Help! Too much slack in the trim wheel chain

[BrianW]

I have a 1979 M20J.  Normally I can rotate the trim wheel about 6" per grab.  My wife can do about 4" per grab because she pulls the seat closer.

The problem is, when I rotate the trim wheel it rotates 3 inches before it actually starts moving the mechanism. When I let go of the trim wheel to grab to continue trimming, the wheel rolls back to the slack position.

This means that for my 6" rotation I only actually move the trim 3".   But for my wife's 4" of movement, she only get 1" of useful trim.  That renders the trim wheel useless for her (thank goodness for the electric trim button!).

Still, I would like to tighten the loose trim wheel chain to fix the problem.

My plane is currently in the shop for its annual at a Mooney service center (owner assisted☺).  The mechanic was not aware of any way to tighten the small bicycle chain between the trim wheel sprocket and the trim mechanism below the floor board.

I researched and came across Mooney service bulletin M20_325_88A .  In the picture of the trim wheel the nuts (screws) labeled "ITEM 2" were the ones that moved the mechanism to tighten the chain.  But I'm not sure.  The chain didn't move when I turned those 2 screws.

How does one get rid of the excessive bicycle chain slack at the trim wheel?

Thanks,

BrianW

M20_325_88A-1.pdf

[BrianW]

~~~~~~~~~~~~~~~~

Yetti said,

The chain is all of 6 inches long.  If you have slack you may want to pull it all apart and look for worn cogs.  I would think you need to check all the rods and u joints if you have that much slack.

~~~~~~~~~~~~~~~~

Hi Yetti,

It can't all be worn sprockets. The chain is very loose.  If it were on a bicycle it would be falling off.

The slack is just in the chain between the trim wheel sprocket and the sprocket below deck.  The u-jounts are all tight with no slop.

With such a superbly engineered aircraft, I would be shocked if there was no tensioner for a chain drive (even a bicycle have an adjustment to tension the chain!).  I just can't see which bolts and screws need to be loosed/turned to make it tight.

 

 

 

[BrianW]

All chain and belt drives have tightening mechanisms.  Belts and chains stretch over time.  

Over time one has to adjust to keep things properly tight.  

Eventually when they stretch to some point, then they get replaced.

But how is it adjusted on the mooney trim wheel chain?

Thanks

BrianW☺

 

Edited November 12, 2017 by BrianW

[Guest]

7 hours ago, BrianW said:

I have a 1979 M20J.  Normally I can rotate the trim wheel about 6" per grab.  My wife can do about 4" per grab because she pulls the seat closer.

The problem is, when I rotate the trim wheel it rotates 3 inches before it actually starts moving the mechanism. When I let go of the trim wheel to grab to continue trimming, the wheel rolls back to the slack position.

This means that for my 6" rotation I only actually move the trim 3".   But for my wife's 4" of movement, she only get 1" of useful trim.  That renders the trim wheel useless for her (thank goodness for the electric trim button!).

Still, I would like to tighten the loose trim wheel chain to fix the problem.

My plane is currently in the shop for its annual at a Mooney service center (owner assisted☺).  The mechanic was not aware of any way to tighten the small bicycle chain between the trim wheel sprocket and the trim mechanism below the floor board.

I researched and came across Mooney service bulletin M20_325_88A .  In the picture of the trim wheel the nuts (screws) labeled "ITEM 2" were the ones that moved the mechanism to tighten the chain.  But I'm not sure.  The chain didn't move when I turned those 2 screws.

How does one get rid of the excessive bicycle chain slack at the trim wheel?

Thanks,

BrianW

M20_325_88A-1.pdf

This should help

[N201MKTurbo]

It will be much easier to do the adjustment if you remove the seat.

last annual I removed the whole gearbox assembly and completely disassembled it, cleaned it and put new grease in the gearbox. It is much happier now and looks much better, especially the chain. It had 40 years of dried lubricant of unknown origin. It all looked brand new when I was done. 

[BrianW]

Thanks for the service pictures and advice!

I loosened the 2 lower mounting bolts no problem. However the 2 upper mounting bolts are difficult to loosen.  I can get the ratchet wrench on the nuts but the chain on the other side blocks me from getting a phillips on the bolt head, so the nut and bolt just spin and don't loosen.

Am I missing a step? Do I need to remove the chain to be able to loosen the upper bolts?

Thanks

BrianW

 

[Yetti]

So just to be technical (I have been through hundreds of bike chains)  chains don't stretch.    The silver rollers and the pins that hold them wear.  This causes the tolerances between rollers to expand.  So replacing the chain would be the first step.   Of course if it is that worn.  The cogs would also be worn.   To something that will look like a shark fin.   I still think you need to look at the whole system stem to stern.

[N201MKTurbo]

To that end, I've never seen any noticeable wear on a trim chain. It is a very robust chain and even a 60 year old plane probably has less than a mile of equivalent bike mileage on the chain. 

