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

[Vance Harral]

Parts came back from the machine shop last night.  The bill was $400.  If I had it to do all over again, I'd just ship them to LASAR for reaming.  $400 seems excessive to me, but to be fair, it was a one-off, expedited job.  Here are the parts on the paint bench after final wash and prep, and a coat of primer.  Color coat goes on tomorrow.

 

[MB65E]

Where was the most of your ware located?

One of mine is close to rubbing. I've played with my rigging it's as good as I can get it. I'd rather not mess with it now but if I can slide out a few bushings with out removing the gear, in entertain installing a few bushings. 

-Matt

[Vance Harral]

In our case, no single bolt/bushing was the obvious primary problem.  All had some degree of slop.  If you could only replace one of the three, I'd do the one in the middle, at the junction between the drag link and the retract truss - i.e. where the castle nut is that rubs the landing gear leg.  However, you're not going to get to that one without completely disassembling the gear as I've shown.

In fact, I'm skeptical you can improve much of anything without removing the gear legs.  First, while the inboard bolt and bushing (red arrow in my second post) are accessible without doing this, that's only one of the three fasteners.  The other two cannot be removed in situ.  Second, understand that the wear is primarily between the bushing outer diameter, and the links into which the bushings insert.  That's the bearing interface, where the grease goes.  The bushings themselves are held captive by the bolts, and the links rotate around them.  This is different from more common bushing arrangements where the bearing surface is between the bolt and the inner diameter of the bushing.  This outer-diameter-bearing-surface design is the reason why LASAR recommends reaming the links and installing oversize bushings.  Just installing a new bushing with the original outer diameter doesn't address the wear on the links themselves.

If you absolutely must avoid removing the landing gear legs, I think the only meaningful action you can take is to remove the shim between the "baby shoe" bracket and the strap that connects to the spar (or use a thinner one).  That will buy you a little more clearance at the castle nut.  But in my opinion it's a kludge fix that doesn't really address the primary problem.

[N201MKTurbo]

3 hours ago, MB65E said:

Where was the most of your ware located?

One of mine is close to rubbing. I've played with my rigging it's as good as I can get it. I'd rather not mess with it now but if I can slide out a few bushings with out removing the gear, in entertain installing a few bushings. 

-Matt

If you want the low hanging fruit, replace the bolts, they usually wear as much or more than the bushings. You can replace them all for a few dollars. Plus labor of course. 

[N201MKTurbo]

5 hours ago, Vance Harral said:

Parts came back from the machine shop last night.  The bill was $400.  If I had it to do all over again, I'd just ship them to LASAR for reaming.  $400 seems excessive to me, but to be fair, it was a one-off, expedited job.  Here are the parts on the paint bench after final wash and prep, and a coat of primer.  Color coat goes on tomorrow.

 

 

I'm curious why you sent them to a machine shop instead of just reaming them your self? Whenever I've got oversized bushings from Laser, they sent me the reamers, I reamed out my parts and then sent the reamers back. It has been a while but I think they charged me $100 deposit on he reamers.

[Vance Harral]

4 hours ago, N201MKTurbo said:

If you want the low hanging fruit, replace the bolts, they usually wear as much or more than the bushings. You can replace them all for a few dollars. Plus labor of course. 

Unlikely in this case.  Again, the links rotate around the bushings.  The bolts do not rotate inside the bushings, they are held captive.

[Vance Harral]

4 hours ago, N201MKTurbo said:

I'm curious why you sent them to a machine shop instead of just reaming them your self?

One of the airplane partners wanted the job done professionally, and had a shop in mind from the get-go.  I won't try to justify the $400 charge, but it's not that big a deal split 3 ways across the partnership.

[MB65E]

Thanks!

Great work Vance!!

-Matt

[N201MKTurbo]

5 hours ago, Vance Harral said:

Unlikely in this case.  Again, the links rotate around the bushings.  The bolts do not rotate inside the bushings, they are held captive.

That's the theory, but I find the bolts warn more than the bushings quite often.

