ADDENDUM MODIFICATION COEFFICIENT

[Frankieboy]

Hi!

I've a question about following.

Transversal pitch = 10 mm
Normal module = 2.75
Transversal module = 3.1831
Pitch angle = 30 degr 14 min 18.04 sec

Three gears in a gear train

Gear 1: Z = 45
Gear 2: Z = 440
Gear 3: Z = 31 - 60 (depending on situation - change gear)

Gear 1 drives gear 2; gear 2 drives gear 3

Addendum modification of gear 3 = 0

What to do with the addendum modification of the other two gears?

Thanks a lot!

Frankie

 

[diamondjim]

Do not know if you have to maintain the
center distance for the set.
I would use 50 percent for the first
and 25 percent for the second and as
you said none in the third.
If you must maintain the center distance
do as above but use 44 teeth in the first
which should give you the same center
distance for the system.
This assumes there is no back driving
in the system.

 

[Frankieboy]

Hi Diamondjim,

Thanks for your reply.

Gear 1 and 2 distance: free to choose and then fixed.
Gear 2 and 3 distance depends on number of teeth of gear 3 (between 31 and 120; in real life max is 80).
Gear 3 is used for driving a cylinder; number of teeth depends on diameter of cylinder used. Bearing console is installed on slider so can be adjusted.

In real life, addendum of gear 3 is always 0.

What do you mean with 50 %? I understand a coefficient of say 0.5; do you mean this with 50%?

Normally there is no back drive; only in case of speed reduction.

Frankieboy

 

[Frankieboy]

Hi Diamondjim!

I forgot something!

The addendum modification coefficient of 50% for the first (Z = 45) and 25% for the second (Z = 440) as you adviced previously.

I assume the first gear has positive addendum modification coefficient and the second gear a negative.

Important is to get very smooth running; it's to drive a printing machine.

Or am I mixing things up?

Frankieboy.

 

[diamondjim]

As you guessed the 50 percent
is .5 and the 25 percent is .25
addendum coefficient. These
would both be long addendums.
This approach uses the recess
action gearing method which may
help to reduce tooth wear.

 

[Frankieboy]

Hi diamondjim,

Thanks a lot for your reply!

A further question: as far as I konw, it always makes sense to use a negative addendum for the big gear and a positive for the small one.

You suggested a positive addendum for the small (gear 1) and the big (gear 2). Why?

What is recess action gearing methode? Can I find more info somewhere?

Thanks a lot!

Merry Christmass and Happy New Year!

Frank

 

[diamondjim]

You increase the first by some amount
then the second (idler) by one half the
amount. (Assuming the third gear is not
modified). The idler change affects both
the center distance between it and the
first gear and also the second gear.
The final gear could be a negative addendum,
but you asked that this not be changed.

Basically you want the first to have the
greatest change, the second to have something
less than the first and the third to be either
less than the second or negative addendum.
I assume the final change gears are standard
with no change, so I used the method as above.
I have seen .6 on the first, .2 on the second
and no change on the third.
Or .40 .30 and 0 on the third.
Note that the first plus twice the second change
equals 1.00.

 

[Frankieboy]

Hi DiamondJim,

Thanks for the info you provided.

I checked the addendum modification coefficient with some (incomplete) data I have and looks like something like 0.63 for gear 1 and 0.185 for gear 2 is not bad.

But why must 1* addendum of gear 1 plus twice addendum of gear 2 equal 1.0?

Thanks, Merry Christmas and Happy New Year!

Frankieboy

 

[diamondjim]

It does not have to equal 1.00
but assumes the end change gears
will be some standard center distance.
You can use a negative change on the
final change gears if you wish.
Have a good holiday!

 

[Spurs]

Frankieboy / Diamondjim

Addendum modification coefficient is a term which is used mostly in Europe but generally misunderstood in North America.

Am I correct in assuming Frankieboy is using a European method?

Addendum modification in general is simply a modification of the tooth thickness.

If X is the addendum modification coefficient then

X = (t/m-pi/2)/(2 x tan pa)

where t is the normal circular tooth thickness
m is the normal module
pa is the normal pressure angle

Therefore as you can see, given a normal module and normal pressure angle, the only variable is normal tooth thickness.

It is customary in Europe to have two gears in a mesh running with a similar magnitude of X except one having a +ve value and the other having a -ve value.

