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Thread: How to fix this play in jackscrew...video attached

  1. #1

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    How to fix this play in jackscrew...video attached

    Found some play in my jackscrew assembly. The nut on the bottom is tight. I put a wrench on it and its real snug. The play seems to be in the jackscrew and the first washer below the threads where it contacts the top of the trim pulley. Can I add another washer or how is this fixed? There is a small amount of play between the 5/16 bolts that hold the yoke to the stab assembly. I understand this is a rotating part and cant be too tight?

    There is some movement in the yoke to the screw as well how much is acceptable there?

    Video attached

    https://www.youtube.com/watch?v=-gjBLLzccEE

  2. #2

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    Replace the yoke.
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  3. #3

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    Add a washer on bottom- common. Pivot bolts should be snug enough to prevent slop, but not tight- you’ll have to tighten them once in a while.

  4. #4
    KJC's Avatar
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    The cast aluminum yoke is worn out and needs to be replaced. The steel threads of the acme screw eventually wear the aluminum threads away. I suggest you get a PA-18 parts diagram for the jack screw assembly. There are 3different thickness washers that need to go in a particular order. Also there are an916L washers that shim the liner tube.
    PA-12 N418BS

  5. #5
    KJC's Avatar
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    The cast aluminum yoke is worn out and needs to be replaced. The steel threads of the acme screw eventually wear the aluminum threads away. I suggest you get a PA-18 parts diagram for the jack screw assembly. There are 3different thickness washers that need to go in a particular order. Also there are an916L washers that shim the liner tube. I explained it here including parts diagram:

    https://www.supercub.org/forum/showt...highlight=Yoke
    PA-12 N418BS
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  6. #6
    Crash, Jr.'s Avatar
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    Seems like half of the play is shimming the bottom of the jackscrew as it's moving up and down a bit.

    The other half of the play is definitely in the yoke threads. Maybe consider replacing it but to be honest probably a large percentage of airworthy Pipers out there have the same or more play in the yoke. To be sure it's probably at the upper limit of safe play so if you have the $$$ and time change the yoke.
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  7. #7

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    Not to mention, according to some, that safety wire is getting ready to saw your liner tube right in two...
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  8. #8

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    Quote Originally Posted by Crash, Jr. View Post
    Seems like half of the play is shimming the bottom of the jackscrew as it's moving up and down a bit.

    The other half of the play is definitely in the yoke threads. Maybe consider replacing it but to be honest probably a large percentage of airworthy Pipers out there have the same or more play in the yoke. To be sure it's probably at the upper limit of safe play so if you have the $$$ and time change the yoke.

    Yup good to know. Will order and change this winter when itís nasty. Itís time to bear hunt now. Tried to shim the bottom just now but to get the shim washer at the bottom of the threads it was apparent I would need to remove the screw itself to do that so we reassembled it and will keep an eye on it.

  9. #9

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    Quote Originally Posted by ak49flyer View Post
    Not to mention, according to some, that safety wire is getting ready to saw your liner tube right in two...

    EH I’ll take my chances. Living on the edge.
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  10. #10

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    You don’t have to remove the screw, just the pulley.

  11. #11

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    Quote Originally Posted by ak49flyer View Post
    You don’t have to remove the screw, just the pulley.

    I just tried that but couldn’t get the large thick washer off so couldn’t add a shim to the bottom of the screw it was almost fixed in place. Don’t have any pics.

  12. #12

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    You have to pull the woodruff key after removing the pulley- it captures the washer(s)

  13. #13

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    Quote Originally Posted by ak49flyer View Post
    You have to pull the woodruff key after removing the pulley- it captures the washer(s)

    Yup did that and still didnít think it would come out. Would try again. So castle nut off and then the washers, pulley and then the woodruff key after all this the thick washer comes off so I can add a washer to shim the bottom of the jackscrew?

