DIN standard confusion!

[EhsanFat]

I am using DIN 58045 to calculate contact ratio of some special gear pair. For calculation of tip circle diameter, we have x and y, x is defined as addendum modification factor and y as addendum factor. I have the value of x from drawing, but I do not exactly know what is y(addendum factor). In the standard it has just mentioned for DIN 58400 use y=1.1 and for DIN 867 use y=1.
Does anybody know the difference between addendum modification factor and addendum factor?

 

[Occupant]

Standard according to DIN 867 (for general and heavy uses), as you already know, is y = 1. y > 1 is usually used for gears with α < 20° in the case of DIN 58400 (for fine mechanics) they decided to use y = 1.1 - beats me why. So the OD, in that case would be PD + 2.2*module and not just PD + 2.0*module. "x" on the other hand is the profile shift (you call it "addendum modification") is used to avoid undercut. So in that case for DIN 867 the OD = PD + 2*module*(1+x).

 

[EhsanFat]

Many thanks for your great information. Just in case I am doing this project in university and I have not access to the DIN 867, so could you please provide some other references for using the y value. Furthermore, do you know any method to calculate the evolute of an angle?
After using Addendum modification, angle pressure value changes and DIN 58045 use an equation including evolute of working pressure angle to find it. What is the evolute of angle? What is difference between evolute and involute of an angle?

 

[tbuelna]

Contact ratio of external spur gear meshes does not change much with addendum modifications. But large changes in pressure angle (from say 25deg to 14.5deg) can result in a noticeable change in contact ratio. Both gears in the mesh should always have the same pressure angle.

When appreciable amounts of profile shift (or addendum modification) are used on a gear mesh, and the standard PCD is retained, you need to be careful about interference between the tips/roots of the mating gears.

 

[Occupant]

The evolute of an involute would be the original curve. So therefore, in case of gears it would be a circle. invα = tan(α) - α, in radians, of course. No, I don't know of any other publications were they discuss this addendum factor "y". It was probably done in DIN 58400 to increase the contact ratio and by doing that get a quieter gear train, but as tbuelna already said, the improvement seems only minor.