[Vance Harral]

Interesting observations about wear on the bolts.  Completely agree that bolts are prone to wear in fittings where the bushing rotates around the bolt.  But when the bushing is supposed to be held captive by the bolt and nut - which I believe to be the case on the landing gear links - wear on the bolt suggests the bolt and nut were not properly tightened in the first place.  Might have worked loose or might not have been tightened properly by a mechanic who mis-understood which surface was the bearing surface.

What is true is that if the bolt rotates at all, it's likely to wear quickly.  There is no hole in these bushings for grease to pass through from the outer to inner diameter of the bushings, and the ends of the bushings are covered by the bolt and nut.  So there's really no way to get any kind of lubricant between the bolt and bushing.

[Guest]

1 hour ago, Vance Harral said:

Interesting observations about wear on the bolts.  Completely agree that bolts are prone to wear in fittings where the bushing rotates around the bolt.  But when the bushing is supposed to be held captive by the bolt and nut - which I believe to be the case on the landing gear links - wear on the bolt suggests the bolt and nut were not properly tightened in the first place.  Might have worked loose or might not have been tightened properly by a mechanic who mis-understood which surface was the bearing surface.

What is true is that if the bolt rotates at all, it's likely to wear quickly.  There is no hole in these bushings for grease to pass through from the outer to inner diameter of the bushings, and the ends of the bushings are covered by the bolt and nut.  So there's really no way to get any kind of lubricant between the bolt and bushing.

You are correct, many maintainers don’t under stand the type of hardware in the assembly.  Many seem to assume that a bolt with a castle nut and a cotter pin is meant to be loose.

Clarence

[Vance Harral]

Just paint today, other activities (including removal of the pitch trim system) took up most of the day.  One quick pic of painted parts.  Re-installation begins tomorrow.

[helitim]

Gentleman,

I would like to tag onto this topic with a similar but slightly different question.  Have any of you found an alternative method of compressing the front rubber discs without the special tool?  

My former Mooney is still owned by our local flying club and they asked that I assist with performing the annual the year.  Our IA suggested placing a tail stand under the rear tie down and then use a floor jack underneath the right side of the nose tire to lift directly on the truss to compress the discs to allow easy removal of the bolt thru the collar on top of the three discs.  This uses the planes own weight to compress the discs using simple physics. Once the bolt is out, jack the plane up normally and finish separating the nose gear to remove the discs. 

Has anyone used this method for nose gear disc replacement?

Tim

[Guest]

6 hours ago, helitim said:

Gentleman,

I would like to tag onto this topic with a similar but slightly different question.  Have any of you found an alternative method of compressing the front rubber discs without the special tool?  

My former Mooney is still owned by our local flying club and they asked that I assist with performing the annual the year.  Our IA suggested placing a tail stand under the rear tie down and then use a floor jack underneath the right side of the nose tire to lift directly on the truss to compress the discs to allow easy removal of the bolt thru the collar on top of the three discs.  This uses the planes own weight to compress the discs using simple physics. Once the bolt is out, jack the plane up normally and finish separating the nose gear to remove the discs. 

Has anyone used this method for nose gear disc replacement?

Tim

I thought there were pictures of someone using a heavy duty cargo strap wrapped around the axle with the tire removed and the hooks on the nose gear braces.

Clarence

[carusoam]

Vance,

Thanks for the great detailed explanations and Pics of this process...

Best regards,

-a-

[N201MKTurbo]

5 hours ago, M20Doc said:

I thought there were pictures of someone using a heavy duty cargo strap wrapped around the axle with the tire removed and the hooks on the nose gear braces.

Clarence

I've done exactly that, I wouldn't recommend it. It is just plane dangerous. I have the tool now (well it is somewhere) and it only takes a few minuets to get the bolt out.

[MB65E]

I'd Be careful of compressing the tail like that. It might not seem like much but compressing the tail manually like that with out a flight load could stress the tail and fuselage . The airplane is only rated to 1.5g negative. Pressing up on the tail would be negative G.

just some thoughts,

-Matt

[Guest]

5 hours ago, N201MKTurbo said:

I've done exactly that, I wouldn't recommend it. It is just plane dangerous. I have the tool now (well it is somewhere) and it only takes a few minuets to get the bolt out.

