## Interference equation valid for internal gear?

## Interference equation valid for internal gear?

(OP)

I'm designing and helical gear set made of and internal gear and a small pinion gear. I have an equation to know if there is interference between gears but this was developped for regular spur gear.

To use it for helical gear I just have to use the transversal parameters but I don't know if it still works for internal gears.

N1=number of pinion teeth

N2=number of large gear teeth

p=pressure angle

to avoid interference, equation must be greater or equal to 0:

(N1**2+2*N1*N2)*sin**2(p)-4*N2-4

Robin B

Mech eng

Montréal, Canada

To use it for helical gear I just have to use the transversal parameters but I don't know if it still works for internal gears.

N1=number of pinion teeth

N2=number of large gear teeth

p=pressure angle

to avoid interference, equation must be greater or equal to 0:

(N1**2+2*N1*N2)*sin**2(p)-4*N2-4

Robin B

Mech eng

Montréal, Canada

## RE: Interference equation valid for internal gear?

I do not know your formula (where it came from?) but if it works for spur gears it also has to be good for helical gears if you use the transverse pressure angle in it. Here is why:

You can think of a helical gear as if it were created from very thin spur gears (with the pressure angle = transverse pressure angle of the helical gear), slightly twisted against each other (by the helical angle) and "glued" together.

If none of these thin spur gears interferes (checked by your formula, I assume that it is correct), then none of them does - therefore no interference...

gearguru

Could you post some more data?

## RE: Interference equation valid for internal gear?

The development for the equation is in a french book but I could scan it.

Do you Know any gear cutter in North America that can cut small internal helical gear? (64dp 325 teeth). I've just got one.

## RE: Interference equation valid for internal gear?

I apologize. The internal gear/pinion should be checked for interference, especially if the numbers of teeth differ just a little; the external gears are checked (sometimes, if the pinion has small no. of teeth - it depends also on the pressure angle) for undercut.

There are three equations for checking the internal gear/pinion for the interference.

For the info about the gear manufacturers look at

www.geartechnology.com

gearguru

## RE: Interference equation valid for internal gear?

and gear data to calculate

parameters. You might want to

use a 25 percent long addendum

on the pinion and a 25 percent

short addendum on the gear. Just

good internal gear practice.

Or use 50 percent long on pinion and

none on the internal gear.

This works well in most cases as well.

Mike Bull has a spread sheet free if

you check out his website. It is an

excell spread sheet, but you do have

to know general gear design parameters.

I have the fairfield gear design program

and find in invaluable especially to

calculate helical gears. It would have to

be a specialty house to cut such a fine

diametral pitch gear. Would be glad to fax

you an output of the fairfield gear program.

e-mail J.GEISEY@juno.com I would think you

would have someone up there who could cut

a module system on the same center distance.

Does it have to be a 64dp system?

## RE: Interference equation valid for internal gear?

We want to find a gear cutter that can make a gear for us. Right now we have a gear set (64 D.P., 14.5 degrees, agma 10) composed of a small pinion in SS 303 (18 teeth) and an internal gear (325 teeth) made of plastic (Ertalyte). Sound and backlash are a BIG issue (there is no load beside a 6in Kaydon bearing) so we plan to try helical gear to drop the sound at a minimum level.

Backlash was proven enough small with the spur gear set but sound is still too high even with tight tolerances. Next step is then helical gear and if you are interested in that small project we can exchange a little bit more and I can send 3D/2D file. Need is immediate and a quotation for a prototype and a prototype would follow.

I've searched for a while to find a gear cutter that can do it but it seems hard to find. If you cannot cut it, can you redirect me to another that could do it?

Thank

Robin Boudreault

Mech ing.

Complement information: Pressure angle was defined to 14.5 degrees (instead of 20) to minimise backlash related to center-to-center distance error but could go to 20 degree (sacrifying backlash for smoothness of operation). Quick research has shown that helix angle decreases sound as the helix angle reach 30 degrees, which seems to be the best angle (over that it does not decrease the sound).

Direction doesn't matter since there is almost no load. My only concern is having a ratio of 18 to 14 (right now the internal gear has 325 teeth and the pinion has 18 teeth, giving a ratio of 18.06). Gear could be metric or english, could have and helical angle between 15 and 30 degree and a pitch angle of 14.5 to 20 degree. Center to center distance is not imposed by the design. All those parameter must be

validated since the system is placed inside a really compact case filled with a lot of stuff.

## RE: Interference equation valid for internal gear?

try to post your requirements on the Gear Industry's forum:

www.geartechnology.com/hot/qanda.htm

Good luck!

gearguru

## RE: Interference equation valid for internal gear?

Is the bearing a slimline syle bearing?

Is the Ertalyte bonded to the inner

race id? Reinforced plastic with canvas

mesh? Was this plastic cut or molded?

You implied that you had an external spur

gear set and it was too noisy.

What kind of speeds are you talking about?

Just from preliminary calculation, the contact

ratios are 1.8 and larger which means that

you are having at least 2 teeth in contact

80 percent of the mesh cycle. How is this

lubed? The greater the helix angle, the greater

thrust load is imposed on the bearings and I assume

greater turning torque.

The modified pinion had a tooth thickness at the tip of .006, i.e. almost pointed. Will run some other

calcs to improve the tip condition.

