. Rotating hardware and housings are common between the two.

The letter after 51 in the model code is the mounting: V is an SAE C, D or E flange, C is cartridge, and D is a DIN ISO 3019/2 flange. Danfoss's example nameplate reads 51V060-R-S1-N-HZ-B1-A-N-C2-NNN-026-AA-E4-03-00 with serial N-99-12-12345, where N is the Neumunster, Germany plant, 99 the year and 12 the week. Many service parts change by that date code, so record the serial before you order anything.

Frame	Max displacement	Min displacement	Rated speed at max / min displacement	Weight with HZ control
060	60 cm³ (3.66 in³)	12 cm³ (0.73 in³)	3600 / 5600 rpm	28 kg (62 lb)
080	80.7 cm³ (4.92 in³)	16.1 cm³ (0.98 in³)	3100 / 5000 rpm	32 kg (71 lb)
110	109.9 cm³ (6.71 in³)	22 cm³ (1.34 in³)	2800 / 4500 rpm	44 kg (97 lb)
160	160.9 cm³ (9.82 in³)	32.2 cm³ (1.96 in³)	2500 / 4000 rpm	56 kg (123 lb)
250	250 cm³ (15.26 in³)	50 cm³ (3.05 in³)	2200 / 3400 rpm	86 kg (190 lb)

Source: 11008567, technical data, pages 21-22. Maximum delta pressure is 480 bar (7000 psi), minimum low side pressure 10 bar (145 psi), and rated case pressure 3 bar (44 psi) with 5 bar (73 psi) allowed at cold start.

Does the Series 51 motor actually need a rebuild?

Most "bad motor" complaints on a Series 51 trace back to the control, its orifices or charge pressure. Danfoss's troubleshooting tables point the same way:

Symptom	Check first (per Danfoss)
Sluggish shifting	Blocked or misplaced control orifices, a high threshold setting, a sticky 4-way spool or control piston, low PC setting, a bad input signal, or low loop flushing relief setting.
Works normally in one direction only	Charge pressure low in one direction means the loop flushing shuttle spool may be sticking. Also check that the control and brake pressure defeat spool are not holding the motor at minimum displacement one way.
Low output torque	System pressure at the motor, loop flushing relief setting, PC setting, orifices, a sticking PC or 4-way spool, a two-position solenoid that will not shift to maximum, and threshold setting.
Running hot	Oil level, heat exchanger, a loop flushing relief set too high or the wrong orifice cartridge, low system relief settings, and air in the oil.
Internal leakage (any of the above)	Install the loop flushing defeat option and monitor case flow. If case flow is excessive, the motor may need major repair.

That last line is the real test of the rotating group. See our case drain flow test guide for the setup, and the hydrostatic transmission troubleshooting guide for the pump side of the loop.

How do you replace the shaft seal on a Series 51 motor?

SAE and DIN flange motors: clean the outside, remove the eight flange screws (8 mm hex on 60 and 80 cc, 10 mm on 110 and 160 cc, 12 mm on 250 cc), pull the flange with a puller, and press the seal out from the flange front. Inspect the seal cavity and the shaft sealing area. Press the new seal in from the rear, cupped side toward the shaft bearings, until it bottoms. Fit a new flange O-ring with petroleum jelly, install the flange over a seal protection sleeve, draw the screws down slowly in a crisscross pattern, then torque them in a crisscross pattern:

Frame	Flange screw torque	Wrench
60 and 80 cc	37 N·m (27.3 lbf·ft)	8 mm
110 cc	63 N·m (46.5 lbf·ft)	10 mm
160 cc	110 N·m (81.1 lbf·ft)	10 mm
250 cc	174 N·m (128.4 lbf·ft)	12 mm

Cartridge motors: remove the seal carrier snap ring, pull the carrier without marking the housing bore or shaft, remove the bore O-ring, and press the seal out of the carrier. Press the new seal in cupped side inward until it bottoms, fit a new O-ring in the housing bore, push the carrier in over a protection sleeve until it clears the snap ring groove, and refit the snap ring.

