basic spur gears questions

[ediushu]

I have a pinnion of 10 teeth, 48 dia. pitch, 14.5 degree pressure angle, brass material which is driving a brass hear sector of 100 teeth. 48 DP, 14.5 pressure angle as well.
Out of the calculation I got the center between axis of 1.1458 inch. In real I measured a bigger distance: 1.1542 This kind of gearing is used in an actuator and we need to reach a particular number of cycles (over 100,000). The thing is I got failures around 25,000 cycles, the pinnion has teeth chewed-up. That happend using brass material for the pinnion. I switched to bronze material for the pinion and I got over 150,000 cycles. No damaged pinion at all. Still the same distance between axis (1.1542)
My questions are:
1. How much are you allowed to vary the distance between axis and how is this going to affect the meshing if is bigger than the calculated distance.
2. Why a diffrent material acts better (bronze in this case) than the brass knowing that is the same tooth profile for the pinnion. I would like to go with brass material because is cheaper.
3. Could you explain what coefficient of profile means and how you calculate this?

thanks,

ediushu

 

[eg9arh]

in anwser to the first part, i think it depends on how much backlash you can handle, as you move the centres apart the backlash will incerease, upto about 0.3mm this shouldn't be a problem, but the gears will become noisier and pitting resistance will have a greater effect on the gear pair.
The material problem could be to do with the hardness of the pinion wearing out the wheel quicker, but i think it may be because is has a rougher surface. If the gears are running at high speed (input > 1500rpm) the latter is more likely the case, as bronze will have a MINOR lubricating effect, similar to how cast iron uses the carbon to act as a lubricant. No idea about the last part, sorry

 

[israelkk]

Do a search in this forum and you will find previous discussions about "Profile Shift".

 

[diamondjim]

It looks like the pinion has a 40 percent
profile shift to avoid undercutting on the
10 tooth pinion. If so, the od of the pinion
would be (10 plus 2.80)/48. Does this match
what you have? Brass is probably more porous
than the bronze and may help to explain some of
the shorter life.
You might even get better life by using a
60 percent profile shift on the pinion
and a -20 percent profile shift on the gear
and use the 1.1542 center distance.
I assume the pinion is always driving?

 

[ediushu]

Yes, the pinion is always driving in counter-clock wise. To have a better image of this mechanism, the pinion is driving the gear sector turning about 360 degrees, so is meshing with about 13-14 teeth on the gear sector. The return is due to a pair of springs attached to the gear sector which pull it back. So in this case, the gear sector is driving now. The wear out shows on the return side of the motion.
Since I am not an expert in gear design, I am trying to understand how can I play with this profile shift coefficent and when? As I said before I would like to use brass material for the pinion, but if I have to change it to the bronze, I'll do it.
My concern is I don't have the right invoulute spur gear data on the gear sector. This part is a fineblanked part. What is the procedure of verifing these parameters?
Thanks for the suggestion!

ediushu

 

[diamondjim]

You could buy a small rack and check out
the gear and pinion by rolling it against
the rack.
The addendum change is great for the
pinion driving the gear, but when the
reverse takes place as you stated you
are getting approach action and a digging
in action like pushing a stick in front
of you instead of pulling it behind you.
You might be able to provide a slight
radius on the gear tip or tip relief in
the reverse drive side of the gear profile
to reduce the digging in action when it
reverses. This would entail special
assembly instructions to make certain that
the sector is mounted in the correct
direction.