FELLOWS STARTING POINT?? 2

[smwdrum]

Does anyone know how to calculate Fellows Starting Point and total Roll for a spline shaping operation?

 

[mfgenggear]

smw

what are to trying to achieve.
do you mean a fellows involute checker.
do you mean the degrees of roll from the TIF.
there is a simple formula I will post as soon as I get free today.

Mfgenggear

 

[smwdrum]

yes I was looking how to calcuate the fellows starting point at t.i.f., p.d., and i.d. as well as total roll?

 

[smwdrum]

I am looking into the Machinery Handbook and it only lists certain things here. Looks like it is not everything, do instance addendum and dedenbum of a involute spline. Is that different that for a regular gear? Is there a good reference besides the machinery handbook that would be good to have for any of these types of formulas?

 

[mfgenggear]

here it is
Good Luck

Mfgenggear

 

[mfgenggear]

yes I like Manual of Gear design, Spur & Internal Gears, by Earle Buckingham, Eliot Buckingham, & of course Dudley, Vogul and more.

Good Luck
Mfgenggear

 

[smwdrum]

Thats for the help!! Also can you use the same formula to find the Total Roll?

 

[mfgenggear]

yes, input the major dia to calculate the degrees of roll.
then calc out the degrees of roll at the TIF. then just subtract the difference.
that would be the total involute roll.
The formula will work at any dia, as long as it is not below the Base Diameter.

HTH
Mfgenggear

 

[smwdrum]

One more question since you are answering me quickly. T.I.F dia, I don't see the formula for that either?

 

[mfgenggear]

smw

This is a little more complicated. when a pair of spur gears are assembled in a set
the major dia of the mating gears must clear. the non involute areas.
the (total or true Involute form) TIF or Involute Form Diameter is set or calculated by the mating gear the major dia of the mating gear must not touch below the TIF. the TIF is given as maximum diameter on the ext spur gear blue print data.
on the formula I gave you if you know the degrees of roll at the TIF you can calculate back to a dia.

I would suggest you buy the books I mentioned about and educate yourself about the subject.


HTH
Mfgenggear

 

[smwdrum]

thanks for the help!!

 

[gearcutter]

Nice explanation and calcs sheet mfgenggear.

 

[gearguru]

Here is a link to an article about the TIF calculation:
Gear Technology magazine has the old issues all digitized, free to download, on their web page. A lot of valuable info there.

http://www.geartechnology.com/issues/0999/Thurman.pdf

 

[tbuelna]

This is a little more complicated. when a pair of spur gears are assembled in a set the major dia of the mating gears must clear. the non involute areas. the (total or true Involute form) TIF or Involute Form Diameter is set or calculated by the mating gear the major dia of the mating gear must not touch below the TIF. the TIF is given as maximum diameter on the ext spur gear blue print data. on the formula I gave you if you know the degrees of roll at the TIF you can calculate back to a dia.

Just one minor comment. I believe what you are talking about is the start and end of active profile. As noted for inspection purposes, unless otherwise stated the start of active profile is defined at the form diameter and the end of active profile is defined at the OD, in degrees of roll. However, in many cases the gear profile is modified with tip relief and tip edge breaks, so the active profile will end a bit before the OD. The profile chart will define the location of the start and end of the tip profile modification and the limit of the tip edge break. The transition from the active profile to the tip profile modification is critical and must be smooth. But the transition from the end of tip modification to the start of tip edge break is not as critical. In fact with gears that are case hardened and finish ground, the tip edge break is usually made prior to finish grinding, so the rounded profile of the final tip edge break often has a truncated shape where it meets the finish ground flank surface.

With gears operating at high PLV and high load, the exact tip profile modification used can be very important for reducing dynamic contact loads.

