Spur gears not running on pitch line 2

[mgb23]

Many gear manufacturers say not to do this, but here is my problem.

I have 2 rolls in which paper is nipped between. The rolls need to be driven. We are currently using spur gears to drive them. Both rolls are the same diameter. The distance between the rolls is ajustable. How much can the center distance change before a new set of gears is needed? Is there a general rule of thumb? The rolls are small and the gears have been overdesigned by a actor of at least 5 to 1.

 

[israelkk]

As long as the contact ratio is over 1.0

 

[Philrock]

One of the inherent advantages of involute gears is that they maintain conjugate action (i.e. smooth running) even with changes in center distance.

Two things to watch out for:

Keep the shafts parallel
Don't overload the gears (since you have a 5:1 safety factor at standard center distance, you are probably o.k. in this regard).

 

[Compositepro]

They will work as long as the teeth mesh. Yours is a very common situation and you don't need to do anything special unless there is a high load on these gears (like when something gets into your nip) or the gap adjustment range is large. Then it is sometimes best to use a serpentine chain or belt drive or a four gear arrangement so the gears are alway in full mesh no matter the gap.

The other thing you will want to do is bevel the top of the teeth so that you don't have a problem with the flat tops hitting each other and preventing meshing.

 

[gearcutter]

Minimizing backlash when spreading centres is difficult; which is one of the reasons why the printing industry generally uses a 14.5 degree pressure angle tooth form.

 

[Javier72]

As it is said, your principal problem when changing the distance is the teeth endurance. Be carefully when your contact ratio is near 1.0 . You can have a little deformation in the tooth head because of the load and the contact ratio is going to go down faster than calculated. In this situation, the endurance is going to be reduced drastically because the new tooth to be engaged is going to hit to its opposite one.

To have a lower angle (14'5º) is a good solution for little loads.

 

[mgb23]

OK guys. Looking at the contact ratio now. The calc that I found says to use theta which is the pressure angle. But as the gears come apart the angle of attack changes. The gears are not really applying force at the pressure angle. The calc that I have also takes into account a pinion and a gear. If both are the same gears does it work the same way? Here's the calc:

(Ro^2-Rb^2)+(ro^2-rb^2)-C(sin D)
--------------------------------
Pc(cos D)

Ro=radius of outside diameter
Rb=radius of base circle
C=center distance
D=theta (? doesn't this change)
Pc=circular pitch

Thanks for your help, guys. If there is something easier for identical gears, I'm willing to hear it.

 

[mgb23]

Do you use the angle from the line of action or do you use the pressure angle? I'm pretty sure its angle perpendicular to the line of action since it changes when center distance changes. I put in just the pressure angle and it does not change the contact ratio as much as the line of action angle does. Any thoughts?

 

[unclesyd]

I think we had a similar problem with the production of non-woven fabric and is was resolved using a Slimb Bearing.

Evidentially the manufacturer doesn't have a website and here is all that I could locate on them.

The Slimb bearings are manufactured by Yorkshire Industries, Inc. Andover, Mass., and are of more fully disclosed in U.S. Pat. No. 3399582. ...

 

[Javier72]

Try with this simple formula:

cos A' = Dt/Dr * cos A

Where:

Dt: Theorical pitch distance
Dr: Real distance between gears centers
A: theorical angle
A': Real pitch angle

 

[donkersdesigns]

MGB23
As the others have said, you will be able to pull the gears further apart from each other. The limitation will be the reduction in your contact ratio and backlash. I would not go below 1.0 for a contact ratio.

You can check out my sight, as I have an online gear geometry calculator. This will allow you to input non standard center distances etc. This will show you all the details with respect to the gears as the center distance changes.

Check it out at

http://www.donkersdesigns.com/gear

calculation.html

http://www.donkersdesigns.com

 

[mgb23]

All the gear design manuals I've seen say that to maintain smooth conjugate action between gear teeth a contact ratio should ideally be 1.4 or higher. A 1.2 ratio for extreme circumstances. You guys keep telling me 1.0. I'm working with fairly small gears with coarse 4-5 d.p. (22-27 gear teeth)approx 5.6 pitch dia. I'm thinking I'll have have more problems if I don't try to maintain 1.4 or higher ratio. Any input would be helpful.

Donkers,

When I input the gear data (on your web site) the contact ratio says NaN. Am I doing something wrong?

 

[donkersdesigns]

Hi MGB23,
I agree that most gear books will tell you that a 1.4 or higher. But this is just a rule of thumb. In fact the higher the contact ratio the better if your designing for low noise. Like HCR gears of 2.0 or higher. I've seen many gear designs that are slightly less than 1.2 that have worked for many years. They are usually of the old stub tooth form.

As for inputing the numbers on the website. You may have gotten this return if your center distance or some other value was way off from the norm.
What were the value you put in?

http://www.donkersdesigns.com

 

[Javier72]

These are not small gears. My bigger one has module 1 (d.p 25.4) and the smaller one has a module 0.3 (d.p aproximately 85) and the pinion p.diameter is 4.2 mm.

I was making different tests this year to know exactly the influence of the center distance on the noise. The distance has not an important influence in the noise if you have a correct engagement. Then, if you respect a minimum clearance, and the minimum contact ratio of 1.0 you are not going to have problems without load. When you have load in the gearing then and, because of the deformation in the teeth head, you need to have a bigger one ratio (with 1.2 is enough). Of course, the endurance of the gearing is going to be reduced, but the noise and, with that noise, the smooth contact between gearings, is not going to change very much. In fact, is easier to have noise problems with low clearances than with low contact ratio. Is much more important to have a correct teeth geometry than to have an exact center distance.

 

[JCJackson]

Don't go less than 1.2 CR. A 1.2 minimum CR accounts for geometry errors. The OD has a tolerance, the teeth have tolerences. I've worked in printing engineering for 40+ yrs. The paper will never come thro' the nip at the theoretical thicknes - that is a minimum! So there will be center distance errors as well. The advice above to minimise the pressure angle is good.

Why are you concerned about backlash, it never of itself caused a problem. It my have allowed other problems to develop.