Comparison - TE 80 and PCS HFFR
For ISO 12156-1 and ASTM D6079 fuel lubricity standard tests, results from the
TE 80 differ to those from the PCS HFRR. The TE 80 (and the TE 77 with low
load adapter) consistently produces larger wear scars than the PCS HFRR. This
effect is particularly marked for the low lubricity reference fluid. This arises for
two reasons, firstly from differences in the method of loading between the two
machines and secondly from the method of actuation. The TE 80 applies a
consistent and absolute load of 200 gm, whereas with the HFRR the load varies
with both stroke position and friction.
The TE 80 uses a mechanical drive mechanism that imposes a precise stroke
regardless of resisting force. The HFFR uses an electro-magnetic oscillator,
which is a force generating as opposed to a displacement generating device.
The resulting stroke length may vary as the frictional resistance of the contact
varies and the control system adjusts the driving force to compensate.
These differences do not represent a problem as the specified test is
comparative and the result is a simple offset bias, with the TE 80 producing a
larger discrimination between fuels with different lubricities than does the
HFRR.
TE 80 Production Test
HFFR Calculations
Certified Reference Fluid
Rolling Average
CEC DF-92-02 (For HFFR Test)
Mean wear scar diameter (MWSD)
426
404 (inferred)
Corrected wear scar diameter (WS1,4)
443
420 (given)
Production Test
Certified Reference Fluid
Rolling Average
CEC DF-70-00 SF 006
Mean wear scar diameter (MWSD)
692
619 (inferred)
Corrected wear scar diameter (WS1,4)
706
633 (given)
MWSD Range (high – low reference)
266
215
WS 1,4 Range (high – low reference)
263
213
HFFR Calculations
Various strategies could be adopted for reducing the bias. For example, the test
load on the TE 80 could be reduced. Alternatively, the test duration (number of
cycles) could be reduced. Both would however have the disadvantage of
introducing variation from the standard procedure, simply to get the results to
fall within the specified HFFR range. A simpler solution would be to recognise
the existence of such bias within the relevant standards.
Static Moment Analysis of PCS HFRR
This diagram shows the HFFR at the midpoint of the stroke. Consider the
contact load R and a maximum stroke length of 0.5 mm. Let x be the
displacement of the centre of mass from the pivot point, with x positive when
the centre of mass is toward the contact side of the pivot.
Moments about the pivot with a displacement of x:
Where F is the friction force, modelled as
and M is the
mass of the reciprocating assembly (vibrator armature and specimen arm).
Rearrange for R:
This demonstrates that R, the resulting load on the contact, is not constant, but
varies with stroke position, friction coefficient and direction of motion, with the
resulting error dependant on b, a, M, μ, N and direction of motion. Specifically:
The friction force interacts with the load because the frictional contact is not in
line with the pivot point.
The friction force interaction reverses sense with the direction of travel.
The centre of mass moves either side of the pivot point increasing or reducing
the resulting contact load depending on the stroke position.
The lower the applied load, the bigger the percentage error.
Units:
R = Newtons
N = Newtons
b = Millimetres
x = Millimetres
Mg = Newtons
a = Millimetres
Further to the above, it will be clear from more complex dynamic analysis, that
the off-set centre of mass of the reciprocating assembly will result in varying
inertia generated forces on the contact and the magnitude of these will vary
with reciprocating frequency.
These load/friction errors were avoided with the now obsolete Plint TE 70
machine, which was an electro-magnetic oscillator device, by rigidly mounting
the oscillator, with a reciprocating arm pivoted on the line of axis of the
frictional contact.
TE 70 Arrangement
The TE 70 design was abandoned because the issue of stroke variation with
varying frictional resistance could not be definitively overcome. Furthermore,
the cost of oscillator, drive and instrumentation was significantly more than the
cost of a simple mechanical drive system and motor.
The load errors are avoided with the TE 80 machine, because the moving
specimen is carried in a linear bearing, with a fixed axis drive system. This
removes the requirement for a pivot. The load on the contact is effectively
constant and does not vary with stroke position, friction coefficient or direction
of motion.
TE 80 Specimen Arrangement
The same applies to the TE 77 machine when used for tests with its associated
low load adapter. The use of a linear bush for carrying the ball carrier was a
modification introduced by David Cusac at Caterpillar Inc and the same solution
was subsequently applied to the design of the TE 80 machine.
One important effect of simplicity of design of the TE 80 is that, once correctly
assembled, it effectively removes the requirement for subsequent calibration,
as there is nothing significant in terms of the control or measurement system to
be adjusted.
Production machines are simply subjected to a test run with the reference fluids
and the results generated appear to demonstrate limited machine to machine
variation, as the following examples demonstrate:
Machine 7671
- High 1
436 x 450 =
443
Machine 7671
- High 2
447 x 419 =
433
Machine 7671
- Low 1
688 x 744 =
716
Machine 7671
- Low 2
Machine 7672 – High 1
695 x 699 =
447 x 407 =
697
427
Machine 7672 – High 2
457 x 422 =
440
Machine 7672 – Low 1
682 x 675 =
679
Machine 7672 – Low 2
696 x 715 =
706
Machine 7770 – High 1
402 x 433 =
418
Machine 7770 - High 2
423 x 390 =
407
Machine 7770 – Low 1
686 x 664 =
675
Machine 7770 – Low 2
693 x 668 =
680
Production Test Examples
Machine: 7671
Operator:
Harris
Measurer: Harris
Machine:
7672
Operator:
Willmont
Measurer: Harris
Machine:
7770
Operator:
Morley
Measurer: Favede
Conclusions
The fact that ASTM and ISO have chosen to sanction a test method using a
specific instrument in which the load on the specimen contact is subject to
uncertainty and is in effect indeterminate is not a matter of concern for Phoenix
Tribology Ltd.
We note that the main purpose of specifying the exact measurements with
different reference fluids appears to be as a means of identifying whether the
HFFR unit is calibrated correctly.
With regard to Phoenix Tribology product specifications and the matter of bias
in the results compared with the PCS HFFR machine, please note that we take
care to point out in our specification that the TE 80 machine will run tests under
the conditions specified in the relevant standards in terms of load, stroke,
temperature and test duration, using the specimens specified in the relevant
standards. We do not state that the results generated will be identical to those
produced by the PCS HFFR machine or as specified in the relevant standards.
George Plint
Phoenix Tribology Ltd
22 May 2010