Pragmatic Maintenance and Reliability

Case History – Identifying oil type in Plastic Extruder Screw Gearboxes

Proper lubrication is vital to reliable plant operation. After suffering a serious extruder gearbox failure, causing significant downtime, our client needed to understand both the condition of the oil in each of 26 large gearboxes and whether the correct oil had been applied.

We implemented an oil condition-monitoring programme and sampled oil from each gearbox. In a number of cases, the oil samples also contained evidence of wear; in these cases appropriate mitigating action (Filtration, oil changes) was taken by the client in response, but how to tell what oil was in which gearbox? The client had conflicting records as to which oil should go in each gearbox, and the plant labelling was confusing.

Our solution was to compare the data for Viscosity at 40°C and Phosphorus of the used oil, with values for samples of clean oil taken from fresh drums in the client’s stores. The graph above shows the resulting data plot. Viscosity is the single most important property for any lubricant and should be within 10% of the clean oil viscosity. Phosphorus was chosen because it is a key additive element as part of a Zinc DialkylDithioPhosphate (ZDDP) anti-wear additive pack or as Phosphate Esters in Extreme Pressure additives used in gearboxes. Comparing viscosity and phosphorus content allowed us to identify the oil in use. The letters and numbers identify samples, the red points show the values for clean oil, linked by the red lines.

Several things are apparent from the data:

1)    Many data points lie close to or between the clean oil data. We can assume these oils are either the same as the clean oil or mixtures of different oils. As the oil inventory in each gearbox is significant, the OEM’s were consulted to determine whether it was acceptable to run on a continued basis with the mixtures or whether the additive packages would react with each other.

2)    A couple of data points show acceptable viscosities but very high Phosphorus levels. These oils are assumed to have been contaminated with supplemental EP additives or degreasers. ZDDP was ruled out because Zinc levels were low. Again advice was sought from the OEM’s.

3)    A couple of data points showed very high viscosities, around 650 – 720 cSt at 40°C. This is clearly the incorrect oil and we recommended replacement of the oil.

4)    The remainder of the oil showed viscosities comparable with SHC 320, but with lower phosphorus values. On investigation it turned out that another 320 cSt oil  was also used that had lower Phosphorus levels.

Control of the lubrication programme can be achieved using simple systems such as dedicated colour coded oil transfer bottles with different dispensers so that only the correct lubricant can be added to the gearbox. These, together with suitable labelling, state of the art storage systems and modern filtered breathers allow verification of the correct oil type and improved contamination control. Lubricants need to be stored correctly away from contamination and moisture ensuring that the new oil when added provides the appropriate level of protection to the moving components. A quality storage system will pre-filter the new oil in line with industry best practice. For further information contact Pragmatic at info@pmar.co.uk