One thing I have seen is the sprockets getting loose on the shafts. If I recall there is a set screw pin on the big end and a roll pin on the small end. Replacing the roll pin on the small end helps.

[jetdriven]

The chain itself is a common size available at McMaster Carr. 

[Yetti]

The mooney owner A&P had his jack screw out working it over and painting it.  I have cleaned and triflowed my whole drive line and then lubed the jack screw installed on the plane for the past two annuals.  We went to do the FR and he said that my trim was much better than his.

Like Turbo said one of the wheels or sprockets is probably loose.

I used to pull bike chains and wash and then relube.   Now I just wipe down and relube.   I don't ride as much.   For the trim chain, mine was gunked with old grease.   One the plane I just sprayed with silicone spray brushed and rotated, repeat, repeat. Then triflowed or spread and rotated and wiped.  Will lube it again this annual.

[BrianW]

Here is the info thus far:

The lower mounting bolts have a normal phillips head on the one side and an 11/32" locking nut on the other side.

The upper mounting bolts are different.  They are called hi-lok bolts.  These have a 3/8" locking nut and use a small Allen wrench on the same side as the nut to hold the bolt from spinning!

Once these are loosened, the trim mechanism body can move up and down by only approximately 3/32". This only allows you to adjust a chain length of only 3/16" of length.  That's not even enough to allow removing one of the 1/4" pitch links from the #25 roller chain.

By the way, since the chain pitch is 1/4", in order to use a standard connecting link (connecting the two narrow ends of a chain), you have to remove a wide and narrow chain segment.  This shortens the chain in increments of 1/2" (ie 2 x 1/4=1/2").

If you use what is known as a half link, or offset link, then you can shorten the chain by 1/4" increments (allows connection from the narrow end of the chain to a wide chain segment).

I found the #25 chain with connector link, a pack of half links and the chain link separator tool shipped direct to the shop for $48.

I figure it is inexpensive enough to just replace the chain with new.  And between the limited adjustment and the new roller chain, I hope the new chain can be snugged up without needing to  replace sprockets just yet.

Hopefully my explanation makes sense for those looking for trim mechanism details.

I'll post the results when done...

BrianW

[kortopates]

Here is the info thus far:
The lower mounting bolts have a normal phillips head on the one side and an 11/32" locking nut on the other side.
The upper mounting bolts are different.  They are called hi-lok bolts.  These have a 3/8" locking nut and use a small Allen wrench on the same side as the nut to hold the bolt from spinning!
Once these are loosened, the trim mechanism body can move up and down by only approximately 3/32". This only allows you to adjust a chain length of only 3/16" of length.  That's not even enough to allow removing one of the 1/4" pitch links from the #25 roller chain.
By the way, since the chain pitch is 1/4", in order to use a standard connecting link (connecting the two narrow ends of a chain), you have to remove a wide and narrow chain segment.  This shortens the chain in increments of 1/2" (ie 2 x 1/4=1/2").
If you use what is known as a half link, or offset link, then you can shorten the chain by 1/4" increments (allows connection from the narrow end of the chain to a wide chain segment).
I found the #25 chain with connector link, a pack of half links and the chain link separator tool shipped direct to the shop for $48.
I figure it is inexpensive enough to just replace the chain with new.  And between the limited adjustment and the new roller chain, I hope the new chain can be snugged up without needing to  replace sprockets just yet.
Hopefully my explanation makes sense for those looking for trim mechanism details.
I'll post the results when done...
BrianW

That’s odd you found only 3/32 movement adjusting the tensioner bolts on top in the cabin. I had much more but I didn’t measure it. I simply raised the tensioner bolts too loosen the chain to make it easier to separate the master link on the chain and then reinstall the master link on the chain. Then turned the tensioner bolts down to raise the assembly and tighten the chain. The tensioner bolts seemed like they had more like 1/2” movement. Same mid body airframe too but a K model.


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[N201MKTurbo]

You can always put in longer bolts. And you can take longer bolts and cut them down to make any length you need. You may have to take the trim wheel off to change the bolts. You need to get the bolts with the short heads so they don't hit the wheel. It probably depends on weather you have the aluminum or plastic trim wheel.

[carusoam]

Normal wear from 1965 to 1994 Mooneys can be measured by movement of the trim wheel.

There is slop when going from up to down. 

If you over trim in one direction, you have to pass back through the slop, before the trim actually begins moving in the other direction...

This slop is a total of barely measurable to a few degrees of rotation, a fraction of an inch of motion. It is slightly annoying, because it is noticeable...

I share this because the OP is describing something that is falling apart.  Not some wear that has happened over decades...

My examples are from a 65C and a 94R...

It is important to have the trim working properly. A stuck trim system can end the life of your plane. MS has lost a K (Marvin K Mooney) this way...  could have been much worse...

Hope this helps.

