3D Printed Gears

[M.B. Naegle]

Thought I had started a thread on this awhile back, but not seeing it. I'm looking for some general 3D printing advise/experience:

Earlier this year we bought our first 3D printer. It's a cheap commodity one (Longer brand), but we bought it with the intent of getting our toes wet learning how the process can fit in with our conventional and CNC machine shop and already have some parts being made from PLA, ABS, and others. We service a specific industry making parts for our own machines and others we rep. and most parts we make are in volumes of 1 to 50 pieces at a time, with repeats of those parts coming every 6 months to 4 years. Low volume, high turn over, etc. Mix of newly designed parts and finding new ways to keep making 80+ year old parts.

Some pieces I'm looking at pushing towards the printer are gears. Currently we machine some specialized gears from grey iron, mild steel, 4140 and A2, and we purchase more generic gears from the usual sources. There's a few specialized gears we sub out to be made from Phenolic, and prices on those gears has been going up prohibitively. What I'm wondering is if we made a gear from 3D printed Kevlar fiber or carbon fiber (I've been looking at Markforged machines), could we expect the same strength and wear qualities as the phenolic gears of the same dimensions? Or does it really depend on the tooth cross sections, maintained and momentary load, etc.?

These phenolic gears are in open lubricated environments running 500rpm or less. I don't think I've ever seen one worn out, but periodically teeth get broken, so we do sell them as replacements in addition to new builds. With a couple exceptions, most are a long tooth design for variable pitch diameter, and were steel 70+ years ago. Phenolic has been an improvement in the wear properties and operating noise level, but it's not getting any cheaper in the quantities we need. Some of the other tool steel and mild steel gears are also candidates for 3D printing in an effort to decrease noise and improve wear properties, but they are higher load bearing so I'm expecting we would have to do some R&D to see if 3D printings would hold up. I'm curious about the Phenolic replacement though as it could potentially pay for the machine investment, while the steel replacement may or may not pan out.

So in short: What if any 3D printing material is a drop in replacement for Phenolic Resin Gears?

 

[GiroDyno]

Can you print/machine a mold and cast your own gears? https://www.smooth-on.com/products/task-13/
I've 3D printed gears using resin and filament printers, had better "shape fidelity" with the resin printer using Siraya Techs tough material
A 50/50 mix of that and Anycubic Eco resin is my go to for durable parts in that printer.

 

[M.B. Naegle]

Can you print/machine a mold and cast your own gears? https://www.smooth-on.com/products/task-13/
I've 3D printed gears using resin and filament printers, had better "shape fidelity" with the resin printer using Siraya Techs tough material
A 50/50 mix of that and Anycubic Eco resin is my go to for durable parts in that printer.

I suppose casting them from resin is another option. I'd imagine it would take a dedicated metal mold to get reasonable** tolerances though?

**reasonable = at least +/-.005" or better. These gears are not super precision, but they do need to have as little axial run out, squareness, and dimensionally accuracy as you would expect machining them conventionally. None of them are hobbed or finish ground, but the ones we make in house are made on a 4th axis VMC.

 

[GiroDyno]

It might be tough hitting and holding that tolerance on finished parts with an FDM printer...

You'd have to check Smooth-On or whoever for shrinkage calculations based on material. With that shrinkage number in mind I'd print or machine and then polish shrinkage-scaled "master gear" and make a silicone cavity from that, then cast the PU or whatever material works for your gears into the silicone mold.
If you have a design finalized 3D printing is a really inefficient process for making parts as simple as gears. That Task-13 resin has a 20 minute cure time (not sure how big your gears are but I bet you can't print them that fast). Even faster if you made a mold with X number of cavities so you can make X gears in 20 minutes vs X*slow printer cycle time.

 

[EmGo]

What size and pitch are the gears you want to make ? All spurs ? The average tolerances on steel or bronze (look up the values for strength and longevity) are a thousandth (.001") or less. That's on crummy, commercial grade, low-cost gears. Gears on which you do not have to do 3d modelling, just make a round thing in the lathe and indicate it on the arbor, touch with the cutter and push the go button.

