Most engineers are familiar with the sight of hardened grease in a bearing that has been opened for inspection. The causes of this are not always well understood.
Hardening can occur rapidly or gradually over a long period of time, depending on the cause. As the grease hardens it causes increased friction, and as a consequence, heat.
As the heat rises the grease problems escalate and the hardening process continues. The problem self-compounds. It will probably be noticed that the bearing is running hotter than normal and that there is a lubrication problem. Attempts to alleviate the problem by re-greasing can actually add to the problem once the grease has reached a condition of deterioration such that hardened grease is caked in the bearing. The main causes, not in any order of likely occurrence, are:
- Grease incompatibility: Mixing incompatible greases will lead to oil separation and grease hardening.
- Contamination: Dirt, dust, ash and similar contaminants in the grease not only cause elevated rates of bearing wear but can also act as thickener in the grease; both lead to higher friction levels and therefore elevated temperatures.
- High-temperatures, particularly in excess of the grease’s drop point, will lead to oil loss from separation and evaporation, causing grease hardening. Excessive temperatures may also cause the oil to oxidise and transform into a thick tar-like substance.
- Over-greasing, mechanical stresses from misalignment and overloading, and under-greasing will all cause elevated temperatures in the bearing. Excess temperature may cause the drop point to be reached, separation of the oil from the grease and lubricant failure. Over-greasing will lead to churning which causes even more heat, and probably lubricant failure. When these problems occur, the end result is that the grease will harden.
- Loss of oil which occurs as a result of mechanical forces imposed by the bearing rolling elements on the grease. The oil is squeezed from the grease base and in some circumstances the grease base may not fully recover and totally re-absorb the oil. This can lead to gradual grease hardening. Spherical roller bearings are more prone to creating this problem than other bearing types.
- Constant pressure applied to grease can cause it to separate. Care needs to be taken with the grease used in single point lubricators and centralised lube systems as some greases are more prone to separation under these conditions than others.
- Continuous vibration and centrifugal force are causes of oil separation and grease hardening. In both cases the correct lubricant must be selected to ensure that the effect is minimised.