| Layshaft Bearings
Layshaft load occurs in both the radial (across the shaft) and axial
(along the shaft) planes. The OEM ball bearing has a moderate load rating
but most of its load capacity is in the radial plane. The roller bearing
used in later model transmissions is stronger, but also only in the
radial plane; it is not designed to accept axial loads. The tapered
roller bearing, as used in the Metric Mechanic Ultimate Transmission,
can accept virtually equal radial and axial loads. The critical benefit
of the tapered roller bearing is its high load rating. The BMW transmission
case imposes a tight limit on the size of bearings and tapered roller
bearings are much stronger than a comparably sized roller or ball bearing.
The only drawback to tapered roller bearings is that they are slightly
noisier than ball bearings. However, it would seem to be a small consolation
to its obvious superiority.
The reason for the tremendous durability of the Ultimate Transmission
is the 3000 lb. margin for safety provided by the use of tapered roller
layshaft bearings.
Synchronizer Life Span
The illustration shows a BMW synchronizer and figure B shows small teeth
on the inside of the synchronizer ring. These small teeth form the clutching
surface between the synchronizer and the gear cone. As the small teeth
wear, the gear takes longer to match the synchronizer speed, resulting
in progressively slower shifts. Normally the BMW synchronizer is considered
worn out when the teeth are worn down more than nine ten thousandths
of an inch (0.0009"). With the Ultimate Transmission, this wear
limit has been extended to 45 ten thousandths of an inch (0.0045")
as illustrated in figure C. These measurements correspond to .031"
& .001" respectively, measured from the base of the synchro
to the top of the gear. Since the tooth form becomes wider as it wears,
the rate of wear decreased geometrically. In other words, by allowing
five times the amount of wear, we allow twenty-five times the amount
of wear area. This increases the clutching action of the synchronizer
to enable very quick up and down shifts, even under racing conditions,
without the need to pause between gears.
Synchronizer warpage and rocking
The factory method for determining whether a synchronizer is still usable
or not is if the air gap between the bottom of the ring and the base
of the gear cone is greater than .031" (usable) or less (not usable).
This method will tell you if a synchro is worn out but it will not tell
you if it is a good one. Eighty-five to ninety percent of the synchros
we see would pass the factory test, and yet none of them work. They
fail because they warp to such a degree that they no longer match the
tapered cone on the gear and therefore no longer grip the gear cone
well enough to stop the gear. Often, new synchros rock on the gear cone
even though they obviously conform to factory specs.
The synchros we use are matched individually to each gear cone to alleviate
warpage and a series of machining procedures are used to increase their
effectiveness and longevity.
Blueprinting the Ultimate Transmission
There is considerably more to what makes an "Ultimate Transmission"
than the simple substitution of superior Metric Mechanic parts for factory
synchronizers and bearings. So much so that a transmission rebuilt in
this fashion would not last. Even two gearboxes of the same model and
same year can be quite different, not only with respect to how they
shift but also as to the actual dimensions of their parts. Therefore,
each gearbox should be "set up" to its own particular specifications
to insure that it performs at its individual best.
Bearings and their Shimming
JUST AS NO TWO TRANSMISSIONS PERFORM OR "FEEL" THE SAME, NEITHER
SHOULD THEY ALL BE SHIMMED ALIKE.
Given the above, it would stand to reason that each transmission must
necessarily be assembled and shimmed according to the exact dimensions
of the parts involved. In theory this is what is prescribed in the factory
procedures, but in actuality, this is not what is done. As different
as they are, these gearboxes are assembled with a "standard"
set of shims, each varying only slightly from set to set and certainly
not according to the variations of the individual groups of transmission
components. Excessive end play (.020" to 025") is always found
in the shimming of the layshaft as well as .005" to .010"
at the input shaft plus an unnecessary amount of distance between the
input and output shafts. Even the output shaft bearing is, in most instances,
not precisely captured.
The set-up method we use is as follows:
To insure the ideal positional relationship among the input, output
and layshafts, all shaft bearings are totally captured, allowing for
no end play.
Specifically - the output shaft bearing shim is chosen so that all clearances
are removed guaranteeing that...
- The output shaft cannot change position relative to the housing
and...
- The bearing itself cannot "free-spin" in its saddle
- The tapered-roller layshaft bearings are machined to fit the individual
housing to capture the layshaft and preload (.0015" to .002")
it to insure stability at operating temperature.
- The input shaft is positioned closer (usually .040" to .060")
to the output shaft for a clearance of .008" to .010", and
then the input bearing is precisely captured in its saddle to maintain
this clearance.
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