Returning to Figure 2, consider the heat exchanger and filter. Only the fluid that makes its way out of the CD ports is cooled and filtered. Depending on the volumetric efficiencies of the pump and motor, the total case drain flow will be about 5% to 20% of the transmission’s power port flow. Is this a reasonable strategy for fluid conditioning?

First, consider the cooling issue. All the case drain leakage flow has been “squeezed through” the small internal clearances at very high pressure, so it has undergone considerable heating. The flow that has gone through the displacement elements also undergoes a pressure reduction, but its energy is converted to torque and sent out the shaft. This flow is not substantially heated, so it requires little cooling.

The port-to-port leakage is another matter. It goes directly from the high pressure port to the low pressure port and is recirculated without any cooling at all. Even though this fluid is not cooled, the method is viable if the heat exchanger is sized to cool both case drain and port-to-port flow. That’s because the supercharge circuit replenishes the transmission’s low pressure side with slightly over-cooled fluid, which combines with the main power port flow.

The issue of filtering only case drain flow, on the other hand, has no absolute answers. If there are any absolutes, they are: First, get your fluid clean, and second, keep it clean. After protecting against catastrophic failures, nothing will enhance component reliability better.

Some advocates encourage placing high pressure, full flow filters in the power ports on both sides of the transmission. This provides a remarkable amount of protection. However, detractors point to high initial cost and continued maintenance. They will further argue that if the fluid has been properly cleaned, and contaminant ingression is under control, then any increase in contamination has to be internally generated, say, from component wear.

Incorporating dynamics

Dynamic effects are easily added to the Type 2 analytical models, shown schematically in Figure 3. In the study of machine dynamics, we are interested in changes in speeds, torques, pressures, and the like — more specifically, the factors that act to prevent instantaneous changes in them. In a hydraulic circuit, inertias of the prime mover, the pump, the output motor, and the load inertia prevent speeds from changing.

These effects are shown in Figure 3 as the curlicue symbols in the mechanical sections. Fluid compressibility and line expansion prevent pressures from changing instantaneously. Those effects are symbolized with electrical capacitors (labeled C, with appropriate subscript) in the schematic. The rule for adding dynamic effects is very simple: add inertia in each torque summation loop (pump shaft input circuit and motor shaft output circuit) and add a separate capacitance at each node in the hydraulic circuit. A node is a point where there is a pressure value different from all others. Four are shown in Figure 3, identified by the four pressure gauges.

We write six dynamic equations to study the transients in the HST: two sum the torques in the pump and motor shaft circuits, and four sum the flows at each of the four nodes in the hydraulic circuit. We would calculate about 30 or 35 different variables in the solutions to the equations. These would provide enormous insight into the operation of the transmission under any manner of dynamic changes, such as loads, displacements, prime mover speed, or any combination of these.

Deeper discussion of dynamics is beyond the scope of this column. But in the end, the Type 2 analytical models are helpful in sifting through and understanding many of the intricacies and nuances of a hydrostatic transmission.

Handbook serves electrohydraulic system designers

Handbook serves electrohydraulic system designers
The newly published fourth edition of the Designers’ Handbook for Electrohydraulic Servo and Proportional Systems contains even more useful information than its predecessor, the highly successful third edition, which has become the defacto Bible for electrohydraulics technology.

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