gear cutting

Never heard of it - neither has Google (in relation to mechanics). Did you mean spur gear? Maybe whatever you're reading has a musprant or pyto.

stub gear.

Regards JD.

STUP could be acronym for several things. I have never seen it used in the relation to a mechanical gear. Then gear has a different meaning all so. So it could be stupid gear or start up gear.

A stub gear is one that has a shorter tooth compared to the standard for that pitch (module for you metric counties). Generally, IIRC, the tooth is about 1/2 the standard height (or is it the addendum is 1/2 the standard height - my manual is at work).

The gear is stronger, but louder, than the standard spur gear.

I just brushed by this rare term the other day when I needed to factor some long addendum (increased pinion pitch diameter) gear sets. It is one that you do not see often.

The "stub" tooth, or Fellowes Stub Tooth Form as it is often called uses an addendum that is (0.5)Diametral Pitch, rather than (1.0)Diametral Pitch. It will mate with another gear of the same diametral pitch as long as the the tooth thickness at the pitch line follows the same ratio of Pi/(2)Diametral Pitch. The dedendum (1.0)Diametral Pitch and clearance (0.157)Diametral Pitch values are the same for both types.

Using the Fellowes Stub Tooth Form the formula for the outside diameter is (Number Of Teeth + 1)/(Diametral Pitch) rather than the one now used that is (Number Of Teeth + 2)/(Diametral Pitch).

It does have a disadvantage when used on gears and pinions with low numbers of teeth as there is a reduced ability to transmit torque due to the issue that only one (1) tooth on each gear may be in contact during the tooth roll. For large numbers of teeth where there are at least two (2) teeth in contact during the tooth roll it is of little consequence.

It is not a tooth form that one sees very often unless one is reverse-engineering some old drive trains built before the 1960's. It is sometimes seen where the space around a gear is very limited or there is some conservation of gear materials.

Wow, this is almost a bit or trivia, save for the fact that some old machinery is still with us.

Best Regards,

Ing. Robert Forbus

Just a small correction.

The Dedendum is also half just like the Addendum of the tooth. Of course the Dedendum needs to be more than the Addendum for clearance as you have said.

This type of tooth form is mostly used for Splines and helical splines where there are various systems used in regards to radial guidance and clearances. The tooth engagement is 1 diametrical pitch, half up and half down with clearance at the root of the tooth in both gears.

Used in gearing, e.g. as in ASA B6.1-1932, at DP of 1, the standard addendum is 0.8, working depth 1.6 standard Dedendum 1.0, whole depth of tooth 1.8, circular pitch 3.1416 and standard circular thickness 1.5708.

Regards

You are correct. I was thinking solely about one of the archived gear tooth forms and not the use for involute splines as it is more commonly found there. I did not cite an exact standard by name-just the one that I was near the other day while I was digging for the long addendum formulae for a helical set I was designing. I don't have to work with splines very much-never have. They are their own science.

That is one of the dangers of dealing with old drive trains that were built decades ago, and that is that the tooth formulae continued to evolve to maintain the strength of the teeth but at the same time reduce the "banging" noise of tooth roll. Since most of the OEM's who built these things are no longer with us, I tend to go with a modern involute tooth form if the tooth strength will pass the test.

I think a lot of the evolution has to do with "noise" i.e. jerky tooth rolls, that is not suitable for higher speeds or sensitive translation. But, the materials used for gears evolved as well with great improvements in blend and consistency, and that no doubt had some influence as well. I know that I have seen this when lower numbers of teeth had to have a smooth roll that the teeth were "tipped" a bit to remove the interference that causes a minute "jerk" as the tooth crest is sliding near and off the crest of a mating tooth.

Off-topic, but that makes me think of the 20-degree tooth form that has basically replaced the 14.5 tooth form due to improved strength, and the 20-degree form has been replaced by the 25-degree form in several newer applications. Since the 14.5-degree form naturally approximates the involute profile so well, I have wondered how these other forms can be as smooth during the tooth roll. But evidently they are very nearly the same.

Best Regards,

Ing. Robert Forbus

Thanks. I also agree with what you are saying.

In the very old days, before the involute system, the driving and driven gear did not have the same angular movements. There was a slight deviation depending if tooth contact was on top middle or bottom of the driving gear.

The involute system fixed that.

The noise can also be reduced by helical gearing, however the gears get larger in diameter by 1/cos (helix angle). If the angle is kept small this is acceptable.

For low loads the 14.5 PA still is around. The gearing is a bit quieter. But as you said, the 20 degree is pretty much standard and 25 degree PA also are being used due to higher strength and load capacity.

If you have an internal-external gear set and a low number (below 12 teeth at 20 degree PA) in the difference of the teeth between them, then a lower tooth height also reduces tooth interference. The stub teeth form allows a difference as low as 7 or 8 teeth. At 14.5 degrees and full height 15 teeth difference are needed.

Another method to avoid interference is to enlarge the internal diameter by .8 tooth, and the external diameter by .5 tooth. There should be at least 16 teeth or more in the pinion doing this.

Best Regards

Rolf

Guest!

I am sure you are not quoting from a written requirement. There is nothing like a STUP gear but surely there is a STUB gear.

I think you heard as Stup and not Stub.

You will find so many links for Stub gear. Go ahead and look up and if you are serious, give yourself a name.