unusual gear tooth profile identification 4

[bsmet95]

I have an application to have a replacement gear made, and I must use an old drawing (1966) to have it made. Anyone with expertise for this particular gear application is either retired or has left the company.

The particulars are as follows:
Tooth type: Fellows 3/4 D.P., 20° P.A. stub
Pitch rad.: 2.50"
Index for 14 teeth on 15 tooth P.D.

This is for a lever arm which is not a precision operation. Two arms operate together, moving gear racks until they grip a workpiece, then must only keep pressure on while lifting (it's a below-the-hook lifter).

My understanding of the description is that it's a 14-tooth gear, only at a 15-tooth P.D., thus having an enlarged space between teeth.

Does anyone have some insight regarding this?

Attached is part of the drawing.

 

[williedawg]

bsmet
The newer editions of Machinery's Handbook have a brief explanation for the Fellows Stub Tooth Form of that type designation; (i.e. 3/4) and it notes that information as to the tooth profile can be found in 18th and earlier editions.
It also mentions that "this type of stub gear tooth is now used infrequently"

According to the 23rd Edition; 1989, the 3 or numerator in that specification is the Diametral Pitch to be used to determine the thickness of the tooth and number of teeth.

The 4 or denominator is the Diametral Pitch to be used to determine the Depth.

In this case, the D.P. is 3 which would give a Tooth thickness of (1.5708/P) = 0.5236.
And, 3 x 5.00 P.D. (Pitch Radius of 2.5) = 15 T

The 4 would be used for Depth; thus a = (0.800/P) = 0.200,
b = (1.00/P) = 0.250, and whole depth then = [1.800/P] = 0.450

The Clearance is more than what would be standard for stub, (the denominator 4 again being used), and is given as 0.25/denominator (0.250/4) = 0.0625 in Machinery's

I think this nominal Clearance is what would be the result of "INDEX FOR 14 TEETH ON 15 TOOTH P.D." and would need no further modification to the profile....keeping the Tooth Thickness in mind.
The manufacture of this "gear" was probably meant to keep costs at a minumum.

Hope this helps.

Willie

 

[bsmet95]

Thank you very much. a few years ago one of the experienced engineers was trying to explain it to me. Your explanation is very clear.

 

[mfgenggear]

Good job Willie

bsmet

in addition to what willie has advice.

the drawing you supplied is missing the span measurement or the radial measurement over one wire.

calculate the span or Meas over one wire from the information supplied using this link

http://www.zakgear.com/Over_Pins.html

for the correct calc just imput the 3 dp and not the 3/4 dp
use .640 wire for the M.O.W.

if there is any actual parts available on hand take an actual measurent of those parts. should be close.

HTH

MfgEngGear

 

[mfgenggear]

I get using tooth thickness of .5236
measurement over 2 .640 wires 5.6583
measurement over 1 .640 wire 2.8291 rounded off

I also notice no root dia specified or form diameter.

MfgEngGear

 

[bsmet95]

Thanks. That drawing is all I have to work with.

 

[dinjin]

If the teeth are cut on a larger blank, the tooth thickness shown is not correct as the tooth thickness would increase on the larger blank. The addendum would increase by .50/P
and the dedendum would decrease by .50/P.
The normal addendum for a 3/4 Fellows Stub is .250
and the dedendum is .3125. P equal 3.
The formula for the addendum and dedendum is
1.oo/P(denominator) and the dedendum is 1.25/P(denominator)

I have found it easier to treat the 3/4 as a ratio and always use the 3 as the P value.
Then the addendum and dedendum become
3/4 times 1.00/P and 3/4 times 1.25/P
Thus there are 2 distinct Fellows Stub gears equal to
the standard Stub gears and that would be the 4/5
and 8/10 Fellows Stub gears.

I think this makes understanding the Fellows Stub
System easier

 

[dinjin]

According to Machineries Handbook the circular tooth thickness would increase by 1/P times the Tangent of the pressure angle where the 1/P designates the change in the outside diameter of the gear blank. This results in a
circular tooth thickness of .644922 inches for the 3/4 Fellows Stub cut on a 1 tooth larger blank.

