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RustyCas

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First of all, is there a technical term for this, John? How hard is it to manufacture a gear with a prime number of teeth, say 89 instead of 88, or a pinion with 17 T instead of 16? I’ve always wondered about this.

But also today I am doing a baseline on a rock crusher with a gear drive and there are 16T on the pinion and 88T on the spur gear. This seems (to me) to be a combination that is designed to fail sooner rather than later. The pinion shaft (358 rpm) is driven via v-belts by a 100 hp motor, so (the final) speed could be easily adjusted at the motor sheave (if the gear ratio was changed slightly).

(denotes info added for clarity)

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Walt Strong

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"rock crusher with a gear drive and there are 16T on the pinion and 88T on the spur gear. This seems (to me) to be a combination that is designed to fail sooner rather than later."

The common factor is 2, so every other tooth would have more wear than the adjacent tooth.  I could see this pattern on a hydro turbine speed increaser gearbox. One can expect that gears with prime number of teeth would last longer. Perhaps more of an issue if gears are not returned to original index position, if for any reason (bearing change) that gears are removed.

Not sure what you mean by changing speed in this situation.

Do you measure vibrations while crushing rocks or with no load?

Walt
John from PA

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Quote:
Originally Posted by RustyCas
First of all, is there a technical term for this, John? How hard is it to manufacture a gear with a prime number of teeth, say 89 instead of 88, or a pinion with 17 T instead of 16?


Something with a prime number of teeth would be called a hunting tooth combination and it is not a problem at all to manufacturer.  It is for instance required by API 613 Special Purpose Gearing for Refinery Service.  Only one of the meshing pair has to be prime to be a hunting tooth combination.   Further, neither gear has to have a prime number of teeth to be a hunting tooth combination.  A hunting tooth combination means the number of teeth of the gear (Ng) and the pinion (Np) simply do not have any common prime divisor.  So a 16 x 27 combination is a hunting tooth combination.  The factors of 16 are 2 x 2 x 2 x 2 and the factors of 27 are 3 x 3 x 3.  Neither share a prime factor. 

A hunting tooth combination is desirable but not absolutely essential, especially if the gearset is lightly loaded (hence slow wear) or in a type of service where the gearset may be frequently changed due to its service. 
 
From a maintenance standpoint, even though a hunting tooth combination may exist, since one doesn't know the design characteristics of the gearing, it is good to match mark them and make sure they go back together with the same indexing.  Going back to the example of the 16 x 27 tooth combination, that gearset would have eight assembly phases.  If you just randomly assembled them every time they were disassembled for some reason, odds are likely that you would on startup have to develop a new wear pattern.  Again, I generalize because we don't know the service.  If a gearset is say rated 400 HP and in reality is transmitting 100 HP, then we may not really care about a wear pattern.

16 x 88.jpg   

Danny Harvey

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Rusty,

For mass-produced, catalog gearboxes (from Falk at least), they will build the ratio's around a common low and intermediate gear sets and change the high speed gearing to alter the ratio.  That way, they can make fewer of the more expensive, big stuff, and stock high speed gear sets designed for that shaft center distance in the housings.  They are very good about their gear-tooth combinations and only on the occasion do they select gear teeth that have a common factor.

Plus, I don't think that the gears both have to be prime numbers. They just have to not have a common factor so if one of them is a prime then the other won't have a common factor unless the ratio is a whole number interger.  You do find that situation frequently with beveled gears.  I'm not sure why but it's a fairly common practice. 

I'm dealing with a gearbox right now that has serious vibration at assembly phase after a rebuild in 2016.  It's double helical gearing and I suspect that the mechanic didn't expect to find a common factor here and didn't match mark the gears. Amplitude at assembly phase was obvious from the first set of data and has gotten steadily worse to the point that 1/2 x gmf is as big as 1 x gmf and both are surrounded by sidebands of secondary shaft speed.  I haven't heard any response since I blew the whistle last week.