How do you service the loop flushing valve, charge relief and limiters?

• Loop flushing shuttle: remove both hex plugs with an 11/16 inch wrench, then the springs, seats and spool. Inspect and clean the spool and its bore, lubricate with clean oil, reinstall, and torque the plugs to 41 N·m (30 lbf·ft). Since SB-2000-044 (N-00-50) the spring and seat clip together; old and new interchange only as an assembly.
• Charge relief, poppet style (older): loosen the 11/16 inch lock nut, unscrew the adjustment screw, and remove the O-ring, spring and poppet. Inspect the parts and cavity, reassemble with a new O-ring, hand tighten the lock nut, and set the pressure (below).
• Charge relief, orificed cartridge: remove the cartridge with a 1 inch hex and strip it with 1 inch and 11/16 inch wrenches. Torque the internal plug to 37 N·m (27 lbf·ft) and the cartridge to 52 N·m (38 lbf·ft). Orificed valves and defeated loop flushing need no adjustment.
• Defeat plug: 1 inch wrench, new O-ring, 40 N·m (30 lbf·ft).
• Maximum angle stop: on Series 51, the four servo cover screws come out with an 8 mm hex (10 mm on 250 cc); on 51-1 the control comes off instead. Torque the stop screw to 23 N·m (17 lbf·ft) on 60 cc or 46 N·m (34 lbf·ft) on the larger frames, and the servo cover screws to 78 N·m (58 lbf·ft), or 110 N·m (81 lbf·ft) on 250 cc. Spacers set maximum displacement in 5% steps; a longer spacer means less displacement.
• Minimum angle screw: removing the tamper-resistant cap destroys it, so have a new one. Measure the screw length before removal. Torque the seal lock nut to 51 N·m (38 lbf·ft) on 60 and 80 cc (17 mm) or 86 N·m (63 lbf·ft) on 110 to 250 cc (19 mm), and verify minimum displacement on a test stand or in the machine.

Minimum displacement changes per turn of the adjusting screw are 1.5 cm³ (60 cc), 2.1 cm³ (80 cc), 3.1 cm³ (110 cc), 4.0 cm³ (160 cc) and 6.2 cm³ (250 cc). Danfoss warns that changing either limiter can create unsafe speed conditions, so check with the machine builder before moving it off the factory setting.

4-way valve and threshold springs (proportional controls)

Remove the control screws (10 mm hex on 60 to 110 cc, 12 mm on 160 and 250 cc), tag every orifice (T1, T2, T3 in the control, T7 and T8 in the sleeve) to its location, and pull the sleeve with a 4 mm screw threaded into it. On reassembly the orifices go back at 4 N·m (2.95 lbf·ft), the G12 plugs at 37 N·m (27 lbf·ft), the G30 plugs at 6 N·m (4 lbf·ft), the threshold screw mounting sleeve at 32 N·m (23 lbf·ft), and the control screws at 78 N·m (57.6 lbf·ft) on 60 to 110 cc or 110 N·m (81 lbf·ft) on 160 and 250 cc. The 160 and 250 cc units carry two ramp springs with an intermediate guide.

What service bulletins matter inside a Series 51 motor?

Bulletin	What changed	Rebuild rule
SB-1996-005	Balance lands moved between end cap and valve segment (1994-1996)	Never use a new end cap with an old valve segment. A new segment works with an old end cap; the new segment has a drill point mark next to the outer drain hole.
SB-1997-030	Stepped spindle valve segment (from N-96-08)	Replaces the straight and retained spindle segments, which are obsolete.
SB-1997-009	Servo piston seal ring (from N-96-08)	New ring numbers 508131 (60), 508132 (80), 508133 (110), 508134 (160); interchangeable.
SB-2001-015	Symmetrical synchronizing shaft rollers (after N-01-39)	Old shaft and rollers replaced by a kit: 517308 (60), 517309 (80), 516485 (110), 517310 (160), 517311 (250). Interchangeable as an assembly only.
SB-2002-001	End cap to control interface went from O-rings to a gasket (2003)	The new gasket does not work on old end caps and the O-rings do not work on new end caps.
SB-2004-004	End cap gasket became rubber coated metal (2004-2005)	New gaskets 515733 (60), 517237 (80), 516124 (110), 519806 (160), 515735 (250); interchangeable.

