The bad news is that the cause of gear failure can range from excessive wear to catastrophic breakage; the good news is that gear failure can be prevented.
In order for gear failure to be prevented, however, one needs to know the type of failure, and compensate accordingly.
The first cause of gear failure is wear. Metal-to-metal contact during gear operation causes the layers of metal to be worn away from the contacting surfaces of gear teeth until a fine, smooth surface develops.
This can be prevented by using a thicker lubricant, by reducing the transmitted load or, in some cases, by increasing the operating speed to produce a better elasto-hydrodynamic oil film.
If normal wear goes undetected for a long time, it can progress to the point where a considerable quantity of the material has been removed uniformly from the surface of the gear, causing the gears to run roughly, which only causes greater wear.
Eventually the surfaces of the gear become so worn out that they are unusable.
When there is foreign material in the lubrication system, abrasive wear can occur.
It produces scratches, marks or grooves on contacting surfaces, but can be remedied, however, by use of a filter, or a finer-grade filter where one is already being used.
The particles may be from the gear and bearing system itself, or rust and sand.
This type of wear is often discovered soon after starting up a new installation, before the filter has had a chance to clean the system.
It can be prevented by ensuring that the gearbox and lubricating system are cleaned carefully before use.
A second cause of gear failure is pitting; a surface-fatigue failure which occurs when the endurance limit of the gear material is exceeded.
It is initially recognised by small pits occurring in localised areas of the gears, and it is caused by gear-tooth surfaces not fitting together properly.
This type of pitting can be avoided by ensuring smooth gear-tooth surfaces and gear-tooth contacts that distribute the load evenly from the start of operation.
Unchecked, pitting can progress until a considerable portion of all the tooth surfaces have developed pitting craters of various shapes and sizes.
As the stress builds up, pitting continues until the tooth profile is completely destroyed, causing extremely rough operation and considerable noise.
This otherwise bleak picture can be avoided, however, by keeping the load on the surface below the endurance limit for the material, or by increasing the hardness of the material, to lift the endurance limit of the gear to the point where pitting will not take place.
A third cause of gear failure is scoring. When the oil film fails due to mesh overheating, metal-to-metal contact occurs, producing alternate welding and tearing which rapidly removes metal from the tooth surfaces.
Once the lubricant has been broken down by the heat, the welding and tearing destroys the gear profile in minutes.
Initial scoring can be recognised by the gear taking on a ‘frosted’ appearance, and can be treated before the gear is damaged irreparably by reducing the quantity of heat in the mesh by cutting down on the load being transmitted, or by reducing the inlet oil temperature.
In some cases, a solid lubricant placed on the contact surface can halt the scoring.