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View Full Version : Replacing trim jackscrew and yoke



Grassroots
11-29-2008, 07:32 PM
I noticed today that when I lift up on the leading edge of the stabilizer on my Clipper I get about 1/4" movement. I peeked inside to look at the trim yoke when I did this and it was moving for and aft more than I would like it to!
Wear seems to be either jackscrew or threads in yoke.
Has anybody replaced a jackscrew and yoke successfully before on a covered plane? Any hints or ideas would be appreciated along with things not to do.

Thanks,
Bill

Steve Pierce
11-29-2008, 09:48 PM
Make sure the slop is not in the jackscrew to the fuselage. 9 times out of 10 this is where I find the play. Pull the cotter pin out and tighten the nut. If it is slope between the jackscrew and yoke take all the inspection plates off back there. Clean any congealed grease out of the threads so you can run the yoke off the jackscrew. Run the yoke almost to the top of the jackscrew. Take the cotter pin and nut loose, loosen the trim cable tensioner that is forward of the jackscrew, pull the pulley off the jackscrew, pull the half moon key out of it's groove with a pair of flush dikes and rotate the jackscrew out of the yoke. Unbolt the yoke from the liner tube housing and slip the jackscrew out the bottom. Easier said than done but it is doable with a little patience. Did it on a Super Cub (which comes out through the top) at a fly-in, with an audience and the jackscrew was bent real bad. He had hit a mesquite tree with the stabilizer in a bad wind. There was a lot of duct tape used to get it there. :shock:

Grassroots
11-30-2008, 09:48 AM
Steve,
It appears that the play is in the yoke itself. When I lift on the stabilizer the yoke rocks back and forth, the jackscrew doesn't move. Thanks for the insight on the process, I'll proceed with caution.

Bill

Brian
11-30-2008, 12:34 PM
Bill,
Could you test the play in different positions of the trim adjust range. I notice mine too has a little play in the area of trim that is used for cruise flight, but is fine or free of play when the trim is adjusted away from this location. I would assume this indicates wear on the jack screw in that location where most hours have been spent. I would be interested to know if yours is worn only in that area as well.
Brian

Grassroots
11-30-2008, 02:58 PM
Brian,
I ran the trim all the way up and down. There is no play the closer you get towards the maximum of up and down trim.
I have a spare trim yoke, and looking at it there is only threads in the middle section of the cavity where the jackscrew threads in. On the top and bottom 1/3 there seems to be a bushing pressed in. The bushings ID would be close to the OD of the threads on the jackscrew. By watching the yoke move on my Clipper when I lift on the stabilizer, I'm guessing that these upper and lower bushings are worn or gone. I could not see any worn part of the jackscrew when I had the trim at maximum up and down.
If these bushings were not worn, I don't see how the yoke could rock back and forth like it does.

Bill

Gilbert Pierce
11-30-2008, 05:56 PM
Look at the first entry here http://www.shortwing.org/forum/showthre ... =trim+yoke (http://www.shortwing.org/forum/showthread.php?t=1028&highlight=trim+yoke)

According to Piper Drawing 11825 note 2: 0.015" maximum verticle play allowable between stabilizer and adjacent fuselage structure.