Field diagnosis
Jerky or crawling movement at low speed: friction, air and stiffness
A function that moves smoothly at speed but stutters when the lever is barely open is fighting friction or air, not power. The cure is usually in the valve, the cylinder seal or the stiffness of the circuit.
Check these in order
| Check | What it looks like | What it tells you |
|---|---|---|
| Air in the circuit | Compressed air acts as a spring: the function moves, pauses and then jumps. Usually worst after a repair or an oil change. | Bleed the circuit at its highest points; check suction and the tank level. |
| Spool lap and metering | An underlapped spool leaks at neutral and starts moving abruptly; an overlapped one sticks until pressure builds, then jumps. | Compare low-speed behaviour in both directions and with similar functions; a spool change or valve service is the cure. |
| Cylinder and rod seal friction | A dry or swollen rod seal grips the rod, releases, grips again - classic stick-slip at low speed. | Inspect the wiper and rod for scoring and dry patches; rebuild the cylinder. |
| Structural and line stiffness | Long unsupported pipe runs, worn mounts and flexible brackets lower the structural stiffness of the circuit, which shows up as oscillation at a particular speed. | Add clamps, replace mounts, shorten unsupported spans. |
| Pilot pressure and control resolution | If pilot pressure is marginal, the first part of the lever travel does nothing and then the spool jumps open. | Gauge pilot pressure at the valve and compare with the machine specification. |
| Oil viscosity and temperature | Cold, thick oil makes stick-slip worse; very hot, thin oil makes the spool unstable. | Check the oil grade for the climate and warm the machine properly before precision work. |
Units and parts this usually leads to
K3V112DTP
20–25 t hydraulic excavator - 112 mL/r (per pump)
K5V140DTP
27–33 t hydraulic excavator - 140 mL/r (per pump)
M5X130CHB
12–15 t hydraulic excavator - 121.6 / 129 / 129.2 cm³ (motor variants)
Spool lap and metering
How overlap and underlap change the first millimetre of spool travel, and why identical valves feel different.
Hydraulic noise diagnosis
Stiffness, mounts and pipe runs: the structural half of stick-slip and shudder.
Field notes on this complaint
The notes our own team works to on this symptom. Each one is a principle you can check on the machine, a step from the case-drain and cleanliness procedure we follow, or a figure transcribed from our catalogue - not advice copied from a video or a forum. Anything we could not check against a catalogue, a manufacturer document or a working machine is left out.
- Time the symptom. Jerking that settles after a few minutes is usually air working its way out; jerking that stays is usually a sticking spool or a worn travel motor. The timing tells you which end to open. (circuit principle, checkable on the machine)
- Pilot pressure first. A dropping pilot pressure makes every function feel jerky, so measure it before opening the motor. (circuit principle, checkable on the machine)
- V-notches in the spool exist to remove the dead band - a step points there A spool with plain lands has to travel through its overlap before any oil passes, which feels like a dead band followed by a jump; that is why most manufacturers cut V-notches into the land so flow starts the moment the spool moves. If a machine has a step or a jerk at the beginning of a movement, the notch and the overlap are part of the diagnosis - together with pilot pressure - and the pump is usually not the first suspect. Compare against the same function on the other side of the machine. (circuit principle, checkable on the machine)
Common questions
Is crawling a pump problem?
Rarely. A tired pump loses flow everywhere, which shows up as slow work at full lever, not as stutter at small openings. Crawl is a control, friction or air problem, and the pump is the last component to condemn.
Why does it only happen when cold?
Cold oil is thicker, so seal friction and spool friction rise and stick-slip becomes visible. If the machine is smooth after twenty minutes of work, the friction is real but still within limits - the useful fixes are warming the machine before precision work and checking the oil grade for the climate.
Can an accumulator cause jerky movement?
A bladder accumulator with low precharge, or a blocked charging valve, reacts too slowly to damp a pulse, so movement can become uneven. Test precharge and the charging circuit if the machine has one.
Why it happens
The mechanism behind this symptom, from our technical articles: reading the principle first usually shortens the checklist.
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