What SI Gear Standards Do You Use?

[Dr.D]

I am an American author, working on a revision for my Theory of Machines textbook. In the past, I used only US Customary units, but now see a need to introduce SI units. This is particularly a problem with regard to gear standards.

An internet search shows that there are dozens of SI-based gear standards, coming from ISO, AGMA, DIN, JIS, etc. They are all basically similar, but they are presumably not quite alike (else there would be no need for so many of them).

This raises two questions for which I would appreciate comments:

1) What SI gear standard is likely to have the most universal appeal?

2) How firmly is the SI gear notation/terminology established?

In the second question, I'm concerned about some notations/terms that strike me as strange, such as z = number of teeth, a = center-to-center distance, "reference pressure angle" rather than "nominal pressure angle," etc. Is the world agreed on these terms, or is there some lea way allowed?

 

[Baluncore]

At some point you need to take a look at this long list.
https://en.wikipedia.org/wiki/List_of_gear_nomenclature#Pressure_Angle

 

[Dr.D]

Thank you for the reply, Baluncore. Unfortunately, the URL you provided does not really address my questions. The questions are specifically about Standards and the notation/terminology of those standards, not just about definitions.

 

[Dr.D]

Is there anyone else out there concerned with SI gear standards? My interest is with spur gearing only, not any of the more complex forms. Responses to the two questions would be much appreciated.

 

[Baluncore]

1) What SI gear standard is likely to have the most universal appeal?

SI is the International System of units, based on metric and decimal systems. SI is a standard managed by the International Standards Organisation = ISO.

ISO standards are worldwide standards but they are not necessarily based on metric or SI units. If you have a standard the ISO will be happy to market it.

EU member's have harmonised the majority of their standards. The previous national standards are still used but the majority of the content has been adjusted to meet or conform with the EU standards. There may appear to be many national standards, but the majority are harmonised and expressed in each nation's language.

The “Metric Module” gear dimensions do not translate directly to an existing “Diametral Pitch”. The very common MM Pressure Angle of 20° generates stronger teeth with less undercutting at the root than did the imperial 14.5° DP system. The separation force on the gear shafts is greater with PA=20°, but bearing quality and lubrication have improved to compensate. DP gears are now often available in PA=20°.

There are so many different gear parameters available that the bigger manufacturers can make it impossible to find a similar standard gear. I only make a gear wheel on a hobbing machine or mill for maintenance when the gear is not available off the shelf. For that reason I have to work with whatever standard was used, or cut a gear pair in an alternative available system. I find that a standard set of DP or MM milling cutters, or a couple of standard hobs, will be available from China for a few hundred dollars, and now at half that price from India. They will grind a custom hob for any specification, to fit a particular hobbing machine. Take a look at their web sites to identify which standards they prefer.

NC gear cutting by the use of wire EDM has revolutionised the manufacture of small runs of accurate gears with arbitrary profiles. One advantage of EDM is that a very hard material can be used without any need for heat treatment and the distortion that results. Custom made hobs have made it easier for manufacturers to control the spare parts supply by avoiding all standards. NC EDM has made it possible for a small operator to make their own replacement, if they can measure the original.

Finding a common standard is difficult. Suffice it to say that a PA=20° gear, in any Metric Module that is a multiple of 0.25 will be available off the shelf. I would look for the DIN standard in English, or the BS, harmonised to EU compliance, and then registered with the ISO. Remember that BSWhitworth and the US Unified threads are both specified in inches, and both are available from ISO. Just because it is ISO does not make it SI.

You must have seen this list. http://www.tandwiel.info/en/gears/international-standards/

2) How firmly is the SI gear notation/terminology established?

Gear notation/terminology is well established.
Re: "reference pressure angle" rather than "nominal pressure angle," That is a language difference. The “pressure angle” is defined by the standard and the profile generation process. The English language meanings of "nominal" and "reference" refer to the target value, from which a real gear will deviate. Each harmonised standard will be available in several languages, with only minor differences in terminology. I believe that every ISO standard is available in at least English, French, German and Italian. It is not surprising that there are differences in the terms, but those differences are not in the actual numerical specifications.

