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Non-Newtonian lubricants
The Newtonian assumption that the shear stress in a fluid is proportional to the shear rate breaks down
under the high pressures of an EHL contact -- inevitably since the viscosity is there so high that the shear
stress would otherwise exceed the yield stress of steel. Polymer-thickened lubricants sometimes exhibit two
Newtonian regimes, for low and high shear rates, with very different effective viscosities.linked by a
non-Newtonian transition. The lubricant film thickness can be a small fraction of that predicted using only
data from the first Newtonian region [1].
In a line contact flow is unidirectional, so the only question with a non-Newtonian lubricant is the flow
rate. But in a point contact, as in a ball-race or a traction drive, the flow is not in the direction of the
shear stress, and flow in one direction affects the shear stress and flow-rate normal to it. As a result,
there is for 2-D flow no equivalent of the generalisation of the Reynolds equation for a non-Newtonian fluid
which exists in 1-D, and the computational procedures become time-consuming. Different approximate equations
have been studied using a hypothetical fluid for which an exact solution becomes possible [2].
Crook initially explained the moderate values of traction coefficients as due to the viscosity reduction with
temperature. Distinguishing between real non-Newtonian behaviour and apparent non-Newtonian behaviour, and
correcting the observed behaviour for thermal effects in order to measure true non-Newtonian parameters,
remains an issue.
Relevant/recent publications
- Greenwood, J. A. & Kauzlarich, J. J. , 'Elastohydrodynamic film thickness for shear-thinning
lubricants' J. Engineering Trib., 212J, pp179-191 (1998).
- Greenwood, J. A., '2D flow of a non-Newtonian lubricant.' J. Engineering Tribology, 214,
pp29-41 (2000).
- Greenwood, J. A. 'Non-Newtonian Lubrication' 27th Leeds-Lyon Symposium on Tribology, Lyon, Sept
2000.
Contact Details
For further information on the work summarised above contact Dr J A Greenwood at Cambridge University
Engineering Department, Trumpington St, Cambridge, CB2 1PZ, UK. Tel: 01223 332733, Fax: 01223 332662. E-mail:
jag@eng.cam.ac.uk
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