Best arrangement for a counterbalance?

I have a machine section that I am working on that is a high maintenance item for our customer that involves raising and lowering a set of rolls every 20 seconds. There is more to it than that, but trying to keep this simple.

One section of rolls is raised / lowered 12 inches by means of a roller chain on two 1 inch pitch sprockets spaced about 10 feet from one another on a common hoist shaft.

Weight of rolls and associated frame equipment is about 2500 lbs and the machine is presently requiring new chains and sprockets about every 6 months.

Operation is:

Lower Load 12 inches over 1 second
Hold Load for about 2 seconds at the lowered position
Raise Load 12 inches over 1 second
Hold Load for 16 seconds at the raised position
Repeat 24x7

This describes 1 side of the machine, there is an identical mirrored set of rolls on the other side of the machine that move in the exact same manner.

Hoist shafts on both sides of the machine are parallel to one-another and are connected to a common gearbox through two additional right angle gearboxes. So a single motor/gearbox + 2 more gearboxes raises and lowers both sets of rolls (which makes keeping them synchronized easy)

The right angle gearboxes require rebuild every 12 months also.

The present motor / drive system is already hitting over 200% of motor current to raise the rolls and my challenge is to speed this whole process up by about 30%.

AND . . . there are multiple identical sets like this. It is a common German Machine design in the industry it is in and I am reworking the entire line to address throughput and maintenance issues like this.

So . . . I am thinking that a counterbalance might be in order for the raise / lower carriages that relieves weight from the lifting chains and doesn't add any inertia to the payload.

I would like to use a pneumatic cylinder or perhaps an air-over-oil cylinder setup. The thing will cycle 25,000 times a year and I would like to get a few years life out of the cylinders. Even if I relieve only half the weight it would seem like this would extend chain life and reduce drive/motor load considerably. Each cylinder would optimally lift about 1000 lbs and with crappy plant air, you cant count on anything consistent above 50 psi . . . so the cylinders would need to be roughly 5 inch bore.

Any thoughts on a good cylinder choice? Seems like a double rod extended cylinder would last longer. Trunnion mount? I am a bit of a novice when it comes to pneumatic / hydraulic cylinder design for high cycle low friction applications.

Switch to Diamond brand roller chain. American Tsubaki comes in a distant second.

Does it have to have the weight pushing down at the bottom of the cycle or can you have a constant up force?

gbent said:

Switch to Diamond brand roller chain. American Tsubaki comes in a distant second.

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Doesn't address the overloaded gearboxes, motor and drive . . .

Bill D said:

Does it have to have the weight pushing down at the bottom of the cycle or can you have a constant up force?

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Good question - constant up force is more desirable. The machine is incredibly crowded in this area but I believe I can push up directly on the carriage bearing cars which would be optimum to prevent any possibility of radial loading of the cylinders.

I'm having a problem picturing the set up.

But

Is a deadweight type counter balance out of the question? .....even well away from the crowded area?

Last set of counterbalances I saw for a similar (probably more demanding) application, I think we used SKF cylinders. They were bog standard single rod cylinders; I'm not sure what going to a double rod gets you.

Use a surge tank. If you use a relieving regulator plumbed straight to the cylinder, you are entirely dependent on shop air pressure, and every time it cycles you'll be pushing a bunch of air through the regulator's relief, which they don't like. You'll also use a bunch of air. If you size the tank properly, you'll get an almost constant spring rate and not use much air. You can then use a non-relieving regulator and it'll only need air for topping up due to leaks. You will, of course, want a pressure relief valve for safety. For example, set your non-relieving regulator to 50psi; the pressure in the tank might vary from 50 to 52 psi. Set the relief pressure just below what it would take to reverse load the drive.

The big killer of service life is going to be misalignment. Trunnion mounts are better then nothing, but are still only 1 DOF. If you can package it, use a ball joint rod end. Also, consider putting the cylinder in tension. With ball joints at each end, a tension cylinder is going to be about as perfectly aligned as it gets. On this short of a stroke, buckling probably isn't going to be an issue, but it may increase your lifespan.

