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Thread: Trim Assembly Overhaul Time

  1. #1
    JDWilliams's Avatar
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    Trim Assembly Overhaul Time

    Looks like it’s time to rebuild the entire jack screw assembly on my PA12-150. Any recommendations on who to send it to for the work?

    (I would have searched the topic before asking but the search function is locked up on me for some reason.)

  2. #2
    www.SkupTech.com mike mcs repair's Avatar
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    depends on if you have ORIGINAL -12 trim system (one groove pullies) or updated dual groove -18 style pullies & cable....

  3. #3
    Colorguns's Avatar
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    I replaced both pullies front and gear original style and that fixed my trim issues. Groove was worn out to the bottom of both.

    Doug
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  4. #4
    TurboBeaver's Avatar
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    98% of the time a single cable trim is slipping , its the pulleys . One or both has its V grooves worn down to the bottom .......... They are now U grooved and will slip constantly. Getting lubrication onto the trim pulleys will certainly spoil your system as well. As Doug suggests; two new pulleys and a clean cable with proper tension does wonders for any "single groove system. Steve Pierces cleaver mod to the "idler tightner" of adding an extra pulley into the original tightner sheath, allows it to pull on BOTH sides of the wire equally, instead of just pulling one side over till its touching itself, so something in the order of doubling the effectiveness of the tensioner.
    The double wire trim system is THE final answer no doubt , and should be done at recover. However a cleaned up single wire, riding high in new V grove pulleys that is correctly tensioned can work for years if properly maintained. Dont forget if the rear thru bushing on your stabilizer has corrosion in it binding the system up, a double pulley will slip as well???
    MMS Mike has converted some over from single/ double with the fabric on
    the plane, and knows all the tricks of the trade. However I would imagine if a mechanic has never done one before; you could easily double the labor bill; over a mechanic that already knows how its done........

    I ferryed a Cub a few years back that was less than 10 hours out of an expensive inspection by a big shop in Maryland. Here is what I found wrong with it the moment I pushed it out.
    Primer would not pull in fuel and leaked when full extended.
    Brake diaphragms both soft so almost no brakes even though fluid was full.
    Tailwheel totally out of adjustment and basically NON functional.
    Tach slipping constantly, Airspeed reading 140 on initial climb out.
    T&B laying over on a dogbox. Ampmeter snaping from charging 30 to discharging 30 constantly just like a wiper blade. Oil preasure below the yellow arc? Carb heat didnt work at all,
    Up to altitude and leveled off, tryed to trim it out......... No luck. Had to hold forward against the tail for next 3 hours. Airspeed reading 60/65 , gps reading 105. '1hr out' both guages showed only 1/4 tanks...... 2hrs out it showed totally nothing? It was quite an interesting flight to say the least.
    Landing it for the 1st time in a 10kt direct crosswind with no brakes and tailwheel basically unhooked, because the connector springs were compleately wrong for that airplane. Was a refreshing reminder of the tap dance and double shuffle of the rudder pedals to their extreame limits.
    Other than that stuff, it flew like a dream!
    2 weeks later into a " real inspection"
    Everything had been repaired or replaced. Both pullys on the trim were totally worn out, had to be replaced.
    The shop in Maryland had entered two pages of total BS in the books and had done really nothing except charge the customer $3k for an oil change and alot of fancy looking BS in a log book.....................
    Good luck with your new double trim system, they work exactly like the single pulley does when they are both set up correctly, but twice as good as a single pulley that is slipping!!! Lol.
    E


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  5. #5
    cubdriver2's Avatar
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    Cheap trick that works, rub some of this on the cable



    Glenn
    "Optimism is going after Moby Dick in a rowboat and taking the tartar sauce with you!"
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  6. #6
    skywagon8a's Avatar
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    Quote Originally Posted by TurboBeaver View Post
    I ferryed a Cub a few years back that was less than 10 hours out of an expensive inspection by a big shop in Maryland. Here is what I found wrong with it the moment I pushed it out............................................... ...................."
    The shop in Maryland had entered two pages of total BS in the books and had done really nothing except charge the customer $3k for an oil change and alot of fancy looking BS in a log book.....................
    Good luck with your new double trim system, they work exactly like the single pulley does when they are both set up correctly, but twice as good as a single pulley that is slipping!!! Lol.
    E
    That reminds me of a Taylorcraft 15 that I flew home after a rebuild and annual for a new owner. I hope that the owner of "your" Cub reported that annual to the FAA. We did on ours, the FAA was grateful as they had been after that IA for years. The IA lost his ticket. The aviation industry is dangerous enough without having this type of maintenance.

    Edit: ps, That IA was an airline mechanic who did annuals out of the trunk of his car for some side cash.
    pps: Just remembered his last name. Crook. How very appropriate.
    Last edited by skywagon8a; 11-09-2019 at 06:29 PM.
    N1PA
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  7. #7
    JDWilliams's Avatar
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    Thanks for the great info. Decided to replace with the Dakota Cub assembly. It’s dual pulley already with the PA18 setup.


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  8. #8
    TurboBeaver's Avatar
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    JD,
    Great choice. I believe their jack screw is a finerpitch thread than the original
    "Cast" yoke has. So it is going to be smoother. The Dakota CNC yoke, is light years
    ahead of a Univar type. You made the right decission for sure.
    Good Luck
    E

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  9. #9
    JDWilliams's Avatar
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    Quote Originally Posted by TurboBeaver View Post
    JD,
    Great choice. I believe their jack screw is a finerpitch thread than the original
    "Cast" yoke has. So it is going to be smoother. The Dakota CNC yoke, is light years
    ahead of a Univar type. You made the right decission for sure.
    Good Luck
    E

    Sent from my LM-X210 using SuperCub.Org mobile app
    Dakota Cubs jack screw installed. Works great. Definitely finer pitch so more turns of the handle to achieve the same degree of trim change. Gives finer pitch trim control but takes longer to crank to the new setting when needed. Very pleased.


