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A question for the gear guys.

If you look at his pic, the extra depth makes it look like it comes from a secondary operation. The radius at the bottom looks too round, like it was milled.
A pre-shave or pre-grind full radius hob or shaper cutter with protuberance, followed by grinding, honing, shaving will all look that way too.

It's not a very good photo, either. Don't most cameras have auto-focus these days ?

The point is, we were given no information, no purpose for figuring out what this was, no problem to solve .... to me this is just an ego display. No point.

I agree with you about posters who post and then disappear. I hate to be blunt.
It's okay, you can be polite and I can be the asshole, we each have our purpose in the world :D
 
Emgo, I'm sorry you think I am "speshul", I have to display an ego, and on top of that my camera has poor focus.

David-M, thanks for the illustrations of the gears. What app do you use to generate them?

The gears had to be sent back to the customer before I could do much else with them but I finally got the back story from them.


The 54 tooth gear is installed on the cam on a John Deere tractor engine and is driven by a 27 tooth on the crankshaft.

The 30 tooth is also mounted on the camshaft and drives the injection pump gear which is the same size.

The part that is probably causing the odd dimensions is the evolution of that series of motors. The original motor was developed in the 50's and of course was inch based. In the 80's the motor was "metricified" but kept all the bore spacings of the older design. The old design used 10DP helical gears to drive the cam and in turn the pump. The newer design kept the center distance but changed to spur gears "to fit".

This is where the question comes in. What process is used to determine DP/MOD sizes and tooth count to accommodate the now odd center distances? And can it explain the odd shape of the teeth?

Thanks, Ed.
 
Since JD aftermarket spares are very common,its just possible Deere&Co made the gears as difficult to produce as possible ..............Deere say these gears are made with sintered powder metallurgy.....formed to finished shape.....as are many other motor parts.
 
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This is where the question comes in. What process is used to determine DP/MOD sizes and tooth count to accommodate the now odd center distances? And can it explain the odd shape of the teeth?
This is what is so frustrating about the subject. A gear in isolation has no nothing. These questions are like, "How loud does a dog bark ?"

There are things people call "standards" but the fact is, they are only conveniences. For example, a gear by itself DOES NOT HAVE A PITCH DIAMETER. All these statements people make "A 100 tooth 10 DP gear has a pitch diameter of 10" ARE NOT TRUE. The only way a gear has a "pitch diameter" is mating it with another gear, measuring the center distance, then dividing by the number of teeth on each part. Then you get the REAL numbers for what is termed "pitch diameter".

Everything about gears is the same way. It's ALL malleable. The so-called 'standards' should be called 'conveniences' because it makes most calculations a lot faster and easier. But all they are is numbers or formulas that people use to make things quick or cheaper (less expensive to make production runs of cutters in accepted sizes than one for each situation).
You can even cut teeth of a different pitch and pressure angle from the cutter you are using, by changing what would be the "normal" cutting relationships.

For anything you can say about gears, someone could come up with an equally true opposite example. Asking a question about "gears" is like asking "Do girls like cheese ?" It's impossible to answer.

If you know the situation, the purpose, the material, possibly the time period, what the user is trying to achieve, then sharp focused photos of the relevant aspects of the part, then you can try to make an educated guess at what the original maker was trying to do and how he tried to do it.

If you want to get in bed with a particular girl, then knowing if she likes cheese and what kind of cheese, is valuable knowledge. But the question as a general case is both unanswerable and useless and frustrating. If you say "doesn't really matter, if it entertains people so what" that's partly true but on the other hand, it leads other people to think these questions are relevant or answerable, which just leads to more of the same misinformation and frustrating nonsense.

As an added note, a lot of stuff goes on that you can't second-guess. For instance, you go to an auction and snag a bunch of hobs cheap, yay ! Then you get them home and they are all oddballs, 7.239 DP 23* PA, weirdass shit, no wonder they were $5 each. But then a job for fifty reducers comes in, hmmm, the dimensions are pretty close, I can use those cutters I got for nothing by scooching the teeth around a little and save $500 on the job, maybe make a little money, yippee ! But what happens fifteen years later when someone tries to make a replacement ? If he goes 'by the book' he's going to be scratching his ass for a long long time. This happens in big companies too. They buy 100 hobs for a project, it gets cancelled, but hmm, close enough for these other parts we're just designing. Or they use some weird thing for the next 80 years because back in 1923 Joe found two shaper cutters at a garage sale and used them for this product that became really popular, now we've got 300,000 in the field, not a chance in hell to change to something current.

