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backlash eliminator(Re: [ST] help! balance shaft mystery)



On Thu, Jul 11, 2002 at 04:00:54PM -0700, Keith Winter wrote:
> 
> ----- Original Message -----
> From: "Brian D" <brd@xxxxxxx>
> To: <st@xxxxxxxxxxxxxx>
> Sent: Thursday, July 11, 2002 3:53 PM
> Subject: Re: [ST] help! balance shaft mystery
> 
> > Still, you can't see where the two meet because it's obstructed by
> > the case.  So, I made a reference mark on the timing gear on the end
> > of the camshaft and a corresponding one on the end of the balance
> > shaft.  They seem to rotate 1:1, but they never line up!
> 
> Actually, if what my manual says is correct, they will only line up once
> in a great while.  The gear you see on the balance shaft isn't the
> driven gear; it's a thin gear in front of the driven gear, called the
> "backlash eliminator" gear.  It has one tooth less than the driven gear.

Actually, I do think what the Haynes manual says is incorrect.

First I went out to the bike to look and see if I could see both gears
on the balance shaft.  Now that I was looking for them, I could.  I
slowly turned the crankshaft through a full rotation, watching the
balance shaft gear the entire time.

Each tooth on the front one had a corresponding tooth on the rear one.
I would expect that if one gear had fewer teeth, I would see them come
out of alignment somewhat, or look even a little staggered.  Unless the
front gear were to rotate independently of the rear one, maybe?  Hrm.

So I came back inside and checked the Triumph Shop Manual that I
borrowed last night.  If you can believe the exploded drawing of the
balance shaft and its components, there are 56 teeth on both gears on
the balance shaft.  Yes, I actually counted them ... 3 times each.  Then
I recounted on another, smaller drawing later on that section (which is
probaly just a copy of the main drawing).  Same thing.

Also, the TSM assembly instructions for the balance shaft make no
mention of the gears having diff # of teeth.

> There's a long description about how to align the eliminator gear with
> the balance shaft driven gear, then detailed description about aligning
> the balance shaft driven gear with the crankshaft drive gear.  

Ditto the TSM.

> They state in one sentence that, if you "rotate the crankshaft one
> turn after aligning the gears, they won't line up anymore, so do not
> be alarmed by it".

The Triumph manual does not mention this anywhere, which I found
strange, as this would seem to be a pretty important thing to note.

Curiosity piqued, I turned to the exploded diagram of the crankshaft.
The drive gear on that, which meshes with the balance shaft gears, has
57 teeth.  Doesn't this mean that 1 rotation of one gear (56 teeth) gets
you one of the other (with 57 teeth)?  I'm not terribly familiar with
gearing.

Thinking about the procedures for checking the balance shaft, that
makes sense.  The check says to turn the engine over to T1 (TDC cyl
#1).  Well, one full rotation of the crankshaft brings the timing back
around to T1 (unless I'm vastly misunderstanding something and turning
the 24mm nut on the end of the crankshaft one rotation does not equal
one rotation of the crankshaft).  If what the Haynes manual says about
turning it more than once and them not lining up anymore being okay were
true, how would you ever know if they were supposed to be in line when
you turn it to T1 to check it?

Lastly, would it even be possible that two gears inline on a shaft with
a diff # of teeth could mesh with the same opposing gear on a different
shaft?  One would have to turn faster than the other, right?  The 2
backlash eliminator gear on the balance shaft is pressed tightly against
the drive gear by some tension washer (the manual states that you may
even need an assistant to compress that "belleville" washer) and a
circlip, but maybe the backlash eliminator gear would be able to wiggle
back and forth a little to still mesh, or to spin a little faster?  But
if it spun faster, why go to the trouble of aligning the marks on the 2
balance shaft gears?  (Plus, I don't think I saw it spin faster when I
watched it rotate.)  And that would cause wear against the washer and
the gear it was pressed up against.

Ugh, head hurting.  Anyway, Keith I know you were just passing that
along and I appreciate the information.  Just want to make sure I
understand what I'm looking at here.

Laters,
Brian
- -- 
'99 ST (black)
Dallas, TX

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