Post Drivers - Effects of Air Pressure, Mass and Springs on Impact

[FoxLair]

Full Disclosure - I am a building contractor with an affinity to design ways to reduce labor and/or allow me to complete tasks in an efficient way.

I have built permanent docks that utilize 4x4 treated posts driven into the lake bottom. Large pneumatic drivers work great but require an 80+ CFM compressor and a floating platform with a hoist of some sort to operate. On repairs. I have used a hand driver that is square metal tube with a cap and handles welded on. To automate this I added a Pneumatic cylinder to raise and lower this capped tube. It operates on a 5-6 CFM air compressor and works great.

Now the question: My goal is to drive a post with minimum blows while maintaining portability. I can add more mass, add springs or change pneumatic design. I understand how free falling mass and velocity affect impact. But, what makes my brain hurt is determining impact force when i add a pneumatic cylinder pushing downward at a specific PSI. Further brain strain is how adding a tension spring affects impact force (Given an extension spring constant (K), and a distance (d), how does this affect impact force of a falling mass)

What I can speculate is that either Air pressure or springs can increase velocity. I don't need exactness here, I just need to understand how I can better predict outcome and lessen trial and error time (of which I have invested a bunch).

Please help my poor brain in its hour of need.

 

[FoxLair]

Not a bad idea Compositepro, however The air hammers I think of would require a large compressor and accessories, which I am trying to avoid for smaller jobs. Actually a gas powered jumping jack tamper might be able to be modified to achieve a self sufficient driver. Just need to look at impact potential there. I will check on the mechanics of one of these tampers and see if they could be modified.

Berkshire suggested an internal combustion driver, however the off the shelf portable internal combustion drivers I have seen do not have the "umph" to do much more than t-posts. Or, there are larger ones that require a crane.

Meanwhile, the one I have is working OK. auto reversing at end of stroke and speed controlled on the up stoke by choking down exhaust at valve, which allows me to avoid overworking my smaller compressor. I think my best bet is to add weigh and play with air delivery speed on down stroke.

If i try the tamper design I would likely build a new tube.

Thanks

 

[Sparweb]

I'm surprised no one has suggested a hydraulic ram. You can work with 1000 psi instead of 100.
I've used these to install fences around my property:

http://www.aggrowth.com/catalogue/category/351/high-heavy-hitters

Only part of the strike comes from gravity. I don't know why some respondents are trying to work this out with gravity alone. This hydraulic pounder uses the hydraulics to raise the striker, and drive it downward. The effect of the impact is mostly due to the inertia of the striker's mass, accelerated through the downward stroke by [a=F/m] where F comes from hydraulic pressure (~1000psi) on a rather small diameter cylinder so that it advances very rapidly. Somebody might be clever and point out that if the piston is free to advance without restraint then the full pressure won't be developed on the piston, well, I said it first, and the pressure will still be much more than 100 psi of air, and besides, the development of pressure on the piston driving down is also dependent on the flow rate delivered. So don't short-change yourself getting a small hydraulic pump. Or maybe an accumulator would help.

You need to grip the 4x4 post to center the striker.

There will be plenty of interesting issues trying to operate this on a floating platform...

No one believes the theory except the one who developed it. Everyone believes the experiment except the one who ran it.
STF

 

[hydtools]

Sparweb, you beat me to suggesting a hydraulic driver. The 5 to 8 gpm, 2000 psi power units are much smaller than a 100 cfm compresser.

Ted

 

[itsmoked]

[party-pooper]
Hydraulics and water is a non-starter.
In the US hydraulic anything near water is personal disaster waiting to happen.
A colored sheen on water is a $10,000 fine.
[/party-pooper]

Keith Cress
kcress -

http://www.flaminsystems.com

 

[hydtools]

Use biodegradable fluid.
On exit: deleted broken link.

 

[Sparweb]

Trust Keith to see the opposite side of the coin. Here I was worrying about water getting IN to the hydraulics.
Is this stuff any good? I've only been dimly aware of it in the past...

https://www.kleen-ritecorp.com/p-48...ase-hydraulic-fluid-5-gallon.aspx#description

The link that Hydtools posted is broken (for me) but may be similar.

No one believes the theory except the one who developed it. Everyone believes the experiment except the one who ran it.
STF

 

[FoxLair]

Hydraulic is out, the equip is heavy/bulky/messy. My current setup is a 60# driver (Total weight) operated by 6 CFM compressor. Even adding mass, I don't intend for the driver to be over 85#. Portability is the key here for small jobs without a crane or floating platform. It works pretty dang good. Just takes a little longer on each post than I would like.

 

[FoxLair]

FYI - For building an entire dock (100-150 posts, depending on size), a Rhino PD140 or PD200 is ideal with a 100 CFM Compressor and a floating platform with a small hoist. Just impractical for repairs/small jobs (20-30 posts)

 

[hydtools]

Apparently RSC bought Terresolve

https://rscbio.com/

Terresolve fluid was subjected to our hydraulic tool testing and performed as well as petroleum based fluids.
Ted

 

[hydtools]

Properly lubricated pneumatic tools are messy.

Ted

 

[hydtools]

Try this

https://youtu.be/x6hD1vS3JAk

Ted

 

[itsmoked]

Fox; Since you are really critically limited by weight due to the working environment it would seem to me that what you have is proven and works well. Why not just double what you have. Another compressor and a second driver? This allows you a scaled approach to any job, it also provides you with redundancy in case one unit goes down.