    can you do this solo and if so how do you keep cables on pulley

  14. #14
    Crash, Jr.'s Avatar
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    I did mine solo it's not that big a deal if you have a decent access panel. The washer isn't held on by anything except a couple decades of hardened grease so you might have to get up behind it with a small screw driver to get it out. Like said before the pulley and key need to come off. It's a very small amount of adjustment so only add a thin washer or replace the existing thin washer with a thick one to take out that play. You need to have a certain amount of play left or you'll bind the jackscrew. Also don't worry about keeping the cable on the pulley, just make sure it goes back on in the same way it came off and test your trim before you fly to make sure it was installed the right way. Look up the tail to make sure you don't have the trim cable twisted around itself before putting it back on the pulley.
    Thanks Bill Rusk thanked for this post

  15. #15

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    Quote Originally Posted by Crash, Jr. View Post
    I did mine solo it's not that big a deal if you have a decent access panel. The washer isn't held on by anything except a couple decades of hardened grease so you might have to get up behind it with a small screw driver to get it out. Like said before the pulley and key need to come off. It's a very small amount of adjustment so only add a thin washer or replace the existing thin washer with a thick one to take out that play. You need to have a certain amount of play left or you'll bind the jackscrew. Also don't worry about keeping the cable on the pulley, just make sure it goes back on in the same way it came off and test your trim before you fly to make sure it was installed the right way. Look up the tail to make sure you don't have the trim cable twisted around itself before putting it back on the pulley.

    Ok great advice thanks Iíll give it a go.

  16. #16
    Steve Pierce's Avatar
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    Adam, verify the washers are assembled correctly per this description:
    The main components of the stabilizer adjustment system are the yoke and jackscrew. The stabilizer yoke has a flat side and a “V” shaped side. The “V” should point down on any Piper with the trim on the side wall. When installing the stabilizer yoke and jackscrew in a Super Cub it is best to use Figure 48 in the PA18 Parts Manual as a guide. The problem with the parts manual is that the part numbers get confusing. Hopefully I can clear some of this up by relaying a little information I have gathered by researching lots of Piper drawings. The parts are as follows starting with the index number depicted on Figure 48:

    11 part# 42961-02 Stabilizer Adjustment Screw
    12 part# 11008-00 Stabilizer Adjustment Pulley
    13 part# 80122-41 Washer measuring 1” O.D. x .375/.385” I.D. x .095” thick
    14 part# 81342-05 Washer measuring 1 1/8” O.D. x .375/.385” I.D. x .062” thick
    15 part# 85012-12 Washer measures the same as an AN960-616L
    16 part# 80122-24 Washer measuring 3/4” O.D. x .328” I.D. x .062” thick
    17 part#81342-08 Washer measuring 5/8” O.D. x .313/.323” I.D. x .032” thick
    Not listed are the two AN5-7A bolts and AN365-528 nuts that hold the stabilizer yoke to the liner tube link assembly index #9 part# 14180-02. There is also an AN280-304 (#2Woodruff key) that is trimmed to .320/.334” long that retains the 11008-00 pulley to the 42961-02 adjustment screw.

    With the above information and Figure 48 in most early PA18 parts manuals the assembly should be pretty straight forward.

    It looks like some of your play is in the fuselage bushing which if assembled correctly you should be able to simply remove the cotter pin on the bottom of the jack screw and tighten the nut a few flats.

    The stock jack screw is not an Acme thread but the Dakota Cub one is which is a finer thread and more surface area to engage the yoke and the DC yoke is through hardened.
    Steve Pierce

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  17. #17
    RaisedByWolves's Avatar
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    Few small zip ties you can hold the cable on the pulley out them so you can figure out the top and bottom when you put it back together.


    Sent from my iPhone using Tapatalk

  18. #18
    daedgerton's Avatar
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    Quote Originally Posted by RaisedByWolves View Post
    Few small zip ties you can hold the cable on the pulley out them so you can figure out the top and bottom when you put it back together.


    Sent from my iPhone using Tapatalk
    This all looks very familiar doesn't it Tom...
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  19. #19

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    Hmmm, it seems I have the same play in my jack screw. Always something I tell ya and like Adam, it’s time to go kill brown bears. ��

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