pump unloading schematics question

[RobertHasty]

Hi,

I have a question regarding the pump unloading valve.

Below is the link to the datasheet:

https://apps.boschrexroth.com/products/compact-hydraulics/ch-catalog/pdf/059006XYZ_RE18310-13.pdf

I cannot understand what is the point of the section (valve) encircled in a picture below.
I believe pump P1 could unload through valve 1 without the encircled part.
So what is the function of that part?

Thank you.

 

[FluidPowerUser]

As a former employee of Bosch Rexroth I can confirm the following.

The highlighted valve is not big enough to handle the flow. It’s a small pilot valve that vents the main stage at the required pressure, as set on the spring in valve 1.

 

[RobertHasty]

Hi, thank you for the reply.

So the highlighted valve has the same pressure as the valve # 1?

My question is : couldn't the system work without it?

In this configuration, the P1 should first overcome the spring of the highlighted valve a then unload through valve 1.
Wouldn't be easier (simpler) without highlighted valve?

 

[FluidPowerUser]

It’s valve 1 that’s too small to handle the flow from P1, not the highlighted valve as I said before.

The pilot flow is very small. There is a control orifice in the highlighted valve to make sure there is a good pressure differential. Otherwise the valve will be unstable when the P2 pressure opens valve 1.

If you just had a big valve to unload the bigger pump, it would be very unstable as it opens and closes. It needs to be pilot operated to be stable.

 

[Jacc]

Another reason for having a pilot operated unloading valve (or relief or anything pilot) is that it becomes more distinct in when it opens and closes and it has a flatter pressure/flow curve, not affected as much by varying flow.