Lifting platform stage with 4 hydraulic cylinders- How to keep in sync 3

[Arniemic]

I am new to hydraulics but I have a unique problem. 4 telescopic SA - power up/gravity down cyclinders are lifting a stage platform. I used a gear type 4 way flow divider (delta) and that works great on the way up. The only problem is (outside of putting an elephant on the stage)the stage does not have enough weight to bring it down. Evidently, it takes a tremendous amount of PSI at each cylinder to start the gears turning in reverse) So I removed the gear flow divider and now it comes down easily but naturally it does not go up evenly at all. The pump is only 3GPM and does not use much pressure to lift the stage. How do I get these cylinders syncronized on the way up and have the stage come down? Any help would be appreciated.

 

[MikeHalloran]

Flow dividers are not known for having a lot of internal friction.

Something else is wrong with your system.

Please post a schematic, and verify that the system is built as the schematic says.



Mike Halloran
Pembroke Pines, FL, USA

 

[MikeHalloran]

Absent other information, hopefully forthcoming, I'd guess that you've got some check valves under the cylinders that should be pilot-operated, or you've got pilot-operated checks already, and the pilot circuit is not being pressurized when you want to lower the platform.



Mike Halloran
Pembroke Pines, FL, USA

 

[PNachtwey]

The big stage productions in Vegas and Macau and use servo hydraulics.


Peter Nachtwey
Delta Computer Systems

http://www.deltamotion.com

 

[hydromech]

Back driving rotary hydraulic devices is notoriously difficult. It takes approximately 80% of the operating pressure to make them start turning. Once they start the pressure required reduces massively.

Other than installing a servo system, which is very expensive, the best way is to convert the cylinders to double acting, install some motion control valves, then use the flow divider to drive the stage up and down.

 

[Arniemic]

Thank you for your response. Yes I believe hydromech is correct it takes a lot of back pressure to get it going if a gear flow divider is used. This is a one time project at low cost. I have already invested in the SA telescopic cylinders each with a 47" stroke. I was wondering if a different kind of flow divider would work and not have the back pressure problem. Say using 3 flow divider/combiner valves. The first one /2 and then each of these would /into 2 so you have 4 outputs driving the cylinders. This way there will be no resistance on the way down since this is a gravity down system. On the other hand placing hand adjusted valves at each cylinder might be the only option.

 

[hydromech]

There are spool type flow dividers...

They are no where near a accurate as rotary flow dividers because they have excessive internal leakage. If you have one cylinder with a higher load, the higher pressure in the cylinder will cause more leakage and the cylinder will be slower than the others.

 

[damienmorton]

Im working on a similar problem, though on a much smaller and simpler scale.

The design I currently have involves two compact hydraulic cylinders, spaced about 100mm apart, lifting a platform as distance of about 10mm.

Planning on using something like this wrench powered screw-pump

http://www.carrlane.com/catalog/ind...6280B1713050245221E0107070F1A3C3B285352405E59

to supply hydraulic pressure.

At this scale, would I still need something like a flow-divider to coordinate the movement of the cylinders?

 

[byrdj]

just thinking... if the geared flow divider will start turning backwards with a lot of weight on the stage, and once it starts go down easily. then install accumulators at each piston so that once end of stroke is reached, the accumulators charge. then on lowering, the accumulator charge will start the reverse rotation

 

[hydromech]

A good idea from byrdj...

The accumulators will have to be fully isolated though as there would be no way to safely top them up, not without potentially jolting the the stage.

Then, when the accumulators come in, they will need pressure compensated flow control valves to regulate the speed, otherwise the whole thing will come down too fast.

It would work, but may be a bit jumpy and very noisy. There is no noise like an over revved flow divider.

 

[byrdj]

assuming here since there has been no values....
the pressure to lift the platform is maybe half of the pump pressure. precharge the accumlators just above this pressure. so as the stage lifts, all fluid goes to the pistons. once the pistons reach stop, then the pressure will increase to pump pressure and fluid will enter the accumulator.

the line from pump to divider is isolated to hold the stage lifted and the accumulators keep the pressure in the cylinders at pump pressure.

to lower, the line is drained. the intial pressure is sufficient to start the flow divider spining backwards. once the volume of the accumulator is exhausted, the pressure in the piston is reduced to just under lift pressure and the platform lowers as the divider reverse spins. the rate of decent is controlled by the opening of the drain valve. there will be a delay from opening the drain till the platform moves

 

[EngineerTex]

Yeah -- I know. This post is nearly five months late. But...

If it works fine going up through the flow dividers and works fine coming down without the flow dividers, place four tees in the circuit - one on each line between the divider output and the cylinder. On each tee, place a check valve that allows fluid flow out of the divider-to-cylinder branch. Run all four of the lines (downstream from the check valve) to the pump side of the flow divider.

There will be no cross-communication between the lines while raising because the check valves prevent any fluid from entering any cylinder and the pump side pressure into the flow divider will always be higher than the pressure in the cylinders, which will keep the check valves closed. There would be no movement when stopped, because the cylinders are fully extended and everything downstream of the work port of the directional control valve is a closed system.

HOWEVER - this requires a directional control valve that has an integral check on the work port when the handle is in neutral.

Engineering is not the science behind building. It is the science behind not building.

 

[HugoTooling]

Hi Guys!

Hey byrdj & EngineeringTex , Can you share a schem of the arrangements discussed?

I have a similar problem, I want to raise and lower a fixture using 4 hydraulic cylinders that need to be n'sync... the challenge for me is that the fixture is used on several stations and the load changes...

Hope you can share.

Happy holidays!

 

[EngineerTex]

Try this, HugoTooling:

[tt]




CHECK VALVES
____________O>_______________________
| |
|____________O>_____________________ |
| | |
|____________O>___________________ | |
| | | |
|____________O>_________________ | | |
| | | | |
| | | | |
| | | | |
SOURCE -----------[DIVIDER] | | | |
| | | | | | | |
| | | |___________________|_)_)_)___________CYL1
| | | | | |
| | |_______________________|_)_)___________CYL2
| | | |
| |___________________________|_)___________CYL3
| |
|_______________________________|___________CYL4

TEES (4QTY)

[/tt]

Engineering is not the science behind building. It is the science behind not building.