There's all knds of things involved in this that don't come out of a book. So trying to fit it into a book is impossible. Sometimes, sure. But you can't limit yourself to thinkig it is, or will, or is even likely to, or you are crippling yourself. Book numbers, plus experience, are an okay start, not the end.
 
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sample size = 1
Now we got something to work with ! Best guess here is a commercial grade material, not too expensive but not cheap junk either. Definitely case-hardened but how deep would take some exposure to determine. Could be a forging, with that shape probably has had some hammering. All soft cheeses on the plate but nothing stinky so atmosphere control should be more pleasant if you get the blank hot. Likes to drink, probably freixenet when by herself but not sure you could pull that off initially. She might want a little more upscale cleaning liquid for first article inspection. Looks like fairly wide pressure angle on the involutes but not a large dp. Good selection of cheeses, not just one type - plus the grapes - so she might be amenable to a variety of hot-working methods and of course the gleason interlocking-tooth infeed system is always good - just set the depth then let the tool plunge to a solid stop, several times per tooth as the blank rotates. Bore size, certainly the best way to tell is with a gage pin but if that fails, any internal measuring method you have handy is better than nothing.

Overall, this one could be quite a project and unless the working conditions are pretty lightly loaded I bet the service life would be extremely short, but if you just need it for emergency use, just as a guess I'd say two quaaludes would get it running very well for at least one night.
 
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This is what is so frustrating about the subject. A gear in isolation has no nothing. These questions are like, "How loud does a dog bark ?"

There are things people call "standards" but the fact is, they are only conveniences. For example, a gear by itself DOES NOT HAVE A PITCH DIAMETER. All these statements people make "A 100 tooth 10 DP gear has a pitch diameter of 10" ARE NOT TRUE. The only way a gear has a "pitch diameter" is mating it with another gear, measuring the center distance, then dividing by the number of teeth on each part. Then you get the REAL numbers for what is termed "pitch diameter".

Everything about gears is the same way. It's ALL malleable. The so-called 'standards' should be called 'conveniences' because it makes most calculations a lot faster and easier. But all they are is numbers or formulas that people use to make things quick or cheaper (less expensive to make production runs of cutters in accepted sizes than one for each situation).
You can even cut teeth of a different pitch and pressure angle from the cutter you are using, by changing what would be the "normal" cutting relationships.

For anything you can say about gears, someone could come up with an equally true opposite example. Asking a question about "gears" is like asking "Do girls like cheese ?" It's impossible to answer.

If you know the situation, the purpose, the material, possibly the time period, what the user is trying to achieve, then sharp focused photos of the relevant aspects of the part, then you can try to make an educated guess at what the original maker was trying to do and how he tried to do it.

If you want to get in bed with a particular girl, then knowing if she likes cheese and what kind of cheese, is valuable knowledge. But the question as a general case is both unanswerable and useless and frustrating. If you say "doesn't really matter, if it entertains people so what" that's partly true but on the other hand, it leads other people to think these questions are relevant or answerable, which just leads to more of the same misinformation and frustrating nonsense.

As an added note, a lot of stuff goes on that you can't second-guess. For instance, you go to an auction and snag a bunch of hobs cheap, yay ! Then you get them home and they are all oddballs, 7.239 DP 23* PA, weirdass shit, no wonder they were $5 each. But then a job for fifty reducers comes in, hmmm, the dimensions are pretty close, I can use those cutters I got for nothing by scooching the teeth around a little and save $500 on the job, maybe make a little money, yippee ! But what happens fifteen years later when someone tries to make a replacement ? If he goes 'by the book' he's going to be scratching his ass for a long long time. This happens in big companies too. They buy 100 hobs for a project, it gets cancelled, but hmm, close enough for these other parts we're just designing. Or they use some weird thing for the next 80 years because back in 1923 Joe found two shaper cutters at a garage sale and used them for this product that became really popular, now we've got 300,000 in the field, not a chance in hell to change to something current.

