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Play in jack screw

skysigns

skysigns

FRIEND
Yelm WA 98597
When is it time to replace the jack Screw and yoke In the trim system in a pa18
what is to much play
 
Where is the play? Between the screw and the yoke or does the screw move up and down in the fuselage? Two different reasons for the stab to move up and down when it shouldn't.
 
There is a spec on the drawing but that is new tolernce. I let my conscience be my guide. The problems I see is that when there is a lot of play in the jack screw/yoke then it transfer out and you get play in the ears of the yoke to the front stabilizer carry-thru tube, the outside elevator bushings and the tail brace wires get loose and have to be adjusted. Bigger engines and longer props tend exasperbate the issues in my experience.
 
Steve Pierce said:
There is a spec on the drawing but that is new tolernce. I let my conscience be my guide. The problems I see is that when there is a lot of play in the jack screw/yoke then it transfer out and you get play in the ears of the yoke to the front stabilizer carry-thru tube, the outside elevator bushings and the tail brace wires get loose and have to be adjusted. Bigger engines and longer props tend exasperbate the issues in my experience.
Click to expand...

Here is a video of the play in my PA-18. Jack screw replacement is on my to-do list (near the top), but honestly I can live with it right now if there won't be any negative effects on other parts of the plane. [video]https://photos.app.goo.gl/q19PFHoEiUVPenvKA[/video]

When I replace the part:
- I plan to use the Dakota Cub STC'd jack screw assembly - and probably the trim handle assembly too. Seems like folks like these.

- Have have the metal belly pans. Can I do the replacement by accessing everything with the pans removed? I also have insection plates on the fuselage sides.

- Is there a good write-up anywhere on the replacement procedure?

Thanks in advance!
 
The Jackscrew comes out through the top. There should be a small metal cap with screws over the hole, I made a X cut over the hole and worked the cap out slowly. You may have to grind/file a bit if the hole does not line up, not a big deal just go slow. Once it was out and new Jackscrew in place I painted the metal cap and put it over the fabric. A dollar patch would work also. Just be prepared for the Jackscrew not to line up. Use zip ties to hold cable onto pulley as needed.
DENNY
 
DENNY said:
The Jackscrew comes out through the top. There should be a small metal cap with screws over the hole, I made a X cut over the hole and worked the cap out slowly. You may have to grind/file a bit if the hole does not line up, not a big deal just go slow. Once it was out and new Jackscrew in place I painted the metal cap and put it over the fabric. A dollar patch would work also. Just be prepared for the Jackscrew not to line up. Use zip ties to hold cable onto pulley as needed.
DENNY
Click to expand...

I'm thinking that I also need to replace the yoke that the screw threads into. How does that come out?
 
Yes replace the Yoke. Depending on which yoke you use it can effect your trim range some of the old ones have a bit of a nipple at the top. Some people will make sure the clean any extra casting material off the top so they have full stabilizer travel. It will come out the inspection holes on the side. I don't have a belly clean out so did the job through just the side inspection holes. Not fun but I goterdone.
DENNY
 
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, zip tie the trim cable to the pulley, 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 top through the hole Denny described. Hopefully whoever installed the part between the center stringer and vert. stab lined the hole up and you don't have to grind it out. Usually you can cut out the fabric and thread the jackscrew thru the hole and then make a round patch out of aluminum and form it over the hole and screw it down. Easier said than done but it is doable with a little patience. Did it on a Super Cub 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 at his previous fuel stop. There was a lot of duct tape used to get it to the fly in.
 
Also look in the parts book and check that the washers have been installed correctly.
These are from the original drawings I have:
80122-41 1" od x .375/.385" id x .095" thick, material 1025ST
81342-05 1 1/8" od x .375/.385" id x .062 thick, material 1025ST CAD
85012-12 AN960-616L washer

I get into this every time I put one together and wrote it in my parts manual. Just fixed one where they were put in the wrong places and the big od washer was rubbing into the support.

Univair has the special washers as 80122-041and 81342-005.
 
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