measuring a worn out gear

Start by tracing out the gear on a piece of paper. Then see if you can define the involute curve on any of the teeth, and from there you may be able to back into the pitch radius and other parameters. The process of designing a gear is explained here:

http://www.profilesmagazine.com/p33/marsalek.html

Basically what I am suggesting is that you reverse engineer it....

In the first picture the worm gear should not have significant wear on the extreme left and extreme right of it. Likewise the wear on the wheel gear should be confined to the center of it. Can you get enough measurements from these locations?

You would be surprised.

The wear in the worm is well extended, so is the wear in the wheel. The abuse this poor gear has suffered is outrageous. Poor baby!

I am still scratching my head on how to measure the angle of the helix on the wheel.

Again, my first idea is using a raster image on AutoCAD, but the imprecision I get is unacceptable. For some reason the resolution is decreased in AutoCAD.

I checked Shigley's for start, and then Casillas'. The problem with Shigley is that it looks great to design (I designed a few in college) but doesn't say anything on inspection. Casillas is oriented to the toolmaker and advices the use of some expensive tools I (still) haven't got.

Ok, I can measure the dedendum circle, count the teeth and make some approx on the tooth width using a standard caliper. How do I go from that?

I don't know, maybe I am just tired. The last two weeks have been crazy.

As I've indicated in earlier replies, as has BAB and some others, there are two basic ways to do this. High tech and low tech. The sophistication of your machine doesn't really warrant high tech. Just ascertain the diameter of the wheel. Easy. Measure the pitch of the 'thread' on the worm. Machine a simple worm on a piece of stock. Easy as making a bolt. Note the thread form, ie buttressed, square, or whatever. This is the determiner of the basic mechanical 'strength' of the finished gearset. If you've made a 'boo-boo' on the diameter of the worm, do it again. Experience will tell you to err on the side of 'large'. You can always remove material. Mill the corresponding tooth form onto the wheel, to the original diameter. Reasonably careful measurement of the element centers will allow reasonable mesh-depth. This is a very basic machine , remember. We're not producing for the next moon-shot.

Stu

Is it possible to get the dimension that represents the distance between the two shaft centres on the original machine; is the orignal machine's general arrangement drawing available, or its Operation and Maintenance Manual, either with the Client or with the Original Equipment Manufacturer?

This equipment was copied from an original Italian Olmia grain dryer. None of the copycats published a manual or anything, and as Olmia isn't manufacturing grain dryers anymore, it is impossible to find a manual on them.

Anyway, I already sent two someone to measure the distances between axes.

I will publish it here as soon as I get the data.

Do you think Olmia went through the expense of a custom designed gear pattern and size? Is it possible that they used, or modified a set of "off the shelf" gears from a gear manufacturer such as Boston Gear? The products offered might be a good starting place for the dimensions you need.

The equipment looks so old that I wouldn't be surprised if it is an original design (i.e. before AGMA and ISO were actually standards) However, I hope it is not.

I tried using the brochures of some manufacturers for this, but they lacked the specific product or they had several ones and I didn't have the dimensions to find the right one.

I already Donwloaded the Boston gear catalogue, so thanks a lot. I am giving it a look.

This type of gear-train is common in winches. Is that worth a try?

The 'back of the tooth on the wheel is the correct shape still.

Carefully trace it onto something and then flop the wheel over and align it again and you'll be able to describe the near-as-dammit correct tooth shape.

Once you've got this it is pretty easy to generate a worm to fit. It's easy to calculate the ratio. Maybe there is an old final-drive from a garden rototiller you can substitute for this one. Most rototillers have worm and wheels and they are certainly able to take a beating (tough).

The 'off-the-shelf' suggestion is good.

Good luck,

Cheers,

Stu

I agree. Given the amount of wear you are describing, "close enough" might be good enough. If you can count the teeth and measure the diameters I would think you will be close enough.

What speeds are you operating at?

