Bearing failures and troubleshooting
All bearings will eventually fail but the length of time it takes is dependent on many factors. The theoretical (L10) life of a bearing is based on:
- Load (actual and load rating)
- Life adjustment factors (material, load, temperature)
Higher speeds, higher loads and difficult working conditions (such as high temperatures and contamination) will all reduce the life of a bearing. Poor maintenance practices such as installation errors and improper lubrication will further reduce its life. Studies have shown that inadequate lubrication, contamination, and installation error account for a large majority (over 70%) of bearing wear and failures
Proper lubrication has been discussed extensively throughout this document. Contamination control is discussed in more detail in the Hydraulics section. Proper installation practices must be used to ensure the bearing is properly aligned, located and loaded to prevent failures. Maintenance personnel must be trained and have the proper equipment for installing bearings. Training and equipment are generally available from the major bearing manufacturers.
Bearing failure modes can be assessed to help determine the root cause in order to prevent repeat breakdowns. Here are the most common failure modes and their solutions:
|Fatigue||Sub-surface crack creation and propagation due to repeated contact stress. Pitting and spalling are the final results.||
|Corrosion||Chemical attack or rust. Bearing at rest under corrosive conditions will show rust (red), staining (black) or etching.||
|Brinelling||Deformation of metal surface due to impact of rollers or balls. Creates roller impact marks on the surface.||
|False Brinelling||Wear due to small vibratory movements between two surfaces. Also called “fretting corrosion”. Often occurs during storage.||
|Contamination||Solid debris leading to abrasive wear. Irregular dents, or material embedded in raceways.||
|Lubrication failure||Fault due to poor lubrication. Accelerated wear rates and high heat (colour change of the metal) are typical.||
|Thrust loading||Excessive thrust loading for the bearing design. Results in accelerated wear on one side of bearing.||
|Misalignment/ Improper fit||Installation error and/or misalignment. Wider wear path or wear path running from one side of race to the other.||
|Electric arcing||Electricity flowing through the bearing. Individual arcs melt the bearing surface and cause fluting.||
Gear failure and troubleshooting
The most common gear wear occurs when pits and/or fatigue cracks initiate either at the surface of the gear tooth or at depth below the surface. The cracks tend to propagate towards each other as shown in the image below.
Pitting is the formation of shallow craters from surface-defect fatigue, and spalling is the formation of deeper cavities by sub-surface-defect fatigue. The image below shows a large single spall over 250µm across. Initial spall particles are typically 30µm to 50µm, but it is common for several particles to be generated from individual spall sites. Note at the sharper crater wall (near the top edge of the spall in this micrograph) there are several cracks associated with the spall.
Although both spalling and pitting are the common forms of surface contact fatigue, spalling results in more rapid deterioration of surface durability compared to pitting. Spalling often induces early failure because it causes severe secondary damage (which is a more destructive surface failure mode for gear contacts). Such secondary damage can result in tooth breakage, initiated from a severe spall on the contact surface, as well as friction or heat-induced surface seizure or complete spalling over all of the contact surfaces.
Gear wear and failure modes
The most common types of gear failure are discussed below:
Adhesive wear (Scuffing)
This is usually caused by insufficient oil film or lack of EP, resulting in metal-to-metal contact. The heavily localised stresses cause smaller metal particles to break away.
This occurs under extreme loads when the yield strength of the material is exceeded. The gear material flows plastically and gets deformed.
This occurs because of high contact stresses. Fatigue is a progressive failure consisting of three distinct stages:
Stage 1: crack initiation;
Stage 2: crack propagation;
Stage 3: fracture.
This occurs when gear material chemically reacts with its environment.
This is when an electric current passes through a gear and damages the gear.
The main causes of gear failures can be summarised as:
- Metal-to-metal contact due to insufficient lubricant film
- Insufficient lubrication due to low quantity of oil, wrong/poor lubricant choice, or lube system delivery failure
- Ingress of contaminants (dirt, water)
- Repeated loading, unloading and/or shock-loading
- High loads or load exceeding the strength of the material
- Misalignment or incorrect dimensions of gears
- Improper manufacture and/or handling
Troubleshooting and corrective actions
|PROBLEM||POTENTIAL CAUSE||CORRECTIVE ACTION|
|OVERHEATING||Wrong oil viscosity or oil level||Check the viscosity or adjust the oil level|
|OVERHEATING||Wrong oil selection (non-EP oil)||Use EP oil or compounded oil|
|OVERHEATING||Mechanical problems||Inspect the gearbox. Fix the problem.|
|FOAMING||Wrong oil viscosity or oil level||Check the viscosity or adjust the oil level|
|FOAMING||Oil contamination||Filter the oil. Improve the breather.|
|FOAMING||Oil additive depletion||Change the oil|
|FOAMING||Mechanical problems||Inspect the gearbox. Fix the problem.|
|NOISE||Wrong oil viscosity or oil level||Check the viscosity or adjust the oil level|
|NOISE||Mechanical problems||Inspect the gearbox. Fix the problem.|
|GEAR WEAR||Wrong oil viscosity or oil level||Check the viscosity or adjust the oil level|
|GEAR WEAR||Oil contamination||Filter the oil. Improve the breather.|
|GEAR WEAR||Oil additive depletion||Change the oil|
|GEAR WEAR||Mechanical problems||Inspect the gearbox. Fix the problem.|
|GEAR WEAR||Restricted oil circulation||Check the oil circulation. Clean the plugs.|
|GEAR WEAR||Plugged filters or orifices||Change the filters or clean orifices|
|BEARING WEAR||Wrong oil viscosity or oil level||Check the viscosity or adjust the oil level|
|BEARING WEAR||Oil contamination||Filter the oil. Improve the breather.|
|BEARING WEAR||Mechanical problems||Inspect the gearbox. Fix the problem.|
|BEARING WEAR||Restricted oil circulation||Check the oil circulation. Clean the plugs.|
|BEARING WEAR||Failure to manually lubricate||Lubricate the bearings manually|
Practical recommendations and rules of thumb
- Cold start-up: The lowest useful temperature of a fluid lubricant is 15°F to 20°F above its pour point at system start-up
- Synthetic versus mineral oil: Synthetics are typically used for high temperatures (above 170°F), low temperatures (below 0°F), or large variances in temperature (> 50°F). If a mineral oil can do the job, it’s usually your best choice.
