Case Drain Flow

Information about case drain flow.

The following article will help you understand case drain flow.

“I tried one afternoon and evening to determine what was wrong with a hydrostatic transmission by monitoring case drain flow and was confused by the readings I was seeing. There was a flow meter in the transmission pump outlet and another in its case drain that always showed charge pump flow, even though the motor was bypassing profusely. The motor case drain went through the transmission pump case to tank.”

Find out more about case drain flow below.

Making Sense of Case Drain Flow part 1

What is a hydrostatic transmission?

A hydrostatic transmission consists of a variable-displacement pump and a fixed or variable displacement motor, operating together in a closed circuit. In a closed circuit, fluid from the motor outlet flows directly to the pump inlet, without returning to the tank.

As well as being variable, the output of the transmission pump can be reversed, so that both the direction and speed of motor rotation are controlled from within the pump. This eliminates the need for directional and flow (speed) control valves in the circuit.

Because the pump and motor leak internally, which allows fluid to escape from the loop and drain back to the tank, a fixed-displacement pump called a charge pump is used to ensure that the loop remains full of fluid during normal operation. The charge pump is normally installed on the back of the transmission pump and has an output of at least 20% of the transmission pump’s output.

In practice, the charge pump not only keeps the loop full of fluid, it pressurizes the loop to between 110 and 360 PSI, depending on the transmission manufacturer. A simple charge pressure circuit comprises the charge pump, a relief valve and two check valves, through which the charge pump can replenish the transmission loop. Once the loop is charged to the pressure setting of the relief valve, the flow from the charge pump passes over the relief valve, through the case of the pump or motor or both, and back to tank.

What is the significance of case drain flow?

When a pump or motor is worn or damaged, internal leakage increases and therefore the flow available to do useful work decreases. This means that the condition of a pump or motor can be determined by measuring the flow from its case drain line (internal leakage) and expressing it as a percentage of its theoretical or design flow.

How does this apply to hydrostatic transmissions?

When applying this technique to a hydrostatic transmission, charge pump flow must be considered. In most transmissions, the charge pump relief valve vents into the case of either the pump or the motor.

This means that in the circuit described by our reader, where the motor case drain flushed through the transmission pump case to tank, you would expect to see the flow meter in the transmission pump case drain line reading design charge pump flow. Here’s why:

Say charge pump flow was 10 GPM, of which 4 GPM was leaking out of the loop through the motor’s internals (case drain) and 2 GPM was leaking out of the loop through the pump’s internals. The balance of 4 GPM must therefore be going over the charge relief – but still ends up in either the pump or motor case, depending on the location of the relief valve. In this particular circuit, because the motor case drain flushed through the transmission pump case to tank, you would expect to see the flow meter in the transmission pump case drain line reading the sum of these three flows (10 GPM).

Before any meaningful conclusions can be drawn, the case in which the charge pump relief is venting (motor or pump) must be determined and the two case drain lines (motor and pump) must be isolated from each other. If the charge relief vents into the case of the pump, then it is possible to determine the condition of the motor by measuring its case drain flow, but not the pump. If the charge relief vents into the case of the motor, then it is possible to determine the condition of the pump by measuring its case drain flow, but not the motor.

It is not possible to determine the condition of the component that has the charge relief valve venting into it because there is no way of telling what proportion of the total case drain flow is due to internal leakage – unless of course the charge relief can be vented externally while the test is conducted. While it is possible to do this on most transmissions, it’s not usually a simple exercise.

Using case drain flows to determine the condition of the components of a hydrostatic transmission, without a thorough understanding of closed circuits, can result in incorrect conclusions and the costly change-out of serviceable components.

ABOUT THE AUTHOR: Brendan Casey has more than 16 years
experience in the maintenance, repair and overhaul of
mobile and industrial hydraulic equipment. For more
information on reducing the operating cost and increasing
the up-time of your hydraulic equipment, visit his
web site: http://www.InsiderSecretsToHydraulics.com

Making Sense of Case Drain Flow part 2

What is a flushing valve?

A closed circuit flushing valve (also called a transmission valve or replenishing valve) usually comprises a pilot operated directional valve and a low pressure relief valve. When the hydrostatic transmission is in neutral, the directional valve is centered and the gallery to the low pressure relief valve is blocked. When the transmission is operated in either forward or reverse, the high pressure side of the loop pilots the directional valve. This opens the low pressure side of the loop to the relief valve gallery.

What does a flushing valve do?

