Where is the swing bearing and why does play develop?
The swing bearing, also called the slewing ring, sits between the upper structure and the undercarriage. It is a large rolling bearing made of two rings: one is bolted to the undercarriage frame, the other to the upper structure. The rotary joint sits in the centre and carries hydraulic oil down to the travel motors. The swing drive turns the upper structure through the gear teeth on the inner or outer ring.
Play develops mainly through wear on the raceways and rolling elements. An excavator loads its swing bearing with constantly changing tilting moments: when digging, lifting and slewing with a full bucket. Every load reversal removes a little material. Wear accelerates with:
- infrequent greasing or the wrong grease
- dirt and water getting in through damaged seals
- impacts, for example using the bucket as a hammer or in demolition
- loose or broken mounting bolts
- overloading with attachments that are too heavy
A new swing bearing already has a small amount of designed clearance. So the question is not whether there is play, but how much it has grown since installation.
Tools for the job: dial gauge, magnetic base and measuring position
Standard workshop equipment is enough:
- dial gauge (dial indicator) with sufficient travel and a fine scale
- magnetic base with a rigid arm, the shorter the arm, the less it flexes
- wire brush and cleaner for the contact surfaces
- the machine's service manual with the measuring procedure and limit
- a log sheet for date, operating hours, measuring point and reading
Place the magnetic base on a clean, flat spot on the undercarriage frame, as close to the bearing as possible. The probe rests vertically on the upper bearing ring or on the upper frame right next to the bolted joint. The further the measuring point is from the bearing, the more frame flex you measure along with it. Preload the probe slightly so it can register movement in both directions.
The machine must stand on firm, level ground. A slope or soft ground distorts the result. On many machines the bearing is reached from the side or from below once covers have been removed.
Measuring swing bearing play step by step (axial and radial)
The usual method is the tilting clearance check. You deliberately reverse the tilting moment on the upper structure, and the dial gauge shows how far it moves relative to the undercarriage. Boom and arm position, bucket height and measuring points are specified by the manufacturer. The sequence below shows the principle:
- Secure the machine: park on level ground and slew the upper structure to the position given in the manual, usually with the boom pointing lengthways over the idler or sprocket. Lower the bucket, stop the engine, depressurise the hydraulics and remove the key.
- Set up the gauge: fix the magnetic base to the undercarriage and place the probe vertically on the bearing ring or frame on the boom side.
- Zero: start the engine and raise the bucket off the ground as specified, so the working equipment tilts the upper structure forwards. Set the gauge to zero.
- Reverse the load: lower the boom and press the bucket into the ground until the front of the undercarriage lifts slightly. The upper structure now tilts the other way. Take the reading. Movements only on a clear signal, nobody under the machine or next to the tracks.
- Repeat: take several readings and average them. Widely scattered values point to an unstable gauge set-up.
- Further positions: slew to the positions specified by the manufacturer and measure again. Raceways wear unevenly, usually more in the main working zone.
- Radial play: place the probe horizontally against the outer face of the bearing ring and repeat the load reversal, or apply side load as described in the manual.
- Record: log every reading with date, operating hours and position.
Take a reading straight after a new swing bearing has been fitted and keep it as your baseline. Bearing manufacturers assess wear by the increase in tilting clearance compared with that value.
Judging the reading: table with recommended action
The binding limit is in the machine's service manual or the bearing manufacturer's data sheet. It depends on bearing size, design and machine, which is why we deliberately give no millimetre figures here. The table shows how to judge your reading against that limit:
| Finding | Assessment | Recommended action |
|---|---|---|
| Well below the limit, little change from baseline | normal | Keep greasing to schedule, repeat the check at the service interval |
| Noticeable increase since the last check, still below the limit | early wear | Shorten the check interval, inspect grease for debris, check bolts |
| Close to the limit | advanced wear | Plan the replacement, identify the part, schedule the swap for downtime |
| Limit reached or exceeded | replacement due | Stop operation as the manufacturer specifies, have a specialist workshop replace it |
| Sudden rise in a short time | find the cause | Inspect bolts, bearing rings and frame for fractures and cracks |
| Reading on the frame much higher than on the ring | movement at the bolted joint | Check the mounting bolts, see next section |
Never decide on the figure alone. Grinding noises, rough rotation or metal particles in the grease justify a closer look even when the reading is below the limit.
Swing gearbox backlash or loose bolts? Common mix-ups
Not all play at the upper structure comes from the swing bearing. Three causes are often confused:
Backlash in the swing drive
If the upper structure rocks slightly in the direction of rotation, or knocks when slewing starts and stops, this is usually rotational backlash. It comes from clearance between the pinion and the bearing's gear teeth or from wear inside the swing gearbox. The tilting check does not capture it. Inspect pinion and ring gear for chipped teeth and the gearbox oil for debris.
Loose mounting bolts
Knocking on load reversal, fretting or rust marks at the joint and missing bolt heads point to the bolted connection. A comparison reading helps: if the gauge shows clearly more movement on the frame than directly on the bearing ring, the bearing is moving against the frame.
Play elsewhere
Worn boom and arm pins, soft cab mounts or slack in the undercarriage can also make the machine feel loose while the swing bearing itself is fine.
When to retighten bolts and when to replace the swing bearing
Loose mounting bolts must be dealt with exactly as the manufacturer specifies: tightening torque, method and sequence are in the service manual. Many manufacturers require loosened bolts to be replaced with new ones, because they have lost preload or may be stretched. If one bolt has broken, the neighbouring bolts are usually replaced as well. For any of this work, secure the machine and depressurise the hydraulics. Measure again afterwards.
The bearing clearance itself normally cannot be adjusted on excavator swing bearings. Replacement is due when:
- the limit has been reached or exceeded
- there are metal particles in the grease or the upper structure turns roughly and grinds
- cracks in the rings or broken teeth are visible
- play returns quickly despite new bolts
- seals are destroyed and raceways corroded
The swap should be done by a specialist workshop with suitable lifting gear. Our guide on replacing an excavator slewing ring describes the procedure. Check the swing drive pinion at the same time, as a worn pinion will damage the new gear teeth.
Finding the right replacement: part number, gearing, bolt pattern
Swing bearings differ in outside diameter, height, bolt pattern, number of holes, internal or external gearing, number of teeth and module. There are often several versions within one model series. For a reliable match, send us:
- machine make and model
- serial number from the ID plate
- the bearing's part number if available, often stamped on the ring
- a photo of the ID plate and of the bearing
- without a part number: outside diameter, bolt pattern and number of teeth
Our guide on finding part and serial numbers shows where to look. Send the details via Request a part. We check the match and send you a quote for a new bearing from our range of slewing rings.
Read more: Slewing rings