Gussosa- I have a suggestion which I hope may be of some use. In the field sometimes I need to solve a problem by first re-establishing the original dimensions. I see that the worm gear is worn, but is the least worn of the pair. Perhaps clean the driven gear well, assemble it to the housing and shaft as it was originally installed. Clean the worm gear, and coat it with a release agent such as floor wax. When dry, assemble it to the housing. Apply Devcon epoxy putty in such a way as when the putty is hard, the imprint of the worm can be seen in the putty yet the putty is stuck to the larger gear. I would do this in two locations opposite each other. The release agent should allow the worm gear to break free, care being taken of course to allow this. Regards, Dan

Good answer. Here I thought Circular Pitch was a product used to fix a leak on a round roof.

Hmm, I would not attempt that. One (1) note about machining a worm with an engine lathe: regardless of whether the worm is of Diametral Pitch or Modular Pitch, it cannot be done unless the lathe is equipped with a quick-change gearbox that has the ratios for these arrangements. Neither Diametral nor Modular pitches correspond exactly to any thread pitch.

The worm is a left-hand single-start and the worm wheel is a right-hand x fourty (40) gashes. This should be a very easy thing (and inexpensive thing) to find from a commercial gear supplier, and the only remaining work may be machining the worm wheel bore to the correct size.

Most machine shops (who are not equipped with hobbers or gear shapers) can cut crude (in my opinion) milled spur gears, fewer still can cut crude milled helical gears, and even fewer can cut milled bevel or mitre gears. The successful generation of a worm wheel is nearly as much art as it is science, and virtually no machine shop has the correct generating hob that has the same pitch diameter as the worm that will fit the worm wheel. Hence, these are produced on special worm wheel hobbers that are designed to feed the hob forward (just like it was a screw) during the roughing cuts, and are then set up so they literally drive the worm wheel during the final cuts.

Best Regards,

Ing. Robert Forbus

Robert,

With respect, we here have been generating worm and wheels on lathes and mills for years.

Come to OZ and see a little ingenuity in operation.

There is NOTHING that can't be done if you have the will to do so.

What's missing mostly in the whole conundrum is not the learned skill to operate a lathe (or any machine for that matter) but that rare quality called, hereabouts anyway, NOUSE.

They call it all sorts of names these days, one of them is: the ability to be able think outside the box.

I call it nouse.

Yes, we all have our preferences. Things we'd Druther have. But when push comes to shove things can be well accomplished in the fashion of the original. I'm continually amazed at what resourceful people accomplish, every day, on very limited machinery.

Be great if we all had machines which did all the stuff for us. But we don't.

No offense meant.

Cheers,

Stu

Stuey-no problem-no offense taken here.I have just lived in the A.G.M.A. 10 and 12 world for more than a quarter century and I am really touchy about the accuracy, tooth roll, and function of gears-and like many people I've seen a lot of well-intentioned messes.

I've cut plenty of those "crude" gears and synchronus sprockets of all types with normal machine shop equipment, and always did so in an emergency when there was no new part to be had quickly. They always worked, but of course we custom-ground cutters and measured until we made them work. And we had an advantage-the larger engine lathe was late model and could translate both modular and diametral pitches. I think the last worm set we made was for a large Schiavi flexographic press and that was many years ago. For some reason I remember it well.

Barring the ability to find what should be a common worm and wheel arrangement, those pieces could be manufactured in most any machine shop with conventional equipment. They will not be "correct", but for the purpose intended they will do the job. Hey, we are not talking 30 arc-seconds of index accuracy here, LOL. Just carve and whittle until they fit.

Best Regards,

Ing. Robert Forbus

Robert,

(Hey, we are not talking 30 arc-seconds of index accuracy here, LOL. Just carve and whittle until they fit.)

It seemed to me that most had lost sight of that fact.