Best regards,

-a-

[Yetti]

For those that have built a single speed bike from a geared bike frame, all the things you mentioned make perfect sense.  There are also Magic gear combinations that don't need half links etc.   There is a huge need to find out how the slack developed.

[kortopates]

These chains even after 35+ years still are in pristine condition. They don’t get the stress or wear like a bicycle does and using the electric trim the chain isn’t even on tension - it’s just turning the trim wheel around. Slack or any improper tension should be easily explained by the position or height of the gear box determined by tensioning bolts in the cockpit. Should not be a mystery nor necessarily a sign of worn spring or sprocket which is very doubtful.


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[mike_elliott]

On ‎11‎/‎12‎/‎2017 at 11:23 AM, jetdriven said:

The chain itself is a common size available at McMaster Carr. 

Wouldn't it need to be a PMA'd part to be legal, or does this important item take a back seat to things like LED lights with the FAA's enforcement division?

[N201MKTurbo]

6 minutes ago, mike_elliott said:

Wouldn't it need to be a PMA'd part to be legal, or does this important item take a back seat to things like LED lights with the FAA's enforcement division?

The difference is there is no way to tell if the chain is PMA or not.

[mike_elliott]

1 hour ago, N201MKTurbo said:

The difference is there is no way to tell if the chain is PMA or not.

Oh, ok.... I thought it would be a $890 part because it had the special mark.

[BrianW]

The electric trim in my plane works fine and doesn't have slack.

The amount of travel in the trim mechanism that I would need to take up the chain slack would be an eighth of an inch more travel than I have.

N201MKTurbo mentioned using a longer adjustment screw.   We thought it was a good solution, and gave it a try.  When we backed out the original screws, we saw they were a standard machine screw design.  So that meant they could only push the trim mechanism down to tighten. With the adjustment bolts removed, and the mounting nuts loose, I moved the trim block from as high as it could go, to as low as it could go. I expected something like a 1/2" travel, but it was only about 3/32". The longer screws wouldn't help because the trim mechanism was already against the limits of its slot.  A longer screw wouldn't be able to push it any further than the current original screws.

FYI, although I use the term "bicycle chain", this is not bicycle chain. It is called roller chain and comes in lots of sizes with various strengths by size.  Bike chain is #41 and 1/2" pitch.  Ours is #25 and 1/4" pitch.  #41 roller chain is twice as strong as our #25 roller chain.  But then the trim wheel didn't need to be that strong and heavy.

I'll have the chance to work on the plane tomorrow. I'll see how it goes 

Edited November 16, 2017 by BrianW

[kortopates]

I’d bet if you pulled the whole gear box out you’d be able to find the problem. Maybe a good time to do Mooney SB M20-88A and eliminate chance of the AP causing the trim to get stuck at the stops.


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[jetdriven]

23 hours ago, mike_elliott said:

Wouldn't it need to be a PMA'd part to be legal, or does this important item take a back seat to things like LED lights with the FAA's enforcement division?

This is up to the installing mechanic but the chain can be called a standard part. It’s easy to determine that. Led paddle bulbs, however, won’t meet the super wide viewing angle like an incandescent without some kind of array. 

Edited November 17, 2017 by jetdriven

[BrianW]

I removed the old chain by disconnecting the master link.

I laid it on a table beside the new chain and tape measure.

The old chain was worn and "stretched out".

(for the nit-pickers, yes, I know the metal doesn't actuallty stretch. The wear allows links to move apart.  But figuratively, stretch is easier to visualize).

The #25 roller chain is 1/4" pitch.  The second pic show the new chain with 1/4" pitch and the old one was longer.

A close up in picture 2 shows the old chain closer to the tape measure and the new chain below it. The new chain is not "cut" to size in the photo, I rotated the link at the same number of links as the original.

Both chains are even at the beginning, but by the 1 foot 4-3/4", the new chain still maintained 1/4" pitch.  The old chain was an 1/8" longer.

I installed the new chain with the same number of links as the original and voila, it was able to be adjusted properly.  Yea, no more slop. I tightened the 4 mounting nuts on the gear box, and then the mechanic inspected the finished assembly. DONE!

 

[BrianW]

By the way, I was sent the drawings for the various parts.  The adjustment does not move the gear box in a slot.  It is simply an oversized hole that the gearbox can slide from upper extreme of the hole to the lower extreme of the hole (allowing about 0.1" of travel).

The stock NAS adjusting bolt allows adjustment for the full range of travel. A longer bolt wouldn't gain any more travel.

[Bartman]

Well done Brian. 

Still the question remains why did your chain wear out, yet nobody not even the A+P has reported a worn out chain.  I have electric trim and like Paul said the chain is just along for the ride when I use it and maybe you have manual only which will wear a little faster.  Only way I ever saw a chain wear too quickly was if it was too tight.  Maybe yours was too tight in the beginning, or maybe some debris got in there in the past and somebody just ham-fisted it till it worked.  Just a thought.