In many hydraulic systems, one of the most frustrating failures is when a hydraulic motor stops turning. It’s easy to assume that the motor itself is broken—but in most cases, the problem lies elsewhere in the hydraulic system. This article will help you fully understand why an old hydraulic motor fails to rotate, how to diagnose the issue, and what maintenance practices can prevent future breakdowns.
1. How a Hydraulic Motor Works
A hydraulic motor is the power-executing component of a hydraulic drive system. It converts the hydraulic energy (fluid pressure and flow) into mechanical rotation and torque. In simple terms, it works as the reverse of a hydraulic pump—the pump generates pressure, while the motor receives it and produces rotation.
Key internal components include:
Rotor and stator (or gear/shaft elements) – generate torque and motion;
Bearings and output shaft – support loads and transmit mechanical power;
Distribution or valve plate – directs fluid between high- and low-pressure chambers;
Seals and housing – prevent internal leakage and pressure loss.
Understanding these elements helps you trace potential faults when your hydraulic motor refuses to turn.
2. Two Major Categories of Hydraulic Motor Failures
Hydraulic motor problems can generally be divided into two groups:
Internal issues — mechanical or sealing problems within the motor itself;
External system issues — insufficient flow, pressure, or contamination from the hydraulic circuit.
Let’s break down both categories in detail.
3. Internal Causes: When the Hydraulic Motor Itself Is at Fault
1️⃣ Broken Drive Shaft or Coupling
A fractured drive shaft or worn-out spline will prevent torque from reaching the output shaft even if the motor receives pressure. Symptom: The motor housing feels pressurized or vibrates slightly, but the shaft does not move. Solution: Inspect and replace damaged couplings or keys; verify alignment between motor and load.
2️⃣ Bearing Damage or Friction Lock
After years of high-load operation, bearings can crack or seize, causing excessive friction that stops rotation. Check for: unusual noise, vibration, or metal fragments in the oil. Fix: Replace damaged bearings, inspect housing for heat marks, and ensure proper lubrication.
3️⃣ Severe Internal Leakage
Long-term wear increases the internal clearances between moving parts, allowing oil to leak from high-pressure to low-pressure areas. When this happens, the motor can spin freely under no load but stalls under load. Diagnosis tip: Measure case drain flow or pressure drop—high leakage indicates internal wear. Fix: Replace seals, restore tolerances, or regrind worn surfaces.
4️⃣ Mechanical Locking or Misalignment
If the motor’s output shaft or coupling is misaligned or “bottomed out” (axially jammed), the rotor assembly may seize. Improper installation or foreign particles can also cause jamming. Fix: Check for axial interference, ensure concentric alignment, and clean the motor interior thoroughly.
5️⃣ Seal Failure or Contamination
Aged O-rings or hardened shaft seals can fail, leading to pressure loss and oil leakage. Contaminants like metal shavings or dirt can block valve ports and restrict rotation. Prevention: Always use clean hydraulic oil (ISO 4406 18/15/12 or better) and replace filters regularly.
4. External Causes: When the Hydraulic Motor Isn’t Getting Enough Power
Sometimes, the hydraulic motor isn’t broken—it just isn’t receiving enough energy to turn. External system issues are often overlooked but are responsible for most non-rotating cases.
1️⃣ Low Oil Level or Air in the System
A low tank level or air leakage in the suction line causes the pump to cavitate, leading to pressure drops and unstable flow. Fix: Check the reservoir level, bleed trapped air, and tighten suction fittings.
2️⃣ Clogged Filters or Suction Strainers
When suction filters are blocked, the pump can’t draw enough fluid, starving the hydraulic circuit. Fix: Clean or replace the filters; ensure the correct micron rating for your system.
3️⃣ Faulty or Stuck Relief Valve
If the main relief valve is set too low or stuck open, system pressure will never reach the motor. Fix: Test system pressure with a gauge and adjust or replace the relief valve if necessary.
4️⃣ Worn or Inefficient Hydraulic Pump
A pump suffering from internal leakage or worn gears can no longer deliver rated flow or pressure. Even if it spins normally, the hydraulic motor may still not receive enough torque. Fix: Test pump flow rate, check volumetric efficiency, and rebuild or replace the pump as needed.
5️⃣ Contaminated Hydraulic Oil
Dirty or moisture-contaminated oil can clog valve spools and jam the motor’s rotating components. Fix: Flush the system, use clean oil, and install high-quality return-line filters. Tip: Keep oil cleanliness within NAS 1638 Class 7 or ISO 4406 18/15/12 for long component life.
5. Typical Failure Symptoms and Diagnostics
Observed Symptom
Likely Cause
Recommended Action
Motor not turning at all
Broken shaft, seized bearing, or no flow from pump
Inspect coupling, bearings, and pump pressure
Turns freely with no load, stalls under load
Internal leakage, low pressure
Replace seals, check relief valve setting
Slow or erratic speed
Contaminated oil, sticky control valve
Clean and flush system
Excessive noise or vibration
Air in oil, pump cavitation
Bleed system, check suction line
Motor overheating
High internal friction or leakage
Inspect bearings and internal clearances
6. Step-by-Step Hydraulic Motor Troubleshooting Process
Initial check: Confirm oil level, oil quality, and filter condition.
Pressure test: Measure pressure at the pump outlet and motor inlet using a gauge.
No-load test: Disconnect the motor from the load and observe rotation.
Component inspection: Check couplings, bearings, and seals for wear or misalignment.
Leakage test: Monitor case drain flow to assess internal leakage.
Reassembly and retest: Reinstall components, purge air, and perform load testing.
Following this logical order—from external to internal checks—helps you isolate the problem quickly and avoid unnecessary repairs.
7. Maintenance and Prevention Tips
Regularly replace hydraulic oil and filters to maintain system cleanliness.
Keep oil levels above the pump suction line to prevent cavitation.
Monitor key parameters such as pressure, temperature, and vibration.
Ensure shaft alignment and coupling clearance during installation.
Use high-quality seals and bearings suitable for high-pressure operation.
Before long-term shutdowns, fill the system with anti-rust oil to prevent corrosion.
8. Final Thoughts
When an old hydraulic motor stops turning, don’t rush to replace it. In most cases, the motor isn’t dead—it’s simply not getting the right pressure, flow, or lubrication. By following a structured troubleshooting process, you can restore performance and extend the lifespan of your entire hydraulic drive system.
Key takeaway: A non-rotating hydraulic motor doesn’t always mean failure— skilled technicians know to check the system first, and the motor second.