It would be way cheaper and easier and faster to get a small hobber or shaper and learn to use them. Honest.

 

[memphisjed]

igus sells almost the same plastic for printers as they use for gears. I got a small sample of it, the little bit that came out right was fairly strong. It takes a sample size to get your printer settings.

 

[EmGo]

I got a small sample of it, the little bit that came out right was fairly strong.

So that'd be about 130,000 psi yield ? 15% elongation ? And you can hold .001" on size ?

I do not understand this fascination with 3d printing gears. Okay, for gyro gearloose in the basement, sure. One part for an 1937 HO scale combine model, he's not going to pay $150 for one and time has no value, it's a hobby.

But for a business, as we are speaking here, which has to stand behind its products :

3d printing is not accurate enough

3d printing requires at the least a profile modelled on a computer. If the user doesn't even understand gears enough to know about base diameters, diametral pitches, tooth thickness, all that stuff, then this is begging for trouble

3d printing makes plastic parts

For what Mr Naegle is talking about here, a hobber or shaper for up to, say, 6" diameter is easy to come by, for anywhere between $2500 and $7,500. Could be twice that if you want to go to a dealer and get a warranty and advice and all that. Nothing is free in life, for some people that's probably good value.

Once you have the machine and a cutter, all you need is a lathe and an indicator and a pair of flange mics or some wires to make good, reliable, accurate, long-lasting, quality gears. Gears that you can warranty with confidence, gears that will work and hold up ... not flaky-ass pieces of plastic that might run a month, or maybe a week. Or maybe ten minutes.

Gear cutting is not a mystery. People have been doing it for over a hundred years. In fact doing it right is easier than this 3d printer stuff and the product is a thousand times better (unless the circumstances are very very unusual, which does happen, so blanket statements can always be countered "I know a guy who ..." but that doesn't change the reality of the situation).

MB ? Just get a little hobber or shaper. Talk about what you need here, if you like, I'm sure zahnrad and a couple others who have a clue will chime in. As well as a bunch of home-shop-harries

 

[memphisjed]

eg, he asked about plastic printed gears. Igus is a company that sells printed plastic gears, with a stellar reputation in product and service. They also sell plastic for printing your own - If his printer can hit tolerance is outside scope of question.
I would say 80 percent of gears do not need .01" tolerance. Wall clocks to microwave oven turn tables and garden shears.. Old bull gears on a cider press meshed with pinion using dirt for lap.
Agree, most things printed are slow and why bother statements. For testing a shape or jointery, quicker than ordering and waiting parts, for caster types, making fixtures for anything, parts for dubbing machines.. printers are handy.

Cost wise, the price of the word gear is ridiculous, if you want gears that go together and assembled you are looking at a down payment for a house.

 

[mhajicek]

could we expect the same strength and wear qualities as the phenolic gears of the same dimensions?

There's one way to find out. Make some and test them. I now have a printer than can do CF Nylon; I could make a short run for you to test.

 

[EmGo]

There's one way to find out. Make some and test them.

I'd be the last one to tell a person not to do something. In fact, I'd happily watch while someone takes a carbon fiber tube, glues some ti to the ends, climbs in and drops themselves down to 13,000 feet. And when it goes kerplonk, nod my head and think, "yeah, pretty much what I expected."

Phenolic and nylon have very different properties. MB Naegle knows how long the phenolic (and steel and bronze) last. Nylon may test out perfectly for "strength" but there are other characteristics which a gear needs.

One is fatigue strength. A big deal in plastic gear design is giant fillets in the roots, because plastic gears like to crack there after a while then split into two halves.

Another difference is, the teeth are designed differently because plastic bends a lot more, so rather than designing for the strength of an individual tooth, they try to get as many teeth in mesh as possible. Different shape, specialized for plastics, does your online gear generator have this ?

Also, because the teeth bend a lot more, the shape should be different because otherwise there's a lot more clanking and clunking when a tooth engages or disengages. And there tends to be a lot of internal heating caused by all this bending which also effects the long-term life of the part.