In a closed circuit, fluid from the motor outlet flows directly to the pump inlet. This means that apart from losses through internal leakage, which are made up by the charge pump, the same fluid circulates continuously between pump and motor. If the transmission is heavily loaded, the fluid circulating in the loop can overheat.

The function of the flushing valve is to positively exchange the fluid in the loop with that in the reservoir. A flushing valve is most effective when it is located at the motor, assuming the charge check valves are located in the transmission pump, as is the norm.

When the hydrostatic transmission is in neutral, the flushing valve has no function and charge pressure is maintained by the charge relief valve in the transmission pump. When the transmission is operated in either forward or reverse, the flushing valve operates so that charge pressure in the low pressure side of the loop is maintained by the relief valve incorporated in the flushing valve. This relief valve is set around 30 psi lower than the charge pump relief valve located in the transmission pump.

The effect of this is that cool fluid drawn from the reservoir by the charge pump, charges the low pressure side of the loop through the check valve located close to the transmission pump inlet. The volume of hot fluid leaving the motor outlet, that is not required to maintain charge pressure in the low pressure side of the loop, vents across the flushing valve relief into the case of the motor and back to tank, usually via the pump case.

How does a flushing valve influence the process of using case drain leakage to determine the condition of a transmission?

The technique is the same as that outlined in the last issue. As explained above, if a flushing valve is fitted to a transmission, it acts as the charge pump relief valve once the transmission is operated in forward or reverse. So if the flushing valve vents into the case of the motor, then it is possible to determine the condition of the pump by measuring its case drain flow, but not the motor. If the flushing valve vents into the case of pump, then it is possible to determine the condition of the motor by measuring its case drain flow, but not the pump.

This reinforces the point that using case drain flows to determine the condition of the components of a hydrostatic transmission, without a thorough understanding of the circuit in question, can result in incorrect conclusions and the costly change-out of serviceable components.

Making Sense of Case Drain Flow part 3

In my previous articles, I described the technique for determining the condition of a hydrostatic transmission using case drain flow, and discussed the role and influence of a flushing valve when doing this.

In response to these articles, some readers were still confused about the influence of the charge pump when determining case drain leakage.

One reader held the view that, assuming the charge pump relief vents into the case of the motor and the motor case drain line is isolated from the pump, then transmission pump leakage is determined by subtracting charge pump flow from the total flow from the pump case. For example, if total charge pump flow was 10 GPM and the flow-meter in the pump case drain line was reading 15 GPM then transmission pump leakage would be 5 GPM (15 – 10 = 5).

This is incorrect because it suggests that a hydrostatic transmission can leak more than the total available flow from its charge pump. It cannot. That is, it is impossible for the flow meter in the pump case drain line to read 15 GPM when the total available flow from the charge pump is only 10 GPM, as in the above example.

The reason is simple. Because the function of the charge pump is to make up losses from the loop through internal leakage, if total losses exceed available charge pump flow, the transmission will cavitate. If in the above example, the transmission was leaking 5 GPM more than the total available flow from the charge pump, there would be a serious deficit of fluid in the loop. In practice, the transmission would destroy itself through cavitation before it got to this point.

Let me explain this another way. Let’s assume we have a transmission that has a volumetric efficiency of 100%, that is, the pump and motor have no internal leakage. The loop has a total volume of two gallons and is full of fluid. Because there is no internal leakage there is no need for a charge pump.

The pump is stroked to maximum displacement, which circulates the two gallons of fluid in the loop at a rate of 50 GPM. Because it’s a closed loop, with no leakage, the flow from pump to motor is 50 GPM and the flow from motor to pump is 50 GPM.

Now let’s introduce internal leakage of 0.5 GPM in both pump and motor. The result is that, with no charge pump to replenish the loop, after one minute there will only be one gallon of fluid left in the loop (the other gallon will have leaked back to tank). Within a second of the transmission starting to leak, the transmission pump will start to cavitate and the severity of this cavitation will increase with each passing second until the transmission destroys itself.

Now let’s install a charge pump with a flow rate of 1 GPM in the circuit. Problem solved, temporarily at least. With 1 GPM leaking out of the loop and 1 GPM being replenished by the charge pump the status quo is maintained… until wear causes the internal leakage of the transmission to exceed 1 GPM.

As you can see, it’s not possible for the internal leakage of a hydrostatic transmission to exceed the flow rate of its charge pump. Charge pump flow rate is typically 20% of transmission pump flow rate. This means that volumetric efficiency can drop to 80% before the transmission will cavitate and destroy itself. The trick is to overhaul the transmission before this point is reached.

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