Reviewed by: Technical Engineering Department – TGB Group Published: September 2026 Technical review: September 2026
A slewing system often shows mechanical warning signs before it fails. Before a slewing ring or slewing drive reaches the point of structural failure, it almost always passes through a period in which wear is already present but the system is still operating. During that period, the machine produces a series of mechanical signals: changes in how it turns, how it sounds and how much it moves. Reading those signals correctly is the difference between a planned intervention and an unplanned breakdown.
This matters because the cost of the two outcomes is not comparable. Acting on early wear usually means a scheduled stop, an inspection and a maintenance action. Ignoring it until failure means unplanned downtime, possible damage to surrounding components, and in many cases a safety hazard. In many cases, a bearing that fails catastrophically has been signalling its condition for some time beforehand.
This article describes the mechanical signals that indicate wear in a slewing system, what each one tells you about the underlying cause, and how to confirm wear before it becomes failure. It deliberately focuses on signals that can be observed and measured mechanically, without relying on instrumentation that may not be available on every machine.
Why slewing systems give warning before they fail
Two characteristics of slewing systems make early detection realistic.
First, they operate slowly. Most slewing rings and slewing drives rotate at low speed, often only a few revolutions per minute or less. Wear therefore develops gradually rather than instantaneously, and the symptoms accumulate over a period that allows time to react.
Second, the symptoms are mechanical and observable. A worn slewing system changes the way it behaves in ways a technician can feel, hear and measure: the rotation becomes harder, develops play, makes new noises or moves unevenly. Unlike a high-speed component that can fail between inspections, a slewing system tends to announce its condition through the machine itself.
The implication is straightforward. The information needed to act before failure is usually present. What determines the outcome is whether the signals are recognised and interpreted correctly.
The main mechanical signals of wear
Wear in a slewing system shows itself through a recognisable set of signals. Each one is meaningful on its own, and together they build a clear picture of the bearing’s condition.
Increased turning torque
One of the most reliable early indicators is a gradual increase in the torque required to rotate the system. As the raceway wears, as lubrication degrades or as contamination enters, friction inside the bearing rises. The motor or drive has to work harder to produce the same movement, and the rotation feels heavier or more resistant than when the system was new.
A gradual, progressive increase in turning torque is one of the earliest signs that something inside the bearing is changing. It points most often to inadequate lubrication, contamination or the early stages of raceway damage.
Developing play and increased clearance
A slewing ring is manufactured with a defined internal clearance. As the raceway and rolling elements wear, that clearance increases. The result is play: the ability of the structure supported by the bearing to move slightly relative to its base before the load is taken up.
In boom or platform machinery, this play is magnified over the length of the boom, where it appears as visible rocking under load. A small increase in internal clearance at the bearing translates into a much larger movement at the end of the arm. Developing play is a clear indicator that the raceway has worn, and its progression should be measured against the manufacturer’s wear limit for that bearing.
Abnormal noise
A healthy slewing system runs relatively quietly. New noises are a signal. Squeaking, clicking, grinding or rhythmic popping during rotation are among the first audible warnings of wear. The type of noise carries diagnostic information:
- Grinding or rough noise often indicates inadequate lubrication or contamination in the raceway.
- A rhythmic knock or popping that repeats at regular intervals during rotation often indicates localised raceway deformation, where the rolling elements pass over a damaged or indented section of the track.
- Clicking or irregular noise can indicate loose bolts, debris in the raceway or damaged rolling elements.
Uneven or hesitant movement
If the rotation hesitates, binds or moves unevenly rather than smoothly, the rolling elements or the raceway may be worn. A particularly informative variant is resistance that increases at specific points in the rotation arc. When the system turns freely through most of its travel but meets resistance at the same point each cycle, that localised resistance often points to a specific defect such as a crack or a deformed section of raceway, rather than general wear.
Vibration
Vibration during rotation, especially when it appears or increases over time, is a signal that the running surfaces are no longer smooth. As rolling elements pass over indentations, brinelling marks or areas of surface damage, they generate vibration that was not present when the bearing was new. Vibration frequently accompanies the rhythmic noise described above and points to the same kind of localised surface damage.
Condition of the grease
The lubricant carries direct evidence of internal condition. Two observations are particularly useful. An increasing quantity of metallic debris in the grease indicates that material is being lost from the raceway or rolling elements, a direct sign of internal wear. Grease leakage, or visible degradation of the grease, points to seal damage and the loss of the protection that keeps contaminants out and lubricant in. A failed seal accelerates every other wear mechanism, because it removes the barrier that was protecting the bearing.
