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 Certified Reference Fluid HFFR Calculations 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 HFFR Calculations 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
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 2 695 x 699 = 697 Machine 7672 High 1 447 x 407 = 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