dump valve question

[muddstopper]

I recently swapped in a 5in cylinder in place of the 4in one I had on my wood splitter. I also added a 28gpm 2stage pump. My splitter valve is rated for 25gpm and hoses and fittings are #12jic in 3/4npt ports. So i know at max flow my valve is probably a little undersized but I dont run the engine at full throttle and probably aint pumping the full 28gpm the pump is capable of. I am wanting to speedup the cyl cylcle times and am thinking that adding a dump valve to the returnside of the cyl would probably be my best and cheapest bet without going for a new control valve. Problem is I havent ever used anytype of dump valve and am not sure exactly how to pump it in. Since I am thinking the dump valve would have to be in line with the retraction side of the cylinder, just how does this work considering this is also the push, high pressure side when trying to split wood. Seems to me that trying to dump returning oil on retraction wpould also dump oil when under pressure. Someone explain this to me please. Right now I am not having a problem with heat, but I usually only work the machine for short periods of time and probably not long enough to get much heat build up, but the heat I will deal with later as modifications continue.

 

[J_J]

Why would you need a dump valve?

The retracting cyl is already pushing fluid out through the rod end port to filter to tank, at very little pressure.

You can set the kick out pressure, but that is achieved when the cyl is almost fully retracted.

You want faster, you need smaller cyl, larger pump, shorter stroke.

 

[muddstopper]

JJ. I was reading somewhere where somebody added a dump valve to their splitter to speed up cycle times. Not sure how that is supposed to work. My thinking is that if I have 28gpm going into the rod end of the cyl to make it retract, I have to have more than 28gpm coming out of the piston end of the cylinder to return to tank. My control valve is rated for 25gpm so I am already upside down since I am using a 28gpm pump. I havent flow tested to see what kind of flow I have, but I dont run at max rpms either, so I think I am safe to say I probably aint flowing the full 28gpm pump capacity thru my control valve. If I can somehow return oil to tank without flowing thru the control valve, this would probably speed up the cylinder without maxing out my control valve and causing over heating. I dont know if this can be done, which is what I am asking. I dont want to buy an expensive regen control valve, but might consider a dump valve if it will actually work to do what I have described. I dont know if this will work, and if it does, how to plumb it into the system, or if the reults will even be worth trying. I am actually pretty satisfied with my cycle times and can speed it up by reving up the engine, just see no need to burn extra fuel if I can get the same results by other means.

Set and thought about this for a minute. Came to realize that my return oil is going to be restricted to the cyl port size, reguardless of what I plumb in down stream back to tank. Since the ports on my valve are as large as the ones in the cyl I cant see how the person that I read about that had done this, is making it work. What am I missing.

 

[J_J]

You need flow to retract, so you can not dump the flow as you are still using it.

When the lever on a valve returns to neutral, flow stops to the work ports.

Retract mode provides the fastest times anyway.

 

[muddstopper]

I like your first reply best, it made me think about it. I might be slow, but I usually catch on after a while. If the person that added the dump valve to their splitter made it work, he has to have some different kind of set up than I am dealing with. I cant see how adding a dump valve to my system is going to work or help. Now I know i can rig up someway to keep the return oil from passing thru the control valve and dump directly back to tank, expensive and useless and I think unnecessary. I think I will leave well enough alone for now.

Edit to add: A big Thank You!

 

[oldnslo]

Muddstopper,
If you have a cylinder with an over sized rod so that you an area ratio of 2:1 then adding a valve that dumps the cap end end oil to tank may help retract speed. This is sometimes used on regenerative systems to achieve equal speeds or velocities in both extend and retract.

Example: If you are putting 25 GPM into the rod end to retract a cylinder with 2:1 ratio you will be getting 50 GPM from the cap end. The 50 GPM may be more flow than your directional valve could handle.

 

[muddstopper]

My cyl is a 5inx2inx24in stroke. Please explain to me how this works and if it will benefit me to add a dump valve. I fail to see if my cyl port is 3/4 and my valve port is 3/4, how it will be possible to dump the oil back to tank using a dump valve any faster than just letting it go back thru the valve. If my cyl ports where larger than the valve ports, I can see where the dump valve dumping oil straight to tank may be benefitual, but the port size is only going to flow, what it can flow. What am I missing?

 

[oldnslo]

Cylinder areas:
5" bore = 19.64 sq inches

2 " rod = 3.14 sq inches

Rod end area = 16.5 sq inches

your cylinder ratio is base area divided by rod end area or 19.64 / 16.5 = 1.19 : 1 so with 25 GPM going into the rod end you have 29.8 GPM return oil from the base end.

Since your valve is sized for 25 GPM the dump valve would probably be of little value.

What has to be considered is the system loss in the return flow not just the port sizes. Most valves have pressure drop of flow curves with flow P to A & B to T. If your dump valve diverts some of this flow from going through the directional valve you will have less pressure drop through the valve.

example: Valve has 100 PSI loss with 25 GPM flow from B to T. Increase that flow to 30 GPM and the pressure increases to around 145 PSI. At 50 GPM the pressure losses are around 410 PSI. This just based on an orifice size of .341. In reality I would expect the pressure losses to increase more than this going through a valve. In the valve the oil has to flow across the lands, around the spool, out another set of lands, through another port etc.

hope this helps.

 

[muddstopper]

Well heres what I am going to try based on a suggestion from another member. I am going to put a Tee and ballvalve at the port at the back of the cylinder. I will just drop a hose into the top of the tank for return. I will extend the cylinder with the valve open and closed and time the results. If I see a significant difference, I have a electric soleniod valve i will just plumb into the circuit and tee it into the return filter, (50gpm filter). I can add a a micro switch to the control valve to activate on return stoke only. Almost zero cost to try and if their isnt any difference with the ball valve test, then nothing lost.

oldnslo, give me your best guess on what results you think I might see with the ball valve test.

 

[J_J]

Your return flow should be through the return filter.

I think you will see no difference. The fluid will exit the cyl port at a velocity/flow caused by the fluid pushing on the other end.

If all these log splitter manufactures could have gained faster times, they would have done so long age.

If you have to have speed, I heard about this one guy who used an accumulator to accelerate the ram speed, and it worked for a short time before it blew up.