You also have however the gear is connected to the shaft - often a keyway. How often do you see a plastic gear that has split at the sharp corner of the keyway and broken in two? The ones that didn't crack at the root and split in two, I mean ?

Aging is another factor. Plastics tend to not age well. Does this part have to live for a year or five years ? Or ten ?

And we have the absorption of water. I have read where people say "no no, nylon doesn't do that" but a friend made rudder bearings out of nylon. It was a 65' boat with the biggest hydraulic steering you could get at the time (not towboat size but huge for pleasure boats). They worked great for the first week, then the whole boat started to groan and shiver and jump up and down just from steering - bushings had absorbed water and swole up tighter than a hog's ass in fly season. He had to skim another 050" out first time the boat was pulled.

These are just the things I can think of. I'm sure there's more - usually the stuff that bites you is the stuff you don't think of.

So, yeah. Make some parts and test. I'll stay up here on the support vessel and listen on the headphones.

 

[johnmontrose]

One is fatigue strength. A big deal in plastic gear design is giant fillets in the roots, because plastic gears like to crack there after a while then split into two halves.

OP, please do not neglect the importance of this point. A gear to be made by an additive process benefits from (perhaps needs) different geometry to one made by a subtractive process.

Above all, that means it would be unwise to take the existing drawing of your gear and send it to your 3D printer.

If you are constrained by the gear having to fit in with and mesh with an existing setup, you cannot start from the ground up and do a total redesign, but the tooth root is one area that offers a great deal of scope for improvement. On a subtractively-machined item, the tooth root is the swept path of the cutter. On an additively-manufactured item, the root can be as big as possible such that it does not interfere with the mating gear.

See:

https://www.researchgate.net/publication/266794877_Optimization_of_Tooth_Root_Profile_of_Spur_Gears_for_Maximum_Load-Carrying_Capacity

Analytical procedure for the optimization of plastic gear tooth root

The increasing diffusion of plastic gears in many industrial sectors makes it necessary to have a better knowledge of the bending strength properties …

www.sciencedirect.com

 

[M.B. Naegle]

Thanks for the insight. I kinda expected that a gear printed from straight plastic, whatever variety, would have different strength properties. It might work with a redesigned tooth profile, but I would be concerned how it would compare in longevity and want to do a lot of testing before I commit, and really we'd rather have gears of the same profile if possible so if one gear of a train breaks, you only need to replace the one. I was hoping that the fiber matrix printing mediums might at least parallel the phenolic properties "if done right". The process to my understanding offers the added benefit of designing how the fiber structure is laid out, rather than everything having the same fabric crisscross grid.

Best case (proper lubrication, no jams or abuse), we expect these gears to last 50 year or better. I'm in no way an engineer and frequently resort to real world testing in leu of their calculation and simulation magic. With my skill set, I'd rather know something was over-engineered than hope it's bare minimum. It seems like every time I work with a part that was engineered to fit exact specs, I find too many fringe cases where it failed for reasons that should have been considered and accounted for.

We're looking at other re-designs as well for some of these gears, such as replacing with serpentine belts, which would obviously be non-backwards compatible without replacing multiple pieces, so we'd still need a few of the old gears on the shelf for legacy support. I'd rather avoid the Haas model of when something is "too old", you have to drop $10,000 to upgrade a part/system that was $100 to repair 10 years ago, but time marches on. It's sucky, but at least it's better than no support at all. Our machines tend to have such long life spans because the jobs they do (Leatherwork) don't see or need as much innovation as other industries, and many of our customers are working a business model where there's no way they can afford to retool their shop every 5 to 10 years just because their stuff is "old." Were always looking at ways to innovate, but it takes some patience and tact to ensure that innovation for the future market doesn't lead to alienating the current market.

The fiber printing machines seem worthwhile enough to invest in, but without standing jobs that will allow it to pay for itself that won't create potential warranty cases, it'll make it a hard sell on the wallet.

 

[M.B. Naegle]

Sorry, it's not a print, but here's a few snaps of what I'm looking at.

This is the long tooth phenolic gear. There's a couple of different gears like this that use the same tooth design, but different diameter and width. 4" to 3" and around 3/4" wide. Never had one of these break in half, but have seen some broken teeth.

