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Thread: Broken Jackscrew

  1. #1
    Todd long's Avatar
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    Broken Jackscrew

    So not mine, but a friend had this happen today while out flying. Said he noticed a vibration then some flutter. No doubt, right? Lucky as hell I'd say. Anyone else had this happen? Pa-12


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  2. #2
    PerryB's Avatar
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    From the picture it looks like the lower bushing and the yoke don't line up very well.

  3. #3
    Todd long's Avatar
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    Appears that way. But that is just from the tension still on the trim pulley.


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  4. #4
    mike mcs repair's Avatar
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    i've only seen them bent from accident.....

    maybe someone tried to straiten a bent one somewhere in it's past??

  5. #5

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    My banner cubs get new ones every 500-700hours depending on how big the signs are.alot of times you look back and horiziontal is moving a lot,a blurr.but never had one break.
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  6. #6
    wireweinie's Avatar
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    Is there a possibility that the flutter occurred before the break? Maybe that is what broke it?

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  7. #7
    PerryB's Avatar
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    Or it could be 70 years old......

  8. #8
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    There is vibration and there is actual aerodynamic flutter. If you can come home and suggest that it might have been flutter, it might not have been?

    But then again.......I wouldn't want to try to duplicate it again to find out if it really was. I'd be happy to get home!

  9. #9
    mike mcs repair's Avatar
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    are the bolts from the yoke to the stabilizer mount tube free moving? not binding?

  10. #10

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    Yep, I had one break on me towing a 65' tall panel, big ass banner, had to fly wide open the first hour til i burned off some gas, over the jersey shore and it broke, trim was stuck til i landed but noprob flying. Pa-18
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  11. #11

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    We have a local AD to inspect these for cracks every annual. They broke a few in the topdressing days I understand.

    Andrew.

  12. #12
    #JoeM8848 #JoeM8848's Avatar
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    Broke recently, but didnít know it at first because in the neutral trim position and just didnít move the stab when turning the handle- fully separated when we took it out of the water and was bounced around by the float truck- new Dakota parts on the way.Click image for larger version. 

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  13. #13
    PerryB's Avatar
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    Well then..... mine broke this morning. I was out goofing off around the valley and it was a beautiful morning. When I got home I decided to make a "carrier" landing, by which I give it about 5 turns up and carry a little power. I can really drag it in that way. Also good for the tailwheel only exercise. Anyway, it didn't perform as expected and touched down in a perfect 3 point at normal speed. Kinda disappointing, but wasn't dwelling on it. As I was rolling out I returned the trim and casually glanced down at the indicator button, which was now all the way forward. My first thought was that the indicator wire had broken. When I got to the hangar, the leading edge of the H.stab was all the way up. At this point things are feeling a little Twighlight Zone-ish, until I pushed down on it and realized what had happened. I guess flight loads kept it in place (down) until after landing when the elevator bungee pulled it up. I don't understand how it broke yet. I keep the threads lubed and it cranks very easy and smooth. I had the screw and yoke replaced 7 years ago at rebuild and it doesn't get used much except when making the aforementioned carrier landings and at annual when it gets a full range. This is strictly a land based plane hangared in the northern Sacramento valley. No salt, modest humidity. It'll be interesting to inspect the break when I get it out. -- I'll probably just replace the screw since the yoke is fairly new and has little usage. Any suggestions to expedite the job are appreciated. Yes I am going to zip-tie the cable to the pulley in a couple places.

  14. #14

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    Think the Charlie Center tail brace would help prevent that? I'm building a 180 PA-18A and heard the 180 really shakes the horizontal. Don't need to add any more weight but I'd rather have something of value in the tail instead of lead.

