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.
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