Calculating Gear "Span" 2

[bbook1]

I have a spur gear that I need to calculate "Span" (Base Tangent Length). I found this website "

http://www.meshingwithgears.com/cgi-bin/

http://wwwboard/span.cgi",

but I am unsure about "Backlash" in this tool. The gear specifies an Agma Backlash Class A, but when I enter this into the tool it gives me a number that is much smaller than the physical measurement of the part over three teeth. I am having a Swiss company make this part, they use Span rather than measurement over wires. Can anyone help me to calculate this correctly?

Agma = 10
Number of teeth = 20
DP = 96
PA = 20 deg
OD = 0.2271/0.2230
PD = 0.2083
Face Width = 0.050
Tooth Form = INVOLUTE
Backlash Class = A

I would like to understand this calculation rather than have someone just give me the numbers (although, I would like to have the numbers).

 

[israelkk]

AGMA backlash Class A is not clear at all. What version of AGMA? (2000-A88, 390.02, 390.03, 2015, etc.) AGMA classes goes by numeric system with preceding and trailing letters, such as "Q" for AGMA 390.03 and AGMA 2000-A88 or "A" for the newer AGMA 2015. For example: AGMA 2000-A88 class Q10C. The "Q" is meaning 2000-A88 or 390.03 the "10" is the accuracy class (Radial Tooth to tooth composite tolerance and radial Total composite tolerance) and the "C" is the tooth thickness tolerance. Added to this you need to specify the dimension over pins, Span, or best is min and max testing radius. Those values are determined by the gear accuracy and the backlash you desire bur AGMA doesn't specify backlash. AGMA is dealing only with one gear dimension tolerances not on the actual gear size.

 

[mfgenggear]

do your self a favor & use the Measurement over two wires.
This fine pitch gear will drive you nuts trying to get an accurate span measurement. besides any gear shop mfg worth their wieght should be able to verify over wires.

Israelkk is correct.

who is designing the gear train?

 

[bbook1]

My customer designed the parts. Is there a cross reference chart for AGMA to ISO 1328?

As I said, a Swiss company is making these parts and I don't have control over how they will inspect the parts. I provided them the gear information above and they asked me what the "Span" measurement was. I have spent most of the day trying to figure out how to calculate "Span" from the infomation that I have. I downloaded KissSoft and it has calculated everything for me based on this info, but the question that I have is that it asks for "Quality (ISO1328). The default is 6, I'm assuming this is similar to the AGMA Quality Number. How can I figure out what an AGMA 10 is in the ISO1328 standard.

Understand, I am not a gear engineer, I'm an Industrial Engineer that was lucky enough to get to field this curve ball, so please go easy on me.

 

[mfgenggear]

bbook1

What is the circular tooth thickness High & low?
Alternatively, what is the max & min M.O.W?
Is it standard tooth thickness, minus backlash?
Is this on standard center distance?

Then I can assist you.

I did a reread of your post! This is a small gear.
Depending on the quantity it may be better to use a master gear to very all of the attributes.

Your are the customer you dictate to your vendor the
method of inspection. & not the other way.

OK AGMA class Q10 will be the quality attributes required.
If you like I can post it for you.

Circular tooth thickness & center distance will
control backlash required.

My profession is gear mfg.

Mfgenggear

 

[Spurs]

Use of span as a measurment tool is usually much more precise and reapeatable than measurment with pins. Span is widely used in Europe and Japan due to this fact. In the USA, industy is stuck on MOW and has not yet recognized the benefits of span.

The swiss company is asking you for tooth span because the tooth thickness specifications you have are incomplete. They obviously realize that a backlash class A is really not a definition.

My guess is that your definition of Backlash Class A is really reffering to a tooth thickness tolerance Class A as outlined in AGMA 2000-A88. Tooth thickness tolerance does NOT define backlash.

This further points out the stupidity many gear designers crank out when they make definitions of tooth thickness using the term backlash. Backlash only exists in a mesh with a mating gear and with a defined center distance. A single gear never can have backlash.

Many gear cutting companies inaccuratly use the term as meaning that you take half the backlash value and reduce the tooth thickness by that amount from half its circular pitch. Its a very dangerous practice.

Sorry we cannot help you. You really need to come up with more information that will define the tooth thickness.

 

[mfgenggear]

Spurs

In America 99.9% of all fine pitch gears are measured with Measurement over Gear Wires.

And i disagree that span measurement is more precise, I have done both and I prefer MOW.
Master gears & spline gages are measure by M.O.W. you cannot get More precise than that.

The only dumb engineer is the one that is not willing to ask questions. & In addition, the ones that are arrogant & think they know it all. This board is for helping each other not to criticize others.

The backlash is dependent on the application, the number of teeth, Diametral Pitch & pressure angle.

 

[bbook1]

Thanks everyone for all the responses to this question, I knew that I would get a lot of good information out of this forum. It is difficult in that I can order gears all day long with just the information that I gave without any questions about any of the things that were brought up here. I guess the simple answer is that I really don't know what I am getting if I can place an order with the little information that I am providing.

 

[Spurs]

bbook1

You are correct - you really dont know what you are getting. My guess is that the Swiss company is a little more on the ball than others and at least knows the questions to ask.

Other companies may take a stab at it and think its good enough but that really is a roll of the dice.


To mfenggear
I agree with you that 99.9% of al fine pitch gears in America are measured with Wires.

I respect your position that you disagree that span is more precise. My extensive experience in this area is based on real gage R & R studies using many operators that does not support your position. While I agree that a very experienced and knowlegable person can "get good at" using wires, and you may be one of them, most people dont have the dexterity required.

The use of MOW in defining Master Gear Specs does not imply precision. It is used as a matter of convenience and communication. For example, would you really use mesurement over two wires to define a master helical gear with odd numbers of teeth? The wires are not even parallel to eachother, but you can get a very accurate span reading.
You would need to add a 3rd wire, but now you need 3 hands to do it.

Many companies overcome this issus by switching to measurement over balls with a ball anvil micrometer. Measurment using this type of device can be much more repeatable and accurate than span, but be aware that balls do not allways have the same specifications as wires.

In terms of being critical, you will note that I was not critisizing the use of wires. I was pointing out the differences between American and European practice. Furthermore, your comments of backlash being dependent on the application is exactly in line with what I am saying!

A single gear does not have backlash, so if you have a drawing of a single gear with a backlash definition, how will anyone that just looks at that drawing realy know anything about its application. In face a single gear can be used with many different mates. This is why I used the term "stupidity". Would you have been less offended had I used the word "error" - My apologies for not being more politically correct.

 

[mfgenggear]

spurs

OK I can agree that we disagree, no problem thats what makes the world go around.

I added a partial wriiten E book
written by Mr GC Bansal, Overview of Gear Manufacturing Process, Page 78 & 79

Please find attached link.

also helical gears with odd # of teeth should be measured
over one gear wire. this is a work around. Happy Gearing.