  15. #15
    Steve Pierce's Avatar
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    Quote Originally Posted by PerryB View Post
    Well then..... mine broke this morning. I was out goofing off around the valley and it was a beautiful morning. When I got home I decided to make a "carrier" landing, by which I give it about 5 turns up and carry a little power. I can really drag it in that way. Also good for the tailwheel only exercise. Anyway, it didn't perform as expected and touched down in a perfect 3 point at normal speed. Kinda disappointing, but wasn't dwelling on it. As I was rolling out I returned the trim and casually glanced down at the indicator button, which was now all the way forward. My first thought was that the indicator wire had broken. When I got to the hangar, the leading edge of the H.stab was all the way up. At this point things are feeling a little Twighlight Zone-ish, until I pushed down on it and realized what had happened. I guess flight loads kept it in place (down) until after landing when the elevator bungee pulled it up. I don't understand how it broke yet. I keep the threads lubed and it cranks very easy and smooth. I had the screw and yoke replaced 7 years ago at rebuild and it doesn't get used much except when making the aforementioned carrier landings and at annual when it gets a full range. This is strictly a land based plane hangared in the northern Sacramento valley. No salt, modest humidity. It'll be interesting to inspect the break when I get it out. -- I'll probably just replace the screw since the yoke is fairly new and has little usage. Any suggestions to expedite the job are appreciated. Yes I am going to zip-tie the cable to the pulley in a couple places.
    Please post some pictures.
    Steve Pierce

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  16. #16
    PerryB's Avatar
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    I'll do that Steve. It broke right at the bottom of the yoke, just like in post #12 above. I'll put up a good pic of the fracture once I get it out. It'll probably be a couple days before I get to it.
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  17. #17

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    It broke at the bottom of the yoke while in a non-typical trim configuration? That may be worth investigating.

  18. #18
    PerryB's Avatar
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    It broke in cruise trim. It happened apparently when I attempted to crank it up. I had no indication it broke, no "pop" through the crank, etc. Just the stab didn't move.
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  19. #19
    Steve Pierce's Avatar
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    Cable probably just started slipping and you couldn't tell the screw wasn't turning.
    Steve Pierce

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  20. #20

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    Personally I'm more focused on the flying wires. My tail is big, the plane is longer and heavier than normal, and it has considerably more the 180hp. The tail shakes and it looks to me like the flying wires take the worst of it. Especially the top wires and that transfers to the fin.

  21. #21
    PerryB's Avatar
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    I have a double wrap trim syatem. It doesn't slip. The screw is broken.

  22. #22
    mike mcs repair's Avatar
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    Brand?


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  23. #23
    PerryB's Avatar
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    Almost certain it was Univair, I'll have to look back through the rebuild documents.

  24. #24
    Steve Pierce's Avatar
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    Quote Originally Posted by PerryB View Post
    I have a double wrap trim syatem. It doesn't slip. The screw is broken.
    I am referring to when it broke and you were trimming, figure the cable slipped and wasn't turning the jackscrew from the binding from the screw being broken.
    Steve Pierce

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  25. #25
    PerryB's Avatar
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    Ah, sorry I misunderstood. Actually the bottom half of the screw still turns just fine, just freewheeling in place. Dammit anyway....

  26. #26
    BC12D-4-85's Avatar
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    With all the stab rocking the vert fin action maybe its bending the screw? Full up trim at the wrong time would be Oops.

    Gary

  27. #27
    mike mcs repair's Avatar
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    Also if you had repairs that might have made stabilizer jam against yoke or screw in up position. Also check under rear stab attach fitting for cracks.


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  28. #28
    Mauleguy's Avatar
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    I know of another broken one so I guess it is not that unusual.
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  29. #29
    mike mcs repair's Avatar
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    Broken Jackscrew

    Quote Originally Posted by Mauleguy View Post
    I know of another broken one so I guess it is not that unusual.
    Strange. Never heard of it, are they all newer parts/manufacture/hardness issue?? Brands??

    Maybe they tried to toughen them up to resist wear but got them brittle???


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  30. #30
    Mauleguy's Avatar
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    The one I know about is out of a certified cub, probably the original one 79 vintage, 150 hp most its life.

  31. #31
    wireweinie's Avatar
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    Both pics,above, show the screw broken at max travel (extension). It looks to me like a pressure point due to side load. Either the screw/yoke are out of alignment or, like MCS Mike says the cross tube is bound and not free to move on the liner tube. If so, that will cause the yoke to 'pry' against the screw at max travel.

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  32. #32
    behindpropellers's Avatar
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    My assumption would be the incorrect hardware is installed on the tailbrace wires. If the wrong hardware is installed they will not pivot and this places loads in other places.

