hi..

i am not sure how wide the area plus the weight you want to lift but if it is not too great yes you are better off using a large valve with flow control and just using one big cylinder lifting the whole plate.

or another combination is using a manifold for four valve supplying your cylinders with individual flow control so you can control the speed but the chances of going out of square is greater than the first set up i mentioned above...

can you give more info on the application including the weight and speed of lift?

dydt

you tweaked something there with the plate thing........I am trying to operate a pair of rails in unison (and run the process between them). at first i was thinking i would use 4 cylinders, but now........if i link the rails and create a "U" shaped profile from the end i could get away with 2 larger cylinders located on the centerline under the connecting baseplates.
The apparatus will be about 8 feet long, 2 feet wide, 3 feet tall and could be made from aluminum etc. With the load i would estimate total at 100-150 lbs. Lets go nuts and say 250. I don't think i would want to try and do this with one cylinder in the center.

If cylinders really are matched, try plumbing cylinders in series.

depending on the pressure available, a single cylinder should be capable of lifting your load.  

If multiple cylinders are used, you'll need to run them slow enough so that the pressure is not flow-limited.  

Additionally, you can mechanically arrange for the load to be driven from a single point.  if you have two cylinders; they can be coupled to a crossbeam that supports your load at its c.g.  through a pivot support.

TTFN

If you end up using 4 cylinders, be prepared for uneven lift. You will need to 'meter out' all cylinders or ensure even load distribution if you want an even lift. A fluid medium will give better control on lift rate.

You can try a series of reed sensors along the cylinders and have a control circuit that monitors and ensurs that each cylinder stops if the other reed switches are not made.  But this will result in a jurking action as well as a drastically increased cost.

Probably your best bet would be to center one flow control valve in the center of the two cylinders and drastically oversize your flow capablities to ensure that pressure does not drop too much during operation.

I too have engineered several projects on machines using four air cylinders in unison.  This was on a gang drilling machine and they needed to all come up and stop at the desired height.  What I did was to create an adjustable stop on each of the cylinder rails.  This system has worked very well in this application.  The only problem I have occured was after three years of heavy use.  Now all of the cylinders need to be rebuilt (now they don't come up at the same time).

If you want to see what I did send me your email address and I will email you that section of the machine.

lesliehowell@howellconsulting.com

The name consulting is for more resonable insurance.

Sincerely yours,
Leslie H. Howell

You'd be better off using 1 larger cylinder. running more that one in unison is more difficult that it seems and a load of 250 lbs. is no problem for a decent pneumatic system.

Use one cyl. pulleys or pantograph/ scissors to keep the load horizontal

<nbucska@pcperipherals.com>

Hey tmox,

You'd be bitter off reconsidering your mechanism.  Using some minimum constraint design techniques, you could develop a system that could operate off of one cylinder.  I could offer support on that or you could refer to a book like Lawrence Kamm's Designing Cost Efficient Mechanisms.

CHUCK
www.pitzerconsulting.com/hidden/index.html

tmox,

I've Just bult a machine with a lift very similar to the one that you describe,  we used a single 80mm bore cylinder & two pivot points to lift a total mass of around 300 kg with a travel of around 150mm, I can email you some photos if you like

Regards Russell Gee

A single cylinder would be best. However if you must use multiple cylinders the next best option would be to mechanically tie them together, put flow control valves on the exhaust side of the cylinder.  Make sure your supply lines are the same length from the common header and the supply to the header is not volume limited.  

Good luck

Hi tmox,

It sounds like you have a very interesting project.  I have developed several multi-cylinder lifting systems (synchronous-lifting) for a variety of industries, and the problems I encountered are identical to those you have described.   I have seen several lifting-system applications that attempted to accomplish the task by tying all the lifting cylinders together to a single valve.  The consistent problem with this approach (as you’ve already realized), is that unless the center of gravity of the load sits directly at the geometric center of the lifting stations, the cylinders will “see” different load values.  Because the cylinders are driven from a single valve, they all see the same pressure, and thus have the same lifting force.  Attempting to raise or lower an “unbalanced” load like this will result in the lightest-loaded cylinder moving the fastest, and the heaviest loaded cylinder moving the slowest.  Not good if you are trying to lift something and position it accurately.  This approach may well be workable for lighter load applications that do not require a high degree of positional accuracy.  However, I have had to develop lifting systems capable of lifting several tons with an accuracy tolerance of +/- 1 mm between cylinders, and the single valve theory just would not cut it.  The only method that I’ve found to provide absolute robust operation of a multi-cylinder lifting/positioning system is to use position sensing of each cylinder and feed that information back to a microcontroller.  The controller in turn operates a single valve per cylinder, servoing on the position of each cylinder from a datum reference.  In this manner, each cylinder is independent of the other cylinders; changes in system pressure; or lifting load changes as the load may shift during the lift sequence.  If your lifting application is a dedicated system (i.e. you’re always going to be lifting the same thing to the same position), then the single valve or mechanical balance system might work.  However if your application will encounter varying loads with different weight distributions, then a “semi-intelligent” lifting system may be the only practical solution.  To answer your initial question of the “best (cheapest) way to ensure simultaneous operation of 4 separate pneumatic components” is usually an engineering contradiction in terms.  We are always searching for the “best” and most cost-effective solution, but those two guys rarely play well together.  You can have the “best” OR you can have the “cheapest”, but trying to get both really makes our profession interesting.  Best of luck with your project!

You can go expensive, or you can go cheap. Your pnuematic cylinders are completely compatible with oil. Use a  needle valve on your oil filled exhaust side to adjust speed and balance (negate any cylinder cushions). You'll need to reservoir your displaced oil to prevent air entry. You may want check return for increased return speed.  It's that simple.

Vertical applications of pneumatic cylinders are often the most challenging.

If you have not already built the device in question, let me know, and I will provide you with all the guidance you want.

I am curious as to why you chose pneumatic over hydraulic. Hydraulic will give the same effect as a mechanical lift and is far less sensitive to load variables. A small hyd. pump and a single cylinder or double cylinders would do the job, and lift the load evenly regardless of weight distribution.

Just curious -- sorry I really don't have a solution for the pneumatic load balancing problem. If the load is identical weight for each lifting operation and is positioned precisely the same each time then all of the folks who have respomded have reasonable solutions.

Goo Luck,
ietech

Thanx very much everyone.

Basically, you have all confirmed my original thoughts on this dilema. There are several good ideas here.

And, you're right "BEST" and "CHEAPEST" don't get together very often.

This project was kiboshed by the customer, but, again thanx for your comments!!!