550 SE spacer washer on bottom of main shaft

[tworst]

In the manual, it shows placing (1) 10x6x0.5mm shim washer on the bottom of the main shaft and then snapping on the C clip. But with just 1 shim, there was a noticeable gap between the shim and the C clip. So, I went ahead and threw a second shim on the shaft and it closed the gap nicely to the C clip. I built my main shaft and gears very carefully, put all the parts in according to the manual and aligned the pinion so it is just about perfect. Just wondering why there was a noticeable gap between C clip and shim when using just 1 shim, but 2 shims came out perfect, but is not called for in the manual. I also noticed that the kit seems to have come with several extra shims, so maybe it just plays out like this for different kits.

[crashjack]

I'm in the middle of my 550 SE build and built a 550 SX last year (same frame, head, etc.). When I built my SX, I think I ran into something like that, but something wasn't right. In the end, everything was right with just the one shim and the c-clip. I'm not saying that is your case, but you might want to re-check everything.

[corrosionate]

That small gap is to allow the main gear to slip up and down a tad on the shaft to help keep proper gear mesh with the herringbone pinion/main gear.

Since the main gear is molded, rather than precisely machined, there can be some small imperfections in the main gear, not only in its roundness, but it's flat-ness as well.

A characteristic of herringbone gears that other gear types don't have is that the center line needs to be lined up so there's no excessive strain or wear. Adjusting the pinion height is obviously necessary, but it helps to let the main gear float a little. I try to center the gear in that gap when I set gear mesh, so it's got room to go up or down a little, whatever it needs.

You may shim the gap out and never have an issue, it should be very small amounts of imperfection, but all the logo pilots I know build per the manual, leaving the gap, and have great results. I believe you'll hear the same from many other folks as well.

[MS60]

I think you want a slight gap. Also note the shims above the gear and the position of the shaft collar on top.

[tworst]

OK, I didn't have any play at all so I started busting down the main shaft to check shims, gaps, etc. I got the main gear off and noticed that my belt seems to ride on the bottom rim of the belt pulley. Shouldn't it pretty much ride in the middle of the pulley if everything is set up correct? I am a little concerned it might wear on the belt. But it doesn't appear that I can change it any because the belt gear is as high as it will go.

In any case, I will make sure I set up for just a tad in up and down play in the main shaft.

[tworst]

There were two shims on top of the belt gear, so I took them out just to do a test fit. Once removed, I found that the belt still rode on the lower lip of the idler pulley. Taking the two shims out, raised the belt gear and so I thought it would lift the belt off the lower lip of the idler pulley as well, but no luck. In addition, the belt no longer rode in the center of the belt gear itself. So I will just put the shims back in above the belt gear as per the manual.

What I think is happening with the belt is that there is a twist of it down the tube and the side riding on the lower rim of the pulley is just kind of on the "low" side of the twist.

As far as leaving a little up and down play on the main gear so it can float a little to align with the pinion, I have decided to keep the main shaft more or less fixed by adjusting the top collar. Not super tight, but very little in the way of noticeable up/down movement. Then, whatever play I need in the main gear for the pinion alignment, I will just use the clip and shims around the main gear to achieve it.

[corrosionate]

Originally Posted by tworst

whatever play I need in the main gear for the pinion alignment, I will just use the clip and shims around the main gear to achieve it.

This is exactly right. The heli frame should be basically "pinched" between the belt pulley below the bottom main shaft bearing and the locking ring above the top main shaft bearing (shimmed per manual). No need to clamp it super tight in there, but take up all the slack before tightening the main shaft clamp down so that the main shaft doesn't move up and down in the frame. The main gear slides up and down between the belt pulley and the C-clip. Usually approx. a mm of total play. Shimming per the manual is usually spot on here as well.

It's pretty normal for the belt to ride off center in the pulley. Seems like on mine it usually rides the top lip, but as long as it's not rubbing so agressively that it's trying to climb out of the pulley, it should be fine. If you spin your tail rotor so that the tension is on the other side of the twisted belt, it will likely work its way over and ride the other side of the pulley. You might double check that your belt is twisted the proper direction, if you haven't already.

