Why bolt preload determines slewing ring life
A slewing ring is fixed to the upper structure and the undercarriage by two bolt circles. Every force the excavator produces passes through these bolts: axial loads from the machine's own weight, radial loads and, above all, the tilting moments generated when digging, lifting and slewing. What really counts is not the torque itself but the preload it creates in the bolt. Preload clamps the bearing ring so firmly to the mounting structure that the joint stays closed even under maximum load.
If preload is too low, the ring lifts locally under tilting moment. The bolt then sees an alternating additional load with every working cycle, and it is exactly this cyclic loading that causes fatigue fractures. Excessive preload is just as harmful: the bolt is stretched beyond its yield point, threads can strip and the risk of failure in service rises.
Typical consequences of incorrect preload:
- loose or broken bolts, often several next to each other
- fretting corrosion on the mounting faces
- distortion of the bearing ring and uneven raceway loading
- increasing tilting play between upper structure and undercarriage
- knock-on damage to the gear teeth and the slew drive pinion
Bolt grades 10.9 and 12.9: follow the manufacturer's data sheet
Slewing rings are fastened with high-strength bolts of property class 10.9 or 12.9 to ISO 898-1. Which class, diameter and length are permitted is specified by the machine or slewing ring manufacturer. Fitting a higher class than specified is not automatically better, because the torque value is matched to the intended bolt.
| Feature | Class 10.9 | Class 12.9 |
|---|---|---|
| Nominal tensile strength | 1,000 MPa | 1,200 MPa |
| Yield ratio | 0.9 (nominal yield 900 MPa) | 0.9 (nominal yield 1,080 MPa) |
| Head marking | 10.9 | 12.9 |
| Matching nut | Property class 10 | Property class 12 |
| Sensitivity to hydrogen embrittlement | present | higher, especially with electroplated coatings |
| Use | only where specified by the manufacturer | only where specified by the manufacturer |
Hardened washers belong with the bolt where the manufacturer specifies them. Spring washers, serrated washers or soft washers have no place in a slewing ring joint, because they settle and reduce preload.
Which bolts, and how many Nm?
There is no universal Nm figure. At the same torque, the preload achieved depends heavily on friction in the thread and under the head: a dry bolt, an oiled bolt and one treated with MoS2 paste reach very different clamping forces. The data sheet therefore always states the torque together with the lubrication condition and often the method as well, such as a torque wrench or hydraulic bolt tensioning. The binding value is in the machine's service manual or the slewing ring data sheet. Do not adopt generic bolt tables from the internet.
Tightening sequence: crosswise and in stages
If the bolts are tightened one after another around the circle, the ring is pulled down onto the mounting face on one side first. This distorts the bearing ring and leaves preload unevenly distributed. Bolts are therefore tightened crosswise and in several stages. The basic procedure:
- Secure: Park the machine on firm, level ground, lower the equipment, switch off the engine, depressurise the hydraulics and lock out against restart. Secure the upper structure with suitable lifting gear.
- Prepare: Mounting faces clean, dry and free of paint, burrs and grease. Check the threads in the mounting structure. Prepare the bolts in the specified lubrication condition.
- Seat: Screw in all bolts by hand until head and washer are seated.
- First stage: Tighten crosswise to a portion of the final torque: first two opposite bolts, then the two offset by 90 degrees, then the bolts in between in a star pattern.
- Further stages: Repeat the same sequence at the next higher torque and finally at full torque. The number and level of stages are given in the manual.
- Check pass: Finally go round all bolts in turn at final torque with a calibrated torque wrench.
- Inspect: Check that the slewing ring turns freely over a full revolution and check the backlash to the pinion.
The procedure applies to both bolt circles. For the full replacement process, see our guide to replacing an excavator slewing ring.
The raceway soft spot: observe the hardness marking
Slewing ring raceways are usually induction hardened. Where the hardened zone starts and finishes, a narrow area remains that is not fully hardened: the so-called soft spot or hardening gap. It is marked on the ring, often with a stamped letter such as "S" or by the position of the filler plug.
Position this area on installation as specified by the manufacturer, usually in a low-load zone, often roughly 90 degrees from the main load direction of the boom. If the soft spot sits in the main load zone, the raceway wears faster there. Some manufacturers also mark the point of maximum runout on the gear, where backlash to the pinion is set. Mark the orientation on the old slewing ring before removing it.
New bolts or reuse the old ones?
Slew ring bolts are preloaded close to their yield point. Many slewing ring and machine manufacturers therefore specify new bolts, nuts and washers whenever the ring is removed. A bolt that has already been stretched will no longer reliably reach the required preload when retightened, even if the torque wrench clicks. The manual is the deciding reference.
In any case, bolts must be replaced if:
- one or more bolts were broken or loose, in which case replace the whole bolt circle, not just the affected bolt
- threads are damaged, corroded or deformed
- the bolt is visibly elongated or the nut runs tight
- the property class marking is no longer legible
- the bolts were tightened with an unknown torque or lubricant
A broken bolt usually indicates that neighbouring bolts were overloaded too. Also establish the cause, such as insufficient preload, uneven mounting faces or the wrong bolts.
Retightening after the first operating hours
After installation, the surfaces under the head, washer and ring settle slightly. These embedding losses reduce preload. Many manufacturers therefore require a check after the first operating hours and at regular intervals thereafter. The interval and the check torque are given in the service manual.
Checks are carried out with a calibrated torque wrench in the specified sequence, with the machine switched off and secured. If a bolt turns noticeably further, it was not sufficiently preloaded. Simply retightening it is then not enough: the cause has to be found and, depending on the manufacturer's instructions, the bolts replaced. Record the date, operating hours and findings.
Slewing ring installation checklist
- machine secured, hydraulics depressurised, upper structure secured with lifting gear
- service manual or slewing ring data sheet with torque, lubrication condition and sequence to hand
- mounting faces clean, flat and checked for flatness deviation
- new bolts, nuts and washers in the specified class and length
- soft spot and gear marking aligned as specified
- torque wrench calibrated and suitable for the torque range
- tightened crosswise in stages, check pass completed
- free rotation and backlash checked, slewing ring greased
- retightening check after the first operating hours scheduled
We recommend having the installation carried out by a specialist workshop with suitable lifting equipment and tools.
Finding the right slewing ring and bolts
Slewing rings differ in diameter, hole pattern, bore size and gearing, often even within one model series. To identify the right slewing ring, please send us:
- machine make and model
- machine serial number
- part number of the slewing ring, if available
- a photo of the machine's ID plate
- photos of the old slewing ring, ideally with dimensions
Our guide to finding part numbers shows where part and serial numbers are located. Let us know in your enquiry if you also need bolts, and we will clarify what we can offer. The quickest route is our Request a part form.
Read more: Slewing rings