 

[mfgenggear]

Terry

very good points. however the OP fail to mention what type of gears. he is trying to manufacture.
but for Aerospace gears you are right on target. one exception. I do not like breaking the tips of the gear, not more than .005 inch max. I like the automatic or CNC gear grinder to obtain the involute profile form and roll, as the parts are being ground. Just my preference.

and I don't prefer protuberance on the gears root diameters, unless the customer requires it, as mandatory. where it is prohibited to grind the root.


mfgenggear

 

[tbuelna]

mfgenggear-

When carburizing gears you need to round the edges of the teeth prior to heat treat. With sharp corners during HT, the edges will through harden, become brittle and fracture. The size of the corner radius required depends on the case thickness, and the corner radius used should be larger than the case thickness so that there is a smooth uniform profile in the finished case/core transition. Also, the thickness of the carburized case is proportional to the size of the tooth. For example, the AGMA recommendations for case depth of a 10DP carburized spur gear tooth is .020"-.035", which would require tooth edge breaks of >.020"-.035" radius.

My experience is mostly with high quality aerospace gears, and they are usually green ground prior to carburizing so that the amount of stock removed during finish grind is minimized. It is also common practice to finish grind both the flanks and roots, which is not difficult since form grinding is typically used. Form grinding the roots results in a smooth and precise fillet profile and transition to the flank surface, which is critical in this very highly stressed area.

There are actually CNC machines designed specifically for making precise edge radii on gear teeth. One other consideration with the size of tooth edge breaks is making sure they are large enough to prevent the edge material being rolled over during shot peening. Normally only the root areas are shot peened, but the nozzle is typically rotated during the process so that the shot spray is directed as close to normal to the root surface as practical. So the tooth tips usually get lots of overspray during the process.

 

[mfgenggear]

Terry

I forgot to mention the Aerospace gears we manufacture are fine pitch gears.
I promise it has been done this way before I was born. and there has never been an issue.
the gear teeth contours are broken before carburize.in addition the new cnc gear shapers and cnc hobbers can easily achieve AGMA class 12 or better. there is no longer necessary to green grind. but that said yes it was standard practice once upon a time to green grind prior to carburize or nitride.
These new machines are very precise. and addition we have no issues with the carburize since it is normally only between .015-.025 effective case depth.

Like you said in an other post all our gears are precisely controlled with very detailed planning.
and strict quality control. all parts are carburized by nad cap approved suppliers. and are extensively tested for met lab requirements to AMS2759/7 or cutomer internal specifications which are more strict. than the latter specification. with course pitch gears it could be a diffrent manufacturing requirement as you stated. The customer drawing have exact requirements for all dimensions and processing. all these attributes have to be held. we work with very critical and precise aircraft gears. The company I work for are know to manufacture some of the toughest parts out there. which I am a lead in the organization.

Mfgeneggear

 

[mfgenggear]

Terry
Just wanted to add, I have the highest respect for you. You have the brains and the experience. My self my time is coming close to retirement.my armor will be hung up. I ready to enjoy life,

Best to you my friend.

Mfgenggear

 

[tbuelna]

Mfgenggear-

I hear what you are saying about the capabilities of modern CNC shapers and hobbers. I have made some spur gears that were rough machined Nitralloy N blanks, quenched and tempered, had the teeth shaped, had the teeth shaved to add some crown, nitrided, shot peened, and finally honed to clean-up the flank surfaces. They came out very nice and no grinding was used. I have also made some drive shafts that had crowned external spline teeth at each end. The crowned spline teeth needed to be case hardened for wear, but the contact stresses were low enough that a thin nitrided case was sufficient, instead of a heavier carburized case. The nice thing about nitriding is that the HT distortions are minimal, so for some applications like spline teeth you can often get away without finish grinding.

Also thanks for the kind words. And while I'm happy to hear you will soon be a full-time man of leisure, I'm also a bit sad that there will be one fewer experienced gear engineer in the industry. I have only been doing gear work for the last 7-8 years. However, I was fortunate to spend some of that time working along side a semi-retired gear expert who gave me an advanced education in gearbox design. Most of what I know about gears I learned from him.

Regards,
Terry

 

[mfgenggear]

Terry

You are wellcome, I have over forty years doing different stuff, aircraft, space and Missile structures . as well as gears.
By the way the new hobbers as well as the shapers can crown. we do it allot.
Keep up the good work.

Mfgenggear