Re: Need help to decide what rack and pinion to use in a 6'x10' table

--- In CAD_CAM_EDM_DRO@yahoogroups.com, "Hector"
<hector_a_quintanilla@...> wrote:

>
> Hello,
> Im confused on which combination of rack and pinion to use for a
> 6'x10' table.
> I know that there are three steps to solve.
> 1.- which presure angle (PA) to choose, 14.5 PA or 20 PA? I know the
> 20 PA will hold more load than 14.5 PA, but how much is more? does
> this size of table really benefits using 20 PA? what is the
> important facts to know to make this decision?

Look at your forces and see what requirements you have first. if you
have a 100 pound gantry, the forces of operation are only a fraction
of the rating.

Look at suppliers and find what is available. you may find only
certain sizes, or one size is a better value.

> 2.- after knowing what pressure angle will be used, the next step is
> to decide the diametral pitch to use. So what is necesarry to
> consider in this decision?

Again, find what is readily available. I found that there is a
supplier who sells stock sizes that are roughly square and in some
proportion to the number of teeth. I could not find a 32 pitch rack
that was 3/4" wide and a reasonable cost.

> 3.- In this step we already know the PA and DP we are going to use,
> now comes to decide how many teeth on the pinion, so what governs
> this decision?
> Is hard to find some 20 PA conbinations i been suggested earlier. So
> if this questions are really hard to explain,

I think this is the easiest to answer. dp is how many inches in a 1"
diameter. so, getting any rack pitch where the tooth count is the
same as the pitch, the spur gear is 1" dia and will move pi inches in
one revolution. (3.14159 inches)

AND, getting a tooth that is the same tooth count as the pitch will
result in a very tight gaps to he point of the gear and rack having a
little bit of a rough movement. A spur gear tooth count that is 1.25
to 1.5 times the pitch will offer a noticeably smoother movement.

general comments;

Of course you would want to design with the ability to swap out spur
gears, so leave mechanical movement for the range of gears you would
want to try using.

Consider the rack mounting requirements. If you want to bolt the
rack, it will need holes in it. either drilled and tapped, or
thru-holes. and of sufficient size for mounting while leaving enough
strength in the remaining rack.

I could not find the link for the rack supplier, but McMaster Carr is
the most common spur gear supplier.

Martin http://www.martinsprocket.com/gears.htm#

has an on-line catalogue with some hp ratings for different gears
widths. that may be of some help.

Dave