A swing assembly is two machines bolted together: a hydraulic motor with an integral brake, and a planetary reduction that turns motor speed into swing speed and multiplies torque. They are sold separately, as a matched pair, or as a complete unit, and those three are not interchangeable across a serial break. Identify by machine model and serial number first, then by the motor nameplate, then by measurement: displacement, control type, relief setting, brake release pressure, reduction ratio, pinion tooth count, mounting pilot diameter and bolt circle, and shaft spline. A motor with the right flange and the wrong displacement will bolt straight on and swing at the wrong speed.
What to record before you ask for a price
| Detail | Why it decides whether the unit is right |
|---|---|
| Machine model and serial number | The serial number, not the model, fixes the build. Swing ratios and brake ratings changed within model runs. |
| Motor displacement and control type | Sets swing speed and how the motor responds to a control signal. Positive control and negative control units are not swappable. |
| Relief valve setting | Caps swing torque and protects the reduction. A relief set for a different ratio will either slip or overload. |
| Brake type and release pressure | Spring-applied, hydraulically released on most machines. The release pressure must match the machine's pilot circuit. |
| Reduction ratio and number of stages | Sets output speed and torque. A different ratio changes swing speed and makes the machine feel wrong even though it fits. |
| Pinion tooth count and module | The pinion meshes with the swing bearing's internal gear. Count every tooth; a one-tooth difference is a different part. |
| Mounting pilot diameter and bolt circle | The mechanical interface. Measure the pilot bore and the bolt circle across the centers, not across the outside. |
| Shaft spline diameter and count | The motor-to-reduction coupling. Spline count, pressure angle and length must all match. |
How to confirm it
- Photograph the machine plate before anything else
Model, serial number and any build code. Everything else is checked against that number, and a photograph taken first survives the cleaning that follows.
- Photograph the motor nameplate in place
The plate carries the maker, the model code and often a build or specification suffix. Clean it only after it has been photographed, because a wire brush removes the suffix that identifies the variant.
- Count the pinion teeth
Turn the assembly by hand and count every tooth, marking the first one. This single number disqualifies more wrong parts than any other measurement, and it cannot be read from a parts list that only quotes the machine model.
- Measure the mounting interface
Pilot diameter with a caliper, bolt circle across the centers of two opposite holes, and the thread pitch of the studs. Note the angular position of the ports as well, because a clocked flange will fit and then foul a hose.
- Read the relief and brake figures from the machine, not the donor
Relief setting and brake release pressure are machine specifications. Copying them from a similar unit on the yard is how a machine ends up with a swing that slips under a full bucket.
- Check the seal interface between motor and reduction
The shaft seal between the two units is a common leak path, and a failed seal lets gear oil and hydraulic oil swap. Note whether the replacement includes it, and order it if not.
What to do about it
Order by serial number and measured detail together. A supplier who asks only for the machine model is guessing, and the guess is usually right on the flange and wrong on the ratio or the brake.
Decide between motor only, reduction only and complete assembly on evidence. A motor that leaks externally but swings at the right speed and holds a load needs a motor; a reduction with metal on the drain plug needs the reduction, and often the motor as well.
Never up-rate the relief to compensate for a weak swing. If a correctly set machine will not hold a load, the fault is in the motor, the brake or the bearing, and a higher relief setting converts a repair into a failure.
Replace the seal between motor and reduction whenever either unit is changed, and check both oil compartments after the first week. Cross-contamination is the most common repeat failure on a rebuilt swing assembly.
Swing motors, swing reductions, complete assemblies, brake plates and seal kits are supplied by Guangzhou Shenyue Machinery Technology Co., Ltd., which keeps 3,000+ part types on the shelf and 100,000+ part numbers available to order, with every line marked genuine, OEM-quality or aftermarket.
Parts you may need, by grade
| Part | Grades commonly quoted |
|---|---|
| Swing motor assembly | Genuine; OEM-quality; remanufactured exchange. Match displacement, control type and relief. |
| Swing reduction assembly | Genuine; OEM-quality. Quote ratio and pinion tooth count. |
| Complete swing unit | Genuine; OEM-quality. Supplied as a matched pair; confirm the serial break. |
| Brake plates and springs | Genuine; OEM-quality. Sold as a set; measure the stack height. |
| Shaft seal and seal kit | Genuine; OEM-quality. Replace at every separation of motor and reduction. |
Need the parts this covers?
Send the machine model, serial number and the part numbers you need. We confirm fitment before payment and state the grade of every line — genuine, OEM-quality or aftermarket.
Request a quoteFrequently asked
Can I fit a swing motor from a similar machine?
Only if displacement, control type, relief setting, brake release pressure and the shaft spline all match. The flange is the least reliable indicator: it is the detail most often shared between variants that differ in everything that affects how the machine swings.
Why does the pinion tooth count matter so much?
Because the pinion meshes with the internal gear on the swing bearing, and that mesh sets both the ratio and the contact pattern. A pinion with a different tooth count will either not mesh or will mesh at the wrong center distance, and either way the bearing and the reduction are damaged.
How do I know whether the motor or the reduction is at fault?
Separate them. A motor that will not hold a load on a bench test is a motor fault; a reduction with metal on the drain plug, backlash beyond specification or a leaking shaft seal is a reduction fault. A swing that is weak but smooth and quiet usually points at the motor or the relief, not the gears.