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Broken Jackscrew

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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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.
 
I wonder if it was just getting into a real dirty part that doesn’t get much use at top

Here’s an old one removed at rebuild

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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.
 
While we’re on the subject, I’m 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? I’m still very new to this trim style and uncertain what’s supposed to move and what’s not.


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Piper did bother to counterbore that hole so it’s 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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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.....

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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.

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KJC said:
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.....

View attachment 53114

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.

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Click to expand...

I really appreciate that and will make it so. I see you have a zerx for the liner tube. I don’t 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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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.

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Like this......
 

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KJC said:
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.
Click to expand...

I was worried it would be that involved. Rebuild is not in the cards soon as it was done 7 years ago and I’m not looking for a project.

As for pulling the tail feathers, I’ll have to just hope it’ll make it to next annual. I’m 3 months into an annual on a plane I bought with a “fresh annual” that needed to be made legit.

Thanks for the info though!!



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It won’t be any cheaper next year and it’s really not that hard. If you find someone who knows how to do it, you’ll be done by lunchtime.
 
KJC said:
It won’t be any cheaper next year and it’s really not that hard. If you find someone who knows how to do it, you’ll be done by lunchtime.
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It won’t be cheaper but I’ll be more flush. I’m a federally employed fire pilot in the lean part of the year facing a government shutdown. I’ll have a local cub guy come have a look if he has time. Was hoping one could tell function/condition sans tail feathers removal.


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Kodiakmack said:
I really appreciate that and will make it so. I see you have a zerx for the liner tube. I don’t 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?
Click to expand...
Judging by all that you've said, you really ought to remove the entire tail. The front where you have been looking does need attention as has been discussed. You also need to look at the pivot tube which connects the two stabilizers at the tail post. Piper just drills an oil hole which rarely gets lubricated with other than moisture. Rust slowly builds in the tube eventually "freezing" the cross connector tube in place. You don't realize this because the angular motion when you move the trim is very small. Because of this design and the fact the stabilizers are rarely removed for any reason, the connector tube becomes rusted into both the stabilizers and the cross tube in the fuselage. Many have welded a short tube or nut over the oil hole in order to install a zerk fitting for regular greasing. Just be certain the zerk doesn't interfere with the elevator control horn. Squirting oil in the hole may make you feel good, but it does a poor job of completely lubricating the entire movable joint. Thus the rust build up.

Do not be surprised to find the stabilizers both rusted to the connector tube and the tube rusted to the fuselage. Sometimes when this has not been addressed for years a hacksaw will be required. A liberal use of Kroil may save some grief.
 
Run the trim and see if the rear liner tube rotates or bends the stabilizer. Pretty easy to tell in my experience. If you watch the Annual Inspection videos I did here a few months ago there is a whole section on the tail. The play you have between the yoke and the carry-thru tube just needs shims and lube. Up and down play there can be a loose screw in the yoke or the screw could be loose on the fuselage and just need the nut on the bottom tightened a flat or two. The side to side play comes from the stabs being fit to the liner tubes with paint and stuff in between, it wears and they get sloppy side to side. safety wire in the groove fixes that. Also check your elevator bushings for play. Tension of tail brace wires. One worn thing seems to compound wear on everything else in short order.
 
KJC said:
The inner part of the yoke which is cut for the acme thread just wears out after years of riding on that steel screw.
Click to expand...
The Dakota Cub jack screw uses an Acme thread which is more threads per inch than Piper or Univair. Takes more turns than stock to trim but lasts a lot longer.
 
Thanks Steve. I’ll watch those videos. Your advice as always much appreciated. The play is minimal but I’m all about minimizing even that.


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A quick easy check for the rear liner tube being stuck or not moving freely is to pull the 2 bolts that attach the front liner tube to the yoke assembly, when you are shimming them, then move the yoke assembly as far to the top of the travel as you can. You can now grab the front of the stabilizer and try to move it up and down by hand. Watch for the rear of the stabilizer to move and it should move freely. It could be rusted solid, but also could be gummed up and moving harder than normal. If not moving freely you can attempt to free it up with some kroil and exercising it up and down. If you can't free it up, removing the horizontal stabilizers is highly recommended.
 
Today I removed the bolts connecting the trim yoke to the horizontal, oiled the liner tubes both front and back and made sure they both have free movement. At first they were pretty tight but after working them a bit and re-oiling they freed up pretty well. Will install new bolts and shim as advised and she should be good to go back there for some time : )


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Well we get to take the horizontal off anyways. As we were re-assembling this “weld” broke off. Weld or new part? We will see.


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Just weld it...

but be careful of making to much extra penetration on inside of tube you will need to grind off. Not fun grinding.


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Good question. As I mentioned this is our first cub type plane so we are learning as we go. I was told a double pulley -18 trim setup was installed at rebuild.
 
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