Hydroviscous Drives

Hydroviscous drives are relatively new in commercial use. There are several manufacturers in the United States who are marketing this type of drive for a wide range of pump applications (Figure 7).

Basic Principle Hydroviscous drives operate on the basic principle that oil has viscosity and energy is required to shear it. More energy is required to shear a thin film than a thick one. The hydroviscous drive varies its torque capability by varying the film thickness between driving and driven members.

Components The following components are common to all hydroviscous drives. The primary variations from one manufacturer to another are in the mechanics of control, the numbers of disks, and the support of the rotors.

The housing serves the same purpose as in other fluid drives, supporting bearings, guarding moving parts, and containing oil and vapors. In addition, one manufacturer uses

FIGURE 7 Hydroviscous coupling (American Davidson)

cored passages in the housing to introduce water to remove heat from the working fluid. Bearings are usually of the antifriction type in small machines, with sleeve and Kingsbury bearings available in large units. Shafts support rotors, transmit driving torque, and, in most cases, are hollow to supply cooling oil and control oil.

The rotors have the driving hub keyed on the inside to driving disks and the driven hub keyed at its inner diameter to the driven disks.

The disks are made of various materials and are usually grooved with some type of pattern to direct cooling oil flow.

Pistons are hydraulic. When moved by control hydraulic oil, they force the disk stack closer together.

Oil pumps are usually motor-driven, but sometimes are driven by the input shaft of the coupling. It is not uncommon to have two separate pumping systems, one providing high-pressure control oil and the other lower-pressure cooling oil.

Oil coolers are usually shell-and-tube water-to-oil heat exchangers, although air-to-oil exchangers can be furnished and, as mentioned earlier, cored housings can sometimes be used.

Operation Oil flow is initiated by the oil pumps, which force cooling oil through the disk stack, draining into the sump. With the control set at minimum speed, the disks are at maximum spacing and the coupling transmits minimum torque. As pressure is applied to the piston, the disks are forced together. This decrease in film thickness between disks increases the force transmitted from one plate to the next. At maximum piston pressure, the spacing between plates is zero and the output shaft is driven at input shaft speed. In the full-speed condition, this device is actually a lockup mechanical clutch; at reduced speeds, it is an oil shear coupling; and in a narrow band between these two points of operation, it must be looked upon as an oil-cooled mechanical clutch. Reversibility can be accomplished by reversing the driving motor if oil pumps are driven by separate motors.

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