The key is once a driver is running on a piling can it run hands-off? If it can't then that is what you should be working to achieve here - unassisted driving - and not worry about faster driving. I suspect you could probably manage two drivers by starting the second once the first is stably driving. If it's close, timing-wise, all you need to do is slow down a driver.

I could see where you might have to start by babysitting a couple of pilings but then those pilings in strategic locations could be used to align and stabilize further pilings to be 'self driven'.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[berkshire]

This device just showed up on another forum today is this of any interest to you, it appears to be based on a gasoline Road breaking unit.

https://www.youtube.com/watch?v=_B4FZ6c_n7s

B.E.

You are judged not by what you know, but by what you can do.

 

[Sparweb]

Hydtools,
Fun video.
I liked the part where the narrator says "...obviously the operator's manual tells you not to get under the machine while it's operating like I am now..."

Berkshire,
In the interest of making an interesting video, I see that they don't actually show the operation from start to finish. Perhaps 10 minutes to watch one post being driven into soft loam isn't everyone's cup of tea. The closeup in the last 10 seconds of the video hints at about 1 centimeter of progress, so should I increase my estimate per post to 15 or 20 minutes? That strapping fellow isn't nearly muscular enough to hold that machine over his head for 4 hours if he has more than a dozen posts to do.

Foxlair,
I see your point. I personally think hydraulic rams deserve more consideration, because they are best suited for the operation you want. You probably can deal the mess issue effectively. I replaced the hydraulics on my tractor 5 years ago. It used to leak, but it doesn't now. Hydraulics that leak are telling you they are damaged in some way or needing maintenance. I could add that if aircraft are sensitive to having excess weight, then why do they all have hydraulic systems, and virtually none have pneumatic actuated systems? Check your prejudices at the door.

That said, I'm sure that everyone else looking at a hydraulic ram on water will say the same thing you just did, and approach it with the same negative bias. So even if you do figure it out, the service is hard to sell to the customer. And the hassles people will give you at the boat launch... And then there's the issue of getting responsible operators who can use your machine without spills or leaks, and that IS a can of worms. More folks who aren't inclined to read operator's manuals. So it sounds like you've thought ahead to the opposition to the idea that you'll have to overcome.


No one believes the theory except the one who developed it. Everyone believes the experiment except the one who ran it.
STF

 

[itsmoked]

Spar you're talking about this type hydraulic driver? All the others look like they use their own weight which is not a boating option.

Hydraulic hammer style

Keith Cress
kcress -

http://www.flaminsystems.com

 

[chicopee]

I personally think that you have not given a chance at examining the use of vibratory pile drivers which are operated by hydraulics and used by contractors on land and over water. Forget the idea of contaminating waterways or the land as such equipment have a good reputation of not leaking at their hose connections. Sure, maintenance is required particularly at threaded joints, nonetheless, such equipment are used frequently in the field. And if you do have a leak over water, means can be employed to contain it and to clean it. We have used vibratory pile drivers on many of our job sites over water with good success.

 

[chicopee]

Also we have used hydraulic water pumps to dewater cofferdams with good success, ie, no oil leaks from submerged equipment and from hydraulic pumps positioned on land.

 

[IFRs]

Coming very late to this discussion, but here are my (very simplistic) 2 cents.

If the driver is essentially in free fall at the bottom of the stroke, the energy available to be transferred to the pile is entirely the result of the mass and final velocity of the driver. If increasing the mass is not an option, then increasing the velocity is a good approach. This final velocity is entirely dependent on the height it falls and the acceleration provided by any force(s) applied as it falls. Ergo, any method that increases the end velocity increases the energy available to perform the work of driving the pile. The end velocity is increased by acceleration, either from the force of gravity or the application of another force. Springs and hydraulics or pneumatics can add to the gravity force. Velocity is also increased by the time duration of the applied force (gravity, springs or pneumatics). This suggests that simply increasing the length of the tube above the pile may be worth considering.

If the device adds force to the driver as it impacts the pile, that force gets added to the available work. This would include springs that are not fully compressed or pneumatics that still have pressure at the bottom of the stroke.

I know it's not much but that's all I've got...

 

[Sparweb]

Hi Keith,
This is what I'm suggesting:

http://www.aggrowth.com/catalogue/category/351/high-heavy-hitters

I posted this link 23 January.

Your link is a miniature version of the type that Berkshire linked to. The "T-Post" driver that you linked to is about as big as I've ever used myself, and it's exhausting to do just a few tiny T-posts posts with one. You are holding the 40 pound machine by hand over your head. Now imagine a 4" post driver, lifting 100 lb over your head, let alone holding it there as it jumps and dances around.

IMO this has to be attached to the boat by an arm or a davit, and have a significant mass to be effective. Kinetic energy is 1/2 * M * V^2. You need lots of M and V!
The hammer of the "HeavyHitter" is a steel plate 4" thick and 12" square so it probably weighs 200 pounds alone, and the slider probably just as much. The stroke is longer, so there's about 100x more energy per blow. Providing a driving force on the downstroke doubles the acceleration, getting 2x more energy again.

No one believes the theory except the one who developed it. Everyone believes the experiment except the one who ran it.
STF