There's all knds of things involved in this that don't come out of a book. So trying to fit it into a book is impossible. Sometimes, sure. But you can't limit yourself to thinkig it is, or will, or is even likely to, or you are crippling yourself. Book numbers, plus experience, are an okay start, not the end.
As EmGo mentioned here.
I recall digging through a surplus dealers used tooling for goodies kept trying to figure out the wording on gear hobs. Nothing written on them matched up to a standard pitch or pressure angle. I recall some of them having a pitch of 8.6529 or a circular pitch of .75439" and so on. Pressure angles of all sorts, nothing standard. Most of this was Military surplus from WWII and possibly automotive surplus, too.
Companies have the tendency of doing things like this to keep their competitors from copying their designs. That's a big part to be using PM on, who knows. They also do these things because they found this pitch/ pressure angle combination to give them the best performance in that gear train, too.
No real way to know the actual answer. Those engineers are probably dead and gone today that design and engineered that gear years ago. Company records destroyed and long gone.
 
I know a little about the evolution of some of the engines we have out there, especially turbodiesels that have been around awhile.

Engines ran helicals forever.

It wasn't a switch to spur gears 'cuz metric.

I bet the gears aren't MOD.

The switch was made to spur gears to lower thrust forces on the accessories.

With lower thrust forces things like high pressure commonrail injection pumps can be made with cheap ball bearings inside. PTO drives for hydraulic pumps and air compressors don't need big beefy pressure lubricated bearings anymore either.

With lighter accessories from lower thrust forces the timing gear housings can be lighter and smaller.

Spur gears can be thinner than helicals.

I have heard a name used for the unique shape of the modern turbodiesel spur timing gears, but I cannot remember it at the moment. It's purpose is very specifically related to NVH. It is a special toothform.
 
This is my best version of the 54-tooth gear. It has a pressure angle of 24.23 degrees (This is based on the over-pin measurements). Also, it is cut .06327 deeper than normal (per side).

I bet the gears aren't MOD.
I agree. I changed my mind on the one I originally had as a 4.5 mod, which I now think is a 5.5 dp.

Has anyone ever run into a Mod 5.03984 dp gear?

174528564.png


174528599.png
 
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Now we got something to work with ! Best guess here is a commercial grade material, not too expensive but not cheap junk either. Definitely case-hardened but how deep would take some exposure to determine. Could be a forging, with that shape probably has had some hammering. All soft cheeses on the plate but nothing stinky so atmosphere control should be more pleasant if you get the blank hot. Likes to drink, probably freixenet when by herself but not sure you could pull that off initially. She might want a little more upscale cleaning liquid for first article inspection. Looks like fairly wide pressure angle on the involutes but not a large dp. Good selection of cheeses, not just one type - plus the grapes - so she might be amenable to a variety of hot-working methods and of course the gleason interlocking-tooth infeed system is always good - just set the depth then let the tool plunge to a solid stop, several times per tooth as the blank rotates. Bore size, certainly the best way to tell is with a gage pin but if that fails, any internal measuring method you have handy is better than nothing.

Overall, this one could be quite a project and unless the working conditions are pretty lightly loaded I bet the service life would be extremely short, but if you just need it for emergency use, just as a guess I'd say two quaaludes would get it running very well for at least one night.
This might just be the best damn post you have ever made
 
This is what is so frustrating about the subject. A gear in isolation has no nothing. These questions are like, "How loud does a dog bark ?"

There are things people call "standards" but the fact is, they are only conveniences. For example, a gear by itself DOES NOT HAVE A PITCH DIAMETER. All these statements people make "A 100 tooth 10 DP gear has a pitch diameter of 10" ARE NOT TRUE. The only way a gear has a "pitch diameter" is mating it with another gear, measuring the center distance, then dividing by the number of teeth on each part. Then you get the REAL numbers for what is termed "pitch diameter".

