Issues Running Hydraulic Cylinders In Parallel?

Don't worry. That won't happen, as the cylinders' travel speed is determined by the load.

In a perfectly balanced system, OK.

If at all concerned, use two identical gear pumps driven by the same shaft, and connect one to each cylinder.

There may be slight variation on the load - there may be more weight on one side of the bar connecting the cylinders. How much imbalance can the system take?

Also, the cylinders will extend in about 1 second and stop with cushion, wait a couple seconds and then retract. The stoke time is pretty fast: ~13 in / sec. These will cycle several thousand times a day.

I just don't want the cylinders to break down after the first month of operation. My main concern is that if the cylinders become out of balance, the rods will place additional strain on the seals and they will fail prematurely.

I was hoping that using 1 gear pump and 1 valve for both cylinders will keep every aligned.

If, and that's a big if, you have large, well aligned tie bars to keep everything straight and aligned, you probably will get away with it.

Any misalignment or binding will seize the mechanism.

The mast of a forklift uses two cylinders to tilt it back. A flow divider is used to control that the cylinder travel is the same. That they will not twist the mast if load is unbalanced.

Another way would to place flow valves on the ports of the cylinder.

Ah, but those cylinders are close together and will have less tilting/unbalanced force.
It's a bit counter-intuitive , but I screwed up making someing with parallel sliding motion with the sliders a long way apart, moved in closer worked better.

So maybe there's more chance of asymetric load on the long beam... dunno just my two pennyworth.
Del

If you call close 3 to 4 feet. It's all relative.

Just came to mind another way I've seen to control two cylinders stroking together set far apart. They were tied together with a shaft, rack and gears.

Any time I have seen this in design stage there was a flow divider. Sometimes later it could be removed it all depends on the resistance being equal. Most of the systems I worked on had 4 parallel cylinders as in injection molding tooling.

Try re phasing cylinders at this link for more info:

http://hydraulicspneumatics.com/200/TechZone/Cylinders/Article/False/21797/TechZone-Cylinders

Hope this helps.

This is a poor design. You would do far better with a single cylinder. The motion should be transferred by a mechanism that will not lock up randomly. If the variation in load at each side is relatively slight then the cylinder itself will control tilt.

From the rate of strokes you suggest an electric motor and 2 chains would be a better and more efficient solution, probably quieter too. Dimension and loads would allow me to do a proper analysis at a price.

by the way, your flow chart has faults.

I've been learning how a Wheatstone bridge works and can see how you may be able to incorporate into your design a hydraulic equivalent Wheatstone bridge with a couple of load cells acting as your ammeter to trigger a pair of valves driving each piston to maintain a balanced output.

This way, you can have a self regulating circuit that will adjust to valve leakage and you can incorporate a trouble condition when the ratio of your parallel circuit goes beyond a selected threshold. As an extra measure, an encoder or position measurement incorporated into the cylinders may provide trouble indication and force feedback on the valves as well.

If this is for a hydraulic drive on a vehicle then you may need to compensate and balance loads with a 3 axis position sensor similar to those used in the nintendo wii for additional feedback loop.

Flow equalising valves are available, or you could use two gear motors on a common shaft. The motors would idle together and balance the flow to the cylinders.

Dear rjd1234, as many expirienced users above wrote, you will definetly need some sequinces devices. There is my range of seq. method depending from speed and accuracy (starting from simplest and cheapest)

1. Double flow pump (twin gear pump) - max deviation 2-5% on stroke

2. Flow divider valve - max deviation 5-10% on stroke

3. Gear flow divider - max deviation 1-4% on stroke

4. Servocylinder - max deviation 0.5-2% on stroke

Also you need to install pressure relief valves to compensate deviation in the end.

But the best in any case to use one cylinder.

Some time it is better to install heat exchanger and after filter (consider max pressure in filter)

A lot of good information - thank you everyone.

1. I agree that the best solution would have been to use one cylinder. The reason I designed this with two cylinders rather than one was because I need to keep the area between them open. The load is on retraction, not extension. Stuff will get smashed as it travels on a conveyor between the cylinders.

2. I haven't included all of the design elements - there is a reason everything is oriented the way it is. I including the elements I thought were needed to have a good discussion.