What the signals tell you about the underlying cause
The value of these signals is not only that they indicate wear, but that, read together, they point towards the mechanism causing it. Identifying the mechanism is what allows the right corrective action rather than simply replacing a component and reproducing the same failure.
| Observed signal | Likely underlying cause | Recommended action |
|---|---|---|
| Gradual increase in turning torque, no play yet | Inadequate lubrication or early contamination | Review lubrication and sealing; inspect grease |
| Rhythmic knock or vibration as elements pass a point | Localised raceway marks (brinelling or false brinelling) | Inspect raceway; review static load, shock loading and vibration when idle |
| Increasing clearance and play, metallic debris in grease | Abrasive wear of raceway and rolling elements | Measure clearance against wear limit; check seals and lubricant |
| Localised resistance at the same point each cycle | Crack or deformed raceway section | Stop and inspect; do not continue under load |
| Grease leakage, contaminant ingress | Seal degradation or damage | Replace seals; assess contamination already present |
| Increased friction with loose or fatigued bolts | Loss of bolt preload, raceway deformation | Check bolt torque and condition; re-tighten to specification |
| Gear noise, irregular backlash, tooth damage | Incorrect backlash, shock loading or gear wear | Inspect gear mesh; review drive conditions and backlash |
The common pattern is important: many of these mechanisms originate not in a defective bearing but in the conditions around it. Contamination points to sealing, increasing clearance with loose bolts points to the mounting interface, and brinelling points to how the system is loaded and parked. Recognising wear is the first step; tracing it to its cause is what prevents it from returning.
What technicians usually notice first
In many maintenance inspections, the first reported symptom is not visible clearance, but a change in behaviour: the drive draws more effort, the rotation becomes uneven, or a new repetitive noise appears at a specific angular position. These early symptoms are valuable precisely because they appear before the bearing reaches its wear limit, while there is still time to plan an intervention rather than react to a failure. Visible play, by contrast, tends to be a later-stage symptom, which is why waiting for it before investigating means losing the early window.
How wear is measured
Beyond observing the signals, wear in a slewing ring is confirmed by measuring the increase in internal clearance, since that increase is the direct mechanical consequence of raceway and rolling-element wear.
The principle is to capture the internal geometric change caused by wear. In typical practice, a dial indicator is fixed to the non-rotating structure with its probe on a clean surface of the rotating ring. A zero reading is established under load in one direction, and then a reverse moment load is applied — often using the machine’s own boom or hydraulics — to tilt the bearing in the opposite direction. The total deflection is recorded at several equidistant points around the bearing, commonly at 0°, 90°, 180° and 270°.
The measured clearance is then compared against the wear allowance specified by the bearing manufacturer for that model and diameter. When the increase exceeds the permissible value, the bearing has reached its service limit and should be replaced before it fails structurally. The permissible wear value is specific to each bearing and must be taken from the manufacturer’s documentation, not assumed from a general figure.
Why acting early matters
The reason to monitor these signals is the gap between the cost of early action and the cost of failure.
Detected early, wear is a maintenance event: a planned stop, an inspection, a lubrication or sealing correction, or a scheduled bearing replacement carried out under controlled conditions. The equipment is taken out of service deliberately, the work is planned, and surrounding components are protected.
Left until failure, the same wear becomes a breakdown: unplanned downtime, potential damage to the structure and drive, emergency procurement and installation, and, in rotating equipment, a genuine safety risk. In many machines, the rotating superstructure presents a recognised hazard, which makes slewing system condition a safety matter and not only a mechanical one.
The economic and operational case for early detection is therefore not subtle. The same component, addressed at the right time, costs a fraction of what it costs when addressed too late.
Common mistakes in monitoring wear
A number of recurring mistakes prevent teams from acting on the signals the system provides:
- Treating increased turning torque as a drive problem rather than a possible bearing signal.
- Waiting for visible play before investigating, when torque and noise changes appear earlier.
- Measuring clearance once and not tracking its progression over time.
- Ignoring grease condition, which carries direct evidence of internal wear.
- Attributing new noises to “normal ageing” without identifying their character.
- Replacing a worn bearing without identifying the mechanism, so the same failure returns.
Each of these turns an avoidable, planned intervention into an eventual unplanned one.
Where this matters most
Wear monitoring is relevant in every slewing application, but its importance increases where the consequences of failure are highest: in lifting and platform equipment where play is mechanically amplified and where safety is directly involved, in cranes and material-handling machinery operating under heavy and variable loads, in solar trackers and positioning systems where developing clearance degrades accuracy, and in continuously operating industrial machinery where unplanned downtime carries a high cost. In all of these, the ability to recognise wear early is a direct contributor to both safety and operational continuity.
How TGB Group supports slewing system reliability
With more than 30 years of experience in motion solutions, TGB Group’s in-house engineering department supports customers not only in selecting slewing rings and slewing drives, but in understanding how they behave over their service life. This includes defining the wear limits and maintenance criteria for each product and advising on how to interpret the signals a slewing system produces as it operates.
