Capacity

Hydrokinetic Drive Being centrifugal machines, fluid drives follow very familiar laws: power varies as speed raised to the third power (Figure 9), as diameter to the second power, and directly as the density of the working fluid.

FIGURE 8 Typical package hydrostatic drive (Sperry Vickers)

0 25 50 75 100

% DRIVEN SPEED

FIGURE 9 Load power varies as speed cubed. This figure represents a pump operating in a system where all the head is frictional, or where head varies as flow squared. In this system, the driven pump operates at only one point on its characteristic curve, and therefore the shape of the pump curve is academic. Although the rapid loss of hydraulic coupling efficiency at reduced speeds is quite obvious, the dramatic decrease in pump power requirements results in a total system power that is most acceptable. Note that the heat rejection requirements in the fluid drive are maximum at about 18% of the total load power.

0 25 50 75 100

% DRIVEN SPEED

FIGURE 9 Load power varies as speed cubed. This figure represents a pump operating in a system where all the head is frictional, or where head varies as flow squared. In this system, the driven pump operates at only one point on its characteristic curve, and therefore the shape of the pump curve is academic. Although the rapid loss of hydraulic coupling efficiency at reduced speeds is quite obvious, the dramatic decrease in pump power requirements results in a total system power that is most acceptable. Note that the heat rejection requirements in the fluid drive are maximum at about 18% of the total load power.

Thus hydraulic capacity is governed by speed, diameter, and operating fluid; mechanical capability is governed by the structural design of housing, bearings, shafts, rotors, and casings; thermal capacity is limited by the capacity of the oil pumps, the specific heat and thermal conductivity of the oil, and the ability of the heat exchanger to dissipate heat.

It should be noted that in scoop-trimming couplings, the oil pumps are usually sized solely for heat dissipation. In the leakoff coupling, the orifices are sized to permit enough oil flow to dissipate heat, and the pumps must handle this plus enough to fill the coupling in a reasonable time. The scoop-control coupling has limited flow and pressure because both are generated by the scoop tube. This may preclude its uses in certain positive displacement pumping applications or where installation of coolers at a remote location is required.

Hydrodynamic Drive This type of coupling varies so much in configuration that it is impossible to establish similar laws. Because any given machine has a definable torque limitation, we can state that power varies directly with speed.

Because standard units have no provision for removal and replacement of the working fluid, all cooling must be provided on the exterior surfaces of the rotating housing. This becomes a decided limitation if the unit is to be used with constant-torque loads and has limited the available sizes to some degree.

Hydroviscous Drive As would be expected, this device also follows the centrifugal laws: power varies as speed raised to the third power and as diameter raised to the second. However, the density of the working fluid has little or no effect; instead, capacity varies directly with viscosity. Thus, hydraulic capacity varies with speed, diameter, and viscosity. Mechanical capability is a relatively simple matter of structural design. However, thermal design is critical. Because power-transmitting capability varies with viscosity, which in turn varies with temperature, disk design is most important. Free area available for cooling oil flow varies with disk spacing, output speed, and heat load.

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