Hydraulic Post Pounder

[Spike-MI]

Okay, so I am cheap (and poor). I could not afford to buy a Vector Post Driver. The best price I could come up with was over $5k plus freight, ouch. So, I built one. As you will see by the photos, I modified Vector's loader-mount model. Theirs uses an offset design, which admittedly is better than mine, but very hard on the loader in my opinion. The big advantage of the offset is that you can drive right down the fence line. With my design the pounder is centered on the loader, so you have to square up to each post; you can't just drive down the line.

I designed the pounder with a 500lb weight and drop of 6'. The "as built" unit came out with a 530lb weight, and 68" of drop. Close enough. It has a 3/4" AR500 independent post cap that travels independent of the weight on the main mast. I also designed a tilt cylinder to adjust for changes in grade, with the hope I can actually drive the posts somewhat plumb.

By now you are probably wondering why I built one this large. Well, I have a fair amount of cattle fencing to do and I want to use railroad ties. This should easily develop enough force to drive a railroad tie blunt.

So, I am not posting to brag, but because my hyraulic design is not working and I need some expertise from you fellas. The system uses a 2" bore x 36" stoke 2500psi cylinder. It is set up as a single acting cylinder, kind of... The pressure line is used to retract the cylinder, which pulls down on block, which retracts the rope, and raises the weight. Once it reaches the top, I throw my auxillary valve in float and down goes the weight, VERY SLOWLY. It doesn't fall with gravity like I expected. I believe that I failed to consider is the rate at which the hydraulic flow is restricted as it pushed back to my tractor's main reservior. There must be enough restriction that the weight (all 530lbs of it) drops at about the same rate that it raises with the tractor at idle. However, even at that speed you don't want anything between the weight and the post cap plate because it sounds like hitting something with a 500lb sledge hammer. Just FYI I am using my factory front 3rd valve to run the pounder. It is plumbed from the rear of the tractor with 3/8" hard lines, and has 1/4" standard quick couplers on the loader. I ran a 3/8" flex hydraulic line with a 1/4" reducing quick coupler.

So my question to you much more skilled hydraulic TBN-er's is what is the easiest, cheapest fix to make this thing slam down like the Vector guys' post pounders do?

Here is a YouTube link to their mini excavator mount model in action: http://www.youtube.com/watch?v=pWnZvijS5nE&playnext=1&list=PL71AD198E95ECC30B

Look forward to your advice. Spike, now in Idaho not Michigan (but my heart's still there!)

 

[bcp]

Needs a FAST oil return.

Some discussion here:

http://www.tractorbynet.com/forums/attachments/115722-post-driver-hydraulic-hook-up.html#post1332914


But if slower would work, make a latch for the weight. Retract the cylinder, raise the weight, latch it up, extend the cylinder, drop the weight, repeat.

Bruce

 

[Redneck in training]

That what I would do too. In addition to it I would connect both sides of the cylinder to the hydraulics and use valve with regeneration feature to increase the speed of extension stroke.

 

[wdchyd]

I have seen guardrail pounder trucks and the cyl/pulley arrangement was setup similar but different....

They used a larger dia single acting ram (5 or 6 " dia IIRC) with the cyl positioned between two multiple pulley sheaves.......they used cable that was wrapped around the pulleys multiple times (4 or 5)to achieve a longer stroke/lift ratio ....kinda like a block and tackle with a cyl between the pulleys...

Google Image Result for http://www.photographers1.com/Sailing/BlockAndTackle.gif

I'm having a hard time to describe it but the principle would allow the cyl that stoked 12" to retract the weight 4-5'.......then using large hoses with large valve block with a direct return to tank (not through regular valve) would allow the oil to return to be unrestricted for quicker drops......

 

[600rider]

But if slower would work, make a latch for the weight. Retract the cylinder, raise the weight, latch it up, extend the cylinder, drop the weight, repeat.

Bruce

Thats what I was going to suggest when I first read your post! :thumbsup:
Something to hold the weight up until you can extend the cylnder all the way then drop it like a 1/4 ton of steel!! :laughing:

 

[SPYDERLK]

Thats what I was going to suggest when I first read your post! :thumbsup:
Something to hold the weight up until you can extend the cylnder all the way then drop it like a 1/4 ton of steel!! :laughing:

Good, but slow due to many needed repetitions. Current OP design should work well with an unobstructed return line ... no QCs, short, large diameter tubing, maybe a larger valve. Alternately a cyl sized for min required lift force would go faster.
larry

 

[arkydog]

I'm not sure how to make it, but make some kind of trip release that when the pull cyl get to the bottom, a trip releases the cylinder from the carriage and the wieght free fall and pulls the carriage (pully and rope) fly up and hits a rubber or harder stop. then you have the cyl go back up to attach to the carriage and again start pull the carriage back down and the weight back up.

 

[Spike-MI]

BCP- I think you are right, I just can't find the info I need to set up a "fast return." The issue that I think most of the replies are missing is that a "latch" and free fall system won't work. You simply can't get 30-40 strokes per minute out of a system like that. The Shaver post pounders and the Vector that I copied are using a similar system. The issue at hand is that fluid is only used on one side of the piston in a system like this one. The other side of the piston is dry. As the weight moves down it sucks in air through the breather which has no resistance. The only resistance (and my inherent problem) is forcing out the same fluid that just raised the weight, back through the pressure line and to the reservoir. If you look at the Vector drivers they don't attach a line to both ports of a cylinder, just the side to raise the weight. Still searching...

 

[SPYDERLK]

The Shaver post pounders and the Vector that I copied are using a similar system. The issue at hand is that fluid is only used on one side of the piston in a system like this one. The other side of the piston is dry. As the weight moves down it sucks in air through the breather which has no resistance.

The Shaver cyls are made differently than you describe. In the Shaver cylinder the thing we normally call a piston is essentially a triangular space frame affixed to the end of the rod. The rod is guided on its exit seal and the "piston". Incoming fluid is able to easily get to both sides of the piston. The net raise force thus achieved is the pressure times the rods area. Very little fluid is moved [and no air] during the 48" stroke -- maybe as much as a quart. ... Still, the flush during fall probably gets as high as 40gpm and needs a large return to reservoir. The speed of downward motion also would definitely be impeded by air being moved on a full area dry side of a normal piston cylinder as you describe. Since you are sucking air the force limits to ~45 pounds x 2 for the pulley arrangement. That 90# adds to the fluid flush loss.
larry

 

[k0ua]

This is in the "just so you know department" not being critical here, but in using the wire rope clamps, (yes I know that is not wire rope) the minimum number is 3 and the way to put them on is so that the "dead end" is adjacent to the U clamp not the saddle part of the clamp. as in the old saying "Do not saddle a dead horse" as a way to remember it. The top ones are correct (but need 3) the bottom ones are incorrect, I am not sure how well they will hold with the nylon rope, but probably OK. Again not trying to start anything, just passing along standard practice.

James K0UA

http://www.sbic.com/smos/may06/smo060503.pdf