The guy just wants to get going again. I see that you actually do talk my language. The first post scared me a bit. I sensed 'complication', and as I now see, it wasn't really so. A well made, 'rough' gearset given sensible lubrication will last for years.

Thanks and Cheers,

Stu

I think you have it here

It occurs to me you simply measure the distance between the shafts.

Then buy a standard worm and worm wheel that match each other, and are of the correct size to fit the distance between the shafts

I don't suppose it matters if the gearing is a bit different

I am still waiting for the miller to send me the distance between shafts.

Thanks. I will check that.

Looking at the pics I estimate 25 MM shaft and 140 mm Dia

The worm looks like 12.5 mm pitch - if so maybe it is really

in inches - when you start looking at parts -

Would you post the Real Numbers -

Look at the backside of the teeth ( if they were left uni-directional ) and you can recreate the curve -

Old balers have the same kind of problems -

Sounds like you have the tools to recreate it if you can not find a set in the catalog.

Luck

rcb

Thanks a lot RCB.

It's not great precision, but I pasted your first photo into my CAD program and scaled it X2.5 to get 10mm on your caliper. If I did that correctly, the ID of the Tooth Base should be very close to 180mm.

I then used my worm gear tool to superimpose a 40-tooth worm gear over the photo. Since the original design was from Italy, it is presumably metric. I found that a pitch diameter between 203.8 and 205.0mm best matched the photo.

I agree with others that all you need is a 40-1 ratio with the correct shaft spacing and a similar gear thickness (tooth width) for strength.

Thanks DK. That's what I tried to do in first place. I guess your computer is better than mine or you are way better than me in CAD.

You're welcome.

For whatever it's worth, I'm using Vectorworks 2008 Mechanical on a MacBook Pro...

Dick

Here in Mumbai, India we have gear and worm gear cutting experts who can cut the worm gear and worm simply by copying the old pair. We have got worm and worm wheel of old elevator replaced around 15 years back, both are still working fine. I can get you replacement from here but cost of shipment and courieer charges will be more than replacing the complete gear box. I think if you have similar experts in your city then better to approach them.

Suresh Sharma

Just as a suggestion, forget the gear for a bit, measure the screw at the roots, then design a gear to match fit. Using the number of teeth will give you the info, and I would suggest you forget form and concentrate on function. Refitting the shafts is easier and safer than guessing, as close always equals wear.

Good luck.

If you have access to an optical comparator, that may to prove to be the best method for extracting "close enough" dimensions from the old set. If you can do a rough sketch and dimension it, perhaps our friends at Boston Gear or another similar manufacturer may have an off-the-shelf set that will suffice - I'd contact them with all of the information I could muster up, and solicit their expert advice. They may at least be able to point you in the direction of someone more familiar with the application.

on another topic,..

As Suresh Sharma has stated, India is one heck of a resource for copy-cat expertise - if you have ever seen the craftsmanship from India that goes into reproducing antique rifles, that shoot with ultra-precision, you might be quite impressed..

GA.

Hear! Hear!

Stu.

Yes, but it is a computer driven, lazer guided, five axis grain dryer. Good ans.

Can you contact the manuf. that the gear came out of? Or find out who has used equip. and look at or buy a better cond. one.

These worm gears are actually an incline plane wrapped around a tube. Measure the INSIDE diameter. Count the "grooves". Measure the OUTSIDE diameter of the gear. Calculater the circumference (pi*D). The number of groves are the match the outside part of the gear teeth. This ratio is the slope or pitch of the incline plane. They use to call this the mechanical advantage.

Similarly, the INSIDE of the teeth of the gear matches the OUTSIDE of the worm gear.

You really only need to know 2 things. To calculate the mechanical advantage. Its the ratio of the outside diameter of the gear divided by the the inside diameter of the worm gear. From the the difference between the inside and outside diameter of the gear is the depth of the gears and the number of teeth dived by the same ratio tells you the pitch or slope

Wrong!

The ratio is the number of 'teeth' around the circumference of the wheel. To One.

