1. Introduction
Danfoss hydraulic motors convert hydraulic energy (pressure and oil flow) into mechanical energy (torque and speed).
Danfoss hydraulic motors are of the fixed displacement high-torque design. For a given oil flow and a given pressure, the displacement (size of motor) determines the speed and the torque. For a given displacement (size of motor) the oil flow rate determines the speed, and the pressure differential determines the torque.
September 2004
Motor basic training

2. How do we convert the energy?
Let’s take a look at the gearwheel set:
The toothed rim in the gearwheel set is stationary and forms part of the motor housing. In the illustrated example, the toothed rim has seven internal gear teeth and meshes with an internal gearwheel with six teeth. The gearwheel rotates in the inside of the toothed rim and circulates around the centre of the gear rim.
The example shows the transfer of energy by means of cardan shafts and illustrates the corresponding gearwheel movements plus the locations of pressure and return chambers (red and blue, respectively). As it appears the circular movement of the gearwheel centre causes the shaft move in the opposite direction at a six times lower angular speed. Thus 1/7 rotation of the shaft corresponds to 6/7 rotations of the gearwheel centre, which displaces the oil from six fluid-chambers. Consequently, one rotation of the shaft results in six rotations of the gearwheel centre and displacement of 42 chambers.
Such utilisation of the gearwheel set results in a hydraulic motor with an approx. six times larger power per revolution than a conventionally designed hydraulic motor. Or seen from another point of view, it results in an approx. six times higher turning torque at a six times lower speed without the use of a reduction gear.
September 2004
Motor basic training

3. Spool valve motors OMM OML DH OMP Spool valve integrated
with output shaft
Cardan shaft
Gearwheel set without
rollers
OML 8 – 32 cm3 2,0 KW bar
OMM 8 – 50 cm3 3,5 KW bar
OMP 25 – 400 cm ,0 KW bar
DH 36 – 400 cm ,0 KW bar DH
OMP
September 2004
Motor basic training

4. Spool valve motors OMR DS OMH Cardan shaft Gearwheel set
with rollers
Spool valve integrated
with output shaft
OMR 50 – KW 200 bar
DS 50 – KW 175 bar
OMH 200 – ,5 KW 200 bar
OMH
September 2004
Motor basic training

5. How do we fill and empty the chambers of the gearwheel set?
Let’s take a look at a motor with spool valve:
OML, OMM, OMP, DH, OMR, DS and OMH motors are equipped with a spool valve: The distributor valve has been integrated with the output shaft. The cardan shaft must therefore rotate the distributor valve as well as transfer mechanical energy from the gearwheel set to the output shaft. This results in a compact motor with few rotating parts.
September 2004
Motor basic training

6. Disc valve motors New OMS TMK OMT TMT OMV Gearwheel set with rollers
Output shaft
Valve drive
Distributor
valve
Cardan shaft
OMS 80 – 500 cm3 21 KW 275 bar
TMKW 160 – 400 cm3 37 KW 325 bar
OMT 160 – 500 cm3 40 KW 240 bar
TMT 200 – 630 cm3 41 KW 350 bar
OMV 315 – 800 cm3 64 KW 240 bar
TMT
OMV
September 2004
Motor basic training

7. How do we fill and empty the chambers of the gearwheel set?
Let’s take a look at a motor with disc valve:
OMS, OMT and OMV motors are equipped with a disc valve:
The distributor valve has been separated from the output shaft and is driven by a short cardan shaft (valve drive). A balance plate counterbalances the hydraulic forces around the distributor valve. This means that hydraulic and mechanical losses are reduced to a minimum. The separate valve drive and the hydraulic counterbalancing result in an extremely precise valve function and high volumetric efficiency at all pressures.
September 2004
Motor basic training

8. DESIGN CONSIDERATIONS
GEAR WHEEL SET
GEAR WHEEL SET WITHOUT ROLLERS
The rollerless gear rim makes these types suitable for long operating periods at moderate pressures, or short operating periods at high pressures.
September 2004
Motor basic training

9. DESIGN CONSIDERATIONS
GEAR WHEEL SET
GEAR WHEEL SET WITH ROLLERS
The rollers in the gear rim reduce local stress, spread the tooth load over their projected area and reduce the tangential reaction forces on the inner gear reducing friction to a minimum. This gives long operating life and better efficiency even at continuous high pressures. Gearwheel sets with rollers are recommended for operation with thin oil and for applications having continually reversing loads.
September 2004
Motor basic training

