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Deceleration and cushioning

Deceleration and cushioning

(OP)
Colleagues
Here is a challenge, I am looking forward to ideas, please advice.
Building a hydraulic tilting table. Table has to tilt by 90 degrees in vertical plane, back and forth. Operated by dual-action cylinder, directly attached to base frame and tilt table.
Please take a look here:
http://files.engineering.com/getfile.aspx?folder=5...
The problem is in changing the direction of action in cylinder. In order to rotate the table by 90 degrees, table has to move the center of gravity thru hinge. That reverses the rotation moment, so actuator starts acting in wrong direction. The table might fall on frame after passing the hinge's vertical axis.
I am sure that actuator speed could be controlled by throttling the return line, however I was not able to find any typical circuit schematics for that. I am afraid there are some hidden stones. It definitely will over-pressurize the return line, since table weight will act as a pump in addition to supply pressure.
What would be the best concept for such a dynamics?
Please advice

RE: Deceleration and cushioning

I have a solution and video using a hydraulic servo control, but I want to see what others come up with first.
The hydraulic and mechanical part must be DESIGNED and not evolved or kludged.
A properly designed and controlled system will not over pressurize anything. All the motion should be smooth.

Peter Nachtwey
Delta Computer Systems
http://www.deltamotion.com

RE: Deceleration and cushioning

(OP)
Thank you Peter
Agree, we definitely do not need kludges here. I visited your web, you have cool technology. There is no doubt it is 100% flexible and sensitive. However it might be a bit of overkill for my little product. Another point, hydraulic science exists for more than 100 years by now, I am sure there is a simpler solution for such a load dynamics. The simplest way is a manual control, properly trained operator can throttle the flow with lever manually, not letting table to accelerate. I really do not want to do it that way, some automation should be added. How much automation is a matter of cost, so I would like to ask professionals on practical solutions to chose from.
Also, Peter, would you provide your direct email, I would like to send an RFQ for your product. I have no idea how affordable your technology is, it might work for us.

Any other solutions? Please advice
Thank you

RE: Deceleration and cushioning

Solving the hydraulic control issue is half the story. The load reversals will still cause issues unless you can eliminate clearance in all actuator linkages and joints.

If possible, you can solve both problems by eliminating the load reversal. This will also reduce the actuator force and hydraulic power required.

To reduce the load reversal you could:
a) Move the pivot point toward the centre of gravity
b) Move the centre of gravity towards the pivot point using a counter weight.
c) Use springs to modify the moment where appropriate

je suis charlie

RE: Deceleration and cushioning

"Also, Peter, would you provide your direct email, I would like to send an RFQ for your product. I have no idea how affordable your technology is, it might work for us."
The forum doesn't want us to post e-mail address and there is no way to private message. Just call me. The phone number is in the contacts section of our website. There are a lot of things to consider. I/O, PLC connections, what kind of position feed back etc.

Here is the link to the video where we cover these kinds of applications. It is from a training class we hold 3 times a year. It is long so download it while getting a cup of coffee. VLC may be able to stream it.
http://deltamotion.com/peter/Videos/Delta%20Comput...



Peter Nachtwey
Delta Computer Systems
http://www.deltamotion.com

RE: Deceleration and cushioning

(OP)
Thank you for suggestions, we thought it thru already. We are talking about 12,000 Lbs of weight and 120" tall skid. Accommodating counterweights and springs will be heavy and not cheaper than servo control.
Thank you for mentioning a backlashes, we definitely have to make all the hinges tight.
What else should I consider?
Thank you for any input or idea

RE: Deceleration and cushioning

(OP)
Thank you Peter, I will try to go thru videos tomorrow, will definitely have questions to you.
Talk to you soon...

Any more thoughts?

RE: Deceleration and cushioning

I do like gruntguru's idea #1
If you use one cylinder it may need to be large to compensate for the mechanical disadvantage during different parts of the stroke. If you had another actuator on the right side mounted in a similar way as the left you may be able to get away with smaller cylinders that will require less flow than the one big one.

It would have been nice to know the time to move and how often it has to move.

Peter Nachtwey
Delta Computer Systems
http://www.deltamotion.com

RE: Deceleration and cushioning

It's very easy: you must use a double counterbalance valve

RE: Deceleration and cushioning

(OP)
Thank you PNachtwey and 73lafuite
"I do like gruntguru's idea #1...." I spent a couple of days trying to find the best concept. I can get away with moving hinge so there is no double action on actuator, however everything gets big and spacious. Double acting cylinder is much smaller concept and I am sure there is a way to manage it.
"It's very easy: you must use a double counterbalance valve" dear 73lafuite, would you able to provide some example circuit? I am not familiar with that. I found a couple of schematics, attached here: http://files.engineering.com/getfile.aspx?folder=5...
Not sure how that would manage pressure in supply line. It looks like it reliefs the pressure to unload the return line. However how would that counter act on falling table?
Please advice

RE: Deceleration and cushioning

That diagram shows relief valves not counter balance valves. Counter balance valves have check valves that will allow pushing the load up but block the flow what would allow the load to drop. To drop the "relief" valve portion of the counter balance valves must have enough pressure on the pilot line to open the valve.

If counter balance valves are used then hopefully you get the desired control without a servo controller. Counter balance valves interfere with servo control. In a servo system there can be only one valve controlling flow.

If using servo control special attention must be paid to the pressure on the cap side of the piston. When the load goes over center the servo valve will be metering out flow on the rod side to slow the system down. This may cause little oil to go into the cap side so pressure will drop and possibly cavitate. The cure would be to get a servo valve with a asymmetrical spool usually,2:1,so that the flow into and out of the cap side is larger than the flow into or out of the rod side. Then the cylinder piston area ration should be 2:1. This is a common technique.

Another possibility is to use a rod that is small relative to the diameter so the areas on the rod and cap side are almost the same but there are restrictions on how small the rod diameter can be based on load and length of stroke. I don't think this second option is feasible especially if there will be any forces trying to bend the rod.





Peter Nachtwey
Delta Computer Systems
http://www.deltamotion.com

RE: Deceleration and cushioning

explanation: http://bernarderic4926.perso.sfr.fr/hydrau/risques....
For safety reasons the connection between counterbalance valve and orifices of cylinder must be rigid (hose forbidden).
This valve is very standard, cheap and available everywhere.

RE: Deceleration and cushioning

(OP)
Thank you gentlemen!
I will contact local component suppliers and hopefully generate a schematics that I confident with. Will post it here for your valuable opinion.

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