Re: [CAD_CAM_EDM_DRO] Leadscrew end thrust calcs for Max mills?

Jon:

At 04:57 PM 5/11/01 -0700, you wrote:

>I'm working on a thrust plate/motor mount for the MaxNC. Going to be
>doing some testing on a bearing, and really don't know how to calc the
>actual thrust loads.

I may not understand what you are doing, but that usually doesn't stop me
from commenting. :-)

I think a reasonable approximation of thrust loading on the table can be
obtained from the spindle horsepower. I just ran a quick calc and I get:

F = 33000 (P/V)

where: F = Cutting Force (table force) (LB)
P = Spindle Power (HP)
V = Cutting Speed (ft/min)

The slower the cutting speed the higher the force. So a 2 HP spindle
cutting mild steel with HSS (100 ft/min) would produce a table force of 660
lb. For harder alloy steels (say 50 ft/min) it would be 1320 lb. This
would be using the 2 HP of the spindle which would be the upper limit and
unusual. It also doesn't take into account friction, etc. but I think for
a design number (plus some factor of safety) it would be OK.

>I'm assuming a worst case of about a 210 oz/in motor and 1200 rpm max
>speed (knowing torque at that rpm won't be that high). I plan to make a
>test rig and run the bearings in a vertical direction using a motor to
>spin the inner race, and hanging a weight from the fixed outer race. I'm
>looking for a ballpark number in actual pounds, that would at least
>equal a worst case load. If some of the servo setups some folks are
>working on using real servo motors and Gecko drives run at a faster rpm,
>I'd like to know about that too!
>(Will also be doing some testing for succeptability to brinelling to
>account for unplanned crashes)

The manufacturers data should have load values for a given bearing. This
needs to be corrected for machine life but this calc should be explained in
the bearing manufacturers literature. Also, you probably need to convert
axial (thrust) load (and any radial loading) to an equivalent radial
loading to get into their tables. I don't think speed is a consideration,
at the speeds were looking at, but bearing selection will depend on loads
and number of revolutions. The static load rating should also be available
which is the max load to avoid brinelling.

I suspect reviewing some of the bearing manufacturers technical literature
would save you doing this test. It is a little hard to dig through but you
might want to lood at www.skfusa.com. They require you to register but I
haven't gotten much spam from doing so. :-)

But then again, I may have miss understood your thinking. If not, I hope
this is of some use.

Hugh Currin
Klamath Falls, OR