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The Optical Viscometer is based on a test rig designed by Professor Cameron at Imperial College, London. Oil film thickness in an elastohydrodynamic (ehd) contact is highly dependent on the effective viscosity of the oil in the inlet zone. This fact allows the simple Optical Elastohydrodynamic technique to be used as a method for determining effective viscosity of oils used to lubricate ehd contacts. The TE 71 provides vivid colour pictures of film thickness profiles in an ehd contact and a method for assessing the viscosity loss in polymer thickened oils at high shear rates and pressures.
The TE 71 consists of a highly polished ball loaded against the coated underside of a rotating glass disc. Sample oil is applied to the disc as a smear film. The zone of contact between the ball and the disc is viewed from the opposite side of the disc through an optical microscope and is illuminated by collimated light from a quartz halogen lamp.
The thin chromium coating reflects 25% of the incident light and the rest passes through the oil film and is reflected back off the ball surface. The oil film changes the path length of the transmitted light and so generates a series of colour interference fringes. The colours seen in the contact zone can be related to the film thickness profile by applying optical principles. The minimum film thickness observable is limited by the wavelength of light and the first fringe corresponds to about 0.12µm.
This method yields results that are easily interpreted and produces vivid colour pictures showing the profile and actual dimensions of the circular elastohydrodynamic contact.
The TE 71 is a compact design and is small enough to fit on any laboratory bench. The instrumentation comprises a motor speed controller and tachometer which are housed in a separate cabinet. Colour photographs of the interference patterns may be obtained with the optional polaroid camera.
The classical Hamrock and Dowson equations for film thickness in point contacts suggest that for a constant oil film thickness, the ratio of the effective viscosity of two oils is given by the inverse ratio of the sliding velocities required to give that film thickness. This assumes, as Sanborn and Winer have shown, that the pressure-viscosity coefficient is the same for both oils. The Optical Viscometric procedure is as follows:
1. Run the apparatus using the base oil (of known viscosity) as lubricant and to note the speed corresponding to the development of some chosen fringe colour in the contact zone.
2. Repeat the test using the additive containing oil and the rolling speed to obtain the same fringe colour is noted.
3. The effective viscosity for the additive oil is then calculated from the ratio of the two speeds and the viscosity of the base oil.
| Disc speed range: | 10 to 150 rpm |
| Rolling Radius: | 60 mm |
| Ball diameter: | 25.4 mm |
| Disc diameter: | 150 mm |
| Maximum Effective Shear Rate: | 2 x 106s-1 |
| Maximum Hertz Pressure | 530 MNm-2 |
| Minimum discernible film thickness: | 0.19 µm |
| Disc coating: | 120 Å thickness chrome, |
| 25% reflectivity | |
| Microscope Magnification: | x 20 |
| Light Source: | 50 W white light with fibre optic light guide |
| Motor: | 190 W dc |
| Electricity: | 220/240V, single phase, 50 Hz, 250 W |
| 110/120 V, single phase, 60 Hz, 250 W |
| Bench-mounting machine: | 600 mm x 300 mm x 450 mm high, 20 kg |
| Bench-mounting cabinet: | 270 mm x 270 mm x 220 mm, 2 kg |
| Packing Specification: | 0.63 m3, GW 102 kg, NW 45 kg |
Copyright © 2002 Plint Tribology Ltd.