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Two roller machines fall into two basic categories, those with mechanical power circulation (rollers back to back with gears) and those with electrical power circulation (mechanically open loop).
Mechanical power re-circulation pre-dates efficient and effective electrical power re-circulation and Phoenix Tribology has three machines that use this technique, the TE 53 Multi-Purpose Friction and Wear Tester, the TE 55 Lubricity Test Machine and the TE 73 High Speed Two Roller Machine.
Electrical power re-circulation first became properly feasible with the advent of the four quadrant d.c. thyristor drive, in which two identical motors are used, one driving and one absorbing, with the drives respectively drawing power from and regenerating power into the three phase supply. This works well, but is not electrically efficient. Because of phase angle effects between the supply current and the regenerating current, the supply has to have a capacity rated at the sum of the supply and regeneration current and not the difference. A non-closed loop version of the TE 73, the TE 103 Open Loop Two/Three Roller Test Machine originally used this system.
In recent years, with the advent of the a.c. flux vector controller, we have ended up with a much more electrically efficient system, in which two vector controllers are supplied from a common d.c. bus, with one drawing power from and the other dumping power onto the bus. This means that the net electrical supply is limited more or less to the system losses, the difference between the driving and the regenerating power, with electrical re-circulation taking place entirely within the machine's power module. This system has replaced the former d.c. thyristor drive systems on all current open loop machines.
With regard to the machines themselves:
These are fairly cheap and simple test machines with limited capacity in terms of load and it is hence not really suitable for rolling contact fatigue tests.
This machine was originally designed for work on materials and traction fluids for continuously variable toroidal transmissions. The rollers are each 150 mm in diameter with a 75 mm crown radius, thus producing similar contact geometry, pressures and component scales as the real application. The specimens are expensive to make!
In addition to the two-roller configuration, there is also a three disc assembly (two rollers on either side of the horizontal disc). This arrangement introduces "spin" into the contact, which is another feature of the contacts in toroidal transmissions, giving rise to a loss of traction coefficient.
The original TE 73 was a back to back machine, with fixed slide-roll ratios set be changing gear pairs.
TE 103 Open Loop Two/Three Roller Test Machine
This is effectively the same machine as the TE 73 but built as an open loop system with two 70 kW motors.
With both the TE 73 and TE 103, the rollers are mounted overhung, in other words, on the end of the test spindles. This does present a bit of a problem in that the overhung load means that the spindle and the bearings must necessarily be large. For higher loads, or indeed smaller test discs, the only solution is to mount the test rollers on a through shaft with bearings either side of the test disc, which leads us to the TE 74.
This is a two motor machine with the test discs simply supported on through shafts between bearings. The standard machine has two 5.5 kW motors with tests shafts at 40 mm centres. A high power version has two 30 kW motors and shaft centres at 70 mm.
In the TE 74 traction is measured by an in-line torque transducer, but this measurement, unlike in the TE 73, includes the losses in the supporting bearings. These can however be estimated by making measurements with the machine running in pure rolling.
This is the most recent addition to the range and combines features of the TE 55 Lubricity Test Machine with the two motor vector drive system of the TE 74 machines.
The specimens comprise a 25 mm ball on a 50 mm diameter disc and the machine has two 0.25 kW motors and servo pneumatic loading.
If none of the “standard” machines meet your specific requirements, the existing designs can readily be used as the basis for new machines with revised specifications.
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Copyright © 2002 Phoenix Tribology Ltd.