Grease and the lubricating oil in it has a finite life, degrading with use over time, according to the operating conditions. If it is not replaced as required, then bearing failures will occur causing higher maintenance costs and plant down-time and production losses.
The reasons why grease – and the oil in it – fails are generally not well understood.
Greases are used as a method of lubricating with oil where oil on its own cannot be practically used. The oil is held in a thickener which acts much like a sponge. Greases fail for a variety of reasons but as the oil provides the lubricating properties of the grease it is important to understand why both the grease and the oil fails. Importantly, if the oil fails then the grease can no longer function as a lubricant.
Oil degradation caused by oxidation which results in a permanent change to the oil molecules, will mean more frequent re-lubrication is required to replace both the degraded grease and any contaminants which the oxidation has created. The main cause of oxidation is elevated temperatures. Generally, the rate of oil oxidization doubles for every 150C above 700C. Therefore, the life of the oil (and hence the grease) is halved for every 150C rise above 700C.
Degraded oils break down into by-products of the reaction and are harmful to bearings. These include carboxylic acids which will corrode surfaces if left in the system and sludge and varnish will form over time.
Higher temperatures also increase the rate of oil evaporation and bleeding from the thickener, which reduces the lubrication properties of the grease.
Additives that enhance the lubrication properties of the oil such as friction modifiers, corrosion inhibitors, anti-wear agents, oxidation inhibitors, EP improvers and adhesion improvers have a finite life and become depleted with use. Once this occurs the grease and the oil in it can no longer perform to its original specification.
Contamination and mechanical effects
Other factors which reduce the grease and oil life and therefore the re-lubrication interval are:
- Materials and by-products created by wear and lubricant degradation.
- Humidity and condensation (daily or operational temperature cycles will cause the air trapped in the housing to become a condensate).
- Water and process-related liquids – The limit for the absolute water content in mineral oil – the lubrication component of most greases – above which problems will occur, is 200 ppm.
- Vibration – High vibration will cause the grease to churn which will cause a grease life reduction. Vibration will also cause the grease to bleed more than normally, resulting in an increase in the hardness of the grease, limiting its ability to correctly lubricate rolling contact bearings.
- Strong air flow or reduced air flow through the bearing (drying of grease, oxidation, removal of microscopic oil droplets).
The factors above will each cause a grease life reduction factor of between 0.4-0.7x the normal grease life in a single row spherical roller bearing. The reductions are cumulative for each cause and are in addition to the effect of temperature on the grease.
Continuous lubrication is the best prevention measure to protect against the build-up of contaminated or degraded grease. If contaminants have built up in the housing, then to fully remove them the housing must be opened, and the bearings and housing thoroughly cleaned. An alternative is to purge the housing however, whilst that may remove most of the contaminants it is unlikely to fully remove them all. If the contaminants are not removed, then the bearing life will be decreased. It is more efficient and cost effective to use continuous lubrication to prevent the build-up of contaminated or degraded grease.