O.T.- Can you help me with a hydraulic system mod?

[Gordon Heaton]

I have a PTO driven wood chipper with a self-contained hydraulic system for the infeed roller which is driven by a single hydraulic motor. I need to tap the pump output to run a low-pressure (200 psi), low flow auxiliary cylinder, but when the infeed roller is free-wheeling (no load) there is little resistance to flow and therefore no 'pressure' developed by the pump. Check me here: I believe if I restrict the flow from the pump with a 200 psi relief valve, and tap ahead of that valve it will provide me with the pressure I need. The 'bypassed flow' from this valve will continue on through the reversing valve, speed control and motor instead of being dumped to tank return. When the motor is loaded, the pressure will rise, potentially to system max pressure and this will also be fed to the aux cylinder. This is not a problem since the aux cylinder is rated 3X max system pressure (but I only need 200 PSI for this application), and the speed of actuation is not relevant.

My main concerns are:
1- A simple ball & spring relief valve (shop-made) will probably be noisy, and may create excess heat. But how much noise and heat at just a 200 psi bypass?
2- The flow and pressure on through the existing components will change, but in what manner and to what extent? I can sacrifice some top-end motor speed since I never run the feed flat-out anyway.

I've looked at commercially available pressure-reducing valves but they only reduce existing pressure and require dumping excess flow to tank return, they are expensive and complicated to boot.

I'm retired, this is my own equipment, my own risk and I don't give a hoot about liability. I just want it to function reliably. Therefore, if you have any suggestions I'm all ears. I have some experience in repairing and maintaining simple hydraulic systems but absolutely no clue as to design issues like this. Tomorrow I plan to start on the relief valve.

 

[gbent]

Do you need the 200 psi hydraulics all the time, or just when the infeed roller isn't running? If its either/or, I'd use an open center valve with relief before the roller valve.

 

[Houdini]

Not a hydraulic guy, but yeah I would be concerned about the volume after still being sufficient.
Your converting not only a potential pressure in the base system, but also a given volume flow.
restricting it to build pressure will yield I would think a lower volume flow.

Yeah its too bad you cant find a off the shelf, I would think there would be something usable to quick test with.

Good luck

Edit: Wonder the availability of a someone or free opensource hydraulic simulator.

Free Opensource Hydraulic sim.

 

[Gordon Heaton]

Do you need the 200 psi hydraulics all the time, or just when the infeed roller isn't running? If its either/or, I'd use an open center valve with relief before the roller valve.

Now that's an idea! I only need the aux unit when the infeed isn't running. I'll have to think it through for unintended consequences. Thank you.
(The aux cylinder is used to raise the infeed roller to accept larger branches. As is, it's supposed to be compatible with limbs up to 4" but in practice the roller diameter is too small and merely bumps against larger stock. Right now I pump the aux system manually, but 25 strokes every couple of minutes really slows things down.)

Not a hydraulic guy, but yeah I would be concerned about the volume after still being sufficient.
Your converting not only a potential pressure in the base system, but also a given volume flow.
restricting it to build pressure will yield I would think a lower volume flow. . .

All true. I was hoping on a hunch that the remaining flow would be sufficient. I'll take a look at that sim you linked. Thank you.

 

[Gordon Heaton]

A follow-up question for gbent: I need the aux cylinder to remain extended until ready to feed (the return stroke is spring-loaded). Are there control valves that will stay 'on' until moved? All I'm familiar with are made to return to center when released.

 

[tree hugger]

Your scheme would probably work but you would be introducing a 200 psi pressure drop (heat generator) when ever the machine runs regardless of function. Best IMO to use a 3 way diverter valve when you need the cylinder. The system will operate at whatever "low" pressure the cylinder needs until it bottoms out. All should be good.

 

[Gordon Heaton]

Thanks. I don't have documentation on system pressure so I'm taking a wild guess at 1500-2500 PSI. I'd measure it, but loading the feed wheel is impractical and blocking flow to observe pump pressure would be a pain. If the system runs closer to 2000 PSI I think I can live with a 200 PSI drop. I'm not certain, but the system for the next size chipper (6X8) is identical as far as I can tell, with the exception of drive wheel diameter so there should be excess capacity in my smaller unit.

edit: this is what I'm working on- https://woodlandmills.com/wc46-4-pto-wood-chipper/

 

[gbent]

Does your tractor have its own hydraulic system? Preferably with power beyond capabilities? As to the valve, yes. You want one with a detent.

