Journal bearing simulator

A method and apparatus for measuring the load bearing capacity of engine oils under determined temperatures and shear rates. An eccentrically rotating cylindrical shaft (rotor) is used in a cylindrical journal bearing (stator) so arranged that the stator can be displaced from the center of rotation by a known amount. Measurements are made of minimum gap between the rotor and stator and of the force required to maintain the eccentrically rotating loaded shaft at a position such that the oil in the gap between rotor and stator achieves a shear rate between 1.times.10.sup.5 and 1.times.10.sup.7 s.sup.-1, typically 1.times.10.sup.6 s.sup.-1.

FIELD OF THE INVENTION 
The invention relates to a method and apparatus for assessing a lubricant's 
load bearing capacity. 
BACKGROUND OF THE INVENTION 
In the automotive industry, knowledge of the rheological properties of 
lubricants or engine oils and in particular the evaluation of the load 
bearing capacity of engine oils under realistic conditions in journal 
bearings is important. 
Currently the measurement of the load bearing capacity of a lubricant is 
carried out by determining the minimum oil film thickness in the journal 
bearings of either a fired engine or dynamically or statically loaded 
motored rig. 
Such tests are necessary for establishing the identity of the rheological 
properties which determine a lubricant's load bearing capacity in an 
operating journal bearing of an engine and for ranking the performance of 
lubricants in journal bearings. 
However, such tests are difficult and time consuming to operate and 
therefore are not suitable for routine use either for product development 
or classification purposes. 
It is therefore an object of the invention to provide a simple and rapid 
laboratory test to assess load bearing capacity. 
It is another object of the invention to provide an easy-to-operate, 
relatively cheap bench apparatus for simulating the big-end and main 
journal bearings of the automotive (gasoline and diesel) engine. 
SUMMARY OF THE INVENTION 
The invention therefore provides an apparatus for measuring the load 
bearing capacity of engine lubricants under temperatures and shear rates 
typical of those applicable to journal bearings of operating engines, 
comprising an arrangement of two cylinders consisting of an outer 
non-rotating vertical cylinder (stator) and an inner, vertical cylindrical 
rotor, which, in use, is rotating about a vertical axis which is 
off-centre from both its own centre axis and that of the centre of the 
stator, a means for measuring the horizontal force exerted by the 
lubricant on the stator, a means for controlling the temperature and a 
means for displacing the stator from the centre of rotation of the rotor 
and a means for measuring a minimum gap between the rotor and stator. 
The invention also provides a method for measuring the load bearing 
capacity of engine lubricants under temperatures and shear rates typical 
of those applicable to journal bearings of operating engines comprising 
the steps of rotating an inner, vertical cylindrical rotor about a 
vertical axis which is off-centre from both its own centre axis and that 
of the centre of an outer non-rotating vertical cylinder (stator) and 
measuring the horizontal force exerted by the lubricant on the stator, and 
controlling the temperature and shear rate of the lubricant.

DETAILED DESCRIPTION OF THE DRAWINGS 
Referring to FIGS. 1 and 2 the apparatus of the invention comprises an 
arrangement of two cylinders, consisting of an inner cylinder (rotor) 1 
and an outer cylinder (stator) 2. The stator was filled with lubricant or 
engine oil 4. The rotor 1 has a centre axis C and, in use, rotates about 
an eccentric axis B which is also off-set from the centre axis A of the 
stator. The graph h.sub.o represents a minimum oil film thickness. The 
dashed line 3 represents the locus of h.sub.o. 
The stator 2 is rigidly attached to a load cell L. The stator can be moved 
in any suitable manner relative to the rotor in any suitable increments 
(for example about 0.1 .mu.m), thereby allowing the eccentricity ratio of 
the arrangement to be varied at will. The minimum gap between the rotor 
and stator is in the range of 0.1 to 10 .mu.m, typically 0.5 to 2 .mu.m. 
Shear rates of the minimum gap position are in the range of 
1.times.10.sup.5 to 1.times.10.sup.7 s.sup.-1, typically 1.times.10.sup.6 
s.sup.-1. 
The operation of the apparatus of the invention is as follows: 
The rotor 1 is rotated continuously at high speed (i.e. up to 3000 r./min). 
In operation, the relative position of the rotation axis of the rotor and 
the centre axis of the stator are fixed and as a consequence of the high 
speed rotation of the rotor the oil exerts a force on the stator, this 
force being a measure of the load bearing capacity of the oil. 
Measurements are made of minimum gap between the rotor and stator and of 
the force required to maintain the eccentrically rotating loaded shaft at 
a position such that the oil in the gap between rotor and stator achieves 
a shear rate of 1.times.10.sup.5 s.sup.-1 to 1.times.10.sup.7 s.sup.-1, 
but typically is 1.times.10.sup.6 s.sup.-1, such shear rates being typical 
of those in main and big-end journal bearings of operating engines. 
In effect, the operation is the opposite to that in, for example, the main 
and big-end bearing of an Internal Combustion engine where a load is 
imposed on the shaft (rotor) and the load bearing capacity of the 
lubricant determines the relative position of the shaft and bearing 
(stator). 
The double eccentricity of the apparatus of the invention imparts "wobble" 
to the rotor thus leading to the type of dynamic motion characteristic of 
that present in journal bearings. In this way the dynamic loading present 
in big-end and main journal bearings of automotive engines is simulated. 
Such dynamic loading is essential for a proper simulation of big-end and 
main journal bearings. 
The relative positions of the rotation axis of the rotor and the centre 
axis of the stator can be adjusted by any means suitable for the purpose 7 
thus providing a means of adjusting the gap between the inner and outer 
cylinder to very fine tolerances (i.e. of the order of 0.1 micrometer). 
Advantageously a combination of cam and lever principles can be applied in 
order to achieve such an adjustment. 
It will be appreciated that the gap between rotor and stator can be 
measured in any way suitable for the purpose. Advantageously capacitance, 
inductance or resistance techniques can be used. For example up to four 
capacitance (or inductance or resistance) probes 5 can be mounted flush 
with the surface of the stator (but electrically insulated from it) and 
the oil film thickness measured by the technique described by T. W. Bates 
and P. Evans in Proc. Jap. Soc. Lub. Engineers, International Tribology 
Conference, Tokyo, Japan, July 8-10 (1985) p. 445. 
Alternative techniques could be applied by those skilled in the art. 
The oil is subjected to temperatures and shear rates typical of those 
applicable to journal bearings in automotive engines - namely temperatures 
of 100.degree. to 150.degree. C. and shear rates of 10.sup.5 to 10.sup.7 
s.sup.-1 ; the high shear rates can be achieved by the use of small (e.g. 
0.1 to 5 .mu.m) gaps and high rotational speeds between the eccentrically 
rotating rotor and stator. 
Preferably, the temperature is controlled electrically or by circulating a 
thermostatically controlled fluid 8 through a jacketed outer stator 6. 
It will be appreciated that the apparatus of the invention can be operated 
in a variety of ways; for example for ranking of load bearing capacity or 
measuring effects of oil rheology on load bearing capacity. 
Various modifications of the present invention will become apparent to 
those skilled in the art from the foregoing description. Such 
modifications are intended to fall within the scope of the appended 
claims.