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# Metric Gear Design Question About Dedendum

## Metric Gear Design Question About Dedendum

(OP)
I am looking at drawings for a paper machine gear train gear set and am trying to understand where some of these dimensions came from. The gears would have been designed by Swedish engineers in the 70's.

Gear Data:
Teeth = 64
Module 12.0000
Pressure Angle = 20 deg
Tooth Form = "Normal Deep Depth"
Helix Angle = 7.5 deg
Whole Depth = 1.091 inch (1.063 STD)
Root Dia. = 29.260 inch
Normal Circ. Tooth Thickness = .722/.717 inch
Normal Chord Tooth Thickness = .722/.717 inch
Normal Chord Addendum = .476 inch
Pitch Diameter = 30.497 inch
OD = 31.441 inch

My questions are:
Does "Normal Deep Depth" give clue to the extended dedendum? I can't find that verbiage anywhere that ties to equations.
I found equations that give me matching numbers for addendum and whole tooth depth. But when I input gear data into a gear generator, the dedendum doesn't match which makes the tooth thickness not match the above. Is this a profile shifted gear? If so, where do the equations come from to match these numbers?

### RE: Metric Gear Design Question About Dedendum

JD PE

Edit: I just notice this is a helical gear please supply the measurement over to balls over even teeth and I will run the number.

there will be normal circular tooth thickness that will be close to spur gear size and transverse circular which will be close to
the size given. thus I can check your numbers. also give me an actual Outside diameter size and minor diameter.

many gear designers will design bastard gears to prevent from easily reverse engineering. a standard gear circular tooth thickness is
(1/dp)/2 = ctt =.2362, standard od = (64+2)/dp= 31.1806, pitch diameter = 64/dp = 30.2357 so it appears the calculations or measurements are way off.
do you have the actual gears measured, or are these calculated data. ?

### RE: Metric Gear Design Question About Dedendum

(OP)
These dimensions came from manufacturing drawings. These dimensions are real though, because that's what we measure on the gears that come in.

It would not surprise me if these dimensions were engineered to prevent reverse engineering. Even then, I would wonder how they got their numbers. Unless they have an internal design criteria.

### RE: Metric Gear Design Question About Dedendum

JD P.E

see my edited comment above give me the actual required measurement over wires. 7.5 degree helix angle should be fine.
the gear calculations are calculated to give both transverse and normal size.

### RE: Metric Gear Design Question About Dedendum

JD pe
Gear Engineers can manipulate the numbers to make the gears with good contact ratio, and no or very little interference, and good efficiency.

### RE: Metric Gear Design Question About Dedendum

(OP)
The only other information I have on this gear is theoretical Span over 8 teeth which is 10.875 / 10.872 inch.

I don’t have actual measurements right now. Would that information be sufficient?

### RE: Metric Gear Design Question About Dedendum

JD PE

based on data provided Normal circ. tooth thickness = .7217396/.7185471, transverse circular thickness = .7279674/.7247474
I converted the Mod to DP and punched that into my gear program. based on normal span.
Pitch Diameter of 30.4966474 inches

so now the gear and pinion would have to be punched in a gear program or calculated based on din or agma spec's
to optimize for the requirements. that's how I would approach this. so normally the pinion is enlarged to strengthen, and the
gear would be reduce by the amount pinion is enlarged to prevent interference and improve gear requirements.

### RE: Metric Gear Design Question About Dedendum

"Normal Deep Depth" usually means that the whole depth of the tool's profile is equal to 2.25 x module. Furthermore, 2.25x12=1.063".
The OD should be 31.442", not 31.441".
Looks to be a gear without profile shift.
Let's take this 2.25 x m profile and drive it -0.711 mm deep to create backlash. The cutter is not cutting the OD, so it does not change.
I get OD 31.442" and root dia. 29.260", which gives whole depth 1.091" and tooth thickness 0.722". All these numbers (except for the OD) match yours.
The tolerance is given by specifying the lower tooth thickness at 0.717", which can be accomplished by driving the tool even deeper, which will of course reduce the root diameter and enlarge the whole depth of the gear, but that's nothing unusual.
A standard cutter would have enough radial clearance to leave the OD untouched.
This specification indicates that there is a lot of backlash.

Is everything clear now?

### RE: Metric Gear Design Question About Dedendum

from ash gear hand book

standard tool is 2.25/DP =whole depth, full fillet = 2.335/dp, pre-grind = 2.35/dp, pre-grind full fillet = 2.400/dp

### RE: Metric Gear Design Question About Dedendum

(OP)
Thank you all for the assistance. This is clear now. Normal backlash new for these gear sets are about .015” - .025”.

Could all of that been calculated from center to center distances? Just trying to understand thought pattern for the existing gear design.

Where did you get the formulas to get the numbers you calculated? Particularly the diameter. Reference manual?

