Home-Built Hydraulic Post Pounder; Speeding up the freefall

[CobyRupert]

You might "rethink" the mechanism.

Use the hydraulic cylinder to raise the weight up to a mechanical latch.
When the cylinder retracts to it;s fullest, it releases the latch by way of a mechanical link.
The weight drops in free fall. Ready to be raised again up to the latched position.

If simple dumb steel works, there is no reason to pump oil ;-)

I was thinking that, before I deleted my post....but that’s “cheating”. I’d be too stubborn, even if it cost me 3 times as much and took twice as long to get it working; there’s a hydraulic solution dang it!

Have we determined where the restriction is yet? Spool valve? Hoses? Cylinder vent? (I assume we can eliminate back into tractor by using short big hose and a bucket? )

 

[Tinhack]

I believe a mechanical drop is what I said in post #9.

 

[KubotaCrossing]

Have we determined where the restriction is yet? Spool valve? Hoses? Cylinder vent? (I assume we can eliminate back into tractor by using short big hose and a bucket? )

Yepper. Back in post #14 I said:
"Several contributors have suggested that the problem is that the hydraulic fluid flow is being unduly restricted. The spool valve has a 3/4" work port. I'm currently reducing that down to 1/2" hose, then finally the port on the cylinder is only 3/8"."

In that post I outlined 3 possible ways of addressing that restriction via "plumbing" changes. I'm was looking forward to getting feedback on those approaches. Sorry for being so wordy it. No hydraulics suppliers out here so trial and error costs me about a week for each pass.

Yeah, I'm still looking to resolve this with a 100% hydraulics solution.

PS
Here's post #14:
https://www.tractorbynet.com/forums...lic-post-pounder-post5902531.html#post5902531

 

[CobyRupert]

Ok, spool port size, hose size, cylinder port size.
My background isn’t in hydraulics, but will a 3/8” diameter (3/8” thick?) “port” have more or less resistance than (say) 10’ of 1/2” hose?
Same with spool valve: What relationship does the port size have to the internal passages?
Where do you think the major restriction is that stopping it from working? Or do you think it’s all of them adding up?

Doesn’t your second option (using 2 cylinder ports without the tee) mean less fittings?

 

[Birdhunter1]

The issue could be the cylinder itself. The only experience I have is the Shaver that I own and a few other model Shaver copy cats. On mine the piston rod does not have seals on it just a nut and an alignment guide. If the cylinder you are using is a dual acting cylinder converted to single acting it is possible the seals on the piston are creating a drag not allowing it to drop fast.

 

[BravoXray]

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.

I agree. We made a driver for shoreline sheeting back in the 90's. We used a hydraulic cylinder to lift the hammer with a mechanical trip mechanism at the top that disengaged the hammer and let it fall. Worked like a charm.

You'll never be able to get the hydraulic fluid the flow out fast enough to make it work to it's potential, as flow restrictions increase the faster you try move it out of the cylinder. You are also fighting the internal friction of the piston and the friction of the glands on the rod.

 

[zzvyb6]

If I'm not mistaken, a post pounder uses the hydraulics to raise the sledge upwards and into the channel where it compresses a very stiff spring. The latch is released and the spring adds quite a bit more downward acceleration than just gravity by itself. Stick a rail car spring in there are watch your posts turn into toothpicks !

 

[KubotaCrossing]

"Yeah I'm Free! Free fallin"

In the last episode of this saga I was stumped by the problem of not being able to get the weight to drop with enough force to actually drive a fence post. After several trial and error attempts I've got it fine tuned and doing what I want.

The problem boiled down to simply not being able to evacuate the hydraulic fluid in the cylinder on the down stroke fast enough. Doing the math, I would have to evacuate something like 64 ounces of fluid in under a second for this to work. It just wasn't doable with the cylinder, spool valve and plumbing I had on hand.

The solution was a simple set of mods to the hydraulic cylinder (see attached image) and converting the spool valve. Originally I started out with an off-the-shelf double-acting tie-rod cylinder. Since it's a tie-rod cylinder it was easy to take apart and reassemble. The modifications were:

1) Plug the hydraulic port on the rod-end of the cylinder, plug the B port on the spool valve and remove the hydraulic hose. My spool valve came as a 4-way with a kit to make it a 3-way. This mod effectively converts the cylinder into a single-acting cylinder.

2) Remove the plunger at the end of the rod. It's held on with a big nut so that was easy. Replace the 2" plunger with a 1"-ish O.D. disk/plunger. This replacement disk is smaller in diameter than the modified cylinder bore (see step 3) but it doesn't act as a plunger in a double-acting cylinder does. In fact, fluid passes by it easily. The purpose of the disk/plunger is two-fold:
a) prevents the rod from shooting out the end of the cylinder when the rod is fully extended
b) centers the rod in the modified cylinder on the up and down strokes

Functionally, I could have stopped here. After the first two mods I was able to get the the weight to free-fall unencumbered. But I wanted to do a little more tinkering and see if I could speed up how fast the rod extended (lifted the weight.) After trial and error, came up with mod 3.

3) Insert a smaller diameter pipe into the cylinder. This effectively reduced the bore of the cylinder and the amount of fluid needed to fill it. Wow what a difference it made.

So how and why does this work? On the up stroke, fluid is pumped into the closed cylinder. The cylinder fills to the point where pressure is finally exerted on the rod and disk/plunger equally. Fluid washes right on by the disk/plunger. Somethings got to give so the rod extends lifting the weight. On the down stroke the valve is opened and the weight pushes the fluid out. In this setup the down stroke is displacing only the equivalent volume of the rod and disk/plunger, not the whole volume of the entire cylinder bore as in the original setup. So instead of evacuating 64 ounces of fluid, it's displacing something like 14-16 ounces.

Thinking about it now. Since the fluid washes right on by the disk/plunger, I can send fluid to either the top or the bottom of the cylinder -- it doesn't matter. I'll need to take another look at that and see if I can further simplify the plumbing.

So what's next is coming up with some scheme to adjust the driver left-right and in-out. I could d it with hydraulics but think maybe first cut will be with simple turn-buckles.

Cheers!

 

[Birdhunter1]

I should have mentioned earlier the rod on the Shaver has nothing on the end except a fluted rod guide to keep it aligned in the bore of the case. Other than that it is just a rod in a tube.