TechArchive

Original article date: March 2000

What exactly is meant by the terms ‘maintenance-free’ and ’sealed-for-life’ when when they are applied to bearings? Derek Hoult of Ina Bearings explains

Maintenance-free bearings are very good for use in areas that are difficult or hazardous to get at for re-greasing bearings, and they also have the added advantage that the equipment user does not have to be concerned about maintenance programs for the equipment.

When bearings are selected for a maintenance-free application it is important that all data regarding the duty cycle of the equipment is considered. In addition to the loads and speeds, the environment – such as temperature and cleanliness of the surroundings – is also important.

‘Maintenance-free’ bearings are only maintenance-free in as much as they do not have facilities for them to be re-greased. Sometimes they are called sealed-for-life bearings, but this term is slightly misleading and can lead to confusion.

Bearings that are sealed need the correct lubricant to meet the conditions in which the bearings are expected to perform. Because it is the life of the grease that generally determines the life of the bearing, once the grease has lost its lubrication properties the bearing will become damaged and will fail.

The sealed-for-life bearing is only as good as the lubricant that is used. It is also important to understand that grease does not last forever, so neither will the bearing – as is sometimes assumed when specifying sealed-for-life bearings. Users must realise this and specify what ‘life’ they expect from the equipment so that the most appropriate bearing can be selected.

Different bearing types

The most common sealed-for-life bearings are deep groove ball bearings and linear guidance systems but these are not the only ones that are classed as maintenance-free. Rod-end bearings used on hydraulic rams, for example, can also be produced with a low-friction PTFE-type lining material that needs no lubrication. Indeed, it is very important with these that they are not lubricated as this severely reduces the life of the bearing. This is because, to operate effectively as a low-friction maintenance-free bearing, some of the PTFE is transferred from the coating to the inner spherical, providing a PTFE-on-PTFE sliding surface. If grease is applied, the material transfer does not take place and the bearing will actually wear much more rapidly.

As was stated earlier, sealed-for-life bearings are only as good as the lubricant used and this is equally true for bearings that can be re-lubricated. Therefore the benefit of being able to re-lubricate is that the grease quality can be maintained over a longer period and the bearing life will, therefore, be extended. However, this pre-supposes that the required re-lubrication is carried out at the correct intervals, using the correct grease, and that the quantity injected is also correct – which of course is a maintenance management issue.

Sealed-for-life bearings, then, if they are correctly specified, eliminate the possibility of poor maintenance affecting the bearing life and subsequently affecting the product’s expected performance.

• Ina Bearings

March 2000