Wear on pnuematic cyinders Rod Clevis
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  1. #1
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    Default Wear on pnuematic cyinders Rod Clevis

    We have an assembly machine, with eight stations.
    There are 7/16" X 6" cylinders. Automation Direct A07060DN with a clevis on them ARC-10-32

    These lift/drop a carrier for syringe bodies.
    One cylinder pushes the syringe and the carrier down to snap the cap onto the end, and other cylinder lifts the carrier, and syringe back up.

    There is a 12mm linear rail, with bearings on it. Then an adapter from the bearing to the carrier for the syringe.

    The issue is the clevis is tearing up the adapter "ear"
    This runs 100 parts per minute, so each cylinder is cycling 12-14 times per minute.

    We had some clearance for the clevis pin in the "ear" to prevent any binding, but that allows it to clatter/hammer.
    There is 60 psi, so that gives 9 lbs of force.

    Trying to think of a way to absorb the shock, and stop ripping the ears off the carrier.
    The ear is .125" thick, and .7 wide 6061 aluminum.
    We could add plastic bushings, but will they hammer out, and provide contamination?
    Also considering a double linkage to replace the clevis, to allow some motion without clearance for the hammering.
    The only force should be tension.. Don't understand why the bottom of the "ear" is getting hammered.
    The lift cylinder is only powered in the Up direction, otherwise free falling.

    Any great ideas?

    My first ever attempt at attaching a picture....
    Attached Thumbnails Attached Thumbnails support-failure.jpg   barrelcartsupport.jpg  

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    My first question is why is that slotted? That is going to allow it to beat itself to death. Is the cylinder fixed rigid on the other end?

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    IIRC there was a binding issue, but I am not sure... It was a little while ago.
    We slotted it a bit but did not anticipate the hammering to be an issue.
    Only one cylinder is activated at a time. Lift or Push down.
    I may have to go visit the customer to see if it can go up and down without any binding, without any slot.

    Since the lift cylinder is not powered down and the push cylinder is not powered up, I am surprised that the bottom of the ear is getting hammered.

    That made me question if simply getting a tighter clearance on the hole would solve it.
    It will go a LONG way to solving it, but that wear on the bottom has me concerned.

    Maybe a plastic bushing, or a soft cushion in there is called for.

    Thanks Tony.

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    I dug through my bucket and found a cylinder that has a type of isolator on it that allows for angular and radial misalignment with no discernible axial play.



    Looking up the part number (FJ-O-10) they are apparently called floating joints or free joints, and are good for both compression and tension.

    I would personally machine off the ears and drill and tap to thread the stud of the floating joint directly into the aluminum block. If you have the space they can be put in between the clevis and cylinder, and remachine the tooling to have tighter pins, but I'm not a big fan of that approach.

    I think Misumi carries them, but I only saw metric threads in my brief search. Absolute worst case buy a metric cylinder.

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    McMaster-Carr

    Click on springs and shock absorbers. We used these on either end of the tooling being actuated to soft stop.

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    Igus is a go-to source for engineered polymer bushings. But 6061-T6 isn't a good choice for any direct-contact loading, especially when pounding and fatigue comes into play. That narrow tab, with clearance, and with line contact to a steel pin was destined to fail.

    If you have room, mill off the tabs, make stainless "pillow block style" bolt-on replacements with appropriate plastic bushings. As long as that works without binding (and you don't need the fancier multi-axis load transfer) you should last a very long time without issue.

    Use Loctite on the fasteners.

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    The failure of the ears is a product of simple fatigue. Aluminum has terrible fatigue resistance to start with. The problem is the design and possibly aggravated by alignment issues. The ears are stupid. Find another way.

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    I will be making new Carriers. ( the part with the ear) regardless. Broken ones won't work.
    Some of the issue with making a slot was to compensate for the accuracy of the cylinder mounting brackets. AMB-4
    The slightest bend and a six inch stroke could move the rod in relationship to the carrier traveling on the linear bearing.
    This machine needs smooth travels without binding to accommodate assembling the syringes.
    Any way for alignments to accumulate and bind could cause a misassembly.

    Plastic shoulder bushings might require new clevis for the added thickness.

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    If you can't guarantee alignment, I would seriously consider going to spherical bushings rod ends. On the end of the design we can see in your picture, you may be able to replace the ear with a spherical bearing. Igus makes some pillow block style ones that look like they might work for you. https://www.igus.com/wpck/3766/igubal_Stehlager

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    Yea, Festo has spherical ends for the cylinders, room might be an issue, hard to tell from your pictures. The axial compensator shown above would be good, but they are notoriously large and extend the length too much. However, you could make one that is sort of build into your bracket by sandwiching two polymer disks or rectangles on either side of a steel piece that you thread for the cylinder rod. This could bolt onto your bracket so easily changeable if required.

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    I used to build pnuematic packaging machinery. A couple of thoughts here. Your cycle time is pretty slow. A 7/16” bore with 60 psi will just about explode into motion, unless the air in the cylinder is controlled. It will be especially bad if the air lines are relatively large and if the valve is close to the cylinder. Put an exhaust flow metering valve right on the cylinder port and turn it down until you get motion just fast enough to do the job. The problem should go away.

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    Big +1 to what garyhlucas said in the above post!
    Get that part managed first if you haven't already. It's great advice IMHO.

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    One of the plants I worked had a similar problem. If space , operation, etc. permits . Would you be able to use a rodless cylinder? McMaster-Carr
    Just a thought.

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    Use Bimba Alignment Couplers. There are Urethane pads inside, but they are isolated and will not contaminate the machine. These will operate for millions of cycles.

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    The damaged eye looks like it was subject to very high loading. If you are using the cylinder to stop the travel of that large block you would expect very high inertia forces. The stop should be on that sliding block. Is it?

    You are using clevis end cylinders. You shouldn't need a slot to deal with misalignment.

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    As mentioned, machine the ears off and replace with a bolted on clevis / bracket.

    Fix alignment issues using spherical mounts for both ends of the cylinder.

    Use cylinders with adjustable internal end of stroke cushions.

    Replace fittings with free flowing in / checked adjustable out (as pictured in Strostkovy’s photo)

    Enjoy having more time to work on paying jobs.

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