This will mean that the gears will mesh at the standard center distance - not taking backlash requirments into consideration.

You can however mate gears of different X factors provided the center distances are adjusted accordingly.

Since gear 3 in your assembly has an X factor of 0 , then any non zero X factor for gear 2 will result in a non standard center distance.

If anyone can explain to me the advantage to using an addendum modification coefficient as opposed to specifying a specific tooth thickness, I would like to have it explained to me.

Incidentally, the software I use in designing gears has an Addundum modification coefficent calculated as a reference. It is available at this link.

http://members.rogers.com/webgear/home.htm

 

[diamondjim]

Spur,
You are incorrect about your information
about the x profile shift as it also
affects the od of the parts as well.
Some manufacturers do use plus and minus
profile shifts to maintain a standard
center distance. Some manufacturers use
only a .5 as the x profile shift on the
pinion and reduce the number of teeth
on the pinion by 1 tooth. This strengthens
the pinions and they can order standard
gears instead of calling for profile shifts
on the mating gear.
The plus and minus profile shifts are called
long and short addendum gears in many of
the gear publications in the US.

 

[Spurs]

Diamondjim

X Modification does not guarentee a shift in the addendum or dedendum from a pure specification point of view.

It is true that just as in cutting gears, when you create a thicker tooth by plunging the cutter to a shallower depth, you will at the same shift your outside and root diameters to larger values (for a given hob).

This however does not automatically mean that some other Basic Rack profile is not being used. For example, if you use an X Modification of .5 on a AGMA course pitch rack form, you will get a different root diameter than using a .5 X factor on an AGMA PT tooth form. However in both cases, the .5 X factor will result in the same tooth thickness for a given module or diametral pitch.


In fact, as is common in Powder Metal and Plastic gears, you can have virtually any addendum or dededum value you want (within reasonable limits) with any X factor.

Remember, that the formula for X factor coefficient as was indicated above is directly proportional to the tooth thickness for a given module (or diametral pitch) and pressure angle.

The addendum and dedendum is proportional to the tooth thickness, pressure angle and Basic Rack definitions.

 

[Frankieboy]

As Spur already suggested, I'm in Europe. With addendum modification, I mean profile shift, in order to get a smooth running set of gears. The quit odd lay-out (44 - 440 - 31 - 44 teeth in a gear train) might require an addendum, because of the fact smooth running is required. Second and fourth gear are connected to fixed cylinders, third gear is a change gear connected to a change cylinder (number of teeth between 31 and 80). Except centre distance between first and second gear, all other distances are adjustable by a sledge. Therefore involute teeth shape is important.

I'll try to use addendum mod coef as suggested by DiamondJim, but in case somebody has additional advices, I'm looking forward to it.

Thanks a lot!

Yours,

Frankieboy

 

[carburized]

Frankieboy

LOng and short addendum are very frequently used in gear industry. Typically a smaller member needs to have plus modification whereas bigger member negative. But, depending on center distances, they both could have plus or negative. Here are some of the reasons.

1. Smaller member makes lot more revolutions than the bigger member. It sees lot more stress cycles than the bigger member.

2. By making modifications, you can have recess action gearing which runs quieter and smoother.

3. Rule which we use in our gear industry is to have enough modifications on the members till contact length on both members is as possible to be equal. This will make your gear set stronger in bending as well as you can have recess action.

4. In essence you are making a tooth thickness wider at the point of contact (long addendum gearing) which helps you in bending of the gear teeth.

Hope it helps.
Good example on the site

http://www.gearsandbearings.com/index.htm

 

[Frankieboy]

Hi Carburised!


Thanks for your reply.

Although I'm looking everywhere, I don't understand what you mean with recess action gearing. Can you please explain in brief?

Thanks!

By the way: link you included is fine, thanks.

 

[diamondjim]

You might try Dudley's gear handbook
section 5-18 where he talks about arc
of approach and also under Low Friction
Gearing later on on the same page.
Earl Buckingham was a great promoter of
this type of action as well. I cannot
seem to find his book and cannot site
the page numbers.

 

[Frankieboy]

Hi Diamondjim;

I ordered Dudleys gearbook two weeks ago and will get it somewhere next days.

Thanks a lot!

Frankieboy