Internal Gear Pinion Profile Shift 2

[hugou]

Wonder if any of the gear experts out there could offer me some advice. I have in internal ring gear module 4 25° pressure angle with 444 teeth and a profile shift of -2.
I want to run a pinion with 37 teeth (12:1).
Now, my initial thoughts were to use a profile shift of +2 on the pinion to keep standard centre distance and pressure angle but looking at my turegear.lsp generated tooth profile that is too much profile shift and results in the involutes actually crossing before the tip of the tooth (reducing the addendum and making a very pointed tooth). So if this is not possible, how do I decide the ‘correct’ amount of profile shift for the pinion?
If someone can recommend a figure and give a brief explanation why this figure was chosen I would be most grateful (I appreciate the technicalities of sliding etc. may be too much to explain in simple terms, in which case I’ll just settle for the figure , thanks).
I think I should be able to work out the correct centre distance for the shifted gears as I think I understand the process of calculating the involute function and working pressure angle to establish the centre distance.
Many thanks.

 

[Occupant]

Why do you have to shift it at all since you say, you can adjust the C.D.?

 

[gearcutter]

How did you come up with xG = -2?
This seems too large a shift.
I'm not surprised to hear that the 25deg PA pinion has ended up with pointed teeth as a result of xP = +2.
Are you sure you don't mean xG = -0.2 & xP = +0.2?
This would yeild a far more realistic result.

Ron Volmershausen
Brunkerville Engineering
Newcastle Australia

http://www.aussieweb.com.au/email.aspx?id=1194181

 

[dinjin]

If you are trying to maintain a standard center distance, I will assume you will make the same profile shift to the gear and pinion. A realistic
profile shift might be .25 times the module often called long and short addendum system or recess action gearing. Are you shifting profile to get better strength? I am surprised at the 25 degree pressure angle as well. Is that a standard in your shop and do you have plenty of 25 degree pressure angle tooling?

 

[hugou]

The internal ring gear is an existing part and is manufactured with a profile shift/addendum modification of -2 (at least this is what it says on the drawing). Apparently it is quite common to use a negative profile shift on an internal gear as it inproves the tooth profile (just what I've read).
I can make the center distance for the pinion what ever I like.
I did think that if I made the profile shift on the pinion inversly proportional to the profile shift on the internal ring gear it would keep things simple in that the gears would run at standard centre distance and pressure angle would remain standard.
I think I may have found the reason this sounds like such a huge profile shift... I had assumed, and it seems others here also, that the drawing is referring to profile shift coefficient. Looking at some other drawings I have, some seem to be quoted as a coefficient and some as a liniar dimension in mm.
How can I convert a liniar shift in mm to a coeficient?
Also on the basis that the ring gear may actually have a lot less negative shift than I had at first thought, would it still be a reasonable idea to make the pinion with inversly proportional shift or cut it with zero shift and calculate the center distance accordingly?
These gears are to run dry and the pinion will be transmitting almost no torque (it's only driving an encoder).
Thanks

 

[hugou]

To answer a couple of specific points people have raised:

Occupant, I don’t have to apply shift to the pinion but as the ring gear has a negative shift I need to decide whether it is appropriate/of any benefit to apply a positive shift to the pinion.

Gearcutter, the drawing states xm2 = -2 I thought this was the coefficient but it is also shown as a linear dimension on the cross section view of the drawing.

Dinjin, yes, I was a bit surprised by the 25° pressure angle too, again this is just what is called up on the drawing for the existing internal ring gear. For clarity we are not the manufacturer I am just designing a compatible gear to drive an encoder off the existing ring gear.

Thanks.

 

[hugou]

Am I being daft? Do you just divide the linear addendum modification by the module to get the addendum modification coefficiant?
So in this case the coefficient is actually -0.5 ???
Thanks.

 

[gearcutter]

Can you post any detail from the drawing showing the internal minor diameter and the internal base tangent length (M dimension) or even perhaps a dimension between pins of a specific size?