Everything about gears is the same way. It's ALL malleable. The so-called 'standards' should be called 'conveniences' because it makes most calculations a lot faster and easier. But all they are is numbers or formulas that people use to make things quick or cheaper (less expensive to make production runs of cutters in accepted sizes than one for each situation).
You can even cut teeth of a different pitch and pressure angle from the cutter you are using, by changing what would be the "normal" cutting relationships.

For anything you can say about gears, someone could come up with an equally true opposite example. Asking a question about "gears" is like asking "Do girls like cheese ?" It's impossible to answer.

If you know the situation, the purpose, the material, possibly the time period, what the user is trying to achieve, then sharp focused photos of the relevant aspects of the part, then you can try to make an educated guess at what the original maker was trying to do and how he tried to do it.

If you want to get in bed with a particular girl, then knowing if she likes cheese and what kind of cheese, is valuable knowledge. But the question as a general case is both unanswerable and useless and frustrating. If you say "doesn't really matter, if it entertains people so what" that's partly true but on the other hand, it leads other people to think these questions are relevant or answerable, which just leads to more of the same misinformation and frustrating nonsense.

As an added note, a lot of stuff goes on that you can't second-guess. For instance, you go to an auction and snag a bunch of hobs cheap, yay ! Then you get them home and they are all oddballs, 7.239 DP 23* PA, weirdass shit, no wonder they were $5 each. But then a job for fifty reducers comes in, hmmm, the dimensions are pretty close, I can use those cutters I got for nothing by scooching the teeth around a little and save $500 on the job, maybe make a little money, yippee ! But what happens fifteen years later when someone tries to make a replacement ? If he goes 'by the book' he's going to be scratching his ass for a long long time. This happens in big companies too. They buy 100 hobs for a project, it gets cancelled, but hmm, close enough for these other parts we're just designing. Or they use some weird thing for the next 80 years because back in 1923 Joe found two shaper cutters at a garage sale and used them for this product that became really popular, now we've got 300,000 in the field, not a chance in hell to change to something current.

There's all knds of things involved in this that don't come out of a book. So trying to fit it into a book is impossible. Sometimes, sure. But you can't limit yourself to thinkig it is, or will, or is even likely to, or you are crippling yourself. Book numbers, plus experience, are an okay start, not the end.
Sometimes things have to be done for a particular reason in the moment. I once had to use a boring bar to bore an ID. It was a long bore with a lot of hangout . So I welded a few pieces of small bar stock to the bar and got the job done. 5 years later I get a call from the shop I was at when I done that asking me how the hell I made that part come out right. They was good people so I told them. :ROFLMAO:
 
Since JD aftermarket spares are very common,its just possible Deere&Co made the gears as difficult to produce as possible ..............Deere say these gears are made with sintered powder metallurgy.....formed to finished shape.....as are many other motor parts.
That makes a lot of sense as the emblem on the part did not look stamped and the finished part did not look forged.

Ed.
 
This is what is so frustrating about the subject. A gear in isolation has no nothing. These questions are like, "How loud does a dog bark ?"

There are things people call "standards" but the fact is, they are only conveniences. For example, a gear by itself DOES NOT HAVE A PITCH DIAMETER. All these statements people make "A 100 tooth 10 DP gear has a pitch diameter of 10" ARE NOT TRUE. The only way a gear has a "pitch diameter" is mating it with another gear, measuring the center distance, then dividing by the number of teeth on each part. Then you get the REAL numbers for what is termed "pitch diameter".

Everything about gears is the same way. It's ALL malleable. The so-called 'standards' should be called 'conveniences' because it makes most calculations a lot faster and easier. But all they are is numbers or formulas that people use to make things quick or cheaper (less expensive to make production runs of cutters in accepted sizes than one for each situation).
You can even cut teeth of a different pitch and pressure angle from the cutter you are using, by changing what would be the "normal" cutting relationships.
OK, I'll ask the question from a different direction. If you have 2 30 tooth gears on 6" centers I believe they are 5DP. What if the centers are 6.075 centers. What process is used to make these gears mesh properly? Do you make the blanks that much bigger and cut the nominal depth marked on the hob or shaper cutter? How do you determine depth to get proper backlash/clearance?

Thanks, Ed.
 
