There is a generic rule for close loop systems, to select the charge pump size: pump displacement should be at least 10% of the combined displacement of the pump and motor. To understand where this value comes from let’s figure-out all factors in the system what influence on a charge flow:
1. The charge pump has to supply enough flow to compensate for leaks due to the volumetric efficiency of the pump and motor. This is why sometimes the charge pump called a “replenishing pump”.
In general, volumetric efficiency of pump and motor in the hydrostatic transmission is around 96..97%, so system overall volumetric efficiency is around 93%. It means 7% of the theoretical pump flow is the leaks which need to be somehow compensated.
2. Addition flow needs for flushing of both main pump case and motor case to remove the hottest oil from the system.
Adding a hot oil shuttle valve at the motor removes the hottest oil in the loop through the motor case drain. Normal practice is to run the motor case back to the bottom drain port of the main pump then from the top pump’s drain port to the cooler (see the picture above).
There no ways to calculate exactly the required flushing flow through the motor. You need some experience with different applications and motor sizes, you have to do tests and check recommendations from the motor manufacturer. The generic rule to find the best flushing flow rate: motor should not be overheated in the worst-case mode.
3. Charge pump itself has huge internal leaks.
Mostly charge pump is a vane or gear type, and the volumetric efficiency of these type of pumps is very low, around 80%.
4. A small qty of flow is required to power the pump’s control system.
This qty of flow is pretty small.
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Of course, different systems require different approaches to the calculation of charge pump displacement. And sometimes we have to make some compromises. If you have doubts – just ask a question!