How to apply air pressure gradually to a pneumatic cylinder 1

[sammcc]

Hi All,

I have a load test machine where I set a regulator to a certain air pressure which translates to a specific force on a pneumatic cylinder.
For example, 10psi on the regulator would equal 100lbf on the cylinder
So, if I want to carry out a load test on a component I set the regulator to the value I require and test the component.
The problem I would like to address is that at the moment the machine instantly goes to the test force as opposed to pulling the load on slowly.
I know that I can use a speed controller to control the speed of the cylinder when the load is applied but how can I control the rate at which the force is applied?

Hopefully this makes some sense.

 

[btrueblood]

Why not open the valve at low pressure and then slowly increase the regulator setting?

 

[1gibson]

An orifice? Or the lazy way, crimp the tube almost fully closed.

 

[ornerynorsk]

There are slow start valves. Just google it.

It is better to have enough ideas for some of them to be wrong, than to be always right by having no ideas at all.

 

[sammcc]

Thanks for the replys all.

btrueblood- this is one idea we have but it is not practical for 100's of the same part I should have mentioned that

1gibson- what do you mean by an orifice? Crimping the tube doesn't look very professional!

ornerynorsk- I had looked at slow start valves- have you ever used these and is there a particular type there seems to be a lot of variation in them.

 

[TheTick]

Or add a needle valve in place of an orifice. Big picture: choke down the air flow.

If you need to load slowly but then open up, you can add a bypass. For speed, you can use automated valves.

 

[LittleInch]

sammcc,

You haven't really given us a lot to go on here, but lets get back to first principles.

You are using a compressible fluid ( air) which we understand is controlled to a certain pressure to create a certain force based on square area of your presumably piston.

What we don't have any idea of is the relative volume of air between your isolation valve and the cylinder end which then pressurises apparently "instantly". Nor do we have any idea of whether this piston actually moves 9 you mention "speed control". Speed control of what? How much volume is involved in this?

I can only think that your regulator is fairly large compared to the volume of the air in the cylinder and hence can pressurise it very quickly. As noted above basically all you need to do in this instance is send the air via some sort of restriction, either fixed (orifice plate / Restriction orifice) or some sort of variable restriction ( needle valve). Once the pressure reaches your required force then open the bypass and then if you need more air in order to make the piston move you won't be restricted.

The devil really is in the detail here.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Jboggs]

Several companies offer soft-start valves and soft-start fittings. I have successfully used Parker soft start fittings in a pneumatic clutch circuit to prevent recurring damage to the clutch from sudden pressurization. Talk directly to the application engineers at whatever manufacturers you consider to find out exactly what they mean by "slow start". How slow is "slow"?

 

[MintJulep]

I would use something like this

http://www.amot.com/products/hazardous-area-instrumentation/current-to-pressure-i-to-p-converters

and

https://www.amazon.com/Generator-Transducer-9-Segment-Programmable-Simulator/dp/B0773JHW14

 

[berkshire]

Several companies notably Parker offer "One way rate valves " which throttle the incoming air to a cylinder, then allow full flow for the return. These are used on actuation cylinders for slow advance ,quick return.
B.E.

You are judged not by what you know, but by what you can do.

 

[itsmoked]

I use exactly what berkshire suggests but was shown by a crack pneumatics salesman that you always want to put the rate valves in the opposite direction hose. So, if you want to slow the extension rate you install the little finger-adjustable valve in the retraction air line next to the cylinder.

BTW these little valves have a check valve in one direction so they do nothing in one direction and restrict to the setting in the other.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[handleman]

Keith's advice is correct in 99% of pneumatic speed controller applications. However, in this case, you've stated that you want the FORCE to increase gradually as opposed to the normal function of controlling speed of extension. Assuming this is correct, and your actuator is already in contact with the UUT, you want to control the flow INTO the actuator.

 

[itsmoked]

Excellent point handleman. Thanks for that.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[sammcc]

Guys thanks for all the input. I will see how I get on and report back shortly.

