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Pneumatic cylinder problems

Discussion in 'Robotics & Mechatronics' started by Hennie Viljoen, Nov 26, 2017.

  1. Hennie Viljoen

    Hennie Viljoen New Member

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    Hallo everyone,
    I am a new member. Interisting comment left above. I am building pneumatic cylinders with feedback (4-20ma) units and external and internal I 2 P's, subject to what I use. The most common dia of the barrels is 8-10" and I use both steel and fibre barrels. The one pro lem that I get from time to time is creeping/ jerking of when low pressure is apply. I.e, I would build 4 cylinders, exactly.the same and 2 of the 4 would jerk or creep.....any idea why?
     
  2. JimB

    JimB Super Moderator Most Helpful Member

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    From my little experience of pneumatics, about 40 years ago....

    Actuators which were used as valve positioners did not have a tightly fitting piston inside a cylinder.
    The piston was a disc which had a clearance of about 1mm inside the piston, on the air side of the piston a rubber diaphragm made a seal so that the air could not bypass the piston. The diaphragm made a completely sealed chamber (a big rubber bag) on the air side of the piston, it was not a sliding seal.

    Doing things this way overcame problems with friction between the piston and the cylinder wall.
    This is where I think that your problem may be, friction between cylinder and piston.

    JimB
     
    Last edited: Nov 26, 2017
  3. jpanhalt

    jpanhalt Well-Known Member Most Helpful Member

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    Welcome to ETO

    What you seem to be describing is called stiction (alternatively, grab and release) that you see with hydraulic and pneumatic cylinder control. Common solution is to use pulse-width modulation (e.g., add a greater pressure for short periods of time) with or without dither (keep it moving so it doesn't grab).

    Here is one of many links, if you search on stiction: https://www.qualityhydraulics.com/blog/what-dither-versus-pwm/

    John
     
  4. dave

    Dave New Member

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  5. KeepItSimpleStupid

    KeepItSimpleStupid Well-Known Member Most Helpful Member

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    Let me ask a few questions:

    1) Are you using lubricated air?
    2) Do the solenoids require lubricated air?

    That solved a lot of problems at work. Another issue was an dry air supply with auto-venters. The shop air supply was 100' or more away from some of the uses.
    The air lubricators were at each machine. They add 10 WT oil to the air supply.
     
    • Agree Agree x 1
  6. shortbus=

    shortbus= Well-Known Member

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    How smooth is the bore of the cylinders? Cylinders are usually finished inside with a very smooth, honed surface, not a lathe tooled surface.
     

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