Feel free to call me tomorrow. 330-562-4000 x238

Cleveland Ohio area.

## RE: Interference equation valid for internal gear?

I downloaded the Fairfield software and played with it but I often get a message saying that the sap diameter is less the base diameter. Is that the signal for undercutting problem? I'm just exploring the software. I'll continue the design and if you know any people that could cut it I would send them a quotation request.

The hardest part of the problem is to have a small pinion to keep the transmission ratio around 18 while maintaining the internal gear pitch diameter around 5.

Thanks for your time, I'mm following this thread with interest and like to design gears.

Robin

Amphitech.com

## RE: Interference equation valid for internal gear?

Glad you have the fairfield program.

When you get that the sap is smaller than the

base diameter, you should enlarge the pinion

say starting at 30 and -30 on the internal

gear. What makes me nervous is the 64 dp

gear only has a working depth of .031 meshing.

A 1 module would be about .079 inches in mesh

but you would have fewer teeth in each member.

I have a hard time visualizing these small

gears. I do not enter the tooth thicknesses

and only enter the percent addendum modification

and the min and max backlash for the system.

Then check the results. You can also stub

the addendum on the internal gear if you had

more whole depth. I like to see the sap angle

about 8 degrees to reduce the sliding velocity

at this critical area. When you start out with

only .031 working depth, you cannot do much other

than change the long and short addendum ratios

and watch that pinion teeth do not become too

thin at the o.d. I am a strong believer in

the recess action that long and short addendum

offer. They run more efficiently and probably

quieter. We manufacture the big stuff and only

get down to around 10 inch inside diameters.

We do not cut helicals in house and send these

out. We have not designed anything this small.

Only know principals after doing much of the

calcs by hand until fairfield offered their program

when we were working together on a joint effort.

I see in catalogues .5 .75 and .8 module pinions

on the shelf and was quite surprised to find them

in the catalogues. I know nothing about these and

the only helical that they offere was 45 degree helical.

Sounds like an exciting project and wish I could

be of some help other than offering some design

guidance. Please keep us posted on your success.

You will also note that the higher helical angles

reduce your contact ratios.

## RE: Interference equation valid for internal gear?

I'm not limited to 64 pitch (me too found it really small) but I'll give you my design guidelines:

1-Internal gear should have 5 inches pitch diameter

2-System has no load and must be as quiet as possible (this is why I use plastic internal gear.

3-Backlash should not be to high but sound is a bigger issue than backlash

4-Pinion pitch diameter should be as low as possible, around 0.280" (here is why it was a small pitch of 64) to get a pinion/gear ratio of around 18.

5-Gear width is around 0.125"

6-Reading shows that an helix angle greater than 30 degrees does not yield in a quieter system so 30 degrees is a maximum.

I will play with the parameters and come back.

## RE: Interference equation valid for internal gear?

## RE: Interference equation valid for internal gear?

Pinion and internal gear: pressure angle 20, helix angle 30, diametral normal pitch of 48, no addendum or dedendum modification.

pinion: 12 teeth

internal gear of 210 teeth

From the run, I do not have any sap problem and slip ratio are between 0 and 1. contact ratio of 1.4. How do I know that I don't have undercutting and that my design will work?

Robin

P.s. The program keeps crashing if I do not select the expand sig fig but it works great.

p.p.s. I got the same addendum height for two different pitch, isn't the addendum supposed to be 1/P so different for different pitches?

## RE: Interference equation valid for internal gear?

The addendums should be different

if you are holding the helical angle

and only changing the diametral pitch.

I do not know why you do not use an

addendum modification for the pinion.

SAP is just another term for the lowest

point of single tooth contact and means

start of active profile, i.e. from this

point to the o.d. is the working portion

of the tooth. To have such a low contact

ratio, you must be using a high helical

angle. Just for you own discovery, change

the helix by 5 degree increments and check

out the differences in contact ratios as

well as center distances.

As you mentioned 1/p is the normal addendum

formula for spur gear. Helical gears are

reduced by a function of the helical angle.

If you are getting the same addendums for

diffent diametral pitch gears, I would exit

the program and come back into it and reenter

the data.

it.

## RE: Interference equation valid for internal gear?

Pardal

## RE: Interference equation valid for internal gear?

I do not have space outside the turning internal gear so the driving device must be inside.

Robin

## RE: Interference equation valid for internal gear?

"and if you are interested in that small project we can exchange a little bit more and I can send 3D/2D file."

Could you send it to

k281969@hotmail.com

Other alternaltive is to use a 2 step reducer by timing belts.

I will be pleasured if I can help you.

Pardal

Pardal

## RE: Interference equation valid for internal gear?

## RE: Interference equation valid for internal gear?

Hope I can help other time.

Pardal

Pardal

## RE: Interference equation valid for internal gear?

I think that it could be a cheaper solution.

k281969@hotmail.com

Pardal

Pardal

## RE: Interference equation valid for internal gear?

This is a pair composed of a small pinion and an internal gear: (Fairfield run is available on request)

DP: 48

pressure angle: 20

Helix angle: 30 (right angle)

Pinion: 12 teeth, addendum offset 50%

internal gear: 213 teeth, no offset.

there is no load on the gear and the only concert is noise and vibration.

Any suggestion?