How the swing system works: motor, brake, gearbox, pinion
The swing system rotates the upper structure of the excavator relative to the undercarriage. On hydraulic excavators it normally consists of four functional groups, often built into one unit known as the swing drive:
- Swing motor: a hydraulic motor, usually of axial piston design, that converts oil flow from the pump into rotation. Relief and anti-cavitation valves are often fitted to the motor to absorb pressure peaks when starting and stopping.
- Swing brake (parking brake): usually a multi-disc brake that is applied by spring force and released hydraulically by pilot pressure. It holds the upper structure when stationary, for example on a slope or during transport.
- Swing gearbox: normally a multi-stage planetary gearbox. It turns the high speed of the motor into high torque at low speed and runs in its own oil bath.
- Pinion: sits on the gearbox output shaft and meshes with the teeth of the slewing ring.
The slewing ring is a separate component: a large rolling bearing that carries and guides the upper structure. The swing system is controlled by the main control valve and the pilot circuit. When the operator moves the lever, the swing spool opens and, at the same time, pilot pressure releases the parking brake.
When the lever is released, the upper structure is mainly braked hydraulically: the spool closes and the valves on the motor absorb the kinetic energy. On many machines the parking brake only engages after a short delay. Understanding this sequence matters for diagnosis, because a fault can sit at any point in the chain.
Why won't my excavator swing? Slow, jerky or no swing
First check whether only the swing function is affected. If the boom, dipper and tracks are slow as well, the cause is more likely the hydraulic pump, pilot pressure or the engine rather than the swing system. If only swing is affected, this overview helps to narrow it down:
| Symptom | Possible causes | First check |
|---|---|---|
| No swing at all, other functions normal | Brake not releasing, broken shaft or gear stage, failed swing motor, pilot circuit fault | Measure brake release and swing pressure, listen for noise at the drive |
| Slow or weak swing | Internal leakage in the swing motor, worn relief valve, dragging brake | Measure case drain flow and swing pressure, compare housing temperature |
| Jerky or notchy swing | Tooth damage on gearbox, pinion or slewing ring, slewing ring bearing damage, air in the system | Inspect the teeth, check slewing ring grease for metal particles |
| Grinding or whining from the swing gearbox | Bearing or gear damage in the planetary gearbox, low oil | Check oil level and magnetic drain plug |
| Heavy knock when changing direction | Excessive backlash in gearbox, pinion or slewing ring | Check play and compare with earlier readings |
| Upper structure creeps round on a slope | Worn parking brake, internal leakage in the motor | Brake test as per the service manual |
| Upper structure overruns after lever release | Relief or anti-cavitation valve, control spool | Check pressures at the test points |
Swing brake or swing motor? How to tell the difference
If the upper structure does not swing at all, a parking brake that fails to release is a common cause. The typical picture: the lever is operated, the engine audibly takes load, but nothing turns. Possible reasons include missing pilot pressure at the brake, a faulty solenoid valve, a leaking brake piston seal or discs that have stuck together after a long standstill.
A dragging brake that only partly releases shows up as slow swing and a noticeably warmer brake housing. In that case, check:
- whether pilot pressure reaches the brake release port when the lever is operated
- whether the relevant solenoid valve switches and receives voltage
- whether pilot lines are kinked, damaged or leaking
- whether fault codes are stored in the machine controller
On the swing motor, internal leakage is the most common form of wear. The motor loses torque and speed, especially when the oil is hot. The most meaningful test is a case drain flow measurement: under defined conditions the case drain line is routed into a measuring container or through a flow meter. High readings point to worn pistons, cylinder block or valve plate. The permissible case drain flow and the specified pressures are given in the manufacturer's service manual, not in general tables.
If hardly any pressure builds up when swinging and the upper structure still does not move, the motor may be spinning freely. That suggests a mechanical break in the drive line, such as a broken shaft, sheared splines or a destroyed planetary stage.
Swing gearbox oil level and metal particles
The swing gearbox has its own oil bath, checked with a dipstick or sight glass. The condition of the oil and any debris tell you a lot about what is happening inside. Proceed as follows:
- Secure the machine: park on firm, level ground, lower the attachment, stop the engine, remove the key and depressurise the hydraulics as specified by the manufacturer. Secure the upper structure against rotation, using the swing lock if fitted.
- Check the oil level: a low level indicates an external leak. If the level rises without topping up, the swing motor shaft seal is often letting hydraulic oil into the gearbox.
- Assess the oil condition: milky oil indicates water, dark oil with a burnt smell indicates overheating.
- Inspect the magnetic drain plug: a thin grey film in small amounts is normal in gearboxes. Chips, splinters or flakes point to bearing or gear damage.
- Keep a sample: if in doubt, take an oil sample for laboratory analysis.
- Inspect pinion and slewing ring: look for chipped teeth and check the grease for metal debris.
If you find coarse metal particles, stop swinging the machine. Loose debris keeps damaging bearings and gears and can destroy the pinion and the slewing ring teeth as well. Also check the filter in the case drain circuit: metal there indicates swing motor damage that has contaminated the hydraulic system.
Decision tree: tracing the swing fault step by step
This sequence saves time because simple checks come before time-consuming dismantling. Pressure tests with the engine running should only be carried out by trained personnel.
- Are all functions slow? Yes: check the pump, pilot pressure and engine output. No: go to step 2.
- Does the upper structure not swing at all? Measure brake release pressure. If it is missing, the fault is in the pilot circuit or solenoid valve. If it is present, go to step 3.
- Does swing pressure build up? High pressure without movement suggests a seized brake or a locked gearbox. Low pressure without movement suggests a mechanical break or severe leakage.
- Does it only swing slowly? Measure case drain flow. High reading: swing motor. Normal reading: check relief valves and control spool.
- Is there noise or jerking? Check gearbox oil and drain plug. Metal debris: swing gearbox. Clean oil: check pinion, slewing ring teeth and slewing ring bearing play.
- Does the upper structure creep on a slope? Test the parking brake as per the service manual.
If the slewing ring is the suspect, you will find the warning signs and the replacement procedure in our guide Replacing an excavator slewing ring.
Swing motor repair or replacement?
Many faults can be put right by a specialist workshop, for example with new seal kits, brake discs or bearings. Replacing the complete assembly with a new part is often the more economical choice when any of the following applies:
- housings, shafts or planet carriers are cracked or broken
- several gear stages show tooth damage
- pistons, cylinder block or valve plate in the motor are scored
- machine downtime costs more than a new unit
- the unit has already been repaired several times
Whatever the damage, check the neighbouring parts too: a destroyed pinion often damages the slewing ring teeth, and debris from a failed motor gets into the hydraulic system. After a motor failure, flushing the system and fitting new filters is standard practice. Installation belongs in a specialist workshop. The motor housing is filled with hydraulic oil before the first start as specified by the manufacturer, and the gearbox is filled with the prescribed oil.
We supply new swing motors, swing gearboxes and complete swing drives. An overview by manufacturer is available under swing drives, swing gearboxes and swing motors.
Finding the right swing drive part
Swing drives differ in flange pattern, shaft splines, pinion and ports, often even within the same model range. To identify the correct part reliably, please send us:
- machine make and model
- machine serial number
- part number of the swing motor or swing gearbox, if available
- a photo of the ID plate on the machine and on the component
- photos of the fitted drive, ideally with the number of pinion teeth
Our guide Finding the part number and serial number shows where to find these details. You can send everything via Request a part. We check the fit and send you a quote.
Read more: Swing drives
