FluidPower.Pro

Semi-closed hydraulic circuits

Have you ever heard about the semi-closed hydraulic circuit?

The idea is to use open loop pumps in closed loop circuit. In the latest eighties, Mannesmann Rexroth (yes, that time Mannesmann yet!) even offered pumps A4VSH, especially designed for semi-closed circuits.

Bosch Rexroth A4VSH pump

Bosch Rexroth A4VSH pump, from catalog RE92110/01.89

The main difference between closed loop pumps and semi-closed loop pumps was in addition block mounted to the ports A and B with check valves which let to supply suction flow from the tank, while mainstream comes from the motor to pump’s suction port:

Since that time Rexroth still builds A4VSH, but there are a limited number of sizes and options available and its no longer a mainstream published option.

But to make a semi-closed loop you do not need some special pump, it’s really easy to turn most standard open loop pump into a semi-closed loop circuit. Of course, you have to have some knowledge about the application and actuators before trying to apply it. If the pump does not have an integrated boost pump and check valves these are commonly added via an external gear pump and external check valves.

Here is the simplest example of the semi-closed circuit schematic where a regular open loop pump with LS control is involved to run two motors with independent proportional control of their speeds:

Parker Gold Cap pumps Interactive Tool

Parker Gold Cap pumps interactive tool

Parker Gold Cap Pumps Interactive Tool

Just have been gotten an interesting link from Parker distributor for their Gold Cap Pumps interactive online software:
https://divapps.parker.com/divapps/hpps/gold-cup-pump/

Here you can:

  • take a look at how different valves work internally in the pump;
  • check schematic for each pump’s option and size;
  • learn the terminology of pump internal components;
  • watch the video and download PowerPoint presentation about these pumps;
  • at the bottom of this software, you can find a link to the eConfigurator.

My notes:

  • Gold Cap pumps are pretty complicated, so any additional info like this is always helpful.
  • It will be awesome if the pumps schematics can be downloaded in DXF format to integrate into my projects.

Charge pump displacement

Close loop circuit

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.

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!

SUN Hydraulics – delivery time issue

SUN delivery issueWorking on the design of new projects I like to involve SUN Hydraulics valves and use them a lot. They are not expensive, pretty reliable and have uncredible understandable paper and online catalogs.

All these major benefits coming to naught as soon delivery time even of very wonderfull products makes impossible to use them in projects. This is exactly what going on these days.

Today I got a quotation from a local vendor for the valve FDEA-LAN with delivery time 31 weeks!!! It is longer than my project needs to be completed!

Helios Technologies (formerly known as Sun Hydraulics) has three facilities in the Americas, two in Europe and already three in Asia-Pacific (one more plant in China has been opened this year). But this fact doesn’t help to provide SUN valves faster.

Because the company, where I’m working now, has fillies in Europe we know the issue with the SUN product delivery time is currently common for the whole world. There no any comments from SUN and its parent Helios at their websites and I see the delivery time is increasing every month.

Therefore, I’m seriously starting to think about solutions on how to proceed. Problem is a lot of manifolds that have already been designed in our projects use SUN cavities. And to re-design of these manifolds is a pain, you know why. I’m seriously thinking to use SUN clones to cover these positions. Currently, I dug the next manufacturers:

  • Winner Hydraulic. This is a manufacturer from Taiwan, but they do not have distributors in North America.
  • PressureBoss Hydraulics Inc. From their website is not clear what country their plant is physically situated. They have distributors in the US but not in Canada.

For new projects, I’m starting to look at Parker and Eaton cartridges. They have a pretty good assortment and actually last months I already used some of their valves as quick replacement options.

How you are dealing with SUN Hydraulic delivery issue?