South African Institute for Tribology
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Garnett Cross - Member since 1985.

There are a few SAIT members who have maintained continuous membership of the SAIT since its inception in 1985. Garnett Cross is one of them, a loyal and long standing member of the SAIT. We have asked for their views on tribology and its importance, and here are Garnett’s thoughts on the SAIT and the value of membership. Here is his story:

I have been conducting hydraulic training courses since 1974, namely the Maintenance Course and the Advanced Hydraulic Course (Design Course).

I was in the Design Office in the Research Department of AECI, designing specialised machinery to automate the handling of explosives for the mining industry. Naturally one was not able to design using conventional methods, i.e. using electrically driven prime movers, hence the use of hydraulics or pneumatics. Way back in 1974 very few people were conversant with hydraulics, so it turned out to be quite a learning curve. As time passed I was offered a position at Sperry Vickers Hydraulics and Pneumatics Division in Johannesburg. I eventually ventured out on my own in 1974. I have also attended hydraulic courses overseas from time to time.

When the SAIT was established I saw the absolute necessity to incorporate the standards of Tribology in the design and maintenance of hydraulic systems.

By requiring companies using hydraulics, to maintain correct standards in their oil handling, they have saved a considerable amount in down time and equipment loss. One mining company was replacing a 460 litre per minute piston pump and a radial hydraulic motor approximately three times per year. The solution was to install ten micron bi-directional filters in the ‘A’ and ‘B’ lines of the closed loop hydrostatic system lines. After seven years the pump and motor were still working, performing without a problem.

When AC solenoid operated directional control valves are used the solenoids fail as a result of contaminated oil. This statement may sound a little strange. When an AC solenoid is energised there is an in-rush current as the electrical power is applied to move the solenoid, once the armature is in position, closing the magnetic field, the power drops to the holding current. As the spool clearances of directional valves average from twelve to twenty micron, it takes a few minute dust particles to cause the valve spool to stick, resulting in the solenoid coil burning-out. Since the spool is unable to move, the in-rush current remains at a high amperage resulting in the loss of the solenoid. The problem is solved by installing off-line filtration systems on the reservoir, plus in-line filters.

One company wanted to replace their thirty solenoid operated directional control valves with another make as they were replacing the solenoids daily. The standing order for solenoids was 125 per month. Once the oil was filtered the company had no more problems. That was the worst incident that was encountered. Solenoid failures are generally two to three per month, once correct filtration is applied solenoid failures almost cease.

Vane and gear pumps are also susceptible to contaminants in the oil. Pumps have been installed and after three or four days the pumps have to be replaced. Score marks from the contaminants are clearly visible on the pumps inner surfaces. Unfortunately it is very difficult to convince the company personnel the oil is not maintained correctly with proper filtration. It is only when the oil is tested by an outside service organisation, are the personnel convinced.

A survey was undertaken by Mobil Oil in Australia some years ago. The results published indicating that seventy percent of failures in industrial plant was the result of contaminated oil. The figure for failures in construction equipment was ninety percent as a results of contaminates in the oil.

The SAIT has been a source of learning for me in that I have learned from their technical meetings their seminars and conferences both local and international. There are many aspecits of the hydraulic industry which rely heavily on tribology……

Garnett Cross

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