pressure angle in a gearing 2

[ediushu]

I have an existing gearing of a bronze pinnion of 10 teeth, 48 dia. pitch, 14.5 degree pressure angle which is driving a brass gear sector of 100 teeth. 48 DP. I have just dicovered that the pressure angle of the gear is 20 degrees. The RPM of the pinion is 6, so very low and I have not noticed a bad wear-out of the gears in the first 100K cycles but I would like to improve the life of this gearing and it looks that I don't have the perfect meshing using those diffrent pressure angles. It seems to me that having a 20 degree pressure angle on the gear affects the life of the gearing. Why would somebody design a gearing using diffrent pressure angles? Is it possible or was a mistake? If not, what are the consequences (advantages) of using diffrent pressure angles in a gearing?
Thanks!

ediushu

 

[diamondjim]

The pinion may have a long addendum
and thus give you the impression that
it is a 20 degree pressure angle.
A ten tooth pinion would be undercut
and they probably cut the 10 teeth
on a 11 teeth blank which would be my
guess to avoid undercut and also to
improve the recess action of the
gearing plus strengthen the week
pinion member.

 

[ediushu]

Is this a common thing to have a diffrent pressure angle in gears? My understanding is that 14.5 degrees is a common pressure angle when you calculate the gearing in imprerial units and 20 degrees when is in metric. So the meshing still can be possible? I thought that by increasing the pressure angle value, the tooth looks fatter, so on the 20 degrees gear the empty space between teeth can't accomodate the 14.5 degrees tooth pinion. Is my judgment right or am I missing somthing here?
"A ten tooth pinion would be undercut
and they probably cut the 10 teeth
on a 11 teeth blank which would be my
guess to avoid undercut and also to
improve the recess action of the
gearing plus strengthen the week
pinion member."
Could you be more explicit on the above statment because this is something completley new for me. I have noticed that I have undercut on tooth pinion when I generated the tooth profile on the computer and I was wonder how are they going to machine it?
Thanks!

ediushu

 

[diamondjim]

If you increase the addendums you are in
a sense changing the operating pressure
angle or the as cut pressure angle. The
pressure angle at the original pitch line
would still be 14.5 degrees. It is kind
of obvious if there were two theoretical
pressure angle being used, there would be
some real problems with the mesh. So I
assumed that the pinion is still a 14.5
degree pressure angle but cut on a larger
center distance. If you withhold the
rack cutter or pinion type cutter, it
cuts a greater tooth thickness at the
pitch line. This is a common way of
avoiding undercut. For instance if you
have a fixed center distance and the pinion
is undercut, you could use the same od blank
for the pinion but cut 1 less number of
teeth and it would still operate on the
same center distance. You have a sine
verses tangent function error and may have
to withhold the cutter even a little more
to maintain your original backlash.