Case drain Issues with pumps and motors: Part 2

Hydraulic motors are more sensitive to problems with the case drain than pumps are. This is because motors are usually required to reverse direction during operation.

Bidirectional Motor
Figure 3. With valve control, neither motor port that has a check valve drain may be at low pressure, so the motor case and shaft seals must withstand high pressure.

In concluding the discussion from last month, motors are somewhat more sensitive to problems with the case drain than are pumps. This is because motors, more often than not, are required to reverse direction during operation. Figure 3 shows the powered and return lands of a directional control valve in a motor circuit. The return land, depending upon its K value, will restrict return flow and cause pressure in its associated motor port to always be higher than the tank pressure. Motor reversals, of course, require input flow to be reversed, so the ports will alternate between high and low pressure. Valves for controlling direction and speed, therefore, cause even the low pressure side of the motor to experience pressures that approach half of the supply pressure. Actual pressure depends on the valve’s coefficient value, KVRL, the load, the application, and the means of control. The case drain becomes more critical if reversing is required.

Bidirectional Pump
Figure 4. The analytical schematic for the hydraulic pump is similar to that of the motor and can help in understanding issues with case drain flow and other topics.

The differences with pumps
The analytical schematic for the pump can be helpful for visualizing issues with the case drain, just as was the case with the motor. Figure 4 shows a Type 2 analytical schematic of the most general configuration for a pump. It is, in effect, a mirror image of the motor. Two mechanical ports represent the hydraulic analogy of the input shaft, and three hydraulic ports. It is necessary in applications where the pump’s A and B ports must operate alternately at high and low pressures. Most pumps do not operate that way.

Most hydraulic pumps operate with a single direction of rotation — they operate with one power port connected to tank and the other to the high pressure elements to be powered. In addition, most variable displacement pumps do not operate, nor are allowed to operate, over center. That is, they do not have to reverse direction of output flow.

In these unidirectional applications, the case drain need only be connected to the pump inlet side. This ensures that the case is always at low pressure, so the shaft seal can be low pressure type and the case need not be designed as a high pressure-containing envelope. The analytical schematic of the unidirectional output pump with external case drain connection is shown in Figure 5, although some pumps are designed with an internal connection.

The above connection allows the pump to be characterized as a Type 1 model. The reason is because with the connection as shown, RLBCD experiences no differential pressure. Therefore, it contributes nothing to the case drain leakage. Also, with the connection as shown, all the case drain leakage is recirculated and is indistinguishable from the port-to-port leakage inside the pump. If the case is drained with external plumbing, then the case drain component of flow can be measured, and RLACD can be determined. On the other hand, if the connection is internal, all the leakage is lumped into RLPP.

The next installment of “Motion Control” will explore applications in which both the pump and motor are reversible. The subject will deal with the hydrostatic transmission (HST) and will show how the Type 2 analytical schematic demonstrates the need for the peripheral equipment to make the HST practical and fully functional.

Click here to view the next article in this series: Understanding Hydrostatic Transmissions.

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