View Full Version : Gears

Fasttrack

11-02-2005, 07:47 PM

Sorry posting this again...i am sure i read a post about it earlier but i couldnt find it...

Can someone tell me how to determine pressure angle and pitch on a miter gear?

speedy

11-02-2005, 08:10 PM

Seek and so shall ye find sayeth the Lord

http://bbs.homeshopmachinist.net//Forum1/HTML/013429.html

http://bbs.homeshopmachinist.net//Forum1/HTML/012616.html

or I will look for you http://bbs.homeshopmachinist.net//biggrin.gif

Ken

Fasttrack

11-02-2005, 09:56 PM

Thanks! Somehow i did a search and couldn't find anything...wonder what i did wrong...oh well! Thanks alot!

Only one problem...the gears are miter gears from a differential. Does that change things? It sure makes the geometry of the teeth funky! What else, besides pressure angle would be needed for a miter gear?

According to a book I have (Elements of Mechanism) the important measurements for a bevel gear are:

Root diameter

Pitch diameter

Outside diameter

Back cone distance

Bevel gear nomenclature is:

Pitch cone = The imaginary cone on which the teeth are made

Cone distance = The length of the side of the pitch cone

Pitch point = The point of tangency of the pitch cones at the large ends

Cone angle = the angle which an element of the pitch cone makes with the axis of rotation of the gear

Face angle = angle which the top surface of the tooth makes with the axis of rotation

Addendum angle = angle which the top surface of the tooth makes with the cone distance

Root angle = angle which the bottom of the tooth makes with the axis of rotation

Dedendum angle = angle which the bottom surface of the tooth makes with the cone distance

Back cone distance = distance along the element of the back cone from the pitch point to the axis (center line) of the gear

Virtual number of teeth = number of teeth contained in a spur gear whose pitch radius is equal to the back cone distance and whose pitch is the same as that of the bevel gear. Since it is imaginary it may be fractional.

I can post a drawing of this for you tommorrow if you wish.

Fasttrack

11-03-2005, 08:12 PM

A picture would be great! Thanks alot!

I'll put it on in the morning.

Here you are. This is from the book Elements of Mechanism, published originally in 1904 by John Wiley and Sons and authored by Peter Schwamb, Allyne L. Merril and later editions with Walter H. James. It deals in detail with all the sorts of mechanisms found in industrial machinery with emphasis on their use in the sorts of machines found in shops as well as dealing with the practicality of constructing such mechanisms.

It is an excellent book for anyone designing machinery and is still available used. It is fairly heavy on the math involved but even without the math it is still useful.

This is from the very complete chapter on gear design.

http://vts.bc.ca/pics/bevel2.gif

http://vts.bc.ca/pics/bevel1.gif

http://vts.bc.ca/pics/bevel3.gif

http://vts.bc.ca/pics/bevel4.gif

http://vts.bc.ca/pics/bevel5.gif

[This message has been edited by Evan (edited 11-04-2005).]

Georgeo

11-04-2005, 10:59 AM

You said these bevel gears are from a differential? In that case I think they are probably Hypoid gears, not regular bevels. Hypoids are used where the centerlines of the shafts involved are perpendicular, but do not intersect.(To visulize,think worm and worm gear, but with the teeth on the side of the gear, not the rim) The teeth are curved in a strange way, something like a spiral bevel gear, but not the same at all. A good gear text would discuss these.

Hope this helps a little.,

George

There are many variations that the above book deals with including crown gears, spiral bevel, skew bevel, helical, hyperboloidal, internal bevel, screw gears and hypoid gears.

Fasttrack

11-04-2005, 03:07 PM

Thanks so much everyone! They are just oridinary bevel spur gears; i neglected to mention that this is a differential from a rinding lawn mower...nothing fancy!

Luckily i found a replacement (with a few modifications anyway...should be fun...get to use my lathe some more!!) from my school. Best thing is, it was free!

Thanks again