helix angle(s) in external helical gears 4

[zanouk]

Hello all,

I work as an application engineer and I have recently taken over a project regarding the construction of gears in general. So I have only a basic understanding of the subject.
I noticed that in the CAD construction the person who was previously responsible of gears has defined more than just one helix angle, i.e. one for each circle: a helix angle for the tip diameter, base diameter and root diameter.
In the actual construction we use only the helix angle at the root diameter (formula = 180/π*rtan(rootØ/referenceØ)*tanß), while in all the other formulas we use the "regular" ß=20° angle. This helix angle at the root diameter actually determines the path of cutting away material from a cylinder with a diameter = the tip diameter (the path of the tool with which we would obtain the gear).
I have read a bit through DIN 3960, but was unable to find a simple explanation of why it is important to use different ß angles.

Can anyone explain as simple as possible the necessity (if there is a real one) of having a different ß angle for each defining circle?

Thank you in advance.

 

[mfgenggear]

spigor

thanks for the reply
I understand some milling houses try to mill gear teeth but the quality surely is not the same as gear shaping & hobbing. no less gear grinding.
I would have see it for myself I guess.
Most of our customers are expecting mirror finish & master gear quality.

but I concede that the geometry is close. but not exact.

Take Care


Mfgenggear
if it can be built it can be calculated.
if it can be calculated it can be built.

 

[tbuelna]

mfgenggear-

I've attached an image of a spiral bevel gear solid model that was produced using GearTrax 5 or 6 years ago. The model was created with specific adjustments for variables such as backlash and root fillet radius. One thing you'll note from looking at this image is just how smooth the surface geometry appears. I ran a quick analysis of a drive side flank surface and the entire working surface has C2 quality (continuous curvature) and has no deviations beyond the CAD system tolerance limit of 0.000039", which means the surface is very high fidelity. I would be quite happy with any finished gear having accuracy equivalent to this particular CAD model.

For fun I also created two curves on the surface. The green curve is a trace of the pitch radius along the face width, while the pink curve is a trace on the working surface connecting the high/low points of contact at opposite ends of the face width. While this example is a spiral bevel gear (which is conical) rather than a helical gear (which is cylindrical), it still provides an illustration of the situation discussed.

Best regards,
Terry

 

[gearcutter]

Here's a video showing a large spiral bevel gear being hard finished on a vertical machining centre.
While I understand the benefits of using this system for large or 'one off' jobs; to me, it just looks like a way of prematurely wearing out the machine.

http://youtu.be/L1x1TRO36gU

Ron Volmershausen
Brunkerville Engineering
Newcastle Australia

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