Torque Loss in Hydraulic Motors: Causes and Diagnostic Methods (Extensive 1500+ Word Technical Guide)

Hydraulic motors are designed to produce high torque at low speeds and deliver consistent rotational force under heavy loads. However, over time, many systems suffer from torque loss, typically caused by internal leakage, wear or insufficient pressure supply.

This guide explains:

how hydraulic motors generate torque
mechanical and hydraulic reasons for torque loss
the differences between orbit, piston and gear motor wear
field-proven diagnostic methods
engineering-level solutions to restore performance

1. How Torque Is Generated in a Hydraulic Motor

Torque generation depends on:

Pressure (p)
Displacement (V)
Motor efficiency (η)

Theoretical torque:

T = (p × V) / (2π)

Actual torque:

T_actual = T_theoretical × η_mechanical × η_hydraulic

When leakage increases or friction rises, efficiency drops → torque drops.

2. Top 8 Causes of Torque Loss in Hydraulic Motors

2.1 Internal Leakage (Primary Cause)

The number one cause of torque loss is internal leakage inside the motor.

Worn components:

rotor
valve plate
pistons
distributor plate
seals

allow oil to bypass from high-pressure to low-pressure zones.

Result:

low torque
reduced starting force
speed fluctuations

2.2 Wear on Motor Components

Orbit Motors:

rotor-to-spool clearance increases
valve plate erosion
housing ovalization

Axial Piston Motors:

piston wear
cylinder block scoring
valve plate scratches

Gear Motors:

tooth wear
side clearance increase
leakage around end plates

Wear reduces volumetric efficiency → lowers torque.

2.3 Insufficient System Pressure

Causes:

weak pump
incorrect relief valve settings
valve blockage
clogged filter

If pressure is low, torque is always low.

2.4 Reduced Flow Supply

Flow issues arise from:

worn pumps
load-sensing problems
clogged lines
incorrect flow control settings

Insufficient flow causes motors to stall under load.

2.5 Overheating

Heat reduces oil viscosity → low viscosity increases leakage → torque drops sharply.

2.6 Poor Quality or Degraded Oil

Bad oil accelerates metal wear and leakage.

2.7 Valve Block Internal Leakage

If the directional or proportional valve leaks:

full pressure does not reach the motor
torque drops immediately

2.8 High Back Pressure

Excess back pressure increases motor load and reduces torque output.

3. Torque Loss by Motor Type

3.1 Orbit Motors (OMP, OMH, OMS, OMR)

Typical failure modes:

valve plate leak
rotor scoring
low-speed instability
stalling under load

3.2 Axial Piston Motors

Failure modes:

piston scoring
cylinder block wear
valve plate leakage

Symptoms:

low starting torque
vibration
unstable rotation

3.3 Gear Motors

Failure modes:

tooth wear
bearing clearance
internal bypass

4. How to Diagnose Torque Loss

4.1 Pressure Measurement

Check inlet vs outlet pressure.

low ΔP → upstream issue (pump/valve)
normal ΔP, low torque → motor leakage

4.2 Flow Test

Measure motor inlet flow under load.

Low flow = low torque.

4.3 Internal Leakage (Case Drain Test)

Measure leakage from motor drain.

Normal:

small motors: 0.3–0.6 L/min
large motors: 1–3 L/min

High drain flow = confirmed internal leakage.

4.4 Thermal Imaging

Hot spots indicate leakage or friction.

4.5 Load Test

Apply controlled torque load.

stall under load → pressure or efficiency issue
high speed, low torque → leakage
vibration → mechanical wear

5. Engineering Solutions to Torque Loss

5.1 Motor Rebuild / Overhaul

Includes:

rotor surface grinding
valve plate replacement
piston set renewal
new seals and bearings

5.2 Correct Oil Viscosity

Match ISO VG rating to operating temperature.

5.3 Adjust Pressure Settings

Ensure relief valves and LS controls deliver full pressure to the motor.

5.4 Replace or Clean Filters

Clogged filters reduce flow → reduce torque.

5.5 Reduce Back Pressure

Enlarge return lines and check for blockages.

5.6 Improve Pump Efficiency

Weak pump = weak torque.

6. Conclusion

Torque loss in hydraulic motors primarily results from:

internal leakage
insufficient pressure
flow shortages
overheating
mechanical wear

Key principle:

80% of torque loss is caused by internal leakage.
Accurate diagnosis requires pressure, flow, leakage and temperature measurements.

This article is part of the Sancoqhub advanced hydraulic engineering series.