### RE: Metric Gear Design Question About Dedendum

(OP)
I see the scope of that AGMA standard is “diametral pitch of 20 through 120”.

Diametral pitch for these gears is 2.1167?

Or does it still apply?

### RE: Metric Gear Design Question About Dedendum

(OP)
So is the standard tooth depth for gear calculation exclusive of any backlash? If so, it makes sense that you’d need to plunge the cutter to create it. I never thought of it like that.

### RE: Metric Gear Design Question About Dedendum

yes it should work except back lash

### RE: Metric Gear Design Question About Dedendum

cutters and hobs can be manufactured & modified to obtain the correct tooth thickness and the correct root diameter.
if the AGMA class requires it to be ground, the tooth thickness of the cutter is made thinner to allow stock on the profiles to be ground.
the gear measurement over wires will depend on the center distance and back lash required, and strength required of the gear.

### RE: Metric Gear Design Question About Dedendum

#### Quote:

So is the standard tooth depth for gear calculation exclusive of any backlash?
JD P.E.
Well, almost. Note that the root diameter changes as the cutter is plunged deeper to thin the teeth and increase backlash. Therefore, the working depth is also increased (provided that the OD is not changed) and those Swedish engineers in the 70s took it into account and put on the drawing (Whole Depth = 1.091 inch (1.063 STD)).
On the other hand, the OD is not modified, which is typical for large module gears with no profile shift. It is possible because the cutter is not meant to clear it. OD modifications usually come from optimization of the meshing parameters.
Case 1: no profile shift, no OD modification necessary (your case).
Case 2: V-0 profile shift, the pinion has a profile modification +X and the gear has -X (X is the same) - no OD modification necessary.
Case 3: V profile shift, the pinion has a profile modification X1 and the gear has X2 - OD modification might be necessary.

#### Quote (JD P.E.)

Could all of that been calculated from center to center distances?
No information has been given on the center distance yet, it was also not necessary up to this point. The Swedish engineers did all the engineering and put the information on the drawing that is needed to manufacture the gear.

#### Quote (JD P.E.)

Where did you get the formulas to get the numbers you calculated?
Up to this point, only the most basic formulas in gear geometry were needed. I saw them many years ago in old books, and then I derived them myself for thorough understanding. A quick search reveals these resources:
https://khkgears.net/new/gear_knowledge/ Gear technical reference section
https://khkgears.net/new/gear_knowledge/gear_techn...
https://khkgears.net/new/gear_knowledge/gear_techn...
You can check all the numbers with these formulas, and you should get the same as I did. If there's any trouble, get back to us for help.

### RE: Metric Gear Design Question About Dedendum

A lot of info here.
When I worked with metric gears, I used metric units for all calculations; one can easily convert the dimensions to inches when its done and if it is needed. The Swedish designers did the same, I believe.
Happy Easter to those who celebrate it.

### RE: Metric Gear Design Question About Dedendum

JD pe

lets make certain facts clear
so according to my calcs I wrote years ago based on buckingham and walter e dalby two gear designers of the past.
standard od for this helical gear is 31.4497 how ever what is important is when assembled where does this od touch on the mating part.
to make sure there is no interference between gears. many designers fail to verify this. the od must not go below the mating gears true involute form (tif)
to verify this one must have both gear data to very this.
also when diving deep into gear designs, the gears must be verified for scuffing, wear, interference, efficiency, # of cycles.
the gear train must be at the center distance derived from the two mating gears pitch diameters.
to strengthen the the pinion and to improve properties it is enlarged because it has less teeth and prone to tooth bending and wear.
and the gear is less prone to wear. due to the larger # of teeth. AGMA calculations do this very well. see this video it will help.
while one can use the basic gear calculations to complete the geometry based on gear formulas, as a designer all of the above is very important.
especially when there are safety concerns.
far as manufacturing while it is important that a gear is manufacturable usually it is best to consult gear manufactures for a review until a gear designer
has plenty of experience.

### RE: Metric Gear Design Question About Dedendum

(OP)
Where do you get calculations for your calculated diameter?

### RE: Metric Gear Design Question About Dedendum

#### Quote (JD P.E.)

Where do you get calculations for your calculated diameter?
I'm not sure what diameter?
First the pitch diameter: d = mz/cos(B) = 12*64/cos(7.5 deg.) = 774.627 mm = 30.4971".
OD = d+2*m = 774.627+24 = 798.627 mm = 31.442".
Std root dia. = d-2*1,25*m = 744.627 mm = 29.316".
Std whole depth = (OD-Std root dia.)/2 = (31.442-29.316)/2 = 1.063".
All these formulas are to be found under KHKgears links above.

### RE: Metric Gear Design Question About Dedendum

(OP)
Thank you both very much. I have a better understanding. I’m not trying to engineer gears.

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