IMO, you should always match mark gears.

Walt,

I think he's talking about using a prime number in the gears and making up the difference in the v-belt drive.
John from PA

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Reply with quote  #5 
Quote:
Originally Posted by Danny Harvey
Rusty,

For mass-produced, catalog gearboxes (from Falk at least), they will build the ratio's around a common low and intermediate gear sets and change the high speed gearing to alter the ratio.  That way, they can make fewer of the more expensive, big stuff, and stock high speed gear sets designed for that shaft center distance in the housings.  They are very good about their gear-tooth combinations and only on the occasion do they select gear teeth that have a common factor.

Plus, I don't think that the gears both have to be prime numbers. They just have to not have a common factor so if one of them is a prime then the other won't have a common factor unless the ratio is a whole number interger.  You do find that situation frequently with beveled gears.  I'm not sure why but it's a fairly common practice. 

I'm dealing with a gearbox right now that has serious vibration at assembly phase after a rebuild in 2016.  It's double helical gearing and I suspect that the mechanic didn't expect to find a common factor here and didn't match mark the gears. Amplitude at assembly phase was obvious from the first set of data and has gotten steadily worse to the point that 1/2 x gmf is as big as 1 x gmf and both are surrounded by sidebands of secondary shaft speed.  I haven't heard any response since I blew the whistle last week.

IMO, you should always match mark gears.

Walt,

I think he's talking about using a prime number in the gears and making up the difference in the v-belt drive.


As far as Falk, what you state is somewhat dependent on the product line and to some degree the age of the gearing.  Hence, always best to count the teeth!  At one time, this was the practice in the gear industry (Falk included) for low speed catalog stuff, typically with REB.

Quote:
It should be kept in mind that a gear box nameplate ratio can vary by as much as ±5% (depending on drive type) so exact ratio setting by tooth combination is preferred.


Some background information...in 1959, AGMA published information sheet #271.02, "Ratios for Helical and Herringbone Gear Speed Reducers" which listed "standard" ratios based on a (1.5)^0.5 progression. It included a table of "Gear Ratio Tolerances": ± 3, 4, 4 and 4% for single reduction, double, triple and quadruple reduction, respectively.  AGMA 420.04 (enclosed drive standard) in 1975 repeated the ratios, but modified the tolerances to 3, 4, 4, and 5%.  Current enclosed drive standards (e.g., 6010 or 6009) have a clause on preferred ratios, but any mention of tolerances has been deleted. The standards only say that ratio is to be indicated on the nameplate, but does not stipulate nominal or exact.  Checking Falk, Flender and Hansen websites it is interesting to see what they do in their catalogs. Falk's standard products state the 3-4-4-5% tolerances apply. The other manufacturers indicate they give exact ratios, but only to three or four significant figures.

Danny, you said "I don't think that the gears both have to be prime numbers." and I had already discussed the issue of prime numbers and common prime factors.  So perhaps read what I wrote.   


Danny Harvey

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Reply with quote  #6 
John,

Perhaps I will. I always do as soon as I see them. 

We must have posted at the same time because this is the first time I have seen your post.  Please accept my apology. I'll try to do better so you won't have to correct me so often.


RustyCas

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Reply with quote  #7 
16 x 88.jpg

   John, I'm confused.  Isn't the gear rotational frequency ~65 cpm for a 16/88 combination with a 358 cpm pinion speed?

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RustyCas

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Reply with quote  #8 
Walt, I ran this unit with and without rock in it.  There was much less "impacting" seen with rock in it than I expected.


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John from PA

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Reply with quote  #9 
Quote:
Originally Posted by RustyCas
16 x 88.jpg

   John, I'm confused.  Isn't the gear rotational frequency ~65 cpm for a 16/88 combination with a 358 cpm pinion speed?


Well the previously attached jpg files are not showing so I've reattached what I believe was the image.  "Yes" your math is correct.  Capture.jpg

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