Spur Gear and Helical Gears

That's a big subject, Naveen.

What will you be doing with your models? Will they be passed to manufacturing? Are you doing analysis with them? Or are they simply a placeholder for getting a good model of a transmission?

Most gear manufacturers have excellent data available. Not to the level of power transmission, but at least the basics of gear design and manufacturing.

I really like the handbooks provided by SDP-SI. http://www.sdp-si.com/Catalogs.htm

They have some download models in Pro/E format that are generated on the fly. They will help but do not count on them for being "gospel".

Also be very careful if you do need precise gears. Hobs are different from Creo tools. I have not seen anything in Creo that does an accurate representation of "round" cutting tools. It makes a difference if you just sweep flat sections in many cases. For instance, the involute of a gear tooth is actually the result of the manufacturing process but we have to work hard to define it. Ground threads, for instance, also creates a different profile than one would think.

Congratulations on your new job, Naveen.

As Antonius says, the intended use of your gear model is important.

For technical drawings used for getting gears fabricated, you normally do not need an accurate model as long as you specify all the needed information the fabricator needs. I.ex for paralel spur gears; Modul size, number of teeth, pressure angle, degree of accuracy and so on...

However, for good fun, I have started on making a parametric model of a straight/parallel spur gear / pinion that have a true involute tooth profile. This is based on the modul system, and for now it look quite good. I was inspired by this document: http://www.cadquest.com/books/pdf/gears.pdf

I have planes for adding correction / profile shifting to this also....

As a gearbox/transmission designer, I agree with Elvind that for most purposes it's not necessary to model the teeth accurately, as the form is generated by the cutting machines, and they take the gear data in parametric form in any case. Most of our gear models just have an arc to represent the flanks - it looks good enough, is easy to model and regenerates quickly.

Gear calculation is best done by purpose-written software (such as Ricardo SABR-GEAR: http://www.ricardo.com/en-GB/What-we-do/Software/Products/SABR/Gear-design/ </shameless plug> ).

Having said that, there have been a few occasions where I've wanted an accurate model to run FEA on. I've found that the simplest and most flexible method for spur gears is to use an equation-driven curve to generate a single involute, which requires just the base circle diameter. You can then take the over-pins dimension to generate a sketch making the pin tangent to the flank, mirror the involute using a plane through the centre of the pin, and then use another sketch to create the root fillet tangent to the root diameter.

Having said that, this assumes that a full-fillet root is a sufficient representation (strictly speaking the root should be a trochoid); and I've yet to create a helical gear successfully - controlling the normal direction and rotation of the section correctly as it sweeps along the helix is difficult to say the least.