- Viscosity: If gear wear is experienced, a high-performance additive system may help. Insufficient viscosity or overloading may also be part of the cause. Increasing the viscosity by one ISO or AGMA viscosity grade above OEM specification can help reduce wear and will usually be acceptable to the gearbox.
- Additives: Unless the gearbox manufacturer specifies an R&O gear oil, an EP gear oil is almost always recommended. Note that EP additives may be aggressive to gears with yellow metal, such as the bronze wheel in many worm-gear drives.
TROUBLESHOOTING CHAIN DRIVES
Chain drives provide a positive and reliable means of operating equipment. However, like most equipment, chains must be matched to the job and properly installed. They also require a certain amount of maintenance.
If you are having chain-drive problems, the following chart may help:
|EXCESSIVE WEAR||Misalignment of drive.||Align sprockets with straight edge or taut string along sides of sprocket faces. Replace or reverse worn sprockets.|
|EXCESSIVE WEAR||Shafts not parallel.||Adjust shafts so that they are parallel; carefully align with a sprint level.|
|EXCESSIVE WEAR||Sprocket wear may be due to excessive axial float.||Adjust axial play with set screws or collar.|
|EXCESSIVE WEAR||Improper lubricant.||Make sure proper lubricant is applied correctly. Guard against over-lubrication.|
|EXCESSIVE WEAR||Badly worn section of chain.||Replace badly worn section at once. Contact Castrol to check correct lubrication is being performed.|
|CLIMBING SPROCKETS||Oil additive depletion||Replace sprockets.|
|CLIMBING SPROCKETS||Badly worn sprocket teeth. Sprocket wrong for chain||Replace worn sections. Contact Castrol to verify that the correct lubrication is being performed.|
|CLIMBING SPROCKETS||Chain too loose.||Use take-up to adjust sprocket centre-to-centre distance.|
|NOISY OPERATION||Bad alignment of shafts and sprockets.||Correct alignment and review other issues.|
|NOISY OPERATION||Badly worn chain or sprockets.||Replace worn sprockets or section of chain.|
|NOISY OPERATION||Wrong size chain or sprockets.||Install chain and sprockets of proper size and pitch.|
|NOISY OPERATION||Improper lubrication or no lubricant.||Make sure proper lubricant is applied. Contact Castrol to check that the correct lubrication is being performed.|
|NOISY OPERATION||Chain too slack.||Adjust take-up or centre distance. Keep tension tight with a small amount of slack. On long drives, adjust shoes, idler sprockets or rollers.|
|NOISY OPERATION||Casing rattling. Bearing wear.||Tighten loose fasteners. See that no loose material rubs against the casing.
Check for proper lubrication. Replace worn bearings. Contact Castrol to check correct lubrication is being performed.
|BROKEN SPROCKET TEETH||Insufficient chain. Obstructions.||Adjust tension of chain and provide adequate clearance.
Check housing and remove anything that might come between the chain and the sprockets. See that no foreign matter can fall on the chain.
|BROKEN SPROCKET TEETH||Excessively worn chain or sprockets.||Replace worn sprockets and worn sections of chain.|
|CHAIN STIFF OR STICKING TO SPROCKET||Poor alignment of shafts and sprockets.||Correct the alignment and review other issues.|
|CHAIN STIFF OR STICKING TO SPROCKET||Badly worn sprockets or chain.||Reverse the sprockets. If badly worn, replace them. Replace worn section of chain.|
|CHAIN STIFF OR STICKING TO SPROCKET||Improper lubrication.||Make sure the proper lubricant is applied correctly. Contact Castrol to check that the correct lubrication is being performed.|
|CHAIN STIFF OR STICKING TO SPROCKET||Rusted or corroded chain joints.||Thoroughly clean chain and sprockets with solvent and brush. Properly apply lubricant.|
|CHAIN BREAKS||Wrong pitch or size of chain.||Make sure chain is the correct size for the load. Match chain and sprockets for proper pitch.|
|CHAIN BREAKS||Excessive loads applied suddenly. Shock loads.||Correct load condition or install coupling to soften the shock.|
|CHAIN BREAKS||Badly worn chain or sprockets.||Replace worn parts.|
|CHAIN BREAKS||Misalignment of sprockets.||Correct alignment as described above.|
|CHAIN BREAKS||Corrosive atmosphere or excessive moisture.||Correct conditions or properly protect the drive.|