Because the permissible wear, clearance limits and maintenance requirements depend on the specific model, TGB provides these parameters in the technical documentation for the selected product and supports engineering and maintenance teams in confirming whether a slewing system has reached the point at which action is required.
If you need to assess the condition of a slewing ring or slewing drive, or to define the wear limits and maintenance criteria for your application, TGB Group’s technical team can help. Contact our technical team to review the symptoms, confirm the applicable wear limits and decide whether maintenance, repair or replacement is required.
Frequently asked questions about slewing bearing wear
What are the first signs that a slewing bearing is wearing out?
The earliest signs are usually a gradual increase in the torque needed to rotate the system and new noises during rotation, such as grinding, clicking or rhythmic knocking. Developing play and increased clearance tend to appear later. Changes in the grease, including metallic debris or leakage, also indicate internal wear. Together, these signals show that the bearing’s condition is changing before it reaches failure.
What does a worn slewing bearing sound like?
A worn slewing bearing commonly produces squeaking, clicking, grinding or rhythmic popping during rotation. A grinding or rough noise often indicates inadequate lubrication or contamination. A rhythmic knock that repeats at regular intervals usually indicates localised raceway deformation, where rolling elements pass over a damaged section of the track. Irregular clicking can indicate loose bolts or debris in the raceway.
Is increased turning torque a sign of slewing bearing wear?
Yes. A gradual increase in the torque required to rotate the system is one of the earliest and most reliable indicators of wear. As the raceway wears, lubrication degrades or contamination enters, internal friction rises and the system becomes harder to turn. Because it appears early, increased turning torque is a valuable warning signal, although it should be confirmed by inspecting lubrication and clearance.
How do you measure wear in a slewing ring?
Wear is confirmed by measuring the increase in internal clearance. A dial indicator is fixed to the non-rotating structure with its probe on the rotating ring. A zero reading is taken under load in one direction, then a reverse moment load is applied to tilt the bearing, often using the machine’s own boom or hydraulics. The deflection is recorded at several points around the bearing, typically at 0°, 90°, 180° and 270°, and compared against the manufacturer’s specified wear allowance for that model.
What causes a slewing ring to wear out prematurely?
Premature wear is most often caused by inadequate or contaminated lubrication, seal failure allowing contaminant ingress, loss of bolt preload, mounting surface deviations, or operating loads and conditions outside the design envelope. Brinelling, a localised raceway deformation, can be caused by static overload or by parking under heavy load for extended periods. In most cases the cause lies in the conditions around the bearing rather than in the component itself.
How much clearance is too much in a slewing bearing?
There is no single universal value. Each bearing has a wear allowance specified by the manufacturer, which depends on the model and diameter. The internal clearance is measured and compared against that allowance. When the increase in clearance exceeds the permissible value, the bearing has reached its service limit and should be replaced. The specific limit must always be taken from the manufacturer’s documentation for that bearing.
Can a worn slewing bearing be repaired or must it be replaced?
It depends on the extent of the damage and the size and value of the bearing. For large, expensive units, repair by a specialised workshop, which can involve re-grinding raceways, replacing rolling elements and re-gearing, may be cost-effective. For smaller or severely damaged units, replacement is often the more reliable choice. The decision is a cost-benefit assessment based on the damage, the cost of repair versus replacement and the criticality of the application.
Why is it important to detect slewing bearing wear early?
Because the cost of early action is far lower than the cost of failure. Detected early, wear is a planned maintenance event carried out under controlled conditions. Left until failure, it becomes unplanned downtime, possible damage to surrounding components and, in rotating equipment, a safety hazard. Since slewing systems operate slowly and signal their condition mechanically, early detection is usually achievable when the signals are monitored.
Can increased backlash indicate slewing drive wear?
Yes. Increased backlash in a slewing drive can indicate gear wear, incorrect gear mesh, loss of preload or internal wear in the slewing ring. It should be checked together with turning torque, noise, lubrication condition and clearance measurement, so that the source can be identified as coming from the drive, the bearing or the mounting interface rather than treated in isolation.
What is the difference between brinelling and false brinelling in a slewing bearing?
Both produce localised marks on the raceway, but their cause differs. True brinelling is permanent indentation caused by static overload or shock loading that exceeds the material’s limit, for example a heavy impact or parking under excessive load. False brinelling results from micro-movement or vibration while the bearing is not rotating, which wears small marks at the contact points, and is common in equipment subjected to vibration when idle or during transport. Identifying which mechanism is involved guides the corrective action, whether it concerns load and impact conditions or vibration and immobilisation.
Does grease condition indicate slewing bearing wear?
Yes. The grease carries direct evidence of internal condition. An increasing quantity of metallic debris in the grease indicates that material is being lost from the raceway or rolling elements. Grease leakage or visible degradation indicates seal damage, which removes the barrier protecting the bearing and accelerates other wear mechanisms. Inspecting the grease is a simple and informative part of monitoring slewing system condition.
