re:Re: Pre load specification (leadscrew bearings)

Hi All,

snips, and re-post of Ron and Wally below

Let's try to clear this up a little.

Angular contact bearings and "face ground, matched pair,FGMP" angular
contact bearings ARE NOT the SAME thing!

Angular contact just means the bearing is DESIGNED to handle THRUST
loads ALONG WITH radial loads. The "angle of contact" is designed by
the mfr. to give more or less of either thrust or radial capacity.

A bearing designed for thrust loads only looks like two flat washers
with balls in between.

A bearing designed for radial loads is two RINGS (one smaller to fit
inside the other) with balls in between.

Let's "fake" this with a couple of small books and a toy ball.
Thrust bearing first:

Lay the first book flat on the table. Then put the ball on the 1st
book. And cover it with the 2nd book. As you hold the top book, you
can see that the "bearing" resists pressure downwards(thrust) but
does NOTHING to keep the top book from sliding all around sideways
(radial).

Now the Radial bearing:

Stand the two books on edge. With the ball in between. Pretend one
book is up against a wall. NOW the sideways motion(radial) is
resisted, but NOTHING stops you from LIFTING the inner book(thrust).

An angular contact bearing is made so the two "books" are at an
angle. If you imagine(or try) this, you will see that with the books
at an angle, there is resistance to BOTH radial and thrust loads.(Be
sure you are still pushing STRAIGHT down or sideways.) If you change
the ANGLE of the books(between the "on edge" and "laid flat" above)
you can see that more or less thrust vs. radial "load" is taken care
of.

Now that we see that an angular contact bearing is just a specific
DESIGN of bearing, let's look at TYPES of angular contact bearings:

Ball bearing type. Tapered roller bearing type. The difference here
is WHAT is doing the rolling, balls or rollers(cylinders)

NOTE: the cylinders are tapered because they are at an angle. If they
weren't, they would be close together at one end, and far apart at
the other.

And now that we see there are two major TYPES of angular contact
bearings, let's get back to our "face ground, matched pair" angular
contact bearings.

These are simply angular contact bearings of some TYPE and ANGLE,
which have had ADDITIONAL machining(face grinding) to make them work
a CERTAIN way. That certain way is:

When they are placed(and held, fixed mount style) in contact with
each other(face grind to face grind), they will HAVE either zero
clearance OR a SPECIFIC level of "designed in" pre-load.(by the face
grinding operation)The manufacturer does this and guarantees it!

If we put EQUAL length spacers in between them (for a rigid mount),
this "designed in zero clearance/pre-load will STILL be the same
designed value.

So Ron and Wally,

UNLESS you SPECIFY face ground, matched pair bearings(angular
contact) you will NOT get the RIGHT thing.

Pre-load is expressed in ounces,lbs., or kilograms. You should
determine what the forces acting on the screw are, and "add-in" some
additional load capacity.

You CAN "make" the regular angular contact bearings WORK like the
more expensive FGMP type, by adjusting the shims/spacers between
them. But remember, they are NOT the same thing. And you get what ya
pay for, usually.

Someone asked which bearing should be "handling" the thrust load? If
you use the bearings as described, one will carry the load one
direction, and the other will carry the load in the other direction.
(true whether they are in contact OR spaced apart)

Hope this helps.

Ballendo

P.S. The FGMP angular contact bearings ARE NOT a "standard industrial
item" in most bearing supply houses. The typical "counter guy" may
never have even heard of them! You MUST let the salesman/engineer
know what you are doing to get the right thing, in my experience.

'******************************************************************

>r_fl_z@h... wrote:
> How are ground pre-loaded bearings ordered/specified?
> or is the preload accomplished by the end user?
> since the torque on the lead screw nut affects the final preload

>>Wally responded:
>>The ground preloaded bearings are not specified by thousandths of
>>an inch. They are found in manufactures catalogs under the name
>>of "angular contact thrust ball bearings" You pick them by the
>>shaft diameter. In the INA catalog there are 2 bearings to choose
>>from for every shaft size. The differance being the size of the
>>outside race. The larger race has slightly better loading. Quick
>>look says loading varies from about the same to 40% better
>>depending on the bearing. Prices for 17mm to 20mm bearings are from

$17USD to $24USD
depending on the shield options.

Wally K.