Converting from air cylinders to ???

[chadzeilenga]

We currently have a testing apparatus which is powered by an air cylinder. This apparatus operates in temp ranges from -40C to 85C. This system pushes a rack at a speed of approximately 0.13 ft/s.

When using this apparatus in the cold temp environment, the valves must be opened up all the way and this is difficult to do considering the configuration. Does anyone know much about electronic acutators for this type of application??

Thanks,
Chad

 

[MintJulep]

How far? How much force?

Are the valves and the cylinder exposed to the test environment? If so, do they need to be?

Would the replacement actuator be exposed to the test environment?

 

[chadzeilenga]

The system travels approximately 6" depending on what parts we are testing. Currently the parts being tested have a cumulative torque of 0.069 N*m.

The valves and cylinder must be exposed to the testing environment due to the temperature chamber configuration. This is unavoidable.

 

[israelkk]

Are you using electro-pneumatic valves?
What is the air pressure?
What is the supply voltage?
What is the cylinder diameter?
What is the valve orifice diameter or the equivalent orifice diameter)?
What do you mean by "the valves must be opened up all the way and this is difficult to do considering the configuration"?

As I see it, you need to push the cylinder against a constant torque (rack and pinion) while moving a mass/ inertia. The 0.13 ft/sec is 1.56 in/sec meaning the 6" travel will take ~4 sec which is quite low.

 

[zeusfaber]

Is this a test you need to do often? If not, perhaps you could stand the expense of sticking with the compressed air system, but filling the cylinder with a less viscous gas?

I've seen flotation bag systems charged with helium, simply because that's the only thing that can be guaranteed to come out the bottle fast enough on a cold day.

A.

 

[chadzeilenga]

The current air cylinders are Bimba "Original Line" cylinders.

The supplied air pressure is between 80 and 100psi.

There is no supplied voltage as the cylinder currently runs strictly on air.

The cylinder OD is 1-1/16", the rod is 3/8"

Not sure about the valve orifice.

There are two supply lines to the cylinder which push the arm forwards or backwards. These air lines each have a valve to control the amount of air pressure and this is currently how we regulate our speed. In normal operating temps the valves are half open, in -40C they are all the way open. This requires us to open the chamber and manually adjust the valves which allows moisture in.

Yes the speed of the system is relatively slow.



We use this test aparatus for multiple tests, so it is used often. We are trying to improve the existing design and thought electric actuators would be better.

Chad

 

[MintJulep]

How do you reverse the stroke direction when the cylinder reaches one end?

What type of valves are your currently using, and where does the air air from one side of the cylinder go when you are pushing the other way?

In general terms, valves do not regulate pressure, they regulate flow. Pressure does not control the speed of a pneumatic cylinder, flow does.

Does the speed need to be controlled to any special value, or is "relatively slow" ok?

You previously stated that the operating range is -40C to 85C. Therefore -40C is a normal operating temperature for this application.

It seems likely that your problems are related to thermal expansion/contraction of the various parts of the cylinder, and thermally induced changes in viscosity of the lubrication in the cylinder. You could try installing some heat tracing and insulation on the cylinder to keep the cylinder at a more constant temperature.

Move the valves outside the chamber, then you don't have to open the door. If you continue to insist that the valves need to be inside, then get some electrically actuated valves, and run the wires outside.



Any electrical actuator would likely suffer the same sorts of thermal effects, and hence similar changes in performance over the temperature range. Also, -40C to 85C is pretty much the "standard" range for industrial electronics, so you would not have much margin.

 

[rmw]

To answer one of the original questions posed, look at

http://www.jordancontrols.com

and find their linear actuators to see if they have one that meets your specifications.

rmw

 

[chadzeilenga]

The cylinder has air plumbed to each end. Line 1 pushes the cylinder in the forward direction. Line 2 pushes the cylinder in the reverse direction.

What type of valves are your currently using, and where does the air air from one side of the cylinder go when you are pushing the other way?

The valves are one-way valves in that when the cylinder is operating in the reverse direction the air gets exhausted. They are adjusted by thumbwheels which open or close the valve.

Does the speed need to be controlled to any special value, or is "relatively slow" ok?

We would like to be able to retain constant speed thoughout the temperature range if possible.

You previously stated that the operating range is -40C to 85C. Therefore -40C is a normal operating temperature for this application.

-40C is the lower temp limit for the testing. It is used to simulate the coldest temp that the part would be exposed to but it is not a constant operating temperature.

It seems likely that your problems are related to thermal expansion/contraction of the various parts of the cylinder, and thermally induced changes in viscosity of the lubrication in the cylinder. You could try installing some heat tracing and insulation on the cylinder to keep the cylinder at a more constant temperature.

Move the valves outside the chamber, then you don't have to open the door. If you continue to insist that the valves need to be inside, then get some electrically actuated valves, and run the wires outside.

I think this will solve my problem, I will try to configure a new layout for the valves.



Any electrical actuator would likely suffer the same sorts of thermal effects, and hence similar changes in performance over the temperature range. Also, -40C to 85C is pretty much the "standard" range for industrial electronics, so you would not have much margin.

 

[FredGarvin]

I used to do environmental, endurance testing on latch systems back in college. We used air cylinders as well. It was always a lovely thing to go into the chamber at -40° to adjust flows to get actuators unstuck. Ahhh memories.

The basics we did were already mentioned. We used heat tape and insulation to try to keep the actuators at a warmer temp. You do run into some other issues, but they are not really anything to worry about. Also, moving your control relays/valves outside of the cell is a big help. It does give you some extra line length to deal with, but at the slow speed you're dealing with, I don't see it being an issue.

One other option that I always wanted to look into but did not have the budget to do, was to look into trating the air source, i.e. heated/dry air. I am assuming that you are running these actuators off of a shop air system that has some kind of dryer. If that dryer is not working or is not maintained (which is the norm for most places I have ever seen), the moisture in your air source will be a problem as well.

 

[chadzeilenga]

After playing with some "low-temp" pneumatic cylinders and having little success I think we are ready to redesign our testing fixture to adapt a 90° gearmotor to the fixture. I have run through some quick calculations.

The -40°C operating torque of our parts is 28 oz-in. With 16 parts installed on our test fixture, this places the force required to move the table around 90 lbs. At 85C this force is reduced to 36lb. These are only approximated forces.
The maximum travel of our text fixture is 8", but we normally only use 2-3" and use end stops to limit this.
Does anyone have any suggestions regarding the hp and rpm rating of the gearmotor that we try out?

The picture below is a quick 3D model of our testing fixture. The holes in the green bars are where our parts sit and mate with the gear teeth on the sliding table. I was planning on mounting the gear motor underneath the sliding table.

Chad