Danfoss also notes that if you find contamination in the motor, it should be fully disassembled, cleaned and inspected. That is the honest answer for most Series 51 motors that come in after a pump failure. A worn shaft bushing, a scored valve segment or a block that does not seat flat is a parts decision, made by eye and experience, because there are no published limits to measure against.

Which overhaul seal kit does a Series 51 motor take?

Motor	Overhaul seal kit (Q210)	From date code	Parts manual
Series 51, 60 cc	11122311	92-07	520L0767, page 88
Series 51, 80 cc	11122322	92-07	520L0765, page 98
Series 51, 110 cc	11122323	92-33	520L0766, page 100
Series 51, 160 cc	11122324	92-33	520L0768, page 138
Series 51, 250 cc	11122325	92-33	520L0769, page 120
Series 51-1, 60 cc	11122326	98-06	520L0770, page 88
Series 51-1, 80 cc	11122327	98-06	520L0770, page 88
Series 51-1, 110 cc	11122328	98-06	520L0770, page 89

Inside each kit, several items carry their own date splits and bulletin references (the lip seal and the end cap interface gasket in particular), so the kit has to match the motor's build date and mounting style, not just its frame size. We do not list these kits as stocked parts; send us the model code and serial number and we reply with price, availability and lead time. Kits we do carry for other Sundstrand units are in the Sundstrand seal kit collection, and the Sundstrand seal kit guide explains how we match them.

How do you start up a Series 51 motor and set charge pressure?

1. Inspect the units, make sure every system component is clean, and fill the reservoir through a 10 micron (nominal, no bypass) filter. Fill the inlet line and check it for loose fittings, restrictions and air leaks.
2. Fill the motor and pump housings through the upper case drain port. Never start the prime mover with either unit not completely full.
3. Put a 0-60 bar (0-1000 psi) gauge in the pump charge port and disconnect the pump control input so the pump stays in neutral.
4. Jog the prime mover until charge pressure starts to rise, then run at the lowest possible speed until it is established. Bleed air from the high pressure lines at the system gauge port if needed.
5. Raise to operating speed and confirm charge pressure matches the pump model code. Shut down, reconnect the control, restart in neutral, then cycle slowly forward and reverse for at least five minutes.
6. Shut down, remove and plug the gauges, and top up the reservoir with filtered fluid.

To check the motor charge relief, put a 0-60 bar gauge in M1 or M2, whichever is the low side of the loop, and another in L1 or L2 for case pressure, then run at normal speed with the pump at half stroke. In most applications the motor setting is 2 to 4 bar (29 to 58 psi) below the pump charge relief setting, referenced to case pressure at 50°C (122°F). On adjustable valves, loosen the 11/16 inch lock nut and turn the plug: clockwise raises it about 3.4 bar (50 psi) per turn. Torque the lock nut to 52 N·m (38 lbf·ft).

Danfoss recommends the first fluid change at 500 hours, then at most 2000 hours with a sealed reservoir or 500 hours with a breather, and cleanliness of ISO 4406 18/13. Our start-up procedure guide and oil viscosity guide cover the general side.

Should you rebuild a Series 51 motor yourself?

Shaft seals, the loop flushing shuttle, the charge relief and the limiters are bench jobs if you have the torque values above and a way to check displacement afterward. The rotating group is different: valve segment and end cap generations that cannot be mixed, synchronizing shaft kits, and a minimum angle that has to be proven on a stand. If case flow is high or the pump that fed it came apart, have it torn down and quoted first. Our hydraulic motor repair page explains how we handle motor jobs, and how hydraulic motors work covers bent axis versus swashplate designs.

Frequently asked questions

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