Do you actually need low-friction seals? If the drive is currently able to handle a 2500# dead and inertial loads, it should definitely be able to handle only inertial loads plus even the worst cylinder friction. I think what you actually want to ask for are low-wear or long-life seals.

Limy Sami said:

I'm having a problem picturing the set up.

But

Is a deadweight type counter balance out of the question? .....even well away from the crowded area?

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In my experience deadweight counterbalances are more of a pain in the long run. They're much more work from a design and maintenance standpoint, and have a worse failure mode. With the pneumatics, maintenance can be "watch that it's moving smoothly and doesn't sound bad" and "check the pressure gauge is in the green" (or have hi-low pressure switches tied to the PLC). With the deadweight, you need to inspect the rope. If that fails, you have 2500# of steel and lead crashing down. Now in addition to a weight, guides, pulleys, rope, etc., you need bumpers or dampers. With the pneumatic system, if it fails, it just stops working and you're back to where you were before you installed it.

I have been looking for a video online of a machine like this and can't find one. I like the idea of a surge tank with top up regulator. Agreed that spherical rod and cap ends of cylinder is better way to go. The double rod end cylinder idea is something that we have done often when misalignment is possible . . . you are right that it is better to design so that it isn't possible to have misalignment (to the best degree that anyone can).

A dead weight counterbalance also add considerable inertia to the system which I don't want to do from a motion control standpoint in addition to all of the reasons you mentioned.

I see some issues with pneumatics in this application. Particularly the 1 sec stroke with a just sized cylinder. That's a lot of air to move somewhere through a regulator in 1 sec. An alternative is a quick exhaust, but then you have to fill that big cylinder in 1 sec also. I don't think you will get the performance you are looking for due to air lag.

Now, if you can fit longer stroke cylinders where you can retain the air in the cylinder (think gas shock) so you aren't having to move it anywhere other than compression / expansion then you may get what you are after. With that cycle rate, you may need to provide cooling for the cylinder to keep the seals in good shape.

Of course, this depends on whether you are using gravity to lower the rollers or are powering them down, your post isn't clear on that aspect, though I suspect they are powered for a 1 sec 12" stroke. How critical is the nip pressure at contact? Can you stand to add some power to the down stroke to make up the compression of the cylinders to give you a relief of the lifting force?

Is the chain driving a cam/crank for lower/raise or is the shaft / chain actually reversing?

If the system doesn't have chain tensioners I would also look into that to improve the gearbox and chain life. The take-up slack from down to up can transmit quite a bit of a shock load back through the chain and gear train and lead to premature gear wear.

Ideally I would suggest a mechanical counterbalance but from the sounds of it there isn't anywhere to put one. Edit:
And after I posted I see you don't want the inertia.

I think compressible gas will not word consistently. If you must use cylinders make it hydraulic. There are many metering schemes and devices possible. Think lock valves and excess flow valves for safety.

Weight of the payload is always on the chain and the hoist shaft is always holding the load up (never powering it downward).

I think Daniel's approach to having a large accumulator charged to working pressure that is piped directly to the cylinders is a good idea - large cylinder ports and tubing should allow proper fill / evacuation of the cylinders. Max cylinder speed < 20 in/sec . . . more like 18 in/sec. Set the pressure to 50 PSI and let the cylinders always sit there at 50 PSI holding up 1000 lbs each.

For the right-angle drives, have you looked into Cone Drive reducers ? More expensive but lots more contact area and longer life.

We have used 100's of Cone Drives with the double enveloping worm set . . . far better than NORD or SEW worm drives for high shock load applications.

This system has an SEW Snuggler drive with a 75mm through shaft about 8 feet long. Each end of the shaft has a 1:1 bevel gear right angle box that powers the two parallel hoist shafts / sprockets that lift the two sides of the roll carriages . . . not really a good place for a cone drive at that location.

