My question is about life expectancy on following linear slide setup:

Two parallel 5/16 rods and a slide that uses a pair of small plain
ball bearings(5mmx12mmx5mm) on the same
axel with the spacer between them instead of a each single grooved roller.
The
ball bearings will roll on the both side of the rod since the spacer will be
shorter than rod diameter.

It's a preloaded design with preload no more than 10-15lb.
Speed is 1 foot/sec.
It's manually driven device and "high mileage" is not expected.

I understand that radial BB are not made this kind of thrust load but since
it's a relatively light load I think that it should work fine.

What do you think? Is it acceptable compromise?

If I were to do this, the answer is no. The bearing contact pressure is too
high and the contact area is too small. It is cheaper and more reliable to
use a plastic sleeve as a bearing. There are many self lubricating plastics
today that easily handle very high loads and do not require hard surface
rods to ride on.
Steve
"Alex" wrote in message
om...
My question is about life expectancy on following linear slide setup:

Two parallel 5/16 rods and a slide that uses a pair of small plain
ball bearings(5mmx12mmx5mm) on the same
axel with the spacer between them instead of a each single grooved
roller.
The
ball bearings will roll on the both side of the rod since the spacer will
be
shorter than rod diameter.

It's a preloaded design with preload no more than 10-15lb.
Speed is 1 foot/sec.
It's manually driven device and "high mileage" is not expected.

I understand that radial BB are not made this kind of thrust load but
since
it's a relatively light load I think that it should work fine.

What do you think? Is it acceptable compromise?

Try it and see. An alternate idea would be to use thrust bearings to take
the side load.
There is a web site with a guy making and selling what he calls a Home Brew
Linear Bearing. Check it out:
http://www.buildyouridea.com/
It is larger than what you describe but maybe...
--

Ron Thompson
On the Beautiful Mississippi Gulf Coast
USA

http://www.plansandprojects.com

Where did everyone go? Oh, yeah.
http://groups.yahoo.com/group/castinghobby/
Y'all come, ya hear?
*******
"Steve Lusardi" wrote in message
...
If I were to do this, the answer is no. The bearing contact pressure is
too
high and the contact area is too small. It is cheaper and more reliable to
use a plastic sleeve as a bearing. There are many self lubricating
plastics
today that easily handle very high loads and do not require hard surface
rods to ride on.
Steve
"Alex" wrote in message
om...
My question is about life expectancy on following linear slide setup:

Two parallel 5/16 rods and a slide that uses a pair of small plain
ball bearings(5mmx12mmx5mm) on the same
axel with the spacer between them instead of a each single grooved
roller.
The
ball bearings will roll on the both side of the rod since the spacer
will
be
shorter than rod diameter.

It's a preloaded design with preload no more than 10-15lb.
Speed is 1 foot/sec.
It's manually driven device and "high mileage" is not expected.

I understand that radial BB are not made this kind of thrust load but
since
it's a relatively light load I think that it should work fine.

What do you think? Is it acceptable compromise?

"Alex" wrote in message
om...
My question is about life expectancy on following linear slide setup:

Two parallel 5/16 rods and a slide that uses a pair of small plain
ball bearings(5mmx12mmx5mm) on the same
axel with the spacer between them instead of a each single grooved
roller.
The
ball bearings will roll on the both side of the rod since the spacer will
be
shorter than rod diameter.

It's a preloaded design with preload no more than 10-15lb.
Speed is 1 foot/sec.
It's manually driven device and "high mileage" is not expected.

I understand that radial BB are not made this kind of thrust load but
since
it's a relatively light load I think that it should work fine.

What do you think? Is it acceptable compromise?

Alex,
Consider that the bearing you are using has a speed rating of probably
50,000 RPM, and a load rating of about 100 lbs, and assuming it doesn't rust
away. The answer is that you'll never wear it out.

Gary H. Lucas

On Sat, 26 Jul 2003 18:59:27 GMT, "Alex" wrote:

My question is about life expectancy on following linear slide setup:

Two parallel 5/16 rods and a slide that uses a pair of small plain
ball bearings(5mmx12mmx5mm) on the same
axel with the spacer between them instead of a each single grooved roller.
The
ball bearings will roll on the both side of the rod since the spacer will be
shorter than rod diameter.

It's a preloaded design with preload no more than 10-15lb.
Speed is 1 foot/sec.
It's manually driven device and "high mileage" is not expected.

I understand that radial BB are not made this kind of thrust load but since
it's a relatively light load I think that it should work fine.

What do you think? Is it acceptable compromise?




Y ears ago, and maybe still, Sears radial arm saws used small diameter
rods along each side of the overarm - and used bearings in which the
outer race was concave on the od - then the bearing was adjusted to
proper force against the rod and rolled along front to back along the
rod just fine. It also supported the weight of the motor, blade,
guard and the forces of cutting. Worked well, so maybe one of those
types of ball bearings would work in your instance.
Ken.

with the spacer between them

As the width of the spacer approaches the width of the 5/16 rod, the paraxial
thrust at the POC will approach infinity by trigonometry, balancing the forces.

Bearing life is proportional to P*V, pressure times velocity. Find a derating
factor for off design loads, then keep the spacer thin to prevent multiplying
the forces too much.

At a spacer of zero, the load is all radial.

a pair of small plain
ball bearings(5mmx12mmx5mm) on the same
axel with the spacer between them instead of a each single grooved roller.

Yeah, but it's not exactly the same, is it, because there's this force, pulling
them apart. Now, you can just use a bearing mount type adhesive to glue the
outer races together, giving you a 5x12x10 bearing with a groove, because the
corners have chamfers. That would roll nicely on a square rod's corner, or on
the corner of an angle section.

You put a little pin through them when you are gluing them up to keep them
aligned. A firm press fit pin. Then you never pull, always push, to get them
off the pin, and then onto your axle. If you pull, they'll separate.

It's a preloaded design with preload no more than 10-15lb.

Um, in which direction? The weight of the traveller will load the rollers
tending to separate them. If the preload is inward, and more, and then the
weight increases they will go from being loaded in to being loaded out, with a
wiggle.

For cheap readily available bearings get the 8x15x6mm or whatever it is, which
are sold as skate wheel bearings in packs of eight, I think.

8mm is very close to 5/16.



Yours,

Doug Goncz, Replikon Research, Seven Corners, VA
Unpublished work Copyright 2003 Doug Goncz
Fair use and Usenet distribution without restriction or fee
Civil and criminal penalties for circumvention of any embedded encryption

While this is true for plaster molds, he is just dipping them to give them a
thin layer for loose sand casting (Lost Foam). Also, his mix has sand in
it. (drywall texture).

--

Ron Thompson
On the Beautiful Mississippi Gulf Coast
USA

http://www.plansandprojects.com

Where did everyone go? Oh, yeah.
http://groups.yahoo.com/group/castinghobby/
Y'all come, ya hear?
*******
"alam" wrote in message
...
On Sat, 26 Jul 2003 16:02:32 -0500, "Ron Thompson"
wrote:

Try it and see. An alternate idea would be to use thrust bearings to take
the side load.
There is a web site with a guy making and selling what he calls a Home
Brew
Linear Bearing. Check it out:
http://www.buildyouridea.com/
It is larger than what you describe but maybe...

I was looking at the section on casting and noticed he was using
plaster to cover the foam. Isn't it dangerous to cast aluminum into
a plaster mold? I thought the moisture content of the plaster posed
a steam explosion risk.

I seem to recall a need to bake plaster molds to drive out the heavy
moisture they absorb.