"Jack W. Drews" wrote:
>
> "Michael D. Porter" wrote:
>
> > "R. Ashford Little II" wrote:
> > >
> > > How often or at what interval should the wheel bearings be replaced?
> > > I've got a TR6.
> >
> > Depends to a considerable extent on degree and type of maintenance,
> > driving conditions and on the type of lubricants used. --
>
> Mike's comments on bearing life agrees completely with my limited knowledge of
> the subject. There is one more aspect of bearing life that I learned during
>the
> several years that I worked with engineers who were designing transmissions
>and
> rear axles. That aspect is "preload", something that is very difficult to
>achieve
> properly on our front spindles, but which is important for all taperd roller
> bearings, including our front wheel bearings and such thoings as pinion
>bearings
> and TR6 rear hubs.
> Bottom line of all that jargon is that if the bearing is too tight or too
>loose,
> it will not live as long as it will if it's "just right".
> All normal folks can now wake up again -- we technofreaks are
>finished.............
Well, not quite finished. <smile> I didn't get into the details of
assembly, simply because those are fairly well-documented in most
manuals. As Jack suggests, preload is very important to bearing life,
and for reasons which seem obvious with just a little examination of the
geometry and materials of, particularly, tapered roller bearings.
Because of the size of the roller, the contact area of the roller is
very small, which means the loads are very high, but the drag is not
high, which makes them, in theory, a very high-efficiency bearing.
Because of this, with too tight a preload, the bearing rollers, under
side load (because they work in coned races) can be distorted or can
fail from too much friction. I think most rollers are surface-hardened,
and those distorting loads can crack the surface, which then contributes
to spalling. If the bearings are too loose, the side loads are increased
due to acceleration before contact, and the rollers and races get
pounded, which results in brinnelling, a condition where the running
surfaces fail from work-hardening. (Brinnelling can also result from
overtightening a bearing retaining nut without sufficient grease on the
bearings, and then turning the wheel.)
My favorite story about preloading comes from the time I was working for
a Toyota garage in Florida in the early `80s. Toyota had, around that
time, just released the Tercel to the American market. It was a curious
car to most Toyota shops, in that it had a new engine, a very curious
arrangement of an inline (not transverse) engine arrangement with an
over-and-under front-wheel drive transmission run through a reduction
gear set. But, most problematic to mechanics was the fact that the brake
rotors were bolted _behind_ the hubs. In order to remove and turn the
rotors, the hubs had to be removed.
And, the car used traditional tapered roller bearings on both the inner
and outer bearings. The bearings, like those on the rear hubs of GT6s,
used a spacer to adjust bearing preload. This was a common practice of
Toyota in the early `80s (same bearing arrangement on the rear hubs of
the Supra).
But, given a couple of hours for a brake job on both sides, mechanics
generally avoided doing the drill on checking preload of the bearings
after assembling the hubs. The procedure was to assemble dry, check
preload to manual specs, and if too high or too low, disassemble, change
the thickness of the adjusting shim, check dry, again, and when right,
disassemble again, properly grease the bearings and reassemble. No one I
knew except me did it.
One slow day, a Tercel pulled into the shop with flames and smoke
pouring out of the left wheelwell. After the fire was put out, the
distraught female owner said that she was driving over the bridge
between Mobile and the Florida border and smoke started coming from the
left wheel. She said she didn't know what to do except to keep on
driving and bring it to us.
And, when it cooled off, they gave it to me to fix (give it to Mikey,
he'll do anything <smile>). On inspection, the bearings had too much
preload, had overheated, grease had wept out of the seals, then the
bearings had gotten hotter, and eventually the hub got hot enough for
the whole mess to catch on fire. When I finally got most of it apart,
the rollers were square in section--obviously, they were so tight that
they wore a flat on them, then, with the clearance, flipped, and wore
some more, etc. And then they welded themselves to the roller cage.
The damage turned out to be so severe that the inner bearing inner race
was heat-welded to the spindle and could not be removed, the spindle was
blued, and the upper part of the suspension upright was showing some
bluing from heat tempering. Had to throw it all away and find a junkyard
replacement.
All because someone didn't properly check preload during a brake job....
Cheers, all.
--
Michael D. Porter
Roswell, NM (yes, _that_ Roswell)
[mailto:mporter@zianet.com]
The gulf between content and substance continues to widen....
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