Replacing over center link bushings to re-establish clearance to main gear trusses

[Vance Harral]

I'm starting a new thread to have a better subject and tags, but this is a continuation of this earlier Mooneyspace thread regarding lack of clearance between the over center link and the main landing gear trusses.

First, a review of the problem.  As shown in the attached photo, with wear over time, the castle nut for the main over-center link will begin to rub the landing gear truss:

 

[Vance Harral]

Here's a broader view of the retract mechanism.  There are three pivot points in the over-center assembly, each composed of a bolt and nut through a bushing.  The red arrow points to the retract truss pivot point, which is supported by the "baby shoe" bracket.  Note that the baby shoe bracket has a shim behind it, outlined in orange, which sets the fore/aft spacing of the retract truss.  The blue arrow points to the pivot point shared by the retract truss and the drag link.  The bolt is held with a castle nut, and this is the nut that hits the main landing gear truss when the mechanism gets sloppy.  The green arrow points to the final pivot point, where the drag link connects to the landing gear truss.  You can't actually see the bolt and nut in this photo because it's behind the truss, but I promise it's there.

[Vance Harral]

It's been discussed here that slop can be affected by adjusting the shims behind the "baby shoe" bracket, but that's just addressing the symptom, not the real problem.  The root cause is wear in the bolt/bushing interfaces, which allow the retract truss and drag link to flex fore and aft.  You can check the slop by partially retracting the gear and lifting up on the wheel to remove all tension, then wiggling the linkages.  As this video shows, ours had gotten quite bad.

 

[Vance Harral]

The bad news is, getting the retract truss and drag link out is a pretty involved operation - you have to remove the entire landing gear.  First, you have to remove the tire, disconnect the gear door, and remove the mud guard, and squat switch.

 

[Vance Harral]

Next, you remove the upper and inboard bolts and nuts on the retract truss.  Don't lose the bushing on the inboard side!

[Vance Harral]

Next, you have to remove the bolt and nut that connect the gear leg to the big spring, which provides tension to help raise the gear. 

 

The maintenance manual specs a special tool to take the tension off the spring so you can remove the bolt.  I panicked when I saw this, because of course we don't have this tool (it's the lever marked "B" in the maintenance manual page below):

 

I called Dan at LASAR for advice, and he said that tool never worked very well anyway.  A trick Dan says he picked up from DMax is to swing the gear down until the springs are at maximum stretch, then jam a bunch of fender washers between the coils to keep them spaced out, then push the gear back in the well until the tension is off the bolt and nut.  This worked well for me, but make sure to get plenty of washers - you have to put one between just about every single coil.  Note that to access the spring to install the washers, you have to remove the inspection plate just outboard of the landing gear wells.  Photo below was taken looking through that inspection plate.  There are 18 washers there!

Edited February 9, 2018 by Vance Harral

[Vance Harral]

Now we're ready to remove the entire landing gear assembly.  The aft end of the pivot shaft is held in place by a wedge-shaped bearing consisting of a pair of steel blocks with a shim in the middle.  In the photo below, I've removed the lower 4 bolts which fasten the primary block to the secondary block, and wiggled it a bit so you can see the shim and secondary.  Due to the wedge shape, the gear won't slide down until you remove the upper two bolts from the secondary block and slide it sideways.  You need to be really cautious about this.  Once the secondary block is slid out of the way, the whole landing gear leg just falls out.  So, you need to support it as you're sliding the secondary block sideways.  Be careful, and don't pinch your fingers!

[Vance Harral]

With the entire assembly out of the aircraft, you can finally remove the third pivot bolt at the interface between the drag link and the landing gear truss (I "reinstalled" it for the photo below, it's the bolt at far left).

Edited February 9, 2018 by Vance Harral

[Vance Harral]

Tomorrow, we'll be taking the retract truss and drag link to a machine shop to have them reamed out to accept the oversize bushings sent to us by LASAR.  I plan to bead blast and paint the parts before re-installing.  As for the effect of the castle nut rubbing on the landing gear truss, there is indeed a shiny spot on the truss where the castle nut was making contact, but just the tiniest amount of wear - much less than 10% thickness and my mechanic is unconcerned about it.  Would have been a serious issue if allowed to continue, though.

One other topic of interest regarding grease fittings... you can see the zerk on the aft pivot point bearing plate in the post two above this one.  I meant to take a picture of the forward pivot point bearing plate as well, to show how there is apparently no grease fitting there.  I always thought this was odd, but ignored it.  Should have trusted my intuition.  There is definitely supposed to be a grease fitting there, too, and in fact the maintenance manual specifically mentions removing it.  Turns out if you look closely, there is a very tiny, 4-40 thread hole there.  I mentioned to Dan that I guessed someone had removed the zerk and forgotten to reinstall it.  Dan say that in fact, these zerks are so small and fragile that it's common for them to get broken off over the years, and a lot of places just don't bother to fix them.  One of my tasks for tomorrow is to see if I can dig out any pieces of zerk from the hole and install a new one.

