post pounder

[lfd250]

I am building a hydraulic post pounder kinda like a shaver, I need to know how to get the hydraulics to free fall back down in order to have the maximum amount of power hitting the post. Is there a special control valve or cylynder needed, I got a cross control valve and a 2.5" x 48" cylinder from dalton hydraulic. Thank you. C.W. Morgan

[Doug Hanson]

It depends how you are building your post pounder, there are a few ways you can do it. Is your cylinder a double acting cylinder? Did you want to rely on any downward hydraulic force, or just the force of the weight? The key to getting a cylinder to retract under the influence of a load with as little drag as possible is to use large porting on the cylinder, large lines and large valves. The type of valve depends on the cylinder being double acting, your pump type, any other circuits etc..
Another solution could be to use a pin cylinder that unpins the lift cylinder from the load at the top of the stroke, then you have no hydraulic drag, or you could size your pump, valve and cylinder to provide an acceleration that would exceed the excelleration due to gravity, or you could use a two pump system, one pump to lift the cylinder, while the other pump charges an accuculator, then when the cylinder reaches the top, the energy stored in the accumulator can be used to drive the cylinder down rapidly. A power down cylinder would have to have some provisions to not hydraulicly lift your macine once the pounder bottoms on the post.
Any one else with ideas? I have never built a post pounder, but there are a few options, I'm not sure what the most common one is.