Home-Built Hydraulic Post Pounder; Speeding up the freefall

[KubotaCrossing]

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I've got a ton of fencing to do this spring that will involve setting several hundred wooden fence posts.

I have a compact Kubota tractor ...


that I have rear remote "hydraulic-enabled" by tapping into the PowerBeyond port on the FrontEndLoader spool valve:

The specs say that the tractor hydraulic pump will produce three gallons per minute. Yeah that pretty low but it should be more than adequate for what I what to do, which is, lift a 300-400 pound weight then drop it letting it freefall. Yeah i know the setup looks fragile but I'm just doing proof of concept right now.

I was driving around in the Root the other day and saw a lonely contraption along the fence line in somebody's yard. Here in the Root, we don't use ForSale signs. If it's on the front lawn, that means it's for sale. I went up to the house and knocked on the door. The rest is history.

Anyway, I picked up a gem. It was probably in a previous life a 3pt-mounted, PTO-driven DanUser post pounder now mounted on a trailer. It was powered by a small gas engine that thru a rube goldsberg (sp?) set of chains and sprockets drove the DanUser friction wheel. The engine was shot. Instead of fixing the engine I removed it and all the friction-wheel/brake stuff. I installed a hydraulic cylinder, spool valve and hoses.

Here's the whole rig:


Shots of the fabricated hydraulic post pounder. Hey! Don't make fun of my welds. I did it with my eyes closed, well kind of, I have cataracts really bad.


The setup of the hydraulic spool valve:
o Port bottom left: From pump (actually the PowerBeyond port on the FEL spool valve)
o Port top right: Return to tank. (actually the one gallon jug for now)
o Port bottom right: Return to tractor. (The other end of the PowerBeyond from the FEL spool valve that goes on to power the 3pt)
o Port top center: B port that goes to the base of the hydraulic cylinder.
o Port bottom center: A port capped. This would ordinarily go to the rod end of the hydraulic cylinder.


So what I've done here is taken a 4-way single bank spool valve and double-action hydraulic cylinder and converted to a 3-way single bank spool valve and single-action hydraulic cylinder. I have a hydraulic line only to the base of the cylinder and nothing but an air vent in the top port of the cylinder. ie I'm depending on the weight of the ram to retract (drop).

For anyone interested, here are the specs on the spool valve:

This monoblock valve is a hydraulic directional control valve. These valves are used to start and stop fluid flow into hydraulic cylinders or hydraulic motors.

Common Uses:

Operate hydraulic cylinders (Double-Acting or Single-Acting)
Operate hydraulic motors ("D" Spool Required)
Install rear remotes on a tractor

Benefits:

1 Spool valve with compact design allows for installations in tight areas
All spools are standard double acting with spring return to neutral position which are ideal for double acting cylinders
Used for open center systems (Pump to Tank Flow) - closed center option available
Anti-drop load checks on each spool
Precision ground and hard chrome-plated spools assure a long life
High-tensile strength cast iron monoblock construction
SAE O-Ring ports ensure solid seal and prevent blockages
Capable of closed center or Power Beyond (conversion plug required)
Vertical or horizontal handle mount
Adjustable Relief Valve Range: 1500 to 3625 PSI
Used as motor or single-acting spool possible (#8 SAE port plug required)

Features:

Max Operating Pressure: P = 3625 PSI, T = 725 PSI, A & B = 4350 PSI
Flow Rating: 11 GPM (40 l/min)
A & B Work Ports: #8 SAE O-Ring (3/4?16) Thread
Inlet Port: #10 SAE O-Ring (7/8?14) Thread
Outlet Port: #10 SAE O-Ring (7/8?14) Thread
2 5/16 Mounting Holes

Here are the specs on the hydraulic cylinder.



So, I put this all together and fired it up.

It works!

Well mostly. The cylinder extends fully. It's not really fast but it is acceptable.

The problem comes in when I retract the cylinder. The cylinder comes down at the same rate that it goes up. It comes down leaving a love-tap ... of course unacceptable for a post pounder.

I'm reaching out to the hydraulics brain trust here. Any suggestions on how to make this freefall?

 

[laurencen]

the pounderI built years ago only used the cylinder to lift, there is no use having the system double acting because you want the pounder to free fall and no restrictions

 

[KubotaCrossing]

the pounderI built years ago only used the cylinder to lift, there is no use having the system double acting because you want the pounder to free fall and no restrictions

So you used a single-acting cylinder? They are pretty hard to find and for some reason really expensive.

 

[KubotaCrossing]

A friend stopped by today and suggested flipping the cylinder over and try powering the cylinder from the rod-end; the rationale being that since the rod-end of the cylinder holds a smaller amount of fluid, maybe it would purge faster.

The hoses are long enough so I tried it. It didn't work in that it wouldn't lift the weight. Powering the cylinder from the rod-end won't lift the weight.

With the cylinder still inverted, I moved the hydraulic line to the non-rod end of the cylinder, so in effect it would be pushing the whole cylinder assembly and weight up.
I tried that. It worked going up and noticeably faster down. But still not fast enough. An advantage to this setup is that the cylinder port is now physically higher than the spool valve so that should give a slight advantage in draining.

The bottleneck still seems to be the rate that the fluid purges the cylinder -- it's being restricted.

Question to hydraulics pros, can these cylinder ports be safely enlarged? The cylinder port is 3/8", the hose is 1/2" and the spool valve port is 3/4". So technically I could go up to 3/4" on the cylinder port and hose before encountering another bottleneck.

 

[old and tired]

...Benefits: Anti-drop load checks on each spool

I don't know anything about post drivers but this listed above seems counter to what you are trying to do. For me, It seems you need a Float valve which lets pressure off the system.

See this video: Post Driver Tractor Attachment - YouTube

 

[Peter 315]

The return port should be 3/4 and dump back to the tractors hyd fill cap..

 

[wdchyd]

Yes, the ports can be enlarged easily.
The commercial units I’ve worked on have a big single acting ram (think big plow ram) and use pulleys under the cylinder and on the top. IIRC the multiple pulleys has cable wrapped around giving it a ratio of cable lift vs ram travel (think block and tackle hoist with a cylinder inside), then a high pressure ball valve with tee used to release the single acting ram and dump directly to tank (return oil not going thru the regular control valve).
If the cable moves at a 4:1 ratio, then the cylinder would need to be 4 times more powerful to lift the hammer.

 

[Tx Jim]

So you used a single-acting cylinder? They are pretty hard to find and for some reason really expensive.

Any double acting cylinder can be converted to single acting by having only 1 hose installed in cylinder & a breather installed in other port. IIRC return oil from post driver control valve needs to go directly to tank(reservoir)

 

[Tinhack]

You still have move air in and out of the open port. It will act as a damper. The only way to reduce that drag is to make the port the same size as the bore of the cylinder.

I think I would have made some type of clicker latch so the weight is falling free. Have the cylinder lift the weight up to point where it's then disengaged from the lift cradle/cylinder. Power return the cylinder to latch the weight in the cradle and the cycle repeats.

 

[Birdhunter1]

The return port should be 3/4 and dump back to the tractors hyd fill cap..

This is the answer to your problem. If you are running the return through your remote valve on your tractor that is your restriction. The return has to freely dump back into the reservoir of your tractor with NO RESTRICTION and the only way to do it is to unscrew your hydraulic fill cap, stick the open ended return hose in there and then operate your driver.

NOTE #2: Do not test your post driver in your garage on your concrete floor. It will drop so fast it will surprise you and it will bust your floor in a manner that will make a jackhammer jealous.