Festo cylinder brake works/dont work !?

Hi all.

I have just had a problem with a Festo pneumatic cylinder where the brake couldn't be adjusted. Even if the adjustment screw was turned all the way in, the cylinder didn't brake and hammered into the end position.
So the cylinder was exchanged and the brake adjusted OK.

The strange things is, when moving the 'defective' cylinder manually, you can clearly feel when the brake activates, and it can even be adjusted weaker or stronger by the screw. As if nothing is wrong with it.
In the past we have had several Festo cylinders with brake problems, but I have never before cared to investigate the defective ones after replacement. I am thinking there is something I am doing wrong or there is something that one can do to fix the problem without replacing the cylinder.

Have anyone else experienced problems like this ?
Anything else one can investigate or do ?

is it a cylinder fitted with a brake or is it the 'Cushion at the cylinder ends. you are reffering to.
the cusshion can only work with restricted return air.
if you are using a 5/3 valve with the centre envelope exhausting the from both sides of the cylinder than the cusshion can never work.
Unless you repressurise the cylinder before you change its direction.
If it is an actual Brake cylinder then the SHaft end of the cylinder has an additional 100mm block on the output side of the Cylinder

Thanks for your reply Ian.

It is an integral cushion brake.

The cylinder is always pressurized in one direction. When starting the movement, the pressure is reversed, so the movement is smooth from one end to the other.

There are throttles on the return air to adjust the speed of the movement in both directions. Even so, IMO the brake should work without the throttles. If you adjust the screw all the way in, a little bit of air is trapped and works as a hard cushion keeping the cylinder from reaching the end position.

the cushions are an integral part of the Piston
these fo an example they might be 20mm dia by 20mm long from each side of the piston.
as the piston approaches either end this reduces the bore size to 20mm
and the screw is an integral speed control.
these are only going to function if there is a restriction in air flow.
if the cylinder has no back pressure it can never work.
adjust the throttle/check valves down to test what I am saying.
even fit a guage to observe the back pressure.
speed control and or cushioning will not work if the back pressure has exhausted before the cylinder reaches that point

The back pressure is there. I believe so because I can adjust the speed by the throttle.
It just continues smoothly all the way at the same speed without braking.

Also, after replacing the cylinder it works as intended, implying that the problem is with the cylinder.

The cylinder has been replaced and the machine is about to be packed for shipping, so I cant do much now.

If I encounter this again, I shall try to fit a pressure gauge as you suggested. Maybe that will reveal something.

i would say the faulty cylinder is exactly that. get Festo to look at it they will usually help

Yes, I shall do that.

Thanks again.

I am trying to have the cylinder setup in a test rig to investigate it more.

http://www.festo.com/cms/en-gb_gb/626.htm
this will help and explains it perfectly

I have used quite a few Festo cylinders, and have seen the cushioning fail occasionally. If I recall, part of the plastic seal holder is a piece of plastic, larger diameter than the rod, that engages a seal in cylinder that routes the escaping air through the end cushion needle valve, then out the working port. This seal can wear out over time and cause the issues you are seeing. On some Festo cylinders you can see the part that engages the end cushion seal through the working port.

I'm gonna suggest the piston seals, since there are two seals.
The seal facing the the cushion adjustment screw is
likely worn out.

I will take your advice and investigate the seals.

After having hooked the 'defective' cylinder up in a test rig, it works perfectly !

Stumped ......

Jesper, what's the model number of the cylinder?
I'm wondering which type cushioning your cylinder actually has.
There should be a 'P', 'PPV', or 'PPS' in the model number.

G.