3. I will reevaluate the design and look at separating the fluid circuits and adding some guide rods.

Again, I appreciate all of the answers and feel like I have a better understanding of what this will take.

You will have one cylinder moving at a different rat than the other, or at least trying to. You have to allow for this in the mechanics.

We have a similar design in our products with very wide spacing (10 feet or more) and pretty high loads (100's of tons) on retraction. We intentionally allow or even force tilt of the loaded cross beam. One cylinder is fixed and the other allowed to tilt, with both attached to the load with pivot pins. We also independently control each cylinder with a closed loop system, as recommended by earlier posts. Also, we always specify low friction seals to reduce variance in drag.

In linear motion systems with parallel guides one guide is always the master and the other is either allowed to float or is mounted to match the master. You have something similar here. Can you allow one cylinder some freedom to follow the other with a bit of lag or lead? If you need to closely maintain parallelism of your cross beam you need to guide independent of your cylinders.

Often designers err in trying to make a system too rigid which causes excessive wear.

A single cylinder driving a big fork will balance your load without fancy hydraulics or any danger of lop sided wear to your expensive cylinders....which you will surely get otherwise. It might look in-elegant, but it would work just fine.

rgd1234, first you can halve the cost by making the working stroke the outward stroke (using the full area of the piston). This also reduces the volume of fluid req'd.

If you hang one cylinder from an A frame over the conveyor then it will 'fall' vertically with no chance of locking and it will be faster (self weight working with the action).

If the "product to be compressed" is reasonably central (one or more lines of product) then the force applied will be even across the width. Better still if a stop is arranged at each side to give the correct squash distance.

Use a pressure switch in the 'Press' line to flip the valve from 'Press' to 'Lift'. An adjustable one will give you more control and reduce consumption. Personally I'd use Air in case of leakage if this is a food product or oil sensitive.

Till nowcoucels were more or less "generic". It is known that a solution has to be found in relation to the real problem. An other machine design principle is to avoid -as far as possible - to give more functions to a part. To the problem there are several simple solutions even usin a single valve. Let us have a quantitative look: stroke is only ≈10".

I do not know the force and the piston rod dimension but if the cylinder is big enough it will have the necessary stiffness. The side bushing wear can be avoided by a cylinder longer than strictly required so that the distance between piston and rod bushing is such that in case of eccentric load the side load is still acceptable. As you see taking into consideration the real problem a solution can be possible. As I mentioned in a precedent comment depends on stiffness. If cylinders are big enough it can work. This could lead to a choice direction lower pressure bigger cylinders with bigger piston rods.

This is also why the change Del mentioned had a positive effect: stiffness is proportional to L^-3 so that if for example the connecting element is halved as length its stiffness will be 8x higher. Most probably the two cylinders were stiff enough and only the connection was the perturbing element

If we respect the second above mentioned principle functions are parted: the cylinders bring ONLY the force and the traverse is on 2 guide rods which have the capability to take the side loads without failure and without seizure. The cylinders can be on pins at both ends so that they work under best conditions without any rod bending or side loads on bushings or piston rings.

As commented below, if you want to come as close to synchronizing the cylinders as possible, a gear type flow divider would probably be your best bet. I have done this in the past with success.

However, what about running the cylinders in series? This has been done before. Pearson made machines where the cylinders were different sizes and the output of the larger cylinder powered the smaller cylinder. You will have to size the cylinders so the output of the big one will move the smaller one the same distance and will give the same downward force to both cylinders. This may mean customer cylinders.

You are right, passing the time soon or later this design will fail due unbalance in the cylinders speed, my recomendation is use a single cylinder with an articulated mechanism to raise both bar ends at the same time (i used this design in a centering device).

Regards

Did you look at the link I provided on reply #16? This looks like a simple and efffective method to synchronize cylinders.

I have faced the same problem in a past life. An engineer friend at the Uni solved it by mechanical means. Imagine if you will a large link from a chain, about 3 foot wide. Now anchor the link to the base/ground and to the arm that goes up and down. One end of the arm cannot go up without forcing the other to go with it and vice versa. Then it doesn't matter how the pump and plumbing work.

The example I worked with was a hundred ton press and the control link was a solid 4" bar with 1" plate arms welded to it.

Jim