  33. #33
    mike mcs repair's Avatar
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    I wonder if it was just getting into a real dirty part that doesnít get much use at top

    Hereís an old one removed at rebuild

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  34. #34

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    Because the aft attachment of the stabilizer is static and the front attach moves up and down the travel of the leading edge is an arc. The tube that carries the stab is hung from tabs on the yoke. Does anyone check to verify clearance of that tube as it moves up and down on the screw? If the clearance was too close the tube could push back on the yoke and put a bending force on the screw. The place that potential is greatest would be when the yoke is high on the screw. I never thought much of it until I looked at how my trim actuator pivots. On standard Cubs the tab-tube connection is the only place the lateral movement of the leading edge can be compensated for.

    Stuff to think about when you have a few minutes of down time.
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  35. #35
    Kodiakmack's Avatar
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    Broken Jackscrew

    While weíre on the subject, Iím curious about some play in my yoke to horizontal connection. The most play is lateral, with very little vertically. The nut is cottered but as you can see the bolt is free to move. Is this intentional to allow movement through the arc of the trim? Iím still very new to this trim style and uncertain whatís supposed to move and whatís not.


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  36. #36
    mike mcs repair's Avatar
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    Broken Jackscrew

    Piper did bother to counterbore that hole so itís still got the taper of the casting making it hard to shim correctly. Univair does. But there should not be slop there. It should be just snug and free to rotate. You can add washers between the parts


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  37. #37
    KJC's Avatar
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    This is a brand new Univair aluminum yoke and the old jack screw. If you look close you can see the light washers between the yoke and the carry thru tube.....

    Click image for larger version. 

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    The washers provide the proper shimming to keep it from moving side to side. Also helps fix the machining on the old original castings. You can see Univair finished them nice and flat on both side of the casting where the bolt goes thru. A finger of greesee on the casting faces, the bolts and the washers keeps it all moving correctly. Snug but not tight on the nuts. It has to rotate.

    Click image for larger version. 

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  38. #38
    Kodiakmack's Avatar
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    Quote Originally Posted by KJC View Post
    This is a brand new Univair aluminum yoke and the old jack screw. If you look close you can see the light washers between the yoke and the carry thru tube.....

    Click image for larger version. 

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    The washers provide the proper shimming to keep it from moving side to side. Also helps fix the machining on the old original castings. You can see Univair finished them nice and flat on both side of the casting where the bolt goes thru. A finger of greesee on the casting faces, the bolts and the washers keeps it all moving correctly. Snug but not tight on the nuts. It has to rotate.

    Click image for larger version. 

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    I really appreciate that and will make it so. I see you have a zerx for the liner tube. I donít have this and doubt this liner tube has seen any love for several years minimum. Recommendations on making sure everything in this area is moving?


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  39. #39
    KJC's Avatar
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    Those yokes should be thrown away and replaced at rebuild. The inner part of the yoke which is cut for the acme thread just wears out after years of riding on that steel screw. Grab the front of your horizontals near the jack screw and see how much they move up and down. The Dakota Cub yoke is harder and solves this problem.

    Regarding the liner tubes, the best would be to remove the tail feathers and pull the tubes and make sure they rotate freely. That way you can clean all the old grease and oil out of the liner. Where my grease zerk is, you have an oil hole. A few drops of a light penetrating oil from time to time might help but I doubt it does much. The liner tube rides in two bronze bushings and I doubt the oil ever makes it out that far.

    Click image for larger version. 

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    Like this......
    Last edited by KJC; 12-19-2020 at 07:17 PM.
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  40. #40
    Kodiakmack's Avatar
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    Quote Originally Posted by KJC View Post
    Those yokes should be thrown away and replaced at rebuild. The inner part of the yoke which is cut for the acme thread just wears out after years of riding on that steel screw. Grab the front of your horizontals near the jack screw and see how much they move up and down. The Dakota Cub yoke is harder and solves this problem.

    Regarding the liner tubes, the best would be to remove the tail feathers and pull the tubes and make sure they rotate freely. That way you can clean all the old grease and oil out of the liner. Where my grease zero is, you have an oil hole. A few drops of a light penetrating oil from time to time might help but I doubt it does much. The liner tube rides in two bronze bushings and I doubt the oil ever makes it out that far.
    I was worried it would be that involved. Rebuild is not in the cards soon as it was done 7 years ago and Iím not looking for a project.

    As for pulling the tail feathers, Iíll have to just hope itíll make it to next annual. Iím 3 months into an annual on a plane I bought with a ďfresh annualĒ that needed to be made legit.

    Thanks for the info though!!



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