Hydraulic Cylinder Controls

[Mule Magoo]

I'm building an attachment for a skid steer to work almost like a grader, the grader portion will need to hang off the side of the machine and pivot when it comes in contact with an obstacle. The cylinder will need to retract at the added pressure from the obstacle and extend when the obstacle has passed. I'm having difficulty figuring out the best way to accomplish this, torsion springs and compression springs didn't work. There was a similar setup on a mower (see pic). It looks to me, that there is a single acting cylinder, followed by a manifold, to an electric pressure switch (just a guess), to an accumulator. Any help would be greatly appreciated. There is also a video

22 seconds to 44 seconds in.

 

[Stick.]

Are you looking to have the sort of relatively civilized and controlled retraction motion that the mower has? Or are you just looking to have something to allow motion so this attachment doesn't get snapped off if it hits something (like the springs on a snowplow)? I'd be hesitant to add hydraulics to the mix unless you need more precise control.

 

[Mule Magoo]

Thanks for your reply. More civilized, there will be people working around it.

 

[FluidPowerUser]

This is commonly known a breakback. It's used on agricultural machines to allow booms to yield if they hit something and then spring back to the normal position when the obstacle has passed.

Contrary to the comment above, the hydraulic solution is the preferred method as it's more reliable and more controllable than using springs or purely electrical solution.

The accumulator absorbs the energy and the spring back is controlled with flow controls so that the system is not too aggressive.

 

[Mule Magoo]

Am I correct in the way this is hooked up?

"It looks to me, that there is a single acting cylinder, followed by a manifold, to an electric pressure switch (just a guess), to an accumulator."

If it is a 12v pressure switch, why is it in there?

 

[FluidPowerUser]

It’s not a pressure switch. It’s solenoid valve that is most likely normally closed to lock the arm in position / isolate the accumulator and stop the break back function. The 12V DC supply is there to turn the valve on/off.

I’ve looked at the video and the operation looks quite slow, so the valve in the aluminium block is restricting the flow and limiting the speed, or there are some orifices in there to control the speed of the cylinder by choking the flow.

 

[Mule Magoo]

Thanks for the response. How do I size the accumulator, the cylinder will be 2.5" with a 10" stroke? Will I need an accumulator with a specific pressure rating?

 

[FluidPowerUser]

It really depends on how fast you want to the system to work.

The smaller the accumulator, the faster it will operate, without using flow controls to slow it down.

The pressure of the gas will double when you halve the volume of the gas.

Calculate the volume of the cylinder at full displacement. When you know this, you can find the volume of gas you need and it will also depend on the amount of pressure rise want.

 

[akkamaan]

10x2.5x2.5xpi/4=49.1 cui