Gearbox bearings: arrangement, lubrication and failures
In a gearbox the bearings position the gears and take the forces from the teeth. A helical gear pushes the shaft sideways and along its length at the same time. The bearing arrangement must fix the shaft axially, with a controlled amount of play or preload.
Typical duty
| Speed | Load | Temperature | Typical life |
|---|---|---|---|
| 100 – 3,600 rpm by shaft | High radial and axial from the gear teeth | Oil 60 – 90 °C | 20,000 – 50,000 h by duty class |
How the bearings are arranged
Parallel-shaft gearbox · tapered roller bearings
Used when: General industrial reducers with combined radial and axial loads
Each shaft has a tapered roller bearing at each end, set so they push against each other. Tapered rollers carry sideways and lengthwise loads together. Thin shims between the cover and housing set the exact amount of play.
Parallel-shaft gearbox · roller and ball pair
Used when: High-speed or high-capacity shafts with defined axial loads
One end has a cylindrical roller bearing that handles a very high sideways load but lets the shaft slide. The other end has a matched pair of angular contact ball bearings that hold the shaft in position and take the axial push.
Common failures and how to prevent them
| Failure | What you see | Why it happens | Prevention |
|---|---|---|---|
| Micropitting and surface distress Surface-initiated fatigue (surface distress) | Dull, frosted bands on raceways and rollers, slow progression to spalling. | Oil film too thin from low viscosity, high temperature or contaminated oil. | Keep oil clean and at the right viscosity, and control oil temperature. |
| Debris dents Indentations from particles | Small dents scattered across the raceway, rough running. | Particles from gear wear, assembly dirt or ingress in the oil. | Flush after assembly, fit and maintain filters and breathers, and use a magnetic plug. |
| Too tight or too much oil Overheating & lubricant breakdown | High temperature, discoloured rings, burnt oil. | Tapered bearings set with too little play or too much preload, or oil level too high. | Follow the setting procedure, check temperature rise after assembly, and keep oil at the right level. |
| Smearing at start-up Adhesive wear (smearing & skidding) | Smeared bands on rollers and raceway. | Lightly loaded roller bearings or sudden acceleration with a thin oil film. | Keep a minimum load on roller bearings, use proper oil viscosity for cold starts. |
| Standstill damage False brinelling | Evenly spaced marks on stationary gearboxes. | Vibration from other machines while the gearbox is stopped. | Turn standby gearboxes occasionally and isolate them from vibration. |
| Loose fit Fretting corrosion | Brown-red powder on the shaft or housing seat. | Worn housing bore or an undersized shaft seat. | Check fits and replace worn parts instead of relying on bearing retention. |
| Housing flex Misalignment & distorted seats (edge loading) | Off-centre wear marks and gear contact pattern shifted to one side. | Casing deflecting under load or misaligned bores. | Check bore alignment and the foundation. Re-check the gear contact pattern after a bearing change. |
Lubrication
Oil type
A mineral or synthetic gear oil, often ISO VG 150 to 320, sometimes with EP additives. Follow the gearbox maker’s viscosity grade.
Method
Splash for slow gears. Pressure circulation with an oil cooler and filter for big or fast boxes.
Level and change
Keep the level at the specified mark. Oil change intervals depend on temperature, from a few thousand hours up to many with synthetics. Oil analysis is the best guide.
Sealing
- Radial lip seals: Rubber or fluoro-rubber lip on a ground shaft sleeve. Replace with the shaft sleeve if the shaft has a groove.
- Labyrinth and PTFE lip: For higher shaft speeds and hotter oil. Less friction, higher cost.
- Breather: A desiccant or filtered breather keeps dust and water out as the box warms and cools.
- Magnetic plug and filter: Collect steel debris. A plug that keeps coming out loaded with debris is an early warning.
What to watch
- Oil temperature, and temperature difference between the shaft ends.
- Particle count and wear metals from oil analysis.
- Vibration at tooth-mesh frequency and bearing defect frequencies.
- Magnetic plug debris at every service.
Questions
Which bearings are used in industrial gearboxes?
Parallel-shaft gearbox · tapered roller bearings: General industrial reducers with combined radial and axial loads. Parallel-shaft gearbox · roller and ball pair: High-speed or high-capacity shafts with defined axial loads.
What are the common bearing failures in industrial gearboxes?
Micropitting and surface distress: Oil film too thin from low viscosity, high temperature or contaminated oil. Debris dents: Particles from gear wear, assembly dirt or ingress in the oil. Too tight or too much oil: Tapered bearings set with too little play or too much preload, or oil level too high.
How should industrial gearboxes bearings be lubricated?
Oil type: A mineral or synthetic gear oil, often ISO VG 150 to 320, sometimes with EP additives. Follow the gearbox maker’s viscosity grade. Method: Splash for slow gears. Pressure circulation with an oil cooler and filter for big or fast boxes.