 

[Ranger Mike]

good advice from Baluncore- i sell gear checking machines from Germany. message me if I may be of assistance.
rm

 

[Dr.D]

Thank you again, Baluncore, for your comments.

The harmonization of standards of which you speak has become evident to me as I have looked at many different standards found on the internet.

I'm not sure where you are, Baluncore, but 14-1/2 deg PA gears have been rare birds in the USA for many, many years (I have never seen a new one, although perhaps in some antique machinery). When I wrote the first edition of my book back in the 1980s, 20 deg PA was the common American standard, but 25 deg PA was also being advanced. It appears that 25 deg PA has lost favor (for the reasons you mentioned, increased bearing loads, etc), although it is a stronger tooth form in most cases.

The English language meanings of "nominal" and "reference" refer to the target value, from which a real gear will deviate.

Actually, it more a case of the fact that a gear pair will deviate from the nominal because of center distance variation.

As you said,

Re: "reference pressure angle" rather than "nominal pressure angle," That is a language difference.

I certainly understand the meaning of these terms, but remember that I am writing an undergraduate textbook, a source for folks exposed to these ideas for the first time. My concern is to avoid confusing the students. If I say, "nominal pressure angle," and then they read five other books that all speak of the "reference pressure angle," they are apt to wonder if they are the same thing or not. I have not been able to look at all the standards that I would like to review (some I cannot find on the net), so my concern is with the existing degree of uniformity of terminology in the literature. In the same way, I wonder about the degree to which z = number of teeth is universally accepted; to me, n = number of teeth is much more natural.

Ranger MIke, thank you for your offer to help. I may very well be sending you a note in the future.

 

[Baluncore]

Dr.D, I understand your problem. You are moving into an impossible situation, a forbidden state. I see no way at the present time you can straddle the Atlantic, or even the 49th parallel, with one edition of the book.

You must decide how to spell the SI unit of length, metre or meter, then standards and terminology will follow that lead. The deliberate misspelling of a name is seen by some as a deliberate insult. If you subtitle the edition as the SI edition, you should use the spelling metre and consider referring only to the ISO version of standards.

In the same way, I wonder about the degree to which z = number of teeth is universally accepted; to me, n = number of teeth is much more natural.

Is this a national difference or a usage dependent issue?

I would expect the integer variables n1, n2 … to be used teaching about gear design concepts such as the ratiometric relationship between torque, speed and power. I would also expect n be used in removing common factors to implement a hunting tooth.

When a mechanic holds a particular gear wheel in their hand, I would expect a parametric Z. They are then a sufficient distance from mathematics where the z might be a complex variable.

Being in Australia may have forced me into a pragmatic compromise between the independent markets and parallel universes of the USA and the EU.

I have not been able to look at all the standards that I would like to review (some I cannot find on the net), so my concern is with the existing degree of uniformity of terminology in the literature.

That inaccessibility is the commercial elegance of the ISO paywall. Most central or university libraries have the ISO standards available, sometimes you need to ask. Some eastern bloc universities have electronic copies on their websites. These tend to be blocked unless you have the regional ip.

There are also regional spelling differences.
https://english.stackexchange.com/q...nglish-users-often-spell-standard-as-standart
A good example is a search engine where the term “standart” is used for searches and as a subdirectory.
https://en.wikipedia.org/wiki/Library_Genesis
See also …
http://www.dtic.mil/dtic/
https://archive.org/
http://www.everyspec.com/

 

[Dr.D]

Very interesting comments, Baluncore! As an old man, I don't have the energy or the life expectancy to complete two new versions, so I will have to try to survive in the forbidden zone. We'll see how that works.

When I started college in the late 1950s, the Russians had just launched Sputnik, and there was a general science/engineering panic in the USA. "We are behind" was being shrieked everywhere. I thought at that point that within 10 years, the US would be fully SI. It did not turn out that way, and today, I see zero movement in that direction in the USA. Companies that deal only in the US are usually entirely US Customary (not quite the same as British Imperial, but certainly similar). Companies that deal in international trade seem to be "bi-lingual" in the sense of working in both USC and SI units.

Thank you particularly for the links at the end of your post. I will have to work with them a while. Paywalls are the life blood of publishers and standards organizations, but they greatly impede the application of their products.