10. GEARWHEEL SET Fixed gearwheel set versus Gearwheel set with rollers
APPLICATION CONSIDERATIONS:
Fixed gearwheel set
Gearwheel set with rollers
Continuous operation
Many start/stop situations
Mainly one direction operation
Reversing operation
Moderate pressure
High pressure
Moderate efficiency
High efficiency
Moderate life time
Long life time
September 2004
Motor basic training

11. DESIGN CONSIDERATIONS
BEARINGS
SLIDE BEARINGS
NEEDLE BEARINGS
ROLLER BEARINGS
September 2004
Motor basic training

12. SHAFT BEARINGS Slide bearings versus Roller bearings
APPLICATION CONSIDERATIONS:
Slide bearings
Roller bearings
Continuous operation
Many start/stop situations
Mainly one direction operation
Reversing operation
Moderate radial and axial load
High radial and axial load
September 2004
Motor basic training

13. DESIGN CONSIDERATIONS
VALVING SYSTEM
SPOOL VALVE
The distributor valve has been integrated with the output shaft. The cardan shaft must therefore rotate the distributor valve as well as transfer mechanical energy from the gear-wheel set to the output shaft.
September 2004
Motor basic training

14. DESIGN CONSIDERATIONS
VALVING SYSTEM
DISC VALVE
The distributor valve has been separated from the output shaft and it is driven by a short cardan shaft [valve drive]. A balance plate counterbalances the hydraulic forces around the distributor valve.
September 2004
Motor basic training

15. VALVING SYSTEM Spool valve versus Disc valve
APPLICATION CONSIDERATIONS:
Spool valve
Disc valve
Moderate pressure
High pressure
Moderate efficiency
High efficiency
Moderate internal leakage
Low internal leakage
Steady speed at low flow
September 2004
Motor basic training

16. DESIGN CONSIDERATIONS
MOTORS WITH CHECK VALVES
MOTORS WITH DRAIN LINE
MOTORS WITHOUT CHECK VALVES
AND DRAIN LINE
September 2004
Motor basic training

17. SHAFT SEAL Seal life depends on PV-factor:
P is hydraulic pressure on seal.
V is shaft rotational speed.
The higher the PV-factor - the shorter the expected seal life time.
What is the pressure Ps on the seal?
MOTORS WITH CHECK VALVES
MOTORS WITH DRAIN LINE
MOTORS WITHOUT CHECK VALVES AND DRAIN LINE
Ps=Motor outlet pressure
Ps=Pressure in drain line
Ps=Average of motor inlet and outlet pressure
September 2004
Motor basic training

18. STATIC LEAKAGE DEFINITION:
The output shaft is rotated at 0.1 rpm. Pressure is applied to port A. The leakage from port B is the static leakage.
APPLICATION CONSIDERATION:
Necessary min. flow to start up motor must be available.
September 2004
Motor basic training

19. DRAIN LEAKAGE DEFINITION:
The oil flow coming out of the drain line at different inlet pressures.
APPLICATION CONSIDERATION:
In closed-loop circuits the charge pump must have capacity to refill the ”lost” flow from drain leakage.
September 2004
Motor basic training

20. DEFINITION: The motor output torque in a static situation (0 rpm.)
STARTING TORQUE
DEFINITION:
The motor output torque in a static situation (0 rpm.)
APPLICATION CONSIDERATION:
Necessary min. pressure to start up motor must be available.
September 2004
Motor basic training

21. FREE WHEELING DEFINITION:
Both A- and B-ports are open and the motor is driven (working as a pump).
APPLICATION CONSIDERATION:
To prevent cavitation inside the motor the motor inlet pressure must be min. 50 % of the pressure loss through the motor.
September 2004
Motor basic training

22. MOTOR LIFE TIME Factors that influence motor life time:
Starting-up conditions
Duty cycle
Extreme situations
Oil viscosity/temperature
Oil cleanliness
Radial/axial load
Typical wear parts:
Shaft seal
Gearwheel set
Splines connections
September 2004
Motor basic training

23. MOTOR LIFE TIME Life time test at S-D Nordborg Continuous operation
At max. continuous pressure and speed.
Fly-wheel test
At max. intermittent pressure.
September 2004
Motor basic training