ETX-125 COVER THREADS, CLUTCHES... Last updated: 20 June 2008

Sent:	Friday, June 20, 2008 09:50:33
From:	Patrick Mullarky (pat@nwce.com)
I asked about the threads on the front cover of the ETX-125 family a while ago.
I have since found that the spec. is  5.625 x 24tpi
 
The thread on the rear of the ETX-125 is 1.375 x 24.
 
The SCT standard rear thread is 2.000 x 24.
 
The 24tpi thread seems to be pretty common!
 
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Also, a discovery about the ALT clutch parts on the ETX-125PE:  (being
an Electromechanical Engineer, this stuff intrigues me)

The gear and the clutch plate (with the rotation-limit-dog) are
oil-filled sintered metal (!) just like an oil-filled bronze bearing,
but of some other white metal.

I found this out when "tuning" the clutches and drives per your
excellent articles.  I ultrasonically cleaned the gear and clutch on the
ALT drive. Then, when gently heating them up to drive off the water, I
found that both components sweated oil! and not a small amount. This is
a bit of a surprise.  It is very clear that the designers intended these
surfaces to be lubricated. So, even though you clean off the
way-too-much grease from the original assembly, the gear and clutch
plate will eventually re-lubricate themselves!

I'm wondering why the designers chose self-lubricating surfaces for the
clutch parts if over-lubrication is a serious problem.

Here's my guess:

The sintered oil-filled parts use a very light oil (seems to be around
5W). High pressure from screwing in the clutch knob will "push-through"
the oil films and create a relatively high static-friction condition.
It's a balancing actthe light oil provides very smooth sliding friction,
but also allows a high static-friction contact when high pressure is
applied. Tightening the ALT clutch knob applies very high pressureit is
essentially a jack-screw configuration.

However, the leaking grease from the poor initial assembly gets onto the
clutch surfaces and will not allow the desired high-friction
contactwhich is why the design fails in the field. The grease film will
not break under pressure like the light oil film.

This is all supposition, but makes sense to me.

One result of this finding might be that perhaps one shouldn't roughen
up the clutch surfaces but simply thoroughly clean the surfaces with
alcohol, lacquer thinner, acetone, or similar solvent. Roughening the
surfaces will definitely improve the static frictionbut it will make the
clutches "grabby" instead of silky smooth.  Eventually the surfaces will
wear smooth and be re-lubricated againlike sintered-metal oil-filled
bearings are supposed to. And, as long as no grease leaks onto the
clutch surfaces, the clutch should operate as designed.

Also, if one uses a water-based cleaning agent to clean the excessive
grease from the clutch parts (like I did), you MUST heat the parts to
drive off the water absorbed by the porous sintered metalor internal
corrosion will surely occur when reassembled. Simply bake at 200F for 5
minutes or so. Wipe off the sweated oil, and you're done!

There's never any need to re-oil the oil-filled parts. There is an
astounding amount of oil in oil-filled partsenough to last a
lifetimeeven if cleaned with solvents from time-to-time.

FWIW
 
Patrick Mullarky
Kirkland, WA

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