OK, I'll ask the question from a different direction. If you have 2 30 tooth gears on 6" centers I believe they are 5DP.
You're starting in the middle :) You don't start out with a tooth size. You start out with what you want to do then fit the size to the need.

Assuming you are creating these parts, you start with a center distance and a ratio and the question, am I transmitting power or motion ?

Fine pitch and coarse pitch gear work is sort of different worlds. I've cut fine pitch teeth but don't have that much experience in figuring them out so won't go there. But if you're working with 32 DP, you can get to just about any center distance pretty easily, just add or subtract a tooth or two.

With power transmission, you don't just pick any ol' size you think looks cool. You want the finest pitch that will carry the load. Finer teeth work better than coarser teeth, for a variety of reasons. The load on the tooth determines what pitch you have to use because, as a general rule, smaller teeth are weaker than bigger teeth. So here we've just started and we're already between a rock and a hard place. Finer is better, bigger is stronger.

Then you start to figure the number of teeth depending on your desired ratio and what size fits most conveniently. (Quick side note, you don't have to use conventional sizes. You can make teeth any damn size you want. It's just a lot easier to buy an off-the-shelf cutter than make one).

There really are pitch diameters but they depend on the ratio and your center distance, not on book stuff.

For example, say you want a 2:1 ration and the centers are 10" ... no, make that 9.842" just to show that it's not about even numbers matching the "standard dimensions".

Imagine you have two disks, mounted on your bearings. The sizes depend on your ratio. They touch each other at one point. One drives the other via friction at that point. In this case, 6.5613" and 13.1226" are the diameters of your two disks at that point. (Another side note, the ratio doesn't have to be even either. It's just easier for my peanut brain to figure out).

Them's your pitch diameters, by definition, period, end of story, doesn't make a damn bit of difference how fat the teeth are or how skinny or whether one is long addendum and the other short or if the little green men came from mars to crank the handles. The only thing that can change that is the ratio and the center distance. That's it, all, end of story. Forget what people say when they start talking "the pitch diameter is bla bla bla ..." *

What if the centers are 6.075 centers. What process is used to make these gears mesh properly? Do you make the blanks that much bigger and cut the nominal depth marked on the hob or shaper cutter? How do you determine depth to get proper backlash/clearance?
Okay so, now to a practical problem instead of a design problem and the answer is -- It depends :)

If you are making antenna-rotators for a space station, you're going to get a lot more carried away. But to make this short enough to tolerate, let's assume these are from your neighbor's broken rototiller and you just want the damn thing out of your hair. It's a simple 1:1 setup, bearing bores were fucked up, guy that welded it up screwed up and the bores are now .075 too far apart. No biggy.

Visualize the gears that were fine at 10" centers, now spread apart another .075". Too much backlash, right ? Quick and dirty way is just to grow each part .0375" on the radius. All you need to do in this case is fatten up the teeth a little to take up that space, and move them out a little to touch each other properly. You can do that until the teeth get too pointy at the outside, and everyone will still be happy. (As you go bigger and bigger with the same number of teeth and the same DP, teeth get pointier and pointier. In the other direction they get scrunchier and scrunchier. Those are your limits to size manipulation : too pointy and too scrunchy).

[If I wuz David M you'd now get a nice graphic illustrating that point. Unfortunately I am using this g-d apple laptop these days and the vmware stuff is such a pain in the ass to start and run that I amn't gonna. Maybe David will wander by and do that for us. Just make a couple of sketches of the same gear spead and scrunched, you can see what the teeth do. Otherwise, you're stuck with your imagination.]

As mentioned, if he screwed up the other way, you can sqursh them down some and they'll still be happy. Until you get to where there's undercut on the roots, for average stuff they'll work fine. That's why the involute curve is so popular for gears - there's other shapes as well, they don't have to be involute - it's really the whore of curves, it'll do anything and still work. And just like other whores, if they are happy they work better so if you have time, try to get the conditions nice. But if you're in a rush, just go for it and they'll still get the job done.

