grease

Greasing is an age-old practice of lubricating machinery (usually made from metal) with oil. The oil is a thickener which acts much like a sponge. The purpose of greasing is to ensure that metal parts do not rust and that they operate smoothly in different weather conditions. However, greasing is not a foolproof method of lubrication and it may fail in certain cases. This is mostly due to the build-up of contaminants in grease which will hinder proper functioning of machinery. This will also lead to degraded quality of products, increased operational costs and a greater rate of depreciation of plant and machinery. So, how does grease and the oil fail? Let us explore further in this blog.

Oil Oxidation

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 oxidation 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 that are harmful to bearings in steel fabrication. 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.

Additive Loss And Other Reasons

Metal fabrication has application in multiple industries, including oil and gas where lubricants are used. 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 in use.  Once this occurs the grease and the oil in it can no longer perform to its original specification. Other factors which reduce the grease and oil life and therefore the re-lubrication interval is: –

• Dust.
• 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) – an example is mining and plant equipment which is exposed to a humid environment.
• 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 above factors will each cause a grease life reduction factor of between 0.4-0.7 times 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.

Final Thoughts

There are ways and means to protect against the reduction of grease and oil life. Continuous lubrication is the best preventative method 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 – hence it is essential that the road to operational efficiency is “well-greased”!