 

[Compositepro]

"One way rate valves" are actually called speed control valves, and they should almost always (I really cannot think of an exception) be placed to regulate the exhaust flow from the cylinder. The point that handleman makes about this case being an exception is not correct. You still want to regulate the exhaust flow. The force generated by the cylinder is due to the pressure difference between the two sides of the piston. There is zero force generated when the pressures are the same. It does not matter if those pressures are 0 psi or 100 psi. The reason for always throttling the exhaust is because 100 psi air is much stiffer than 0 psi air. Throttling inlet air rather than exhaust air results in a variable delay in cylinder movement. This is because it will not move until the slowly increasing inlet side pressure rises enough above the rapidly falling exhaust side pressure to move the load. Also, movement will be jerky because of the "springiness" of low pressure air.

When the exhaust flow is throttled, the cylinder will move smoothly as the air volume on the exhaust side decreases with high pressure air on both sides of the cylinder. If you want the force to build slowly, this occurs naturally as the piston cannot move any more. The exhaust side pressure will start to fall slowly from full pressure, which adds force to the load.

Now, if the cylinder is not near the end of its stroke you may see some strange cylinder movement or delay when you reverse the stroke. This is for the same reason as mentioned before. Throttling the inlet flow instead would only make this worse. Normally when using pneumatic cylinders both sides are always pressurized unless the cylinder is at the end of its stroke and there is very little volume on the unpressurized end. In cases where there is much volume on the unpressurized end, there will be strange behavior when switching direction.

I did learn all this the hard way, through experience, when in my first job. I was very simple to correct but it was a very valuable learning experience.

One complication is that the cylinder speed adjustment and the rate of force increase are not separate adjustments. It is quite likely that the throttle setting for setting the desired force increase rate results in a very slow cylinder speed. The rate of force increase will depend on the volume in the exhaust side of the cylinder when it hits the load. So a more sophisticated control strategy may be required.

 

[SwinnyGG]

I really cannot think of an exception

The exception would be when you want to control the rate at which force builds against a load which does not move, with a very consistent and precise force vs. time relationship that is independent of maximum test pressure.

Flow control on the exhaust controls extension rate until the cylinder and load are in contact- after that,if the load is fixed, flow control on the exhaust side does little. That air will bleed to ambient at a controlled rate while pressure builds at a different rate, dependent on test pressure, on the other side of the piston. So for every change in test pressure, which results in a different piston speed, you have to adjust the exhaust side flow control to maintain a consistent force over time curve. That's clumsy.

If OP cares about force over time being as close as possible to the same ramp for varying test pressures, what he wants is flow control on the pressure side, and the least possible amount of flow restriction on the exhaust side.

 

[handleman]

I don't just write random words for no reason.

... Assuming this is correct, and your actuator is already in contact with the UUT, you want to control the flow INTO the actuator.

Perhaps you're not familiar with the term "UUT"?

 

[Compositepro]

"after that,if the load is fixed, flow control on the exhaust side does little."

That is not true. It does exactly the same thing as flow control does when applied to the inlet side. And your argument for throttling inlet flow only addresses the pressure ramp rate after contact with the load. You still cannot control speed separately from pressure ramp rate.

For this application it does seem that the best solution would a single acting rolling diaphragm air cylinder with spring return and an instrument grade pressure regulator. In this case one would restrict the inlet flow because that is all that can be done and the spring return force stays constant. It is still a problem trying to control speed when both air pressure and restriction are variable.

Few people are really aware of how bad standard air cylinders and pressure regulators are at precise force control. Piston seal friction is petty significant, particularly on small cylinders because the friction is a larger percentage of the max force. Utility grade pressure regulators can drift 5-10 psi after being set.

 

[handleman]

Dude. Read the OP. Pressure ramp rate after contact with the load IS THE DAGGUM PROBLEM STATEMENT! Speed of actuator motion is never mentioned.

 

[Compositepro]

So why have you have decided to ignore this part of his question?:

"I know that I can use a speed controller to control the speed of the cylinder when the load is applied but how can I control the rate at which the force is applied?"

Your arguments for not throttling the exhaust side of an air cylinder are still wrong. My main concern in my answers to this thread, is make sure that engineers new to pneumatics do not pick-up the common, and erroneous, impression that speed control valves can be used to throttle to inlet flow to an air cylinder as you and others have said or implied. This case is not an exception as you stated. A speed control valve contains a needle valve in parallel with a check valve.

If one were to place a restriction in the line in this case, it would be only a needle valve, not a speed control valve.