[Dennis Everett]

if the belt were twisted in the wrong direction your tail would turn backwards .
you want no up and down movement of the main shaft , if you do the heli could be unstable at hover , twitching up an down ...
don't overthink it , do like the manual says (most time ) and you will be fine ...

[usaf_aggie]

What did you ever figure out about the belt riding on the belt tensioner lips? The heli I have not flown yet looks exactly the same.

[RedJoE]

Originally Posted by usaf_aggie

What did you ever figure out about the belt riding on the belt tensioner lips? The heli I have not flown yet looks exactly the same.

Be careful with the belt riding on those tensioner lips. They are plastic. In my experience, eventually the bottom lip will break off from the constant pressure. Then the belt comes off and gets jammed up and you go down. It will sometimes break the tensioner receptacles on the frame side as well, possibly costing a new frame side.

What I did on mine was I took the front tail pulley and grinded down the top ring, like a mm or two so it's down to the second lip. Installed the pulley with one thin shim on top, then a bunch of shims under it to take up the space. I had shims lying around from other helis. Then the main gear, etc.

This brought up the belt just enough to where it still rides on the lower lip of the tensioner, but very very softly.

In my opinion the belt is slightly out of alignment in the stock configuration.

[usaf_aggie]

Thanks Joe. I tried taking one shim off the top (too nervous to start grinding stuff!) and like others chimed in about, it only changed the height of the belt on the belt drive pulley, not the frame guide pulleys. I also flipped my boom upside down (plastic talk case) and that didn't change anything. It is weird to look at old posts and see all of the folks who have the issue. Has Mikado never addressed the problem (sorry I am new to this brand)?

I wonder if it is a defect in the way the tail boom holder fits in the frame, i.e. it is slightly angled. That might explain why the 600 doesn't have the problem. If you look closely it does appear that the belt/boom and main tail pulley are not perfectly square.

EDIT: I tired every combination of shims and boom orientation. Only real change is where the belt rides on the main tail pulley. When examining the belt tension pulleys with a flashlight, I can see a minuscule amount of space between the belt and pulley rim.

[RedJoE]

Yeah, even with my mod of grinding the pulley down a mm or so, the belt still rides on the left idler lip. however, it doesn't ride on it very hard anymore and I doubt it will break off now.

It's because the belt goes from horizontal(on the pulley) to vertical( in the boom) that causes it to ride the idlers. Might be better to move to metal idlers to solve this. Don't think they offer this option.

It took 200 flights before the left idler first broke on me, that was enough to get me to make the mod, and I've modded three more pulleys since then. So no problems since.

This mod works very well for me so far.

[usaf_aggie]

I have a few flights on it now, and it looks like the belt is a few thousandths of an inch above the rim, but it is not much space! Of course who knows what is happening when the system is loaded up in flight. I still would like to figure out why the 600s supposedly don't have the issue. Maybe the twist is further away since the boom is longer, or the boom mount is angled slightly different. I will keep an eye on it. I swore I saw some metal tension pulleys somewhere. Speedix I think?

[Dennis Everett]

i think mikado usa has them , under upgraded parts ...

[RedJoE]

Lol, right under our noses. Why oh why are these not stock on the heli.

[usaf_aggie]

Lol yep sure enough.... I figure if Dennis had his way with the heli for a while and there is no sign of wear, they must be fine.

[simplechamp]

I noticed on my 480 the belt riding to the bottom of the pulleys and side of the tail as well. If I spin the rotor the opposite direction, sure enough it changes to the top of the pulleys and other side of the tail.

I did buy the alloy pulleys for my build. I didn't even know about the belt thing when buying them, just figured they would be more durable and also may be a little more concentric to reduce vibrations (although the plastic ones that came in my kit seemed fine, no wobble or anything)

I notice on my Compass machines the belt seems to favor a side and not perfectly centered, but they don't actually ride on the lip.