ISS-Machine Health
Used Grease Analysis
The analysis of used grease
samples from machines has been a very useful adjunct to the ISS-MH condition
monitoring programs. Grease sampling is
useful because, like the oil filter or magnetic
chip collector (MCC), it retains a history of any wear process. Most other techniques are simply a “snap-shot
in time” of the events occurring at the time of sampling.
The used grease samples are
broken down into a liquid and processed to extract any entrained wear
particles. The wear particles are
collected in two formats; a Ferrogram
which is prepared to show the smaller <100µm particles, while any larger
particles >100µm are transferred to an MCC
sample card.
Used grease analysis has
been shown to be the only reliable technique for determining the condition of
the low speed bearings of sugar mill drive gearboxes. The vibration noise floor of these gearboxes
is typically very high making the acquisition of useful vibration data a very
time consuming process.
1. Grease Sampling.
Grease sampling is very
sensitive to the sampling procedure and care is required to collect a useful
sample. The basic requirement is to
sample the grease only in contact with the active bearing track or rolling
elements. This is because the majority
of the grease in a rolling element bearing does not take part in lubrication (other
than to act as a reserve) but functions as a seal to prevent the entry of dirt
and moisture into the moving contacts and so is static. Any wear particles generated by a bearing are
only going to be trapped in the active grease immediately adjacent to the
rolling elements and it is this grease that must be sampled. Access to this active grease is going to vary
with design of the bearing, its housing and the sealing arrangements.
The majority of large
spherical roller bearings in use are dual row and it has been shown that wear
particles generated by one side of the bearing does not always appear in
samples of grease taken from the opposite row.
For this reason it is better to sample both side of a dual row
bearing. Two or more samples from the
same bearing are treated as a single sample when charged.
1.1
Sampling Instructions – Typical Spherical Bearing. Eg Conveyor pulleys/ Sugar
Mill Drive Gearboxes.
1. Clean up the area around the bearing
to avoid contaminating the bearing.
2. Remove the side shield off the
bearing or take the top cap off (Safety - Consider direction of any residual load –
strap the shaft etc).
3. Using a non metallic spatula collect
samples of the used grease immediately adjacent to the rollers. As little as
5ml – a heaped tea spoonful is adequate provided it comes from the right
position in the bearing.
4. Put grease samples in a self sealing
polythene bag, Correctly label sample with Plant, Contact name, Email address
for results, Machine, Sample Position, Date and any comments.
5. Close bearing to avoid
contamination.
1.2
Sampling Instructions – Typical Slew Bearings. Eg Sugar Mill Rotary Bagasse
Reclaimers
1. Clean up the area around the bearing
to avoid contaminating the bearing.
2. Purge the bearing with fresh grease
as the used grease exits past the bearing seal collect samples using a non
metallic spatula. As little as 5ml – a
heaped tea spoonful - is adequate provided it is taken from the used grease
(usually discoloured).
3. Put grease samples in a self sealing
polythene bag, Correctly label sample with Plant, Contact name, Email address
for results, Machine, Sample Position, Date and any comments.
1.3
Sampling Instructions – Typical Slew Rings. Eg Excavators Cranes etc.
1. The grease flow around an excavator
ring is insufficient to transfer wear particles around the ring and damage is
often localised and not dispersed. When
we collect grease samples from excavator slew rings, ISS-MH samples from eight
positions evenly divided around the slew.
2. Because the constant flow of fresh
grease onto the ring dilutes any wear particle populations large (typically
250ml) grease samples are scraped from the ring at each sample point.
3. Put grease samples in a self sealing
polythene bag, Correctly label sample with Plant, Contact name, Email address
for results, Machine, Sample Position, Date and any comments
Notes:
·
Grease
sampling kits are available from ISS-MH on request, these comprise a wooden
spatula and a self sealing bag complete with an identification label to be
completed.
·
Collecting
used grease samples as it is purged from a bearing is possible with virtually
all low speed bearings, but it is not the preferred techniques. If possible the side shields should be
removed from the bearing.
·
Take
care not to over grease higher speed bearings as this will cause overheating –
see bearing supplier catalogue for details.
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requested by JM.