Helical Gear Hob Runout

[SKinzel]

I have a integral helical pinion that has a larger bearing diameter behind it. I can't find how to calculate how much length beyond my effective tooth length the cutter will need. I would like to minimize/eliminate the hob cutting into the bearing diameter and space is a concern. Does anybody know how to calculate this?

Thanks for any help.

Stuart Kinzel

 

[BillPSU]

You could use a solid model program such as rhino and build yourself a model of the solid part. Draw yourself a cylinder to represent the hob. Cant the hob cylinder at the proper hobbing angle. Move the hob cylinder in the tooth depth and then moving the hob cylinder down axially until it runs into the surface behind.

You could calculate it but the visualizing the geometry would be the most difficult problem.

 

[SKinzel]

I thought about that, but was hoping to not have to make a 3D model. I was thinking that I might just figure the runout as a spur and then use the cosine of the helix angle to approximate the distance along the centerline. It won't be 100% accurate, but it might be close enough

I appreciate the help.

 

[israelkk]

I would sujest you call a gear manufacturer and ask about standard hob diameters for helical gears. For spur gear there is way to use fellow gear machining but I am not sure about helical. You can look at gear catalogs like PIC, SDP, KHK etc. and look for similar gears/pinions to see if they have something close and what are the dimensions.

 

[BillPSU]

Skinzel,
Slice the hob cylinder at the difference between the depth of cut and the larger diameter calculating a rectangle. Slice the large diameter cylinder with same the depth of cut creating another rectangle. Cant the hob rectangle at the hobbing angle. Intersect the 2 retangles and calculate the center line height from intersection points.

 

[EnglishMuffin]

It depends partly on knowing the hob details, because hobs have a lead-in at the end before you get to the full tooth form section. If you know the length of that lead-in, you can figure out how much "offset" the hob will stand, assuming that you want to minimize the distance from the end of the tooth to the shoulder. Then lay out the hob cylinder on the part in plan and elevation - I've done this in 2D CAD several times - not too difficult. Even easier with 3D.

 

[SKinzel]

Thanks everybody for the tips. I ended up changing the gear geometry & heat treat to give me a better overall design and the runout became less critical in the process. I don't know why I didn't make the changes earlier, but I've been through so many iterations on the gearing that I've lost count. I think its turned into a "can't see the forest for the trees" situation now %-)

 

[hobmonger]

Contact Scott Atkinson at a company called Ash Gear.
They sell tools and he does calculations like that every day.
I sure he would walk you through it.
scott@ashgear.com or 248 374 6155
-dwr