The swing bearing is held to the upper structure and to the undercarriage by rings of high-strength bolts, and those bolts carry the whole machine's swing loads. They relax over time through a combination of embedment of the joint faces, vibration and thermal cycling, and a bolt that has lost preload is not doing its job even though it is still there. The consequences are severe: a loose ring lets the bearing move, which loads individual bolts in bending, cracks the bolt or the mounting face, and turns a scheduled torque check into a structural repair. The check is straightforward and takes a couple of hours: clean the threads and faces, confirm the specification, torque in the specified sequence in stages, and record the values. Anything that turns more than a small amount under the wrench has lost preload and should be replaced rather than simply retightened.
What you will notice
| Symptom | What it usually points to |
|---|---|
| Swing feels loose or has a knock at the start of movement | Backlash at the start of a swing can be the bearing itself or a mounting that has lost preload. Measure both before condemning either. |
| Paint cracked or fretting marks around the bolt holes | Rust streaks and fretting dust around a bolt circle are the clearest visual evidence that the joint has been moving. |
| Bolts found loose at an unrelated service | A bolt found loose is not an isolated event. The whole circle is checked and re-torqued, and the bolts that were loose are replaced. |
| Bearing grease contaminated with metal particles | Movement at the mounting face produces particles that reach the bearing. The sample is a cheap indicator of a structural problem. |
Most likely causes
| Cause | How to confirm it |
|---|---|
| Embedment of the joint faces | Paint, scale and surface irregularities compress under load over time, which releases bolt preload even though nothing has stretched. |
| Vibration and thermal cycling | Swing duty is repetitive and the joint cycles in temperature. Both effects work the preload off over hundreds of hours. |
| Bolts that have already yielded | A bolt torqued past yield, or cycled after losing preload, will not hold preload again. It is replaced, not retightened. |
| Improper re-torque practice | Tightening in a circle rather than the specified sequence, or in one stage rather than several, leaves some bolts carrying more than their share. |
What a re-torque check involves
| Step | Detail that decides the outcome |
|---|---|
| Confirm the specification | Torque value, sequence and lubrication from the model documentation. A dry torque applied to a lubricated bolt over-loads it by a large margin. |
| Clean the threads and the face | Wire-brush the threads and remove scale or paint from the joint face. Contamination under the head and in the thread accounts for most scatter in the readings. |
| Mark the bolts before starting | A paint mark on each bolt head and the adjacent structure shows at a glance which ones moved, and by how much. |
| Torque in stages, in sequence | Use the specified pattern and bring the ring up in two or three stages rather than one pass, so the preload is distributed around the circle. |
| Replace what moved | Any bolt that turns more than a small amount is replaced, and the same sequence is repeated. A bolt that has lost preload has usually stretched. |
| Record the result | Note the date, hours, values and replacements. The next check is only meaningful against a record of this one. |
How to confirm it
- Park the machine level with the swing locked and the attachment grounded
Relieve the hydraulic pressure and lock the swing before anyone goes near the bolt ring. Working under a suspended structure is the one part of this job where the risk is not to the machine.
- Clean the ring and mark every bolt
Degrease, wire-brush the threads and mark each bolt head and its adjacent structure with a paint line. Marking first is what converts a torque check into evidence about which bolts have moved.
- Check torque in the specified sequence, in stages
Use a calibrated wrench and the specified lubricant condition. Bring the circle up in two or three passes. Note every bolt that turns beyond the mark, since those are the bolts that had lost preload.
- Replace rather than re-torque the bolts that moved
Replace any bolt that turned significantly, and re-check it at the same value. A bolt that lost preload has yielded and will lose it again, and the cost of the bolt is trivial next to the cost of the bearing.
- Check bearing clearance and grease at the same time
With the area accessible, measure swing bearing clearance and inspect the grease. The two checks answer different questions and are far easier to do together than separately.
- Record and set the next interval
Write down the date, machine hours, torque values, replacements and the measured clearance. The interval for the next check should follow the manufacturer's schedule and be shortened where the bolts moved.
What to do about it
Re-torque to specification and replace the bolts that had lost preload. Where several bolts in a ring have moved, treat the joint as having been working loose and re-check it at a shorter interval for the next few hundred hours.
Where cracks or fretting are found at the mounting face, stop and assess before retightening. A cracked upper structure mounting is a fabrication repair, not a torque check, and continuing to work the machine makes it worse.
Keep the bearing greased on schedule, since a mounting that has been moving sheds particles into the bearing and the grease is the only way to remove them. Guangzhou Shenyue Machinery Technology Co., Ltd. supplies swing bearings, swing gearboxes, mounting bolts and grease hardware from Guangzhou, holds 3,000+ part types on the shelf with 100,000+ part numbers available to order, and declares the grade of every line. The five brands for which the company holds authorized dealer status are Garrett, Mitsubishi Heavy Industries, TONGMYUNG, NIKURA and Marusan, and the Mitsubishi authorization covers MHI engine parts only. Kobelco is not among them, and Kobelco parts are supplied through original channels or as quality alternatives.
Add the check to the routine rather than treating it as a one-off. A bolt ring that is torque-checked on a schedule is a bearing that reaches its design life; one that is not is a bearing that fails early with an expensive repair attached.
Parts you may need, by grade
| Part | Grades commonly quoted |
|---|---|
| Swing bearings and slewing rings | Genuine; OEM-quality; remanufactured exchange where the specification allows. |
| High-strength mounting bolts | Genuine; OEM-quality; matched aftermarket to the same grade and coating. |
| Swing gearboxes and pinions | Genuine; OEM-quality; remanufactured exchange with core return. |
| Swing circle grease and grease hardware | Genuine; OEM-quality; specified grade for the application. |
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
How often should swing bearing bolts be re-torqued?
Follow the manufacturer's schedule for the model, and shorten it where a check has found bolts that moved. The first check after a bearing replacement should come earlier than the routine interval, because a new joint settles as the faces embed and the bolts lose preload in the first hours of work.
Why can a bolt that looks tight still be doing nothing?
Because torque and preload are not the same thing. A bolt can feel tight and still have lost the clamping force that holds the joint, particularly after the faces have embedded or after the bolt has yielded. That is why bolts are marked before a check: the ones that turn beyond their mark are the ones that had lost preload and are replaced.
What happens if the bolts are left alone?
The bearing moves relative to its mounting, individual bolts are loaded in bending instead of tension, and cracking starts at the bolt holes or in the structure around them. The repair then involves the bearing, the mounting faces and often fabrication, which is a great deal more than a torque check and a set of bolts.