Machine Design


Hydrokinetic drives are typically rated from 1 to 5,000 hp but higher ratings are common. Largest units are rated at 25,000 to 30,000 hp.

Because there is no mechanical lockup between the driver and driven elements, the drive has a constant 2 to 4% slip that, although reducing efficiency, provides good shock protection to the driven and the driving equipment.

To remove heat generated by this slippage, all hydrokinetic drives require a heat exchanger. However, constant-speed hydrokinetic couplings used for soft starts are totally self-contained and do not need heat exchangers. Only variable-speed hydrokinetic couplings are equipped with heat exchangers. These drives are controlled by adjusting the amount of oil in the casing. Most scoop-tube controlled hydrokinetic drives use electrical, hydraulic, or pneumatic controllers, but a few drives are manually controlled.

Because the drive operates by the impeller input circulating the oil, the hydrokinetic drive is particularly vulnerable to inaccuracies at low speeds. Thus, minimum output speeds are usually specified. The minimum output speed at which the drive will continuously operate is given usually in terms of a percent of input speed. Operation below this minimum ordinarily is not permissible except in the transient conditions of clutching and declutching. These minimum output speeds are typically 20 to 30% of input speed for variable torque loads and 35 to 45% for constant-torque loads.

In theory, maximum torque is transmitted at 100% slip when the turbine is stalled and the impeller turns at motor speed. However, in a standard constant-speed hydrokinetic fluid coupling, maximum torque is not exactly at 0% output speed. Maximum torque occurs at about 10% slip. This can be significant and must be considered if a fluid coupling is used as a torque limiter in a power train. Recent developments in fluid couplings allow some to produce soft starts and low torque limits.

Torque limitation can be as low as 130% but normally is about 150%. Without the soft-start feature, fluid couplings can only offer torque limitations of 170 to 180%, usually too high for material-handling equipment.

Studies from power utilities suggest that for large power transmission, hydrokinetic fluid drives require less maintenance than hydrostatic or hydroviscous types. This is because a fluid, rather than a solid surface, transmits power from impeller to turbine.

Typical applications include conveyor drives, to provide smooth starts and minimize belt stretch; on diesel engines, to protect gearboxes from torque fluctuations; and on automatic transmissions.

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