Page 48 - CEN 13 Apr.indd

18
APRIL 2013
technology
T ypically, the presence of lead in an oil sample
comes from the wear of plain bearing material,
as it is a metal used in the construction of such
bearings. As lead is toxic, there is a move to use
‘greener’ materials and the metal, bismuth, fits the bill.
So says John Evans, Diagnostic Manager for WearCheck,
who has over thirty years’ experience in the oil analysis
industry, and currently heads up WearCheck’s team of
diagnosticians.
He explains, “For many years ‘white metal’ (plain or
hydrodynamic) bearings have been made of a lead/
bronze alloy, usually containing 10% lead, 10% tin and
80% copper.
“Typical uses of such bearing can be found in big-end
and main bearing applications in internal combustion
engines, and bearings in hydraulic pumps.
“The alloy is very effective as a bearing material and
relies on the fact that lead is insoluble in the copper/
tin (bronze) alloy. The lead forms small pockets in the
bronze matrix that deform and smear easily across the
surface of the bearing.
“This lead coating has very low shear properties and
forms a solid lubricating surface at high speeds. The soft
lead is also good at minimising the effect of abrasive wear
and contamination particles as they become embedded
in the lead layer.”
Bismuth analysis first for WearCheck
Condition monitoring specialists WearCheck,
recently became the first oil analysis laboratory in
South Africa to offer the spectrometric determination
of bismuth levels in used lubricating oils.
Continuing, he adds, “Typically, these bearings would
consist of a lead overlay on a bronze matrix for fatigue
resistance with a steel backing for strength.
As effective as these alloys may be, John adds, “Engineers
are under pressure to find alternatives to the use of heavy
metals such as lead. This has been primarily driven by
the European Union under a directive concerning the
Restriction of Hazardous Substances. Lead-based alloys
for bearing materials are currently exempted from this
directive, but the race is on to find an alternative to lead.
“Some success has been found with the metal bismuth
replacing lead in an alloy containing 3% bismuth, 10% tin
and 87% copper. The bismuth is also insoluble in the bronze
matrix and operates in exactly the same way as lead.”
Bismuth is a white crystalline metal with a pinkish
tinge and a melting point of 271°C. Like lead it is very
dense, but considerably less toxic. The major producing
countries are Peru, Bolivia, Mexico, Canada and Japan.
According to John, since introducing the test in its trial
phase, the use of bismuth has been noted in some engine
oil samples, usually accompanied by the absence of lead.
He says, “The analysis of bismuth will be useful in
determining the presence of abnormal plain bearing wear
in engines, and WearCheck can now offer this service.”
In conclusion he comments, “Interestingly, bismuth
naphthenatehas alsobeenused to replace leadnaphthenate
as an extreme pressure (EP) additive in greases, and it
sometimes shows up in grease analysis and helps to identify
the additive chemistry of the grease.”
John Evans , Diagnostic
Manager for condition
monitoring specialists
WearCheck, recently
confirmed that theirs
is the first oil analysis
laboratory in South
Africa to determine
bismuth levels in used
lubricating oils, a
development which
spells good news in the
long term for ‘greener’
bearing material.
“Engineers are under pressure to find alternatives
to the use of heavy metals such as lead.”