Re: [CAD_CAM_EDM_DRO] DETERMINING THRUST BEARING LOAD REQUIREMENTS ?

The thrust loads in normal operation consist of cutting loads,
acceleration loads, friction loads, and shock loads.

For cutting and friction loads you can make a guestimate by measuring
the force on the handle when cutting manually. You can do this
with a fish scale or just guess.

Turn this into torque by multiplying by the distance the
crank handle is from the centerline.

The force is just 2*3.14* torque/screw lead.

Example: You find that you apply a radial force of 10 oz on
the handle when manually machining. The handle is 3" offset from
the leadscrew centerline.

Then torque= 10 oz * 3 inches = 30 0z in

Let's say the screw has 5 turns per inch. That is a lead
of 0.2 in / rev.

So force is 2*3.14*30/.2= 942 0z or 59 lb!

That covers cutting and friction.

Now acceleration.

How much is the moving mass and how fast do you want it to start and stop?

The force for this is simply mass * acceleration.

So if the mass is 20 kg and you
want to start and stop at .1g (1 meter per sec per sec) the force
from that is 20*1= 20 n or about 5 pounds. Sorry for switching to metric
but I don't want to confuse the issue with slugs or lb mass.
Anyway small enough to ignore in this case.

Now finally leadscrews and bearings. To handle the shock and vibration
we would like to keep the bearing load to 10% of the maximum rating.
Loaded like this it will last almost forever if clean.

Let's look at a standard r4 .25" bore bearing. It can handle about
200 lb thrust by dynamic equivalent radial load calculations in
C0 standard internal clearance. That is good enough for the forces
we calculated but violates the 10% rule for shock and vibration.

Ok lets try an R4A. Still .25" bore but bigger o.d. About 600 lb thrust
load capacity.
That will do it as long as it is moving! (for the example numbers)
The static ratings are much lower, but it has to move to crash into
something. Some crashes that stop the drive might damage the
bearings by brinneling, but as I said these are $3 bearings!

Now the 12" ballscrew... can it handle the forces without buckling
or whipping?

The table I have here says about 600 lb with one end fixed
(two spaced bearings) and one supported (one bearing free to slide a little)
So, for the example numbers you are ok!

Critical speed is not a problem.

Now this analysis is a very simplified one but enough to get you
there as far as ballpark stuff.

Whether the ballscrews and/or ordinary deep groove bearings will work
depends on your particular machines numbers.


Les




Leslie Watts
L M Watts Furniture
Tiger, Georgia USA
http://www.alltel.net/~leswatts/wattsfurniturewp.html
engineering page:
http://www.alltel.net/~leswatts/shop.html

----- Original Message -----
From: "Kayla Sanders" <tattoos4unme@...>
To: <CAD_CAM_EDM_DRO@yahoogroups.com>
Sent: Wednesday, October 02, 2002 1:48 PM
Subject: [CAD_CAM_EDM_DRO] DETERMINING THRUST BEARING LOAD REQUIREMENTS ?


> HOW DO I DETERMINE THE BEARING THRUST LOAD REQUIREMENTS FOR MY
> GRIZZLY MINI MILL??