Safety first: relieve hydraulic pressure and support any raised or rotating load before disconnecting hoses or removing an actuator. A drifting load can move unexpectedly when a load-holding valve is disturbed. This guide is for diagnosis; pressure testing and internal repairs should follow the machine service manual.
TROUBLESHOOTING
Rotary Actuator Troubleshooting: Drift, Weak Torque, Leaks & Noise
When a swing function drifts, loses force, leaks oil or moves roughly, the actuator is not always the culprit — supply pressure and the load-holding valve cause many of the same symptoms. Work from the outside in: confirm pressure and the valve first, then look at seals, shaft and housing. This page maps each symptom to its likely causes and the checks that separate them.
Last reviewed: October 2026
Symptom-to-cause table
Drift / will not hold position
Likely causes
Internal seal leakage; leaking or mis-adjusted counterbalance valve; over-running load beyond the valve rating.
Checks
Confirm load-holding valve type and setting; check for internal bypass; verify the load matches the original duty.
Weak or lost torque
Likely causes
Low supply/relief pressure; worn piston or shaft seals causing bypass; slipping or sheared drive coupling.
Checks
Check pump and relief pressure first; look for internal leakage; confirm the shaft still drives the load positively.
External oil leak
Likely causes
Worn shaft seal; loose port fitting; damaged housing or flange face; scored shaft cutting the seal.
Checks
Clean, cycle and locate the first oil source; torque fittings to spec; inspect shaft surface and housing faces.
Won’t move / moves only one way
Likely causes
No pilot or supply pressure; blocked or reversed ports; failed counterbalance valve; seized load or actuator.
Checks
Verify supply and pilot pressure; confirm plumbing; isolate the valve before condemning the actuator.
Noisy or jerky motion
Likely causes
Air in the oil; cavitation from a restricted or cold supply; contaminated fluid; damaged splines or bearings.
Checks
Bleed the system; check fluid condition and temperature; inspect mechanical internals if symptoms remain.
Ends short of full angle
Likely causes
Internal stop-tube or mechanical stop set for a special angle; reduced flow; internal leakage near end of stroke.
Checks
Read the tag for a special angle code (e.g. base/final); check flow and pressure at end of stroke.
Diagnose outside-in before opening the unit
01 · Supply & pressure
Verify pump flow, relief pressure and that the correct pilot/supply pressure reaches the ports. Many “weak actuator” faults start here.
02 · Load-holding valve
Check the counterbalance valve for leakage, adjustment and the correct part number. A drifting load is frequently a valve issue, not a body issue.
03 · External signs
Wipe clean, cycle and locate the first oil. Inspect port fittings, flange faces and the shaft surface, not just the shaft seal.
04 · Internal condition
Only after the above, inspect seals for bypass, the bore for scoring and bearings/splines for wear — this decides rebuild vs replacement.
Once you have confirmed the fault is inside the actuator, the next decision is whether a seal kit will restore it or whether wear to the housing, shaft or bearings makes a replacement the more reliable route. See our rebuild vs replacement guide.
ENGINEERING CHECK
Send the symptom and the tag
Tell us what the actuator is doing and send a tag photo. We help you separate a supply/valve problem from an actuator fault and confirm whether a seal kit or a replacement unit is the right course before you commit to downtime.
Or email directly: hm@hmhydraulics.com
Find your actuator reference
Search by model code or OEM part number across our cross-reference library.
Browse by series
Heavy-duty & specialty Helac-family actuators
Other Helac series & variants
Questions
Frequently asked questions
Why does my actuator drift or will not hold position?+
Position drift usually means pressure is not being held. Common causes are internal seal leakage past the piston or shaft seals, or a counterbalance / load-holding valve that is leaking, mis-adjusted or mismatched. Confirm the valve and its setting before assuming the actuator body is at fault.
What causes weak or lost torque?+
Low supply pressure, internal bypass leakage from worn or damaged seals, or a slipping load coupling can all reduce output. Check pump and relief pressure first, inspect for internal leakage, and confirm the shaft is still positively driving the load.
Is an external oil leak always a failed shaft seal?+
Not always. Oil at the shaft can be a worn shaft seal, but it can also come from a loose port fitting, a damaged housing face or a scored shaft surface that cuts a new seal. Wipe the unit clean, cycle it and watch where the oil first appears.
Why is the actuator noisy or moving roughly?+
Air in the oil, contaminated fluid, cavitation from a restricted supply, or mechanical damage to splines, bearings or the shaft can cause noise and jerky motion. Bleed the system and check the fluid before opening the actuator.
Should I rebuild or replace a faulty actuator?+
If only seals are worn and the housing, shaft and splines are sound, a seal kit can be enough. If the bore is scored, the shaft worn or the bearings failed, replacement is usually more reliable. Our rebuild-vs-replacement guide walks through the decision.
These are general diagnostic indicators, not a service procedure. Always relieve pressure, secure the load and follow the equipment manufacturer’s service manual before testing or dismantling. Final diagnosis and the rebuild/replacement decision depend on the actual unit condition. Helac® and all OEM names are trademarks of their respective owners; HM Hydraulics is an independent manufacturer and is not affiliated with or endorsed by them.