wheels: do bearings necessarily rotate or just support ?

http://www1.mscdirect.com/CGI/NNSRIT?PMPXNO 75012&PMT4NOf31670

I don't really understand your question.  Generally a bearing has two
components, one of which in theory moves with the axle and the other of
which moves with the wheel.  If the fit on the axle is loose and the
lubricant is stiff then under light load the piece that is supposed to move
with the axle may slip so that the axle turns inside it.

--
--John
to email, dial "usenet" and validate
(was jclarke at eye bee em dot net)

Hmmm .... I guess a much better question for me would be to ask is "when is a
bearing *locked* to the wheel ? " , so that both it
and an inserted axle *must* turn when the wheel turns. The point being that in
that case I would probably drive the axle in order to
transfer power from a motor to the wheel.

Now, when I see a wheel advertised to have "roller bearings" or "ball bearings"
I intuitively understand that those wheels will spin
freely even when an inserted axle is locked in position - ie - held in my hand
or attached to a robot frame. So in those cases I
would need to attach a pulley or gear to the wheel in order to transfer power
from a motor to the wheel. But when I see a wheel
advertised as having a "plain bearing" I an not sure it

Another question to help me understand the terminology would be: for the lawn
mower wheels I have used so far, which are the cheap
kind that has what seems to be rubber molded onto a plastic hub and absolutely
no other parts, I have been mounting them directly to
the motor shaft. Are these type wheels considered to have a "plain bearing" ?
Are they considered to have a bearing at all ?

If you want to do that then you do not want a bearing in the wheel, you want
the bearing between the axle and the frame or other supporting structure.


If they are the kind that press onto the axle and turn with it then they
don't have a bearing.  If they are supposed to turn on the axle but don't
have a movable component in the wheel itself then they have a bushing that
uses the axle itself as part of the bearing.

--
--John
to email, dial "usenet" and validate
(was jclarke at eye bee em dot net)

That helps to clear things up - - - thanks !
JCD

goes, and it "bears" the weight of the rest of the

The bearing supports the *load* on the wheel if it's free-wheeling, and
the load of the motor (or other driver) if it's driven. Depending on the
torque of the motor, the load on the bearing is significantly more than
just the weight of the wheel and other mechanical parts.

-- Gordon

axles goes, and it "bears" the weight of the rest of
the

Thanks Gordon. Another poorly worded posting on my part (nothing unusual there!).

I guess a better question to have asked is: are bearings ever locked to the
wheel, so that they must turn when the wheel turns ? I
am trying to determine when a wheel is advertised to have a "plain bearing", how
will I need to transfer power to that wheel from a
motor ? I have been mounting the lawn mower wheels direct to the motor shaft,
with a screw drilled into the hub to make sure they
are locked to the motor shaft.

Is the hole the shaft/axle goes through considered to be a "bearing" of *any*
kind ?

Thanks !

wheel, so that they must turn when the wheel turns ? I

how will I need to transfer power to that wheel from a

with a screw drilled into the hub to make sure they

I usually refer to "plain bearings" as bushings to differentiate the
two, though technically bearing is also correct. The bushing (bearing)
provides mechanical support via its physical properties, while offering
a low-friction surface for the axle to rotate. To work properly the
bushing should not rotate within its cavity.


kind ?

It could, yes. The hole provides what a separate bearing/bushing would
(mechanical support, etc.), except maybe not as well.

-- Gordon

Now *that* makes sense to me !

Thanks !

To extend somewhat from your original question, you want to minimize the
surface area of the rotating parts. This minimizes friction, which
reduces heat and wear. Therefore, you want the smaller axle to rotate
within the body of the bearing, and not the "bearing" fixed to the axle
rotating within the frame.

That said, exceptions abound. Most ball bearings are a combination of
this: they rotate against the axle *and* the frame (well, the frame of
the bearing, which holds all the balls in place, but you get the idea).
Because the balls are spherical there is very little surface area of
contact, so the goal of minimal heat and wear are achieved.

-- Gordon

For those interested, I found this description of "Plain Bore" in Albion's PDF
catalog, page 9:
link: http://www.albioninc.com/AlbionCatalog.pdf  ( 21MB )

    Plain Bore
    (41)
    In this option, no bearing material is used. The
    wheel runs directly on the axle or spanner
    bushing. This is the least expensive bearing
    option, but also results in a higher rolling
    resistance.

So I guess now my question becomes when does a wheel distributor mean "plain
bore" when they say "plain bearing" ?
The more I know the less I know I knew ...

bore" when they say "plain bearing" ?

"Plain bore" simply means "hole." The exact definition will vary between
manufacturers.

-- Gordon

bearing rotates independent of the wheel ? ( That is

)

goes, and it "bears" the weight of the rest of the

determine it from this info ?

A wheel with a rolling element bearing (ball, tapered roller, needle
roller etc) has a 'place to put the axle' that will rotate independantly
of the wheel. The bearing outer is supposed to be a tight (non rotating)
fit in the wheel; the inner grips the axle. This is acheived by the use
of grub/set screws, interference fits, or Loctite.  In between the
bearing races is the rolling element that allows low friction rotation.

To use a roller bearing in wheel setup, the axle is usually fixed to the
chassis and the wheel rotates on the fixed axle driven by various means
but always directly to the wheel.

A 'plain' bearing is a bush, a tube that is fixed to the wheel, through
which the axle passes.  The plain bearing is often made of some white
metal, a bush is often bronze; bearing bronze is a self lubricating
'plain' bearing metal. The purpose of this type of bearing is to provide
a sacrificial element that can be replaced as it wears away.

A plain bore is usually a hole through the wheel hub that is a specific
size, but has no bearing elements as such. A pilot bore is a small hole
through the hub centre - or a hole drilled from each side with a centre
drill - to provide a guide for further machining.

The wheel pictured in the link appears to have a steel sleeve in it,
which is essentially a plain bearing or bush - but the terms are usually
used to describe different things.

A wheel with a bush may be used on a fixed axle; or if you want to drive
the axle, use pillow blocks to attach ball bearings to the chassis,
through which the axle passes.  The axle can then rotate freely - wheels
and sprocket/gears may be attached to it.

To attach wheels/sprockets etc to a shaft or axle it is common to use a
'key', this is a piece of hardened steel that fits in a groove machined
half in the bore and half in the shaft.  If low power is transmitted a
threaded axle with double locking nuts on each side of the wheel can be
used.

Other methods of attaching things to a shaft are; grub/set screw,
collar, roll pin, taper lock and press/interference fit.

GREAT writeup! It goes into my notes.
Thanks again !
JCD