The tooth thickness is what determines your backlash, and there are formulas to determine what thickness you want for x lash on y centers with z teeth. Many texts will have all the fomulas you need, maybe even Machinery's Handbook (I never looked in there for gear stuff, had easier ways. The handbook has gear stuff but it's kind of a mash, gear books are better organized and clearer.) For this stuff I think the best is Jones and Ryffel, Gear Design Simplified - it's a little green pocket tall skinny thing with JUST drawings to illustrate what the formula is for and the formula and an example or two. Pictures and numbers, no big words, my kinda book. For running these numbers on a daily basis, you need this.

If you are interested in the subject, the two more generalized books you must have are both by Darle Dudley - he was a guy who did actual gear design and manufacture, not some twat i a university playing with his dick - Gear Handbook and Handbook of Practical Gear Design. Let's see if we can confuse people with the titles, eh ? Nice. Anyway, I'd start with Practical Gear Design - it's the one with the blue graphics on the slip cover. It goes into depth less but covers a wider variety of applications and manufacturing methods. I find it a little more interesting and less technical. The Gear Handbook was not written by Dudley, he edited a collection of chapters by specialists in each type of gearing, and you best bring your calculator. It's still readable and understandable without a physics degree but not exactly a light romance novel. It's a good thing there's only two really good texts because you can take out a loan to buy either one. Luckily again, Practical is less expensive. Maybe they'll take your three year old pickup in part-trade.

And oh yeah, if you are really lazy the HP-41 calculator had a measure over wires program on the little plugin card that would calculate everything you needed to do that, so you could totally forget what you were doing and just punch in numbers. That's my kinda math :)

Last but not least, as a practical trick .... for 14.5* pa teeth, each thousandth of cutting depth gives you a half thousandth of backlash. For 20* it's about 2/3 (or 1 anda half, if you go the other way). So if your mesh is tight, the teeth are 14.5*, and you want three thou more lash, drive the cutter in six thou. I do that in a couple of tries because unless you have a deposit tool, backing the cutter out when you took too much ain't gonna do shit, and you can toss the part in the scrap bin :)

(As noted, that trick would theoretically work the other way but as a practical matter, it's a one-way deal).


* Note because I know this is going to create confusion and opposition. People grow up and they learn stuff in high school or a trade class and in general practice it's totally common to say "the pitch diameter of a 50 tooth 10 DP gear is 5" " and people all accept that and move along with life.

But that's a convenience, not the fact. When a simple beginner text says this, what they mean is, "if the tooth proportions are exactly to the conventional ones, and if the centers are exactly on target and if the mating part is exactly perfect, then the "operating pitch diameter will be blablabla."

Those of us who have cut things know this is never true. So it's okay to say this, as long as you understand that it's only sorta true. Kinda like saying Joe Biden is capable and experienced. Even the people who like him have to realize he's never been better than a c student and now that he's senile, it's worse. This is common in China too, lots of times you know something is not true but to make things go easier, you just pretend it is then everyone is happy and on the same page. There's an expression - cha bu duo - means "close enough" -- but we know that underneath, nope it ain't. Not really.

That's the situation with a lot of what people say about gears. So you can take the conventional words and use them to solve a problem but if you try to figure out theory from them, you'll drive yourself nuts. Figure out theory first, then accept that practice is a rough approximation and you'll be happy.
 
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30.375/5 = 6.075

Because it is profile-shifted only 3/8 of a tooth, there isn't much tooth pointing.
Sorry, I didn't say what I meant .... not talking about these parts, I meant in general. Many people think "this is what a X DP tooth is supposed to look like" but again, that's not the complete situation. For example a 30T 10 DP tooth will look like what your software draws on "standard" centers with "standard" addendum and dedendum.

But they don't have to be that way. You can adjust teeth all over the place, and it's not weird or strange or even unusual. With the same 30 teeth and same diametral pitch you can sqush them down until you get undercut or spread them out until they get pointed. Those are the practical limits.

Like in his question "what if the centers are spread by .075" ?" Until they get pointed, with standard cutters one method is to just spread the teeth out. (Another part of that limit is the standard-dimension cutters. Otherwise we'd probably be making 8.379 DP or 6.432 DP stuff all the time and reverse-engineering teeth would be really miserable).

I was hoping for two drawings of the same gear, one with teeth spread to where they are pointed, the other squshed to where you can see the undercut. And maybe a third to show the generally-accepted appearance. Hint hint :)
 








 
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