Fixed Plant Grease in the Mining Industry – has it changed for the better?
Prior to the 1980’s, the most common grease used on mining processing fixed plant was a lithium 12 hydroxy stearate thickener.
This would be used for general purpose greasing on all bearings, bushings and hinge pins for low temperature ranges, less than 60ᵒ C.
For high temperature applications, bentone grease would be used, this is a clay-based grease and used extensively on high temperature in excess of 60ᵒC, it was a favourite for front wheel bearings on vehicles with disc brakes.
So typically, there would be 2 greases used on site, a low temperature lithium 12 and a high temperature bentone or clay-based grease. The risk with having two different greases was that they are incompatible and cannot be mixed. If a clay-based grease is added to a lithium grease the thickening agents react, resulting in a breakdown of the thickening agents, in most cases becoming very fluid, resulting in the lubricant running out of the bearing and lubricant starvation.
This was a standard practice until the mid-1980’s when Lithium Complex soap gained a significant share of the market. In general terms, these greases operated at low and high temperature ranges, allowing sites to reduce the number of greases on site.
Apart from its limited upper temperature range, the lithium 12 soap displayed very good properties in regards to:
- Shear stability
- Lower oil bleed
- Maintained consistency and retention
- Good flow or rheology
Rheology is a key characteristic of non-Newtonian flow of semi solid liquids such as grease, and impacts on the grease performance. In short, it is the greases response to sustained loading, such as continued system pressure, continuously rolled, shock loading and shear forces as it is worked in a bearing or bush and is paramount to its performance.
Grease needs to maintain its consistency, have good retention and memory. As the load is applied, the grease is squeezed out of the load zone leaving an oily film to lubricate, as the load is relieved, the grease pulls back into the void, by its consistency, how well it continues to do this with little change, demonstrates its stability and memory or retention.
Over the last 35 years, our experience is contrary to the marketing aspirations of the oil and grease industry. We have found that Lithium Complex soaps demonstrate inferior performance in a number of areas and consistently demonstrate poor performance characteristics such as:
- Poor shear stability
- Excessive oil bleed
- Poor pumpability or flow
- Poor memory or retention characteristics
Oil bleed and shear stability is critical to a grease performance, as the oil bleeds and the soap structure is sheared, the grease consistency changes, resulting mostly in the grease consistency thickening. This thickening then changes the way in which the grease flows and moves within the bearing or bush, effecting its retention or memory.
This change in grease characteristics can result in channelling and grease starvation in the load zone of a bearing, race or bush.
Poor pumpability and oil bleed results in grease separation in lines, we see free oil at one end of a grease line and hard soap at the other end. This characteristic requires greater pressure to maintain grease flow and ensure the grease is reaching the bearing.
The use of Lithium Complex greases in Single Point Lubricators which operate at relatively low pressures (75psi) suffer with reliability issues due to the poor pumpability and oil separation characteristics.
To discuss any problems, you may be having with your grease systems, contact Integrated Reliability Solutions on 02 4903 8900. We have over 35 year’s experience in the field of lubrication technology, dispensing and maintenance systems.