Hi,
I am looking for 2 pneumatic cylinders that woukd be able to lift a total weight of 36 pounds vertically up with a seven inch stroke. The total length of the pneumatic cylinder at compressed position has to be about 10 inches...I hope someone would be able to advice me on this. Thank you.

Trying to explain the requirements for a circuit like you are attempting to make is beyond the scope of a Forum like this. It might be possible if you were trained in Pneumatics and worked with the equipment on a regular basis.

Air Cylinders Lifting and Lowering a LOAD can be very sluggish lifting and VERY QUICK when lowering if not hooked up correctly and are usually relegated to hydraulics after trying it once.

You can see a way to successfully make air cylinders do what you are asking by going to the Circuits Ebook at a link from the "Back t HydraulicsPneunatics.com" link at the top of this forum page. Look in section 8 at Fig's. 8-51-64 to see wht can happen when cylinders lifting and lowering a load are plumbed wrong and what must be done to make the cylinders operate smoothly.

Your best option for help to hook up the circuit is to contact your local Fluid Power Distributor and ave their salesman design a circuit and give you a list of parts required. That is the way most Fluid Power equipment is implemented.

Thanx for the advice Bud..so do u think it is easier and more feasible to use a hydraulic cylinder? Do you have any recommmendations for the hydraulic cylinder?

Your circuit requirements of 36# will work fine with pneumatic cylinders plumbed with the dual pressure circuit I referred to in the previous post. I've sold many air cylinders in my sales days to do a circuit like you are asking about.

The only problem with the circuit is when the load is not consistent. That requires a pressure adjustment each time the weight changes.

A hydraulic circuit is simpler since the non-compressible oil will not allow runaway or pause actions but can still require some pressure modifying especially with heavy loads. Heavy loads do not run away but can make some loud compression noises and some jerk when retracting. This should not be a problem with your light load.

Most Fluid Power circuit design is handled by the local Fluid Power distributor salesman. They usually do the circuit design and make a parts list on an Order Sheet to get business. They can also see your application and ask the necessaty questions about the circuit that allows them to make a qualidied judgement. That last part is difficult and sometime impossible in a Forum like this.

After seeing some request's and talking it over with the customer I have advised against using Fluid Power for their requirement. Lost an order but eliminated the hassle of a job done halfway with equipment that was not designed to do the work satisfactorily. Usually got clled again anytime there was a possible need for a Fluid Power application.

Once again thanks for your reply..was wondering whether you do have any good cylinders to recommend..Thank you..

For what you are looking at I would use some inexpensive throwaway type like Bimba.
www.bimba.com

Usually you wind up with the product the Distributor sells. The main difference is in the price.

Surplus Center has a lt of Fluid Power stuff if you plan on winging it. www.surpluscenter.com

Hi Bud,
Thank you very much for your advice, i have decided to use 2*39 lbs ( capacity of the piston at 100 psi ) to lift my dead weight of 42 lbs. I am using a 35 feet, 1/4 inch line with a flow control valve. My air source is about 100-115 Psi, I am expecting a pressure drop of about 10-20 psi in the hoses and the fittings and thus would be able to have about 70-80 psi to lift my dead weight. Do you think that would suffice?

Two cylinders that are both capable of lifting 39 Lbs. will lift your 42 Lb. load no problem.

The 35 Ft. of hose will have a pressure drop depending on the speed the cylinders are moving. The biggest problem with the long lines is there will be a pause when the valve shifts since it will take some time to exhaust the long lines and get a differential pressure across the cylinder piston.

The other situation when using long lines is the amount of wasted air, especially if they are oversize to reduce pressure drop. These long ines must be filled and exhausted each cycle without that AIR doing any work. That means the Compressor must run longer than required to do the work. Air waste is mainly a problem with high cycles rates.

A much better situation would be to place the valve at the cylider and use some mechanical linkage to operate it or use a Solenoid Operated Valve and a Switch with electrical wires to run the distance.

Control and action would be at least 100 tmes better.

Hi Bud,
Thats the plan, i am planning to place the control valve close to the cylinders. I am going to use a double actiuated cylinder. Do i have to put the flow control valve in both ports. If one port is good enough, which direction would you better? and also do i have still have to use an orifice if i do have a control valve? Thank you.

Take a look at Fig. 8-54-61 in th circuits design book here.
http://www.hydraulicspneumatics.com/200/eBooks/Article/True/82476/

This shows what happens to an air cylinder operating loaded in one direction and how to use a Spool type 5-Way valve to operte it smoothly.

I would use Meter Out Flow Controls in each direction of travel. a single Speed Control Muffler on the xhaust Port of the valve may work if the cylinders are equally loaded. Otherwise use separate Meter Out Flow Controls at each cylinder port. They are a little tricky to adjust but will keep an uneve load from trying to run away or transfer air to the opposite cylinder.

Hi,
I am actually looking for a sequence valve, which would lock my load in place after the cylinders have raised the load to the required level. I was thinking of using some air-clamping cylinders ( single acting with spring return) to lock them in place. the air supply would be the same for the load carrying cylinders as well as the air-clamping cylinders. I was wondering where i would be able to get such a valve. Thank you.

Contact your local Fluid Power distributor to get some help. What you are attempting may not be possible using Air alone. You may need an Air-Oil system or a full Hydraulic setup.

What you re trying to do is not something that can be worked out satisfactorily on a Forum like this. At least not in my opinion.

mechboy1, you should state
How fast.
How accurate.
Are the two actuators do be synchronize?
Consider electro-mechanical actuators.

And, Infinitely positionable or 2-4 preset, fixed positions?????