Single acting cylinder with dual acting pump

[Ray2210]

I bought an ex grain truck that had the hoist removed and am in the process of putting a deck on it with a really good scissor hoist I picked up used really cheap. I really want the dump feature. I completely didn't notice that the pto pump that's still on the truck is double acting. The cylinder for the hoist is single acting and I'm not wanting to drop 300 bucks for a new one. I am no hydraulics expert.
The pump functions in a manner where when the pto pump is engaged there is pressure to either the extend or retract side of a cylinder. There is no neutral position so when you engage the pump it's one or the other. Generally when you engage the pump the selector will be in down. In such a case the pumps relief valve must bypass as it will when the selector is up and the bed reaches full height. When in up I have verified a clear return to the tank for the down side and vice versa. I have concluded from this that I can plug the down port on the pump and it should relieve and bypass internally just as it would if the bed was down with a double acting cylinder that was now bottomed out. When the selector is in the up position the up port will pressurize and the bed will rise and once again the relief sholuld bypass at the top. At this point I assume that I should be able, even with the pump engaged, to switch to down and the bed will lower with a gravity return and no harm to the pump. I should also be able to simply disengage the pump and let the bed return with gravity the same way. Am I missing something or are my assumptions likely correct?

 

[kennyd]

Am I missing something or are my assumptions likely correct?

Most likely you are correct. But one possibility to explore is that the cylinder on the dump may be a DA cylinder with just a vented plug on the rod side. If it is, then you can just run a extra hose and use the pump as intended.

 

[Ray2210]

The cylinder is a snap ring cylinder with a small weep hole/vent and no port. The gland is really only a dust seal to the rod and would not sustain hydraulic pressure if a port were retrofitted. Thanks for your input.

 

[mmurphy]

I would like to see a picture of this pump. If it is like you say, then plumb the one "port" to tank. All grain trucks I was around, you disengage p.t.o., push your valve knob opposite direction, and gravity would lower it.

 

[Ray2210]

The pump has two ports. With the knob in if you spin the pto shaft fluid comes out one of them and you can blow air into the tank and it will bubble. With the knob out fluid will come out of the other port and you can blow air into first port and it will bubble in the tank. Therefore pressure to the down side with Pto engaged and up is in return. If the hoist is all the way down then the pump will be in pressure relief bypass at that time or else something will blow. Pull the knob out and up will be in pressure with down in return. Because of this I'm making an educated assumption that plugging the down port will not cause any condition that would not already exist when the hoist was down with the pump engaged. Pull the knob out for it to go up. Kill the Pto and push the knob in and it should go down. Even with the Pto in it should go down fine and all should be well as long as the pump is not allowed to bypass for an extended period of time and overheat. The known quantities suggest that my assumptions should be correct. If somebody can add anything I'm missing I'd appreciate it.

 

[kennyd]

If it is like you say, then plumb the one "port" to tank.

That is a great idea, plumbing the "down" port to the tank will keep the system from going into relief when the bed lowers.

 

[Ray2210]

Thanks. I've considered plumbing a return to tank but the unit is a bit tough to get at and installing a bung in the tank will mean metal filings in the tank so it will need to come out to do it. I've been kicking around the idea of adding a return to the cap so as to not have to drill the tank.