 

[Gordon Heaton]

My tractor does have power beyond, and I considered tapping that but would much prefer having everything self-contained in the chipper. I'll look around for a suitable control valve. Being an old retired guy I don't want to spend any more than necessary to make it work. Meantime, I may make the ball/spring relief valve and give it a try. Maybe it'll actually work!

 

[boslab]

The true hydraulic engineer is coming out there Gordon, that’s what all the ones I’ve had dealings with seem to do!, split the line, stick a tee in and pray!, I think they all went to the same school myself.
Damn think I visited there as well, good news is if the lines are marked it’s easy to put back.
Getting a cheap hydraulic test set and hoses is handy too, insert test point, screw on guage. I had one for the fluid power regulatons over here ( they are very touchy about stored power in systems before working on them due to lots of accidents with things moving about and crushing things and people)
Mark

 

[true temper]

I am thinking installing a cheap power beyond single spool valve in the system. I don’t think you will need a detent valve because the work ports will be closed off dead when the handle Is the middle. Fluid will flow through when the handle is in the middle “neutral “.
It will be cheaper to build and try the valve you are talking about, but it will creat heat how much I don’t know.
I built a valve like you want for another application using an inline hydraulic check valve and changing the spring to get the pressure I wanted.

 

[dkmc]

Being an old retired guy I don't want to spend any more than necessary to make it work. Meantime, I may make the ball/spring relief valve and give it a try. Maybe it'll actually work!

Have a look around EBAY, lots of low(er) cost components there.

 

[boslab]

I think it’s either a 3/2 or 5/2 spool valve you need, ie 3 port 2 position or 5 port 2 position , but I’m guessing!
Tried looking on the rexroth site, all sorts of help there for valves.
Mark

 

[Gordon Heaton]

. . .I built a valve like you want for another application using an inline hydraulic check valve and changing the spring to get the pressure I wanted.

I like that idea, it would sure go quicker for a test!

 

[svs]

You could use an “adjustable priority flow control” to split off a small flow to a spool valve to operate the cylinder.

 

[Gordon Heaton]

That's something I wondered about. Every flow divider I've encountered was 50/50. If such a device exists it seems like it would fit the bill, provided it isn't too expensive! I'll search those terms- thanks.

well... these devices from Parker or Sun are very expensive. Since I do already have a cumbersome method in place to get the job done, I'll pursue a budget solution for the lazy man in me.

 

[tree hugger]

Check these guys out. Good stuff at reasonable prices.

Brand Hydraulics Co. | Hydraulic Valves & Pumps | Electronic Controls

Brand Hydraulics of Omaha Nebraska designs, manufactures, and assembles a diverse product line of hydraulics and electro-hydraulic components including hydraulic valves, electronic controllers and hand pumps which it sells to customers world-wide.

www.brand-hyd.com

 

[ViktorS]

Flow priority valve is the way to go, they're made to solve exactly this problem.

 

[ViktorS]

That's something I wondered about. Every flow divider I've encountered was 50/50. If such a device exists it seems like it would fit the bill, provided it isn't too expensive! I'll search those terms- thanks.

well... these devices from Parker or Sun are very expensive. Since I do already have a cumbersome method in place to get the job done, I'll pursue a budget solution for the lazy man in me.

If you would go this way, make sure that you put a pressure relief valve after the flow divider. If it's a gear divider, it can also act as a pressure multiplier!

Not applicable if it's a valve divider.

 

[Gordon Heaton]

Running out of time, I purchased a cheapo ball/spring relief valve from Tractor Supply. I replaced the spring to bring the adjustable range down to 200 PSI and it works beautifully! Turned out to be a very clean installation and after 20 minutes of running full flow bypass the temp rose 15F. I suspect nearly all of that is from normal pump & original valving losses. There is no discernible noise, I get full infeed speed and the only thing left to check is whether there will be any loss of torque (I don't have any large limbs left at the moment to test). I suspect there won't be any loss because the reversing valve incorporates a pressure regulator that is set more than 200 PSI below the pump bypass so... I'm a happy guy. The simplest and cheapest won!

Thanks to everyone who contributed.