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The High Speed Two Roller Machine is a research machine for the study of highly loaded lubricated power transmitting contacts and rolling contact fatigue of materials.
The successful operation of machine elements such as gears, rolling bearings, cam/follower systems and traction drives is vital to industry. The common feature to all these is that fact that power or motion is transmitted through highly loaded lubricated contacts with rolling or combined rolling and sliding motion.
The high loads (or contact pressures) result in elastic deformation of the surfaces in contact and the components operate under elastohydrodynamic lubrication. The performance of the gear, cam or bearing is dependent on both the material and the lubricant withstanding the highly stressed conditions in the contact.
In the case of gears, the motion of rolling and sliding at a given instant in the meshing cycle can be reproduced by two circular rollers of radii equal to the pitch circle rotating with equal and opposite angular velocity about fixed centres. This is the basis of the two roller machine first developed by Merritt in 1935 to simulate conditions in the gear contact. Since the radii of curvature of the teeth at the contact point are the same as the rollers the contact stresses under a given contact load are also simulated by the roller machine.
For gear contacts, the obvious departure from complete similarity arises from replacing the cyclic behaviour of tooth meshing by a steady motion which reproduces the conditions at only one instant in the meshing cycle. However this does at least mean that one condition can be studied at a time and only transient effects are ignored.
A similar picture exists in the simulation of the cam/follower contact. In a cam cycle there is a range of sliding and rolling velocities of the contact point.
In traction drives even more attention is focused on the lubricant since the power transmitted by the drive is limited by the coefficient of traction of the fluid. Petroleum based oils possess neither the lubricating power at high pressures nor the chemical stability for the high temperature/stress conditions in the contact. Therefore new lubricants with piezo-viscous properties and good molecular stability are being developed.
The traditional method of studying the performance of such fluids under controlled conditions is by using a two roller machine. In the TE 73 the two rollers are driven at different speeds through a back-to-back gear arrangement to give varying amounts of slip in the contact and therefore to transmit increasing amounts of traction. The maximum traction coefficient is obtained at a particular value of slip and this is a characteristic of an individual fluid.
In modern variable ratio gear systems there is an additional complication. It is an inherent property of this kind of mechanism that there is a velocity gradient across the width of the contact zone in a direction perpendicular to the rolling direction. In other words there is spin in the contact zone. The effect of spin is to modify the traction-slip characteristic. The distinction between the high and low slip regions is lost and instead there is a more gradual growth in traction as slip increases.
This phenomenon can be studied in the TE 73 by introducing a third disc between the two rollers that is free to rotate about a vertical axis. This is the TE 73/S Contact Spin Adapter.
The TE 73 is a closed-loop device. The two roller specimens are mounted on the ends of parallel test shafts which are connected to a helical gear pair at the rear of the machine. The lower shaft is driven by a thyristor controlled variable speed dc motor through a belt drive. The motor supplies only the losses in the loop, not the full tractive power which is locked into the loop.
Slip percentages from 0% (pure rolling) to 8% are selected by installing an appropriate gear pair. The test rollers have a crowned profile giving a well defined circular contact zone and high Hertzian contact pressures.
The lower shaft housing is fixed while the upper housing is hinged to allow motion in the transverse and vertical directions. A static compressive load is applied manually via a lever to the roller contact and this value is measured by an in-line strain gauge load cell. A separate load cell mounted in the transverse direction measures the traction forces in the contact. Both shaft speeds are measured by inductive pick-ups.
The upper shaft and casing are insulated electrically from the rest of the machine. This allows a small potential to be applied across the roller contact. Variations in this voltage give a clear indication of the frequency of inter-metallic contact between the rollers. These variations in voltage can be displayed on an oscilloscope and the rms potential may be fed to a chart recorder.
The test rollers run inside an aluminium housing with a glass front cover. At low speeds, dip lubrication of the lower roller is acceptable. At higher speeds, pumped lubrication by jet into the exit side of the contact is provided. The roller temperatures may be increased above the steady-state test temperature with infra-red heating through the front window. Bulk temperature is measured with a sliding contact thermocouple.
motor speed controller and mains isolator
digital tachometers for upper and lower rollers
digital read-out of applied load and traction force
temperature controller with indicator
Lunn-Furey Electrical Contact Resistance Circuit
One major application of rolling contact power transmission is in continuously variable transmissions. In such devices, of whatever configuration, the geometry of contact is such that there is a degree of rotation of one surface relative to the other and not pure rolling/sliding. This relative rotation or spin has a profound effect on the performance of the transmission:
1. substantial losses occur, even when no power is being transmitted
2. flash temperature effects become significant
3. the degree of slip for maximum traction is greatly increased
4. the overall efficiency is strongly dependent on the degree of spin
In the TE 73 relative motion of the two rollers is limited to combinations of pure rolling with varying rates of sliding in the rolling direction. Spin is introduced into the contact by running the machine with a third disc, whose axis is vertical, between the two smaller diameter test rollers. Two contacts are formed, one on the upper surface of the third disc and one on the lower surface and the contact shape is elliptical.
The degree of spin in the contacts may be varied over a range encountered in transmissions by changing the radius at which the two rollers contact the third disc. The TE 73/S is interchangeable with the standard two-roller configuration.
| Speed Range: | 100 to 7,500 rpm |
| Rolling Velocity: | 0.75 to 60 m/s |
| Load Range: | 0.25 to 15 kN |
| Maximum Hertz Stress | 2.8 GPa (based on steel rollers) |
| Roller Diameter: | 152 mm |
| Crown Radius: | 76 mm |
| Slip Rates: | 0% supplied with machine |
| Optional Slip Rates: | 1%, 2%, 4%, 6% and 8% |
| Motor Power: | 11 kW dc |
| Roller Diameter: | 120 mm |
| Crown Radius: | 20 mm |
| Contact Elipticity: | 2.36 |
| Spin Ratio: | 0.8 to 1.33 (4 steps) |
| Maximum Hertz Stress: | 2.5 GPa (based on steel rollers) |
| Electricity: | 480 V, three phase, 50 Hz, 12 kW with neutral and earth |
| mains water and drain |
| Floor-Standing Machine: | 1,450 mm x 1,100 mm x 1,400 mm high, 540 kg |
| Bench Mounting Control Unit: | 520 mm x 520 mm x 400 mm high, 80 kg |
| Packing Specification: | 2.2 m3, GW 760 kg, NW 630 kg |
Copyright © 2002 Plint Tribology Ltd.