shocks

[keyur trivedi]

Hello everybody,
Previously my problem was there was a jump(shock) found in my machine tool cylinder during the movement of my DSG 01 3C 60 yuken make solenoid valve's spool. It was well solved by using the DSG 01 3C 6 yuken make valve(shock proof) as per guidance of MR. DOUG. But now my problem is these 3c 6 valves are special type and take too much time to be delivered after placing order(4-5months). So If there any other option for my problem. Can i use counter balance valve to tank line for that? or any thing else? I don't want to go for propotional or servo valves. I know the best option is 3C 6 but if there any other option to remove these shocks it would be great for me. These shocks must be accurately removed for machines having wood packing cycles only! So i can not place a bulk order and that's why it takes long time. I hope I can get the best answer here, since it's the best website for freshers like me in hydraulics.

[Doug Hanson]

I don't think a counter balance valve will help too much in this applictaion, just do to the regen effect of the open transition type spool, it may still surge. If there were a surging problem not do to regen effect, it would help, but I think this is a bit different.
I can think of three options, 1. find another supplier of valves like Hystar, or Eaton ..etc... 2. Get yuken to ship you the spools seperately so that you can change them out. 3. get a similar profile spool (catalog may show profiles) and machine them to provide the proper closed transition spool function.
You could try a counter balance. Trial and error or calculate the cylinder ratios, pressures, counter balance pilot ratio. would need to know the back pressure induced during the transition, which will be hard to read because it is so brief. If you had a pressure transducer and a way to record peak pressure, that would be the easiest.
For the other readers, the original post is here: viewtopic.php?f=4&t=43

[keyur trivedi]

hello mr. doug,
Thanks for your reply. I am trying to do trial and error with counter balance. Hystar and eaton don't have suppliers in my region, though it's giant market for hydraulic parts. Here only 2 big names are there, Rexroth and yuken. I thought some other possibilities to solve this problem. (1) If i change my meter out circuit to meter in and place a counter balance at T line? or only meter in? (2) If I use a 2-way pilot operated modular(sandwich) check valve with my dcv?(3) Right now i am using P-T connected dcv in center position, should i change that? Which of the above 3 looks more feasible?

[Doug Hanson]

1) If you go meter in with a counterbalance on the T port, the counterbalance valve will generate back pressure so that when the directional control valve shifts through the open transition, it will cause the cylinder to lunge in regen fashion. If you go to just meter in control you will loose the stable feed rate that you have with meter out.
2) I don't think PO checks will help in the transition stage, as the open transition and nature of the P to T valve will keep the PO check open. The PO check may cause a chattering problem. You could maybe get away with a sequence vlave in the P port, set slightly higher than the amound of back pressure seen during the transition shift and a anti reverse flow check valve in the tank port.
3) Changing the system design is always an option if your open to it. Let me think on it a bit and I will let you know. I'l be working out of town for a couple of days, so it might take a bit of time. Would you be open to changing to a closed center, variable displacement, load sensing system?

[keyur trivedi]

hello mr. doug,
Thanks for your reply. Yes i would like to change my design since this has became sever problem for me now. Closed centered vavles, will it work? Should i use P blocked and A-B-T(tandem) connected dcv with PO check valve? It would be great if you suggest me some other ideas also! Thanks for your help.

[Doug Hanson]

If you wanted to go to a closed center, load sensing system with a variable pump I would suggest the following diagram. The advantage in this system is that the output of the pump is proportional to the adjustment on the variable orifices. The pump will try to maintain A constant pressure drop of 220 psi across the orifice. When the feed valve is in neutral, the load sense line is vented through the A+BtoT spool, so the pump destrokes at 220 psi, creating very little heat. The reason for the counterbalance valve on the extend side of the cylinder is because of the A+BtoT valve. When the valve shifts to neutral, if there were only a counterbalance valve on the retract side, the cylinder would jump back (retract) until the pressure/force balance was equal across the piston, so the extend side CBV will prevent this.
The two way solenoid valve is there to give you your rapid feed. There is a reverse check valve parallel to the orifices so that retract will be rapid.
A similar system can be made with a fixed displacement pump and a Load Sense unloading valve.

[keyur trivedi]

hello mr. doug,
Thank you for your valuable suggetions. Now i think the problem can be solved!

[Doug Hanson]

Here is a fixed displacement load sensing version. Should be able to use your existing pump?

[keyur trivedi]

hello mr. doug,
Thanks for the help. In last circuit i have few questions. when the main double solenoid valve is in neutral (p-blocked) the vertical two way(pilot controlled) valve will be opened, so pump line will be tanked directly, is it right? now if p connects a or b then how the cylinder will be actuated since p is connected with tank already via pilot 2 way valve? I am confused about the working of two way(pilot controlled) valve, how it senses the load? Since i haven't used load sensing device before! You may find these questions silly! in short if p is blocked(neutral) what will be the position of pilot 2 way valve and how it achieves? And if it is p to a or b what will be the position of pilot 2 way valve and how it achieves? load sensing is infact a new technique for me. is it possible to use it in giant presses of more then 200 tons capacity? Thanks for your guidance!

[Doug Hanson]

The two way two positon pilot operated valve is a comepensator valve, or some people call them hydrostats. It is used as a load sensing unloading valve in this application. the purpose of the valve is to unload the pump at the sensed pilot pressure plus the pressure value set by the bias spring. If the bias spring had a value of 200psi and the circuit pressure was 1000 psi, the pump would try to achieve 1200 psi. If the differential of 200 psi was achieved, the pump would unload.

So when the control valve is in neutral and the pilot line is vented, the spool will remain blocked until the pump pressure reaches 200 psi, the unloading valve then unloads the pump. When the control valve is shifted, that 200 psi is now available at the actuator and sensed through the appropriate check valve, and into the unloading valve pilot line, the pilot pressure plus the bias spring shifts the spool to the closed position and will remain in the close position until the pump pressure is 200 psi higher than the load pressure. The load sensing system combined with the variable orifices in the system create a pressure compensated flow control, the unloading valve will always maintain a constant pressure drop of 200 pis across the orifice regardless of load pressure. Constant pressure drop across an orifice = pressure compensated flow control.

There can be some draw backs to load sensing systems, reponse time being the greatest challenge.