I’ve never done it either as I have the tools as well.

Clarence

[Vance Harral]

Well, the plot thickens.  I reinstalled the starboard side gear this morning and couldn't be happier - smooth operation, no slop, plenty of clearance at the bolt, not to mention everything looking cosmetically much better with the newly painted parts and new hardware.  When I started putting the port side together - which was the side that was rubbing - the problem is still there.   All the slop is gone from the links, but the center pivot point just comes down too far, and bumps the landing gear truss.  The video below shows the problem.

 

 

[carusoam]

Is that the right bolt length?

looks like it might be an extra thread or two long?

the safety pin looks a little tall in the saddle.  (Wish I had the vocab working to describe this.)

check what the other side looks like for a quick reference...

PP observation only, not a mechanic...

Best regards,

-a-

[Vance Harral]

For grins, I tried temporarily installing the linkages with the shim behind the "baby shoe" bracket completely removed.  This does provide just a tiny bit of clearance, see video below.  But I'm still not convinced using a thinner shim (or removing it altogether) is anything but a kludge workaround that doesn't really address the root cause.  I'm now wondering if the problem is wear at the interface where the retract truss and the drag link come together over center.  That would allow the parts to drop further, and maybe that's why they contact the gear truss.

My plan is to call LASAR again tomorrow for further advice, but any thoughts here tonight are also appreciated.  I confess I'm a bit demoralized.  We're about 10 days into this effort, and it's taking all my free time and some of the time I really should be working at my day job, too...  The pitch trim systems are out of the airplane while my partner works on them, so that's hanging over our head, and a lot of the "normal" annual stuff (compression checks, cleaning plugs checking mags, etc.) hasn't even been started yet.

 

[Vance Harral]

Thanks for your thoughts, Anthony.  The bolt is spec'd in the parts manual, and there is only one size: AN26-17.  That's what LASAR shipped.  But I agree It looks like it could be shorter.  The rule of thumb for lock nuts is at least one full thread showing.  I'm not sure what it is for castle nuts, but there are at least two full threads showing here, and as you note, the cotter pin is barely on the edge of the nut rather than snugged down inside the castle groove.  Perhaps the answer is simply going to an AN26-16 bolt, which would be 1/8" shorter.

[carusoam]

Find out from Lasar when you talk to them...

What the stack height of all the bits and pieces should be...

The drilled hole for the safety device should be in the castellated nut...

From the photo the hole is a little above the castellated part...

Makes it look like a spacer or washer may be missing under the bolt head...

Do you have a drawing that defines what the stack of hardware should look like?

Reminds me of the hardware discussion on the battery terminal the other day.  Some stacking of hardware has changed over the years... or people have forgotten how it’s supposed to go.  Or something like that.

 

PP thoughts only.  Not a hardware expert...

Best regards,

-a-

[Vance Harral]

I do have a drawing that defines what the stack of hardware should look like - the parts manual for my airplane.  There are only four parts: the bolt, nut, bushing, and a single washer that goes on the nut side which you can see in the videos above.  I'm pretty confident the stack is assembled per the parts manual, and that it matches what we found during disassembly.  But that doesn't necessarily mean it's "correct".

As I write this, it occurs to me to look in the M20J parts manual. Our 1976 M20F is kind of a funny airplane.  That was the last year of the F, and it was built in the middle of the transition to the J.  We've found a few things here and there on the airplane where the assembly corresponds to drawing and parts in the M20J manual rather than the M20F manual.

So... the J manual specs the same bushing, nut and cotter pin.  But it lists an NAS1106-12D bolt with no washer.  The NAS1106-12D is 1-9/64" long, just a skosh longer than the 1-7/64th AN26-17.  So doesn't seem like that would help.

 

[MB65E]

You could file the bolt threads with a dremel for clearance. 

(Due to the PITA to washer the bolt.)

-Matt