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My understanding is there are two distinct styles of TCM starter adaptor, new and old.
When starting, the engine crankshaft is turned by a clutch shaft. This shaft is geared to the engine and always turning when the crankshaft is turning.
The clutch shaft is engaged by the clutch spring when the electric starter motor is energized. The spring is like the chinese finger trick. Twist the spring and it constricts drawing tighter over the shaft. The spring has a big brass gear on one end which is engaged at a right angle by a small sliding worm gear that transfers the power from the elctric motor.
Very important this sliding worm gear completely disengages the brass gear or else the spring cannot relax enough to release the clutch shaft. Or else the engine will drive the starter mechanism.
Actually the distress point is the spring/clutch shaft interface.
The old style adaptors used a clutch spring which is designed to be a loose fit over the 'clutch' shaft. One end of this clutch spring fits into a bushing providing a slight bit of drag. So, when the electric starter motor began turning, the spring constricts due to the friction in this bushing, the spring tightens around and turns the clutch shaft.
The new style adaptor's design removed the spring end bushing and the spring was changed to be an interference fit over the clutch spring. The redesign depends on oil pressure throught the shaft to float the spring hydrodynamicallly so it disengages itself while the engine is running. Before oil pressure floats the spring, there is enough friction for the spring to constrict when the electric motor is energized.
One may notice this while trying to turn a propeller backwards by hand. The new style provokes resistance.
The old Delco electric starter motor has low turning force. After being de-energized, it will rotate a bit in reverse and allow the spring mechainshm to unwind and relax. The new permanent magnet motors have higher turning drag and will not allow a bit of reverse rotation.
Aircraft Specilaties's test cell video shows the sliding worm gear staying partially engaged, in some instances, even at high cruise rpm!
The new motors will work okay on the new style adaptor because because oil pressure floats the new style spring. But the premanent magnet motors do not allow the old style spring to unwind.
The common distress point in the starter adpators is at the spring/clutch shaft interface. Usage causes wear and the shaft becomes slippery. Various methods have been tried to keep the shafts serviceable. Rebuilding the dimension by chrome plating was a failure. The chrome was not tough enough. Today, the scheme is to grind the clutch shaft a slight smaller size and use a different sized clutch spring to keep the tolerance dimension. There are a list of sizes available.
regards
Kent Felkins
----- Original Message ----- From: "Kevin Stallard" <Kevin@arilabs.net>
To: <lml@lancaironline.net>
Sent: Tuesday, December 09, 2008 7:43 AM
Subject: [LML] Re: Starter adapter drive
Hi All,
I just spoke with another Legacy owner, who recommended that I speak
with the folks at Niagara Air Parts (http://niagaraairparts.com/).
They will rebuild starter adapters and put the adapter back to the
pre-improved design (which is good). They also recommended that I use
the old Delco/Presta Light/Teledyne Energizer (one of the many names
used for the same motor?) starter motor until such time as the other
lighter weight starters are beefed up. This is what Kent (below)
recommended as well.
The story is that TCM tried to improve the starter adapter by knurling a
shaft and shortening the spring (if I recall my conversation correctly).
This was supposed to increase the friction of the spring and make the
system more reliable. However, this had a net effect of shortening the
life of the starter adapter. Normally the worse thing that happens is
that the adapter fails to turn the engine over.
The adapter coming apart in flight and dumping metal into your engine is
usually caused by starter motors that will not turn in reverse. The
Iskra is notorious for this because it is so hard to turn statically.
Apparently, the old TCM manual used to specify that the torque needed to
turn an un-energized starter motor had to be 5 inch-pounds (or was it
inch-ounces, I don't remember) or less. This basically means that you
should be able to turn the output shaft of a brand-new starter motor
with your fingers.
Alas, this isn't the case with the Iskra's and other geared, lower
weight starter motors. They don't allow the spring in the adapter to
unwind, and this causes a lot of the starter adapter failures.
So, I guess I'm going to be flying around with a 12 pound starter motor,
but hopefully it will prevent starter adapter woes....we'll see. I may
have missed or gotten some of these details wrong, please feel free to
correct errors mentioned above.
Kevin
----Original Message-----
From: Lancair Mailing List [mailto:lml@lancaironline.net] On Behalf Of
Kent
Sent: Saturday, November 29, 2008 5:51 PM
To: lml@lancaironline.net
Subject: [LML] Re: Subject:,Starter adapter drive
hello
The TCM starter adaptors are stressed by the permanent magnet motors,
like
the Iskra brand and others.
The permanent magnet motors have high starting torque and more
importantly
do not relax after being de-enegized. The start adaptor needs to be
able
to unwind a bit to totally disengage.
Aircraft Specialties Services found it was being blamed for warranty
problems that they knew was not their fault. So they made a cutaway
starter
adaptor and made video of the mechanism on a running engine.
regards
Kent Felkins
Tulsa
----- Original Message ----- From: "Douglas Brunner" <douglasbrunner@earthlink.net>
To: <lml@lancaironline.net>
Sent: Friday, November 28, 2008 2:25 PM
Subject: [LML] Re: Subject:,Starter adapter drive
I also had to replace the starter drive adapter on my IO-550 (non
turbo)
after about 50 hours. In my installation we only had to loosen the
bolts
holding the engine, not pull the engine.
--
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