This gear mates with a steel gear at a fixed pitch and used to be made of phenolic, but switched to nylon and while they do wear out eventually, the teeth don't break like the old phenolic ones. These ones we don't have any problem making or sourcing, but good case study.

These are currently case hardened steel, and will likely be A2 in the next run. Lots of shock load on it and speeds up to 1000 rpm, and has an intentional loose mesh with its mating helical gear. Years ago we made some from Delrin (I think) and they worked fine under normal conditions, MUCH quieter as the steel ones ring, but hit a jam and teeth break off, so went back to steel. It takes 50 years of use, but they do wear out, otherwise the only way we hope 3D printing could help is to decrease the ring and make them cheaper to make. The tooth profile on these would need to stay largely the same as it has to be backwards compatible. It mounts on a fine thread shaft, so a steel hub would likely be nessisary. We've also considered setting O-rings in the sides to help with the ringing.

 

[mhajicek]

Best case (proper lubrication, no jams or abuse), we expect these gears to last 50 year or better.

50 years? I'm very surprised if a phenolic gear can last 50 years. It must be under pretty light usage.

 

[M.B. Naegle]

50 years? I'm very surprised if a phenolic gear can last 50 years. It must be under pretty light usage.

Compared to metal working applications and higher horse-power gear boxes, I imagine it's pretty light. Most of these machines have a 1/2hp motor driving them. They're like a light duty rotary press with other cutting and forming functions going on. Used for making leather belts. The phenolic gears are to drive upper and lower rolls that will spread apart (1/8" to 1/4") as a strap goes through the machine, which is the reason for the long gear teeth. Some machines in factories will see 40 hours a week continuous usage, while others maybe 40 hours a month. For a long time the machines would be set up to operate at a fixed "high" speed of 600 to 800 rpm, but more recently the new ones operate via inverter and can be set to run anywhere from 100 to 500 rpm as the operator sees fit.

 

[sfriedberg]

Never had one of these [phenolic gear] break in half, but have seen some broken teeth.

I would not expect a phenolic gear like the one you showed to break in half unless something simply crushed it to death. It's basically a fiber-reinforced composite material.

I don't have enough experience to say if the Markforged carbon fiber reinforced 3D printing has the same strength and lifetime as the fabric/phenolic resin. I am pretty confident it won't have the same bore, keyway, and tooth flank accuracy and smoothness unless you do some cleanup machining or grinding. At the HS robot lab, they use a Markforged printer with carbon fiber to make parts that get a lot of physical abuse, but I really can't say whether they are more durable than comparable aluminum parts. (Not enough samples/time/experience.) Nobody has complained loudly in my hearing that they snap apart, at least.

 

[sfriedberg]

Why ...

Spammer reported

 

[dgfoster]

I am only an occasional visitor to this subforum. So, I am unaware of prior discussion of 3D metal printing. But, it might be interesting to at least do a cost/benefit analysis of getting a metal wire or powder 3D printer.
I suspect that for your inhouse relatively small number of gears needed, you might not be able to fully justify one. On the other hand, if you have a small space to dedicate to a machine and the interest to personally learn or hire an operator, perhaps you could be bookending the machine age---by supplying 100-year-old-technology parts using cutting edge methods and perhaps becoming a regional prototyper for others needing printed metal parts.

I, personally, enjoy resin and filament 3D printing (I just moments ago loaded up a file in my Prusa MK3 to make a decent-sized PETG pattern piece) but I doubt that filament or resin printers can produce the toughness and dimensional accuracy you need for your parts.

If Elon can print complete rocket engines from metal, we oughta be able to make a gear!

Denis

 

[dgfoster]

I meant to include a link to a site that makes some general cost comparisons:

Metal Printer Comparisons

Denis

 

[john.k]

Phenolic gears were often used in woodworking machines in days gone by ............I used to get lots to repair ,as often as not ,stripped keyways ,and generally in a small diameter motor shaft pinion ............these gears were often helical ,too...............Quality cast iron woodworking machines are long gone now ,replaced by die cast and plastic.