I'll try to come back and update this thread with more photos as I complete the re-installation process.  Will likely be well into next week as we wait on the machine shop to install the new bushings.

[RLCarter]

Good job on walking through the process @Vance Harral

[N201MKTurbo]

I believe the front grease fittings are press fit. The grease gun fitting won't let loose and while trying to get it loose it pulls out the fitting, I have the grease fitting which I put on the grease gun, then push it in the hole, grease it up pull it back out and put them in the tool box till next year.

[Vance Harral]

31 minutes ago, N201MKTurbo said:

I believe the front grease fittings are press fit.

Dan said the newer ones are indeed press fit.  Older models have (or had) threads, but if you can get any leftover bits out, you can apparently install a press fit zerk in the same hole.  Will be looking at this today.

[MB65E]

Thanks for the wirte up Vance!! You've answered a lot of questions for people have. I've had above average luck with grease needles in spots where the zerks were damaged or missing. Love to see another video when complet of the slop. 

A few funny notes I had to your gear project-

1. the preloads on the main change the preloads on the nose. 

2. the main gear door alignment changes a lot if the gear door brackets are moved around. I was having trouble after obtaining correct preloads and the gear doors rubbing on the trailing link when down and locked. Moving the brackets around slightly gains clearance, then you can adjust it out. With the rod ends. 

To be honest the geometry of the gear is rather amazing. Everyone talks of how simple it is, but the angles and tensions that the thing works at are rather clever.

-Matt 

[Vance Harral]

Drawing for the machine shop.

[Vance Harral]

Parts ready for delivery to machine shop, after bead blasting and with new hardware.

[N201MKTurbo]

4 hours ago, Vance Harral said:

Parts ready for delivery to machine shop, after bead blasting and with new hardware.

You might as well do the rest of the landing gear parts while you have it apart. Those beautiful new parts will look out of place with that old tired landing gear!

[Vance Harral]

6 minutes ago, N201MKTurbo said:

You might as well do the rest of the landing gear parts while you have it apart. Those beautiful new parts will look out of place with that old tired landing gear!

I hear you, but it's a piecemeal job as the years go by.  I did the lower half of the landing gear trusses last year, which you can see here.  To do the upper trusses I'd need to compress the shock disks and separate the upper and lower trusses again.  That was a tough enough job with the gear mounted on the airplane, lots more trouble with it out.  I think we'll make do with a thorough cleaning.  I've only got so much time off from my real job, and the airplane has never been a beauty queen anyway.

[Yetti]

[Vance Harral]

Decided to go ahead and pull out the idler arms and walking beams for inspection, bead-blast and re-paint.  They've looked awful for years, despite our mechanic's assurances they're "fine".

[Vance Harral]

A lot of what I thought was corrosion turned out to just be "goo" - decades of grease, lubricant, and brake dust mixed with a healthy dose of dirt and other detritus from runways and taxiways.  The parts looked much better after a good solvent scrub.  But still plenty of missing paint and surface corrosion.

[Vance Harral]

Nice and clean after bead blast.  If you're contemplating doing this yourself, make sure you tape off rod ends and springs for blasting.  I don't personally think it's that big a deal to get a bead or two in the moving parts as long as you wash them thoroughly in solvent afterward and lubricate them properly before re-installation.  But you don't want to directly blast the rod ends or the moving surfaces under the springs.

You may or may not be able to tell by zooming in, but the parts aren't really "pristine" with respect to corrosion.  There's some very minor pitting here and there, but nothing close to 10% thickness.  Mechanic says they look fine to him, and a healthy coat of prime and paint should protect them for another decade or two.  I'll start on that when the retract and drag links come back from the machine shop later this week.

[Stephen]

Thanks Vance, appreciate the detailed write-up. Would be interested to get costs for all parts/machining when said and done.

Unfortunately I have run into an issue with my engine (metal in filter) that trumps this and has me grounded indefinitely and may have my plane out of service pending sale due to the 30-40AMU required to rebuild, I canceled my annual and, unless I can find a cheap, serviceable IO-360-A1A will put the plane in a hangar pending sale. 

[Guest]

4 minutes ago, Stephen said:

Thanks Vance, appreciate the detailed write-up. Would be interested to get costs for all parts/machining when said and done.

Unfortunately I have run into an issue with my engine (metal in filter) that trumps this and has me grounded indefinitely and may have my plane out of service pending sale due to the 30-40AMU required to rebuild, I canceled my annual and, unless I can find a cheap, serviceable IO-360-A1A will put the plane in a hangar pending sale. 

That sucks.  The overhaul price seems high, was there more internal damage?

Clarence

[kortopates]

It's the holes for the roll pins that tend to become enlarged resulting in the rod end becoming loose that can cause their demise.


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[Vance Harral]

Sorry to hear about your bad engine luck, Stephen.