I boiled down some video from my phone - you can see the orange hoist shaft in the upper right. The chains are inside the white tube structural columns.

[video]=Deleted[/video]

I'll delete the video after this clears up a few questions.

Pneumatic rubber bellows springs (think tractor trailer air springs) with a properly sized accumulator charged with nitrogen or argon. You would have no seals to leak/maintain. If you wanted to you could have a small nitrogen/argon bottle regulated down to keep things topped off, but that shouldn't be needed. Consult engineering data for proper column height/diameter to prevent outward bulging or put them in circular telescopic way covers.

I suggest an air counterbalance to relieve 90% of the load. It could be a closed system with an accumulator with a regulator if the pressure drops. 90 PSI on a pair of 4" cylinders with a 12" stroke (with the rod end pressurized) would be a good starting point. Put them anyplace handy that offers good attachment.

Not changes to chain or driver necessary but the load is mostly relieved. If there has to be a downward force on the roll in service it can be applied Via air or air'oil on the closed end of the cylinders or mechanically. via a latch.

Your best bet if the budget will stand it is to reconfigure with hydraulics

motion guru said:

Weight of the payload is always on the chain and the hoist shaft is always holding the load up (never powering it downward).

I think Daniel's approach to having a large accumulator charged to working pressure that is piped directly to the cylinders is a good idea - large cylinder ports and tubing should allow proper fill / evacuation of the cylinders. Max cylinder speed < 20 in/sec . . . more like 18 in/sec. Set the pressure to 50 PSI and let the cylinders always sit there at 50 PSI holding up 1000 lbs each.

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I used to build pneumatic powered packaging equipment. The pneumatic concept with multiple identical cylinders and a single large tank should work very well. We got a few million cycles on the large cylinders that oscillated the carriage about 12” every two seconds! We stopped installing lubricators, just put a little oil in the cylinders at startup. Lubricating the air filled the hoses with oil and made a mess. Cylinders were all Bimba non-rebuildables. I wouldn’t worry about crappy shop air, you only need to make up leaks. Install a tiny compressor for that and run it at 100 psi with smaller cylinders. With a tank volume 10 times the total cylinder volume the pressure change and force change will be just 10%. Use cylinders in compression if you can, then only the piston seal matters and it doesn’t see dirt like the shaft seal. Spherical rod end couplers take care of alignment and work best in compression. Do NOT use push-in type connectors if the cylinders have any side to side motion, they wear out and leak.

Hope this helps.

Do you have pictures or video of the access points around the whole machine?

David_M said:

Pneumatic rubber bellows springs (think tractor trailer air springs) with a properly sized accumulator charged with nitrogen or argon. You would have no seals to leak/maintain. If you wanted to you could have a small nitrogen/argon bottle regulated down to keep things topped off, but that shouldn't be needed. Consult engineering data for proper column height/diameter to prevent outward bulging or put them in circular telescopic way covers.

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Beat me to it.

Many options available here and since designed for over the road and many years service life the cycle count not relevent.

One would likely need to make an articulating arm of some sort to both multiply the length of travel and keep in alignment.

A plc can control pressure and deflate to allow for better control as the moving arm would change the volume in the "air bag".

You would inflate to lift then deflate to lower and the air control would need to be tuned a bit to not over lift the action.

Next option would be air servo.

Our motor home has both pneumatic throttle and tranny shifter that are servo drive so there could be a possible pneumatic servo solution that could be used to supplement the existing system.

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Probably not in the budget, but have you ever considered just replacing that whole mess with EXLAR servo actuators and be done with it?

The problem with a pneumatic cylinder is moving its volume of air in and out in less than a second (each way) through the port of an off-the-self cylinder--right?

This is what I'm thinking, now: A highly under-stressed torsion bar mounted up out of the way. To get constant force, use a pie section of a cam shaped sprocket. Have a roller chain anchored to the sprocket section just past its most rotated position with the other end attached to the load.