Hydraulic system fault tracing

TRACING FAULTS IN THE LOESCHE MILL HYDRAULIC SYSTEM

A Field Guide for Roller-Loading and Grinding Pressure Troubleshooting

If you've spent any time around a Loesche mill, you already know the hydraulic system doesn't announce its problems politely. One day the grinding pressure is dead steady, the next day the roller lift alarm is chattering and production is asking why throughput just dropped. The good news is that despite how intimidating the pipework and accumulator station look, almost every fault in this system comes back to one of five causes: air trapped somewhere it shouldn't be, a valve that's sticking instead of shifting cleanly, a worn seal, a tired accumulator, or oil that's past its working condition. This article walks through how to actually find the fault, not just guess at it.

1. Know the System Before You Chase the Fault

The Loesche hydraulic system does one job: it pushes the grinding rollers down onto the table with a controlled, adjustable force, and it lets them lift and float over tramp material without smashing the table lining. Everything else in the circuit exists to support that one function reliably.

The main players you'll be dealing with when tracing a fault are:

• Hydraulic power unit (HPU) — pump, motor, reservoir, filters, and cooler

• Loading cylinders — one per roller, these apply the grinding force

• Nitrogen accumulators — smooth out pressure pulses and absorb shock loads from tramp metal or surges

• Proportional / directional control valves — regulate flow and pressure to each cylinder

• Pressure transmitters and switches — feed the PLC for interlocks, trending, and alarms

• Rod position / lift sensors — confirm roller lift-off during startup, tramp events, and low-load condition.

2. The Five Usual Suspects

a) Air in the system

Air is the most common troublemaker and the easiest to misdiagnose as something mechanical. Trapped air compresses instead of transmitting force cleanly, so you'll see a soft, spongy pressure response — rollers that lift or settle slower than normal, or pressure that oscillates gently instead of holding a flat line. Air usually gets in after any maintenance where a line was opened: filter change, hose replacement, cylinder reseal. If the fault started right after a job like that, bleed the system before you suspect anything else.

b) Sticking or worn control valves

A directional or proportional valve that isn't shifting cleanly gives you jerky pressure changes, a roller that hunts instead of settling, or a cylinder that won't respond at all to a setpoint change even though the pump is clearly running and delivering pressure elsewhere. Contamination is the usual cause — a spool that's picked up varnish or fine particulate won't slide smoothly in its bore even though it isn't fully jammed.

c) Seal wear in the loading cylinders

Worn rod or piston seals show up as a roller that slowly creeps down under load even with the valve holding position, or as oil weeping around the rod — sometimes visible, sometimes only noticeable as a slow reservoir level drop over days. This one is mechanical, not electrical, so no amount of PLC or valve troubleshooting will fix it; the cylinder needs a reseal.

d) Accumulator losing precharge

This is the one people forget to check because it's a gas-side problem hiding inside a hydraulic fault. When nitrogen precharge drops, the accumulator stops cushioning the circuit properly, and you'll see sharper pressure spikes on tramp events, faster cycling, and sometimes a hydraulic hammering noise you can hear from the platform. Precharge should be verified against the OEM spec with the system depressurized — never estimate it from behavior alone, confirm it with a gauge.

e) Oil condition

Degraded or contaminated oil silently drags down performance across the whole circuit — sluggish response everywhere, elevated operating temperature, and filters that clog faster than they should. If more than one roller or more than one valve is showing a vague, similar symptom at the same time, stop suspecting individual components and pull an oil sample first.

3. A Practical Trace Sequence

When you get called out, resist the urge to start opening fittings. Work outward from the symptom in this order — it saves time and it keeps you from chasing a mechanical fault with an electrical fix, or the other way round.

• Step 1 — Confirm the symptom on the trend, not just the alarm. Is it one roller or all of them? Constant or intermittent? Did it start after a specific event (maintenance, tramp metal, power dip)?

• Step 2 — Check pump discharge pressure at the HPU. If it's low or unstable at the source, everything downstream will look faulty even though it isn't.

• Step 3 — Verify accumulator precharge against spec. Do this early since a weak accumulator can mimic valve or seal problems downstream.

• Step 4 — Isolate to one cylinder circuit at a time. Compare pressure at the valve inlet versus the cylinder port — a difference here points straight at the valve.

• Step 5 — Command a small setpoint change and watch the actual response on the position/pressure feedback. Sluggish or no response with good supply pressure confirms a valve or wiring issue rather than a hydraulic supply issue.

• Step 6 — If pressure holds fine under no-load but drops under load, or drops slowly over time with the valve closed, that's a strong seal-wear signature — get eyes on the cylinder for weeping.

• Step 7 — Pull an oil sample if two or more independent circuits show similar vague symptoms, or if it's simply been a long time since the last analysis.

4. Quick Reference — Symptom to Likely Cause

Symptom	Likely Cause	Check First
Spongy, oscillating pressure	Air trapped in system	Bleed points near cylinder and valve
Jerky response / hunting	Valve spool sticking	Valve response to a manual step command
Slow pressure creep-down under load	Worn cylinder seals	Visual check for oil weeping at rod
Sharp spikes, hammering noise	Weak accumulator precharge	Nitrogen precharge with gauge, system depressurized
Sluggish response across multiple circuits	Degraded / contaminated oil	Oil sample and filter condition
No response, pump running fine	Valve coil or wiring fault	Coil resistance and command signal at valve

5. A Field Example

A useful case to keep in mind: a roller that showed a slow, steady pressure droop only under grinding load, with the system holding perfectly steady during no-load checks. Every valve test looked normal, and pump pressure was rock solid. The giveaway was that the droop only appeared under load — that's the classic signature of a seal that's sealing fine at low differential pressure but starting to bypass once real force is applied. A borescope check on the cylinder confirmed light scoring on the rod, and a reseal cleared it completely. The lesson: always test behavior under actual grinding load, not just at standstill, because some faults simply don't show themselves until the system is working.

6. Safety Reminders Before You Open Anything

• Never crack a fitting or remove a gauge without confirming the circuit is fully depressurized — hydraulic oil under pressure can penetrate skin.

• Isolate and lock out the HPU before working on cylinders or valve blocks, not just the mill drive.

• Nitrogen accumulators must only be discharged through the proper charging valve, following OEM procedure — never loosen a accumulator fitting directly.

• Keep a spill containment tray under any fitting you open; even a small hydraulic leak on the mill floor is a slip hazard.