 

[bsmet95]

Thanks again for your help.

 

[williedawg]

Spent some more time on this.... ran the 3 DP 15T 20° & 2.5 P.R. Stub Gear specs through a Gear Profile generating program I wrote some years ago. A DXF was imported into AutoCad. Then, after some modification to the program to get the Fellows "3/4" D.P. figured in, the modified profile for a 2nd DXF was brought into the same AutoCad Dwg for comparison. See attached.

Even though there is a nominal 0.050 Clearance, Machinery's says under Fellows Stub Tooth that "the clearance is made greater than in the ordinary gear-tooth system and equals 0.25 / denominator of the diametral pitch (4) "...which, as noted above is 0.062

What is a bit puzzling, is that the OP's print calls out a 2.75 R for the outside radius. If this large a Radius is used, the Clearance between (2) gears made to this spec would be 0.0

Either the nominal centerlines would have to change to accomodate this Clearance and the Clearance ignored, or the centerlines kept the same & the outside Radius reduced to say, 2.688 which would result in the 0.062 Clearance recommended.

As to the "INDEX FOR 14 TEETH ON 15 TOOTH P.D." the right side of the attached jpg shows that when this INDEX is used, the space becomes 0.5979 when using 25.714° for 14T - as opposed to 0.5236 when using 24° for 15T circular spacing, and keeping the Tooth profile intact.

When another corresponding gear tooth is placed in the space, the tooth side-clearance or backlash becomes apparent.

 

[dinjin]

williedawg

The tooth thickness for the 14 tooth pinion should be measured at its normal pitch radius of 2.33333 which
should result in the .644922 circular tooth thickness.

The pressure angle at the 2.5 radius is not 20 degrees
but it is at the 2.33333 pitch radius and that is
where the 14 tooth circular tooth thickness should be measured for the enlarged 14 tooth gear.

 

[williedawg]

dinjin
The OP & print state that the 14T INDEX is to be at the 15T Pitch Radius of 2.500 (Keep the 15T Pitch Radius, but cut only 14 Teeth....thus a wider space between any two sides of Teeth on this part)

Additionally, Machinery's Handbook explains the 20deg Stub Gear and the Fellows design, although in brief.

 

[dinjin]

I have an old 15th Edition which has the Fellows Stub info in it. I do realize that the new versions has very limited information about the Fellows Stub which is almost considered obsolete now. The comment about hobbing or indexing at the 15 tooth radius is correct but somewhat misleading as the pressure angle of 20 degrees will be at the 2.333333 radius. Since the standard stub hobs give a little deeper dedendum, I have often recommended that they use that tooling to replace the Fellows Stub but simply cut it on the Fellows Stub OD size blank. I do not know how to post the simulated hobbed profile that I have in ACAD to the site. I do appreciate your posting yours there.
If the mating gear for the pinion is withheld the same amount as the change in the pinion, they will both operate at the 20 degree pressure pitch line.

 

[dinjin]

From my layout and using .625 dia. pin
the measurement from the center to farside
of the .625 dia. pin is 2.91791
Since there are only 4 teeth, I would put
a 3/8 pin in the center pilot hole and dimension
over the 3/8 center pin and the .625 dia pin would be
3.10541. I assume this will drive or ride in
a Fellow Stub standard rack with a 20 pressure
angle. This should give bsmet enough information
to see if this is close to the part that he
submitted with the 4 teeth drawing.

 

[bsmet95]

Thanks, guys, I really appreciate the help since I need to have these parts made and wasn't sure how to go about it.

 

[mfgenggear]

one option

is to have ash gear make you a milling cutter, supply them with the gear data you have & send them a sample part.

the gear teeth can then be manual or cnc milled with 4th axis indexing head.

your gear teeth will then become the same as your sample part. that is why it is important to measure the exact ctt
with measurement over one wire.

MfgEngGear