Take a closer look at the pics.

The wheel has 40 teeth. One complete rotation of the worm ( one helix) will advance the wheel one tooth.

Therefore one turn of the worm will result in one fortieth of a turn of the wheel.

Come on, Guys.

This is, luckily, a single start helix.

This is also a simple problem to sort.

By way of further explanation, without changing the relative diameters of the elements a different ratio can be arrived at by making a 'finer' helix on the wheel, which will need a corresponding shape change to the teeth of the wheel, say half the existing pitch, then the ratio would be 80:1. Remember, same diameters. Just results in a 'weaker' thread and tooth form and so will not have the same mechanical strength.

In a worm and wheel gearset the diameters are a matter of convenience, nothing to do with ratio.

You're right , though, about it being the 'theory of the inclined plane'.

Cheers,

Stu.

Just results in a 'weaker' thread and tooth form and so will not have the same mechanical strength.

Please clear up your above statement to me. When axles on automobiles need strengthening for racing, it is common for the number of splines to be increased. I was led to believe that even though each spline was smaller, the fact that the sheering action was shared between more splines would make for a stronger axle.Both diameters being the same. On a gearset that was used on an off road truck we had, they used a wheel gear that had only six splines on it. Hard use would split the gear when a crack would form between the outside of the spline, and the low point of the gear. When we went to a local machine shop we were told again that if the inner spline was ground with more smaller splines it would share the load between more, but smaller splines, and last longer. Would your example be sharing the drive load between twice as many gear teeth? Although each tooth would be weaker, the combined gearset should be stronger. Thank you.

Yes, Bob, in your examlpe you're correct. However, in a spline coupling, each of the mating elements assemble to form a continuously connected structure. That is, each spline is supported by it's neighbour, simultaneously.

With the worm I was describing, only ONE helix is working one ONE 'tooth' at a time. With a reduced cross-sectional area in the 'finer' example it will have much reduced shear strength.

It was my intention to describe the simplest of worm-and-wheel couplings, and not one where the worm is profiled to fit more of the circumference of the wheel, and so pick up more of the wheel teeth, and so share the load. However, in this configuration also the robustness on the coupling is limited by the cross-sectional area of the elements, working in shear.

Quite separate problem to a spline coupling. Although it must be understood that a spline's strength also is limited to its total cross-sectional area, minus any stress raising aberrations in the tooth profile (hence the use of root radii)

Hope this clarifies it for you. Sorry about my ambiguosity.

Cheers

Stu

True fact - if the box and shafts are as worn as the gears shown, it would behoove you/them to replace the whole works... Why did it take until now to think THAT up?!?

It's not just the relative diameters of the two elements which determine the ratio.

In a worm and wheel it's the PITCH of the worm. Like a fine thread versus a coarse thread, the nut (circumference of the wheel) will travel different distances in one revolution of the worm.

Now why did it take until NOW for that to come up?

Far as I can see, everyones been talking about spur gears so far, as far as ratio is concerned.

Stu.

Thanks to all of you. You have made very valuable comments. I am using all of them!

Thanks a lot.

I usually just send such parts to my gear cutter to figure out. No point to draw anything up in CAD when the gear cutter can check the teeth with a simple template.

It is very simple.

1. Measure the centre distance of worm and wheel.

2. Now, pitch circle dia. of worm wheel = (module * no. of teeth)/cosine of helix angle.

Pitch circle dia. of worm = no. of starts/sine of helix angle.

Half of pitch circle dia. of worm + half of pitch circle dia. of worm wheel = centre distance which you have measured.

3. Helix angle can be found by protractor or by rolling the soiled teeth over a blank paper.

4. Solve the equation and you get the Module. There are standard hobs or milling cutters available for particular module.

NOTE: To get most accurate worm & worm wheel pair, make cutter exactly of the shape of the worm.

Contact me if you still have any problem. nitin47@vsnl.com