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.

 

[LD1]

I understand the question completely, but agree that you probably wont see much difference. And if you put a tee in the base end, And put the valve on the line that returns to tank, you want that valve CLOSED when you extend. Otherwise you would never build pressure as the fluid going to the rod end from the valve would just return back to the tank. The open/closed test needs to be on the return only.

Dump valves are used on pneumatic cylinders ALOT to increase speed. I actually have used them on can crushers to increase cycle time.

IF the dump valves are similar, they are usually labled. They are like a tee with a check valve in them. And labled C (cylinder), E(exhaust) and P or I (pressure/In). In which case you would put cylinder to cylinder, P or I would come from the valve, and the E would be the return to tank.

A dump valve works the same way you are describing with the tee. In which when you are extending the cylinder, the pressure is comming from the valve and entering the P or I port of the dump, and coming out of the Cylinder port and into the cylinder. When retracting, and the pressure/flow is coming in the Cylinder port, the "check valve" inside closes off the P/I port and opens the Exhaust port and offers an unrestricted path back to the tank without having to go through the valve.

 

[muddstopper]

The whole ball valve thing would just to be for a test. I would manualy open and close the valve, before operating the control valve. I would only be timiing the retaction stroke of the cyliner, not the extention stroke. For the testing, I would just drop the return hose into the tank, but if I endup using a dump valve, it would be connected to the return filter.

Now i dont know if I will gain more speed doing this or not, but It cant hurt to try and wont cost me anything. If it does work, then I just have to buy a couple of fitting to plumb it properly. It will either help or it wont.

JJ, I know I can get more speed by increaseing pump size, replaceing the current ports on the cyl with bigger ports, bigger hoses, changing out the control valve, and I even have a accumilator laying around I could plumb in. I am not wanting to spend the money to do any of these things. I am just trying to see if I can get a little more speed out of what I already have, without spending a lot of cash.

oldnslo, What you suggested about the control valve being rated for 25gpm and the return oil being about 29gpm, makes me think that I might gain some speed by simply bypassing the return away from the control valve. I realize return pressure wont be great, but any restriction would increase pressure and reduce speed. Reduceing restrictions and pressure build up should also reduce oil temp.

 

[J_J]

You seem locked into this against all data given so far.

If this is a manual gate valve, you lose time just moving the handle, and you are going to do this on a repetitive basis. The valve doesn't even have to be pressure rated as long as you don't ever block the return line to tank.

Just seems like that will add more time for a full cycle.

The best you could do is to mount an oversize gate valve on the cyl port in a tee, and dump directly to tank.

 

[LD1]

You seem locked into this against all data given so far.

If this is a manual gate valve, you lose time just moving the handle, and you are going to do this on a repetitive basis. The valve doesn't even have to be pressure rated as long as you don't ever block the return line to tank.

Just seems like that will add more time for a full cycle.

The best you could do is to mount an oversize gate valve on the cyl port in a tee, and dump directly to tank.

I think he is just doing the manual valve to test it out, before he spends the money on a actual dump valve.

I know they do work on pneumatics. I dont know about hydraulics, or even if you can find them. Just a quick google search, and the biggest I could fine was 1/2" ports. If you can find them large enough, and if they work like the pneumatic quick exhaust valves, then there is no valve to switch manually. Its automatic.

But let me ask this question...Since you are in the quest for more speed, why did you up to the 5" cylinder?? Did the 4" provide sufficient splitting force?? IF so, why not use that cylinder, with a larger pump. Going with a larger cylinder AND a larger pump kinda defeats the purpose

 

[oldnslo]

Muddstopper,
is the pressure at which your hi-lo pump is set to switch from high flow low pressure to low flow high pressure. As long as the accumulated system loss is lower than this set point you will not see any speed change. As long as you guides on the splitting head are not binding and your lines, filters, etc. are sized for the 25 GPM flow rate I would not expect to see any speed change.

With 25 GPM flow into the rod end your retract speed is 5.8 inches per second or 4.1 seconds for 24" travel
with 28 GPM flow into the rod end your retract speed is 6.5 inches per second or 3.7 seconds for 24" travel

stop watch is easier than wrench flipping in this case. If you are close to these times it is as good as is gets.

Like LD1 stated,
If you looking for speed your options are limited to a different cylinder, or different pump.

A 5" bore with 3.5" rod cylinder would provide very fast return.

25 GPM would give 10 inches second retract speed
28 GPM would give 11.2 inches per second.

This would require some for of dump valve to achieve these retract speeds.

 

[Scooby074]

Have a look about 1/3 the way down here: A + Log Splitter parts hydraulic pumps valves cylinders

They are offering a 30gpm dump setup for $120.

Must be a viable option for a splitter. Maybe call them?

Prince also offers a LSR valve that offers a fast extend (not retract) that works with a fixed, single displacement pump so it likely is of no value to you. http://www.princehyd.com/Products/MonoBlockValves/RapidExtendLogSplitter.aspx

 

[LD1]

Have a look about 1/3 the way down here: A + Log Splitter parts hydraulic pumps valves cylinders

They are offering a 30gpm dump setup for $120.

Must be a viable option for a splitter. Maybe call them?

Certainly looks like it would work. Allthough I dont see why you couldnt mount it right at the exit of the cylinder as well. Less clutter up by the valve.

 

[J_J]

Maybe add some retract springs to the cyl.

If you don't need 24 in logs, add a 8 in plug and accept 16 in firewood. and that will reduce the cycle time.

What do you want, speed or force.

 

[Scooby074]

Certainly looks like it would work. Allthough I dont see why you couldnt mount it right at the exit of the cylinder as well. Less clutter up by the valve.

I guess they must have mounted it there to save money on hoses?

 

[George G]

My guess would be, They are using a pilot operated check valve. When the cylinder extends the check valve closes. When the cylinder is retracting, pressure from the rod end opens the check valve. Allowing the oil from the cap end to freeflow back to tank.