It is not at all unusual for ring gears to be 25deg PA. Apart from the added bending strength; if we were talking about a planetary set, a higher pressure angle can also help to keep the floating members in the set centralized while under load.
All of the planetary gear components we make for the mining and steel making industries are all 25deg PA.

Ron Volmershausen
Brunkerville Engineering
Newcastle Australia

http://www.aussieweb.com.au/email.aspx?id=1194181

 

[hugou]

I can't post the drawing unfortunately (it would be against our customer/supplier confidentiality agreements) but the only other dimensions pertaining to the gear are the internal diameter (to the top of the teeth) of 1756mm +0.4/-0 and the PCD of 1760mm. I doubt the I/D tells you a lot as it could be enlarged for clearance reasons.
The only other thing on there I have not already mentioned is that the basic rack profile is to DIN867.
The dimension xm=2 is shown radially on a cross section on top of the PCD. Then, in a seperate box with the module (4), tooth count (440), pressure angle (25°), PCD (1760) and addendum modification (-2).
What do you think to my above post about the linear dimension converting to a coefficient? I have another drawing (differant supplier) which shows addendum modification: 2.4mm and addendum modification coeffient: 0.6
This is what lead me to believe the coefficient can be calculated by dividing by the module?
Thanks.

 

[dinjin]

If your module is 4 then xm=2 suggests
x = .50 which would be the coefficient
for the profile shift. Since you do not need the strength, I would suggest using the smaller pressure angle and use a .50 profile shift on both parts and use the standard center distance. I think the smaller pressure angle would operate more smoothly. The .50 coefficient suggest that the user was trying to create a recess action gearing design. When the pinion is always the driver, this seems to work well and helps to avoid involute interference in the root of the driving pinion.

 

[hugou]

That makes sense.
Yes, the ring gear is diven by a pinion (two pinions in a pair in fact). I'm meerly trying to add a seperate pinion to drive this encoder as it's not possible to drive it from one of the existing pinions due to the shape to the drive motor gearbox.
Thanks.

 

[Occupant]

Something doesn't make sense to me. You are saying Ring Gear Modul=4, 444 Teeth and also give the PD as 1760. That doesn't go together 440 Teeth is more likely. Normal ID of that gear would be 1760 - 2*m = 1752. To that you'll have to add twice the profile shift 1752 + 2*x*m = 1756 - to get to your number. Therefore x = -0.5.

 

[hugou]

Sorry for the confusion that was a typo, yes it is indeed 440 teeth!

 

[israelkk]

hugou

Am I correct that you are trying to use the ring to drive the pinion which is attached to an encoder? If so, then the profile shift of the ring makes it more recess action and this is not favorable for a gear to be used as the driver. You may experience an erratic and non smooth movement of the pinion.

 

[hugou]

Yes, that's exactly what I'm trying to do. The body of the encoder will be fixed and the pinion on the shaft of the encoder will be turned as the ring gear rotates.

What you are saying about the recess gearing does not sound good from the point of view of what I'm trying to achieve but what you and dinjin have said makes sense as the primary function of the ring gear is to be driven by a pair of pinions on the bottom of a fixed motor in order to rotate a large assembly attached to the ring gear.

As the encoder pinion doesn't have to transmit any torque is there anything I can do with the design of the encoder pinion (with profile shift) to minimise the smoothness problems you are describing?

Thanks.

 

[gearcutter]

There is something wrong with the gear data for the ring gear.
It appears as if the drawing has been put together by an inexperienced designer who has no knowledge of the fundamentals of basic gear design.
If you were to ask me to manufacture the ring gear I would simply tell you that it can’t be done based on the lack of information.
You are going to have to get hold of either the measurement between pins, the M dimension or a chordal tooth thickness.
Without this data there can no guarantee that any pinion you make will correctly mesh with the gear.
Because of the lack of this basic information; we're all just guessing at what it is that is required.
You need to push your customer for this vital information or get hold of a sample for you to measure yourself.



Ron Volmershausen
Brunkerville Engineering
Newcastle Australia

http://www.aussieweb.com.au/email.aspx?id=1194181