3.1 Viscometer Mechanism.

Operation of the viscometer mechaalsm is now described, see figure 3; Complete engineering  drawings are shown in Appendix C. All major parts were machined from Dural, with the exception of the stirrer: the corrosive nature of the lubricants under study necessitates the use of glass for this component. In all experiments the assembly was suspended above the test fluid by a retort stand.

A small DC motor (A) drives the disk (E), the outer edge of which is removed over 180 degrees, as shown in the detail. Disk (F) is also cut in a sirhilar way. Disks (E) and (F) may rotate independently about the same axis, due to the bearings in (E) and lower plate (C). The outer end of spiral hair spring (1) is bolted to (E), while the inner end slots into part (F). Therefore the lower disk (F) is indirectly driven by the upper disk (E), via torque spring (I). Connected to (F) by means of a pinned push-fit joint is the chuck (G). Three nylon screws in this component clamp the glass stirring rod (J) securely. The dimensions of the cylindrical, stirring end of (J) restrict the instrument to measurement over a certain range of viscosity. Different ranges can easily be obtained by altering the size of this stirring cylinder. In the following experiments two stirrers were used, one with a cylinder diameter of 2cm and length of 2cm, the other with a cylinder diameter of 1 cm and length 2cm.

An infra-red light beam is emitted by the LED (L), and passes through the mechanism before detection by the photo diode (M). The LED cover (D) ensures that the beam is sufficiently narrow that reflections from parts of the mechanism do not disrupt readings.

The mounting plates (B) and (C) are supported by four corner pillars, (K). Side plates (H) are fitted; these perform the multiple functions of increasing the rigidity of the structure, excluding dirt, and preventing ambient light from corrupting the infra-red beam measurements.

In operation, the motor rotates at a speed up to 300 rpm, determined hy the control electronics (see section 3.2). The stirrer is turned via the spiral spring, which provides a torque proportional to its angular extension. Thus the relative angular displacement between disks (E) and (F) depends directly on the viscosity of the fluid and rotation rate (see section 2.3 theory). As described, parts (E) and (F) are cut away over 180 degrees; this causes the beam to be interrupted once per revolution. Calculation of the mark/space ratio results in the angular displacement, whilst the motor speed may be accurately determined from the period. Figure 4 illustrates the mechanism operation with liquids of different viscosities, and also shows the corresponding expected photo-diode outputs.
 

A serious problem occurred with the spiral spring. Such a component is difficult to obtain commercially, consequently one was wound from a strip of plate brass approximately 30 cm long, 2 mm wide and 0.5 mm thick. Unfortunately the quality of the spring thus produced was unsatisfactory: only about 3 turns were possible and the spiral shape was difficult to muintain. In addition it was impossible to keep the spring planar: the result of this was that when mounted in the mechanism, the spring scraped on disks (E) and (F) (refer to figure 3). Undoubtedly the instrument's measurement accuracy was seriously affected by the additional friction, hysteresis and non-linearity of this component. Nevertheless useful results were obtalnable, as described in section 4, proving the principle and justifying future pursuit of a suitable spring.

2: Theory of viscosity measurement

Contents

3.2 Microprocessor control system