Hydraulic system design advice, closed system of 2 single acting cylinders

sheys · Feb 13, 2014

I'm doing some design work for a kinetic sculpture and I'm considering using a simple, low pressure hydraulic system for actuation. Ideally I'd like to use a pair of single acting cylinders attached to each other with just a 10' hose and a fitting to bleed the system, no reservoir. Pressure would be LOW <200psi. One side would be a commercial cylinder (like this: 1.625X9 SA HYD CYL 5800 PSI), the other i'd make out of honed ID AL tubing with a custom piston and seals. The commercial cylinder would be driven by a motorized screw and the custom cylinder would be slaved. Return is by gravity as the screw is backed off. This is a SLOW motion, a few minutes to retract a few inches.

Simple, right? Well, I'd have a couple dozen of these master cylinder/slave cylinder/hose assemblies in parallel driven by the same motor to lift a number of objects evenly. Each pair acts as its own closed system, no oil mixes anywhere. I need all of the cylinders to be exactly in sync over a long period of time. I'm not aware of an easier way of keeping the position of 24 cylinders coordinated without a closed loop control system.

My concern is potential leakage past the piston seals, I don't know how much to expect. This piece would be running for many months so even a very tiny leak rate would eventually be a big problem. Any oil that escapes from the closed system will end up on the floor and cause the slave cylinder to lose position. A drop or 2 of loss a month is probably OK but more than that would be a big concern.

So, is this a bad design? How much oil will I lose past a pair of 1-2" ID piston seals at 200PSI after a month? I've been searching around for references to similar systems (master/slave with NO reservoir or valving) and haven't found much. I plan on setting up a test ASAP but wanted to get some opinions here first.

Thanks for any input!

RayG · Feb 13, 2014

sheys said:
I'm not aware of an easier way of keeping the position of 24 cylinders coordinated without a closed loop control system.

I think the safe option is to assume the worst and that the system will leak over the months, and so, at this stage of the project it would make sense to design the system to allow for leakage.

If you don't, then you have no plan B to fall back on when (if) the system leaks and loses position.

There are simple self levelling systems around, imagine a pendulum that controls valves. The pendulum is free to swivel in any direction, there are 4 valves ( or micro switches) which operate the cylinders to oppose any tilting.

Seeing as it's an artwork, maybe you could make a feature of the self levelling mechanism..

Ray

sfriedberg · Feb 13, 2014

Can you arrange for this sculpture to return to a "home" position each night or each week (presumably during a maintenance window when the public won't be viewing it)? If so, you may be able to adapt the trick used for "synchronized" cylinders to (automatically) refill everything to a known level. I'm thinking of the setup where several cylinders are mechanically parallel, but plumbed in series, with overflow valves to allow everything to be pushed to an extreme position, refilling any leakage and also resetting the extension of all cylinders in the set.

sheys · Feb 13, 2014

Yeah, I am uncomfortable with not having any recourse if something leaks. This is very low load, low duty cycle and it can be rebuilt every time it is displayed (likely to be for a few months max) if necessary so it's not a demanding situation. Still though, anything more than a very small leak will cause problems.

The overall arrangement is more complex than just keeping an object level. There are 8 objects with 2-4 cylinders each (26 cylinders total) and I need them to all change height accurately and simultaneously within a very space constrained environment.

I have a 50 year old super heavy engine hoist with a single acting cylinder on the boom and I've never seen a drop leak from the rod. The boom slowly sinks over a month or 2 but I presume that's all going back through the valve since there isn't a way for any leaked oil to get back to the tank. Seems like a similar situation to me but maybe I am missing something?

sheys · Feb 13, 2014

sfriedberg said:
Can you arrange for this sculpture to return to a "home" position each night or each week (presumably during a maintenance window when the public won't be viewing it)? If so, you may be able to adapt the trick used for "synchronized" cylinders to (automatically) refill everything to a known level. I'm thinking of the setup where several cylinders are mechanically parallel, but plumbed in series, with overflow valves to allow everything to be pushed to an extreme position, refilling any leakage and also resetting the extension of all cylinders in the set.

Would a configuration like this work if I needed all the cylinders to move completely in sync with each other? It's obviously easy to have them match up at the travel extremes but I need them to be the same during motion. The only synchronized cylinders i'm aware of are different sizes to allow the rod side area to match up with the opposite side area of the next cylinder.

All of the cylinders will return to zero a few times an hour.

RayG · Feb 13, 2014

sheys said:
I've been searching around for references to similar systems (master/slave with NO reservoir or valving) and haven't found much.

Try researching aircraft control systems, some of those are fully sealed systems.

Ray

Heavey Metal · Feb 13, 2014

You need a rotary gear flowdivider

Hdpg · Feb 13, 2014

Why not look into making it an all mechanical system? You can get cable systems that can both push and pull: Push-Pull Controls, Low Friction Controls, Control Cable Assemblies from Cablecraft Motion Controls

Push-Pull Cable Control Systems

These would require you to make a master 'pusher' connected to all the cables and this could be either hydraulic or an electrically driven linear actuator.

If the force you need to move your sculpture is greater than any cable system can push you could do a hybrid system where a cylinder pushed while a mechanically controlled cable restrained the motion.

sheys · Feb 13, 2014

Crap, i meant to put this thread in general!