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  10. #10

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    My trim cable and pulleys have 6700 hours on them. Always worked great. I shot some Tri-Flow on the jack screw in October, and the trim system stopped working in November.

    I assumed the cabl or pulleys had finally taken a break. Not so - the jack screw and yoke were being held by a substance not unlike JB Weld. Cleaned it all off, applied engine oil as I have for almost six decades, and presto - a really well functioning trim system again.

    Maybe there really is something to this business of compatibility of lubricants?
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  11. #11
    BC12D-4-85's Avatar
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    Plain non-detergent motor oil or fine electric motor oil is safe. Tri-Flow contains oil plus about 24% solvent and a couple of anti-corrosion anti-wear components (http://www1.mscdirect.com/MSDS/MSDS0...5-20161002.PDF). If there's any residue on the part it may get loosened and then can recombine when the solvent evaporates.

    Gary

  12. #12

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    My PA20 single groove pulley system would freeze up when the temperatures were well below freezing... bought the old school cloth electrical tape...same stuff piper used to tape the wiring to the fuselage...tears like duct tape...fed a 1/4” wide strip into the pulley groove at the trim handle a couple three wraps. No more slipping


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  13. #13
    BC12D-4-85's Avatar
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    Trim screws can fret-wear from vibration and asymmetric loads on the horizontals. Watch a Cub tail on takeoff shake like a dog passing worms (big engines and props are worse) and it's no mystery why the trim assembly wears and gets looser. Oil helps - LPS-2 and whatever - but temperature appropriate Moly additives (Molybdenum Disulfide based lube) in a grease can add an additional layer of protection.

    Clean the trim parts periodically with a narrow long rag soaked with solvent. Wrap it fully around the screw (with trim up then down) and pull the ends in a sawing up motion and down the screw to clean off debris. After it dries apply lube of choice. Also lube the cross tubes that connect the front and rear horizontal stabilizers at their connections or fittings with grease or penetrating oil. Recent tubes are chrome plated and can resist corrosion.

    Gary

  14. #14
    TurboBeaver's Avatar
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    Great advice Gary, and the friction tape is also a bandage for the problem. Of course a new front pulley with the unmolested V groove does wonders to fix 95% of them.
    A friend of mine decided Piper screwed up trying to use existing tail brace wires on the 18 stabilizers. And felt it was a major culprit of excessive
    "Tail Shake" so he had some longer ones made up that where attached out where they should have been in the 1st place, from memory about 7" outboard of factory holes. He flew it that way for years and claimed it was
    A HUGE improvement over factory wires. I flew it for fun, and it had NOTICEABLY less shaking going on back there. Great mod if your experimental. SuprCrow has 4 wires on his tail and long flaps but his tail is nearly ROCK solid.
    These new CNCed trim yokes are light
    years ahead of the old cast ones. And the finer pitch thread gives a "lower gearing". Corrision in the rear tube bushings is pretty common and one old time Cub mechanic told me he has used nothing but Vaseline to lube them for years with stellar results.
    Food for thought.
    E

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    skywagon8a's Avatar
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    Quote Originally Posted by TurboBeaver View Post
    SuprCrow has 4 wires on his tail and long flaps but his tail is nearly ROCK solid.
    My Cub has the stock tail wire setup and the long flaps with a rock solid tail. I think that it is the long flaps eliminating turbulence from the outboard end of the stock flaps reaching the tail which does the trick.
    N1PA
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  16. #16

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    My experience is even with flaps up, a full power climb out will make them shake pretty good. Most don't know that because it is really difficult to turn around and look - especially for older folk.

    If you don't keep a drop of oil on the interface between stab and tube, you will have a difficult time getting them apart at overhaul. I pulled mine with great difficulty when the aircraft was 25 years old and had been covered twice. Now it is 63 years old and they slide right off. Vaseline sounds like a horrible idea- isn't it water soluble?

  17. #17

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    Of course you all knew about the young newly married mechanic who didn't know the difference between vaseline and window putty?

    Yes, all his windows fell out.
    You can't get there from here. You have to go over yonder and start from there.
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  18. #18
    BC12D-4-85's Avatar
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    Question: Does the PA-18 tail shake worse than the PA-12? I had both and it seemed like it did but can't exactly recall. If so maybe the flatter fuselage sides of the -18 create some of the turbulence on the tail. Or maybe it's fuselage truss design?

    Didn't CG Taylor notice the fabric buffeting on the sides of the original Cubs he designed and then built the tapered side Taylorcrafts for better air flow along the fuselage and speed? May be an Urban Legend worth exploring.

    Gary

  19. #19
    TurboBeaver's Avatar
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    Not sure if the 18 shake is more than the 12 or not? Can tell you for sure
    the PA-20 feathers on a 12 have noticeable less than 18 feathers. They surprised me how well they work. Only place they seam to show much inferiority is when loaded heavy AFT on a hot day they will sag more.....
    But for normal flying they work darn well. I had 18 feathers on my PA-11
    But if I was doing it again I would probably look hard at PA-20 tailfeathers if there was good set cheap........
    On a real nose heavy 12; then18 feathers would be mandatory.
    Or if you were constanly hauling gross loads.
    I am thinking the shorter the prop the less the shake back there?
    The big long props just pushing more air around fuselarge?
    E

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  20. #20
    skywagon8a's Avatar
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    Quote Originally Posted by TurboBeaver View Post
    I am thinking the shorter the prop the less the shake back there?
    A stock -18 with a 74" prop shakes the tail quite a bit.
    N1PA

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