What is a slew ring on an excavator?
The slew ring, also called a slewing bearing, swing bearing or swing circle, is the bearing between the upper structure and the undercarriage. Unlike an ordinary rolling bearing, it does not sit on a shaft. It is bolted directly to the frames of the upper and lower structure through two bolt circles. This leaves the centre free for the rotary joint, which carries hydraulic oil down to the travel motors.
A slew ring has to handle three kinds of load at once: axial loads from the weight of the upper structure and the payload, radial loads from side impacts and acceleration during slewing, and tilting moments created when the boom and bucket are fully extended. This combination is what sets it apart from most other bearings on a construction machine, and it is the reason the upper structure does not lift off the undercarriage when the boom is reaching out.
Slew ring design: rings, rolling elements and gearing
Slew rings look very different depending on machine size, but they are built from the same basic components:
- Inner ring: on excavators it usually carries the gear teeth and is bolted to the undercarriage.
- Outer ring: bolted to the upper structure and rotates with it.
- Rolling elements: balls or rollers run in machined raceways between the rings and transmit all the forces.
- Spacers or cage: keep the rolling elements apart so they do not rub against each other.
- Gearing: cut into one of the two rings, either on the inside or the outside. The swing drive pinion meshes with it.
- Seals: sealing lips between the rings keep grease in and dirt and water out.
- Grease points and filling plug: grease nipples supply the raceway. On ball slew rings the balls are often inserted through a filling hole that is then closed with a plug.
The rings are normally made of quenched and tempered steel with hardened raceways. Raceway wear cannot realistically be machined out in the field. Once the raceway is damaged, the slew ring is replaced as a complete component.
Slew ring types: single row, double row, ball or roller
Slewing bearings are classified by the type and number of rows of rolling elements. The table below shows the common designs and where you typically find them:
| Type | Rolling elements and contact | Characteristics | Typical use |
|---|---|---|---|
| Single-row ball slew ring (four-point contact) | One row of balls, contact at four points on the raceway | Compact, handles all three load types, low overall height | Mini excavators, compact excavators, many mid-size excavators |
| Double-row ball slew ring | Two rows of balls, sometimes of different diameters | Higher capacity for large axial loads | Cranes, machines with a high dead weight |
| Crossed roller slew ring | One row of cylindrical rollers, alternately crossed at 90 degrees | Line contact, high stiffness with low overall height | Mid-size to large excavators, material handlers |
| Three-row roller slew ring | Three rows of rollers, separate for axial, radial and reverse load | Very high load capacity, large and heavy | Large excavators, port cranes, heavy equipment |
The bearing type is chosen by the machine manufacturer at the design stage. A replacement must match the original in type, diameter, height, bolt pattern and gearing. Fitting a different bearing type simply because the mounting dimensions happen to match is not a sensible solution.
Internal or external gearing on excavators
Most hydraulic excavators use a slew ring with internal gearing. The toothed inner ring is bolted firmly to the undercarriage while the outer ring carries the upper structure. The swing drive pinion reaches down into the ring and meshes with the teeth from the inside. This layout has several advantages:
- The gear teeth sit protected beneath the upper structure, away from stones and most of the dirt.
- The gearing can run in an enclosed grease chamber that keeps it lubricated.
- The design is compact, because the swing drive sits within the bearing diameter.
With external gearing, the teeth are cut into the outer ring and the pinion meshes from outside. You find this on some cranes, rotating attachments and a few excavator models. The teeth are easier to inspect but more exposed, and they are usually greased in the open.
When identifying a replacement, gearing type, number of teeth and module are among the key data. They must match the swing drive pinion, otherwise both parts wear out quickly.
Where is the swing bearing located and how does it work with the swing drive?
The swing bearing sits between the upper structure and the round mounting face on the centre section of the undercarriage. Look sideways underneath the upper structure and you will see the outer ring and the heads of the mounting bolts. With internal gearing, the teeth are only accessible through inspection openings or after removing a cover.
The swing drive provides the force for rotation. It usually consists of a hydraulic motor, often an axial piston motor, a planetary gearbox and the pinion. When the operator moves the joystick, the hydraulic pump delivers oil through the control valve to the motor. The gearbox converts the high motor speed into low speed with high torque.
Because the swing drive is mounted on the upper structure and the ring gear on the undercarriage stays still, the pinion rolls around the ring gear and takes the upper structure with it. Holding is normally done by a spring-applied multi-disc brake that is released hydraulically. Valves in the motor cushion pressure peaks when starting and stopping the slew.
So slew ring and swing drive are two separate components: the slew ring carries and guides, the swing drive provides the drive. Matching drives are listed under Drehantriebe, Drehgetriebe, Drehmotoren.
Typical loads and wear points on a slew ring
In service, the slew ring is exposed to constantly changing forces. Particularly damaging are impacts when digging in hard ground, hydraulic breaker work, harsh braking of the slew and lifting heavy loads at full reach. Typical wear points are:
- Raceways: pitting from material fatigue and indentations from heavy impacts.
- Rolling elements: flat spots, spalling or corrosion, often caused by poor lubrication or water ingress.
- Seals: cracked or displaced sealing lips let dirt in, which then damages the raceway.
- Gearing: wear on the tooth flanks, often concentrated in the sector where the machine does most of its work.
- Mounting bolts: loose or broken bolts lead to uneven load distribution.
- Mounting faces: uneven or distorted frame surfaces load the ring on one side.
Greasing at the intervals specified by the manufacturer is the single most effective measure against premature wear. A reliable indicator of condition is the tilt clearance.
Checking tilt clearance: the method
- Secure the machine: park on firm, level ground, align the upper structure with the undercarriage, switch off the engine, secure it against restarting and depressurise the hydraulics as described by the manufacturer.
- Set up the dial gauge: fix a dial gauge with a magnetic base to the undercarriage and place the probe vertically on the upper frame close to the slew ring on the boom side.
- Reverse the load: with the engine running, a second person reverses the tilting moment, for example by pressing the bucket down to lift the front of the machine slightly, then holding the boom clear in the air. Nobody stays in the danger zone during this movement.
- Read and compare: record the difference between the readings and compare it with the limit in the manufacturer's service manual and with earlier measurements.
If tilt clearance rises noticeably, or you hear grinding and find metal particles in the grease, replacement is due. Our guide on replacing an excavator slewing ring explains the process. Fitting should be carried out by a specialist workshop.
Terminology: slew ring, slewing bearing, swing bearing, Drehkranz
The same component goes by many names, which often causes confusion when searching for parts, especially across languages:
| Term | Language | Meaning |
|---|---|---|
| Slew ring, slewing ring, slewing bearing | English | General term for large rolling bearings of this kind |
| Swing bearing, swing circle | English | Common in excavator manuals and parts books |
| Turntable bearing | English | Older or more general term, same function |
| Drehkranz, Drehkranzlager | German | Standard term in the construction machinery trade |
| Drehverbindung, Kugeldrehverbindung, Schwenkkranz | German | Bearing engineering terms and less common names for the same part |
| Swing drive, Schwenkwerk, Drehwerk | English and German | Not the bearing but the drive with motor, gearbox and pinion |
Finding the right slew ring for your machine
Slew rings differ in diameter, bolt pattern, height and gearing, sometimes even within one model series. To identify the correct part, please send us:
- machine make and model
- machine serial number
- slew ring part number, if available
- a photo of the machine's ID plate
- if there is no part number: outer diameter, inner diameter, height, number of bolt holes and number of teeth
Our guide on finding part numbers shows where the serial and part numbers are located. With this information we check the match and send you a quote. Simply use the Request a part form.
Read more: Slewing rings