I would normally not use hydraulics as my expertise is in electromechanical systems but for a variety of reasons I wasn't able to come up with a solution that fit within the space constraints. I only have a small amount of hydraulic experience, thus the questions.

The flow dividers are cool but their internal leakage seems like it may cause drift over time. Plus I'd need a ton of them to hook up 20+ cylinders in parallel.

I guess I'm really just looking for someone familiar with hydraulic piston seals to chime in. I'll set up a leak test as soon as I can and watch it over a month but in the meantime I want to hear if anyone thinks it's a terrible idea. I don't have time in this project to make huge mistakes!

machtool · Feb 13, 2014

sheys said:
I guess I'm really just looking for someone familiar with hydraulic piston seals to chime in.

At 200 p.s.i, I wouldn't overlook the humble O-ring. You can still have a reasonable clearance between your piston and the cylinder bore, yet be at no risk of extruding the ring into the clearance. At that pressure, I'd even come down to Shore A 60 hardness, and because you seem to want little friction, I'd back down to about 8% of mechanical squeeze.

The problem with most piston seals, is that they are engineered for pressure to energize them. At 200 p.s.i, your only lubricating them.

Regards Phil.

sawmill · Feb 16, 2014

There are a number of european truck manufacturers that uses hydralics to control the steering of the last axel on some chassis configurations.
That axel is basically a front axel mounted at the rear and at least Scania has a closed system with a cylinder at the front following the steering and a cylinder at the back that controlls that axel.

They use a closed system like you describe, but also use a compressed gas accumulator in the system.
This design has been around for many years, and as far as i can tell causes very little problems even in that environment with dirt and salt etc.

Gary E · Feb 16, 2014

sheys said:
I'm doing some design work for a kinetic sculpture and I'm considering using a simple, low pressure hydraulic system for actuation. Ideally I'd like to use a pair of single acting cylinders attached to each other with just a 10' hose and a fitting to bleed the system, no reservoir. Pressure would be LOW <200psi. One side would be a commercial cylinder (like this: 1.625X9 SA HYD CYL 5800 PSI), the other i'd make out of honed ID AL tubing with a custom piston and seals. The commercial cylinder would be driven by a motorized screw and the custom cylinder would be slaved. Return is by gravity as the screw is backed off. This is a SLOW motion, a few minutes to retract a few inches.

Simple, right? Well, I'd have a couple dozen of these master cylinder/slave cylinder/hose assemblies in parallel driven by the same motor to lift a number of objects evenly. Each pair acts as its own closed system, no oil mixes anywhere. I need all of the cylinders to be exactly in sync over a long period of time. I'm not aware of an easier way of keeping the position of 24 cylinders coordinated without a closed loop control system.

My concern is potential leakage past the piston seals, I don't know how much to expect. This piece would be running for many months so even a very tiny leak rate would eventually be a big problem. Any oil that escapes from the closed system will end up on the floor and cause the slave cylinder to lose position. A drop or 2 of loss a month is probably OK but more than that would be a big concern.

So, is this a bad design? How much oil will I lose past a pair of 1-2" ID piston seals at 200PSI after a month? I've been searching around for references to similar systems (master/slave with NO reservoir or valving) and haven't found much. I plan on setting up a test ASAP but wanted to get some opinions here first.

Thanks for any input!

As others have said...yes it wil leak, eventually...

so... take a look at the boats in your local boat sellers yard... they probly use TELEFLEX push pull cables... and they also have hydraulic steering, if you must... But the ALL CABLE systems WILL NOT LEAK and since you can rig up the push/pull cables to all start at the same time, everything stays in sync...

vegard · Feb 16, 2014

I would say it is easily doable. Your shock absorbers on your car can cycle millions of times without leaking through the rod seal. I agree that an o-ring type seal would probably work well for your application due to low speeds and pressure. Maybe a quad ring to avoid the o-ring getting twisted.
Make sure you get hydraulic cylinder tubing material or aluminum air cylinder material, as it will have the required finishes on the ID.

There is a chance the commercial cylinders will not be built for zero leakage across the piston (especially at low pressure). You should investigate it before buying.

You will need to have an accumulator somewhere in the system to account for the oil expanding or contracting with temperature.

Vancbiker · Feb 16, 2014

Suspa Movotec do something similar to what you describe. I built ergonomic cleanroom machinery using their product with only one cylinder failure in dozens of 4 and 8 cylinder systems. Maybe they have a solution for your application. SUSPA MOVOTEC »

sheys · Feb 18, 2014

Thanks everyone for the advice. It's very helpful to get everyone's ideas.

A push/pull cable arrangement would be great but I need 12" of movement @ 150lbs or so which seems a bit past the practical limit.

Very good points about fancier seals not working properly at a lower pressure, this is something I will test out ASAP. It'd save some time to use commercial cylinders paired with custom ones but if the commercial seals aren't appropriate I can make both cyls myself.

Luckily this will be indoors in a temp controlled environment so I'm not too worried about thermal expansion. Once everything is at temperature I can bleed or add oil to each pair to get everything in the correct positions.

The Suspa Movotec link is great. I actually have two tables here in my shop that I got at auction with these systems installed. I need to take a close look at them.

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