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Is there a filament strong enough to work for high torque gears?

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This is my Hitec HS-9380TH servo. I have 36 of them in each of my robots. I will be building thousands of robots as a product. At the moment I am prototyping.

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See the specs for Torque, and understand that 472 oz.in is about as much force as a ten year old trying to arm wrestle.

This is the twenty toothed gear that fits precisely over the spline shaft of the servo.

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I now need to create a second gear that has the same twenty teeth for a 1:1 ratio but it needs to be longer - around 50mm long to fit into a rotating adapter. I need to design a few extra parts to it so I need to be able to print it at home.

IT NEEDS TO BE STRONG.

Each gear needs to be 11.5mm diameter and have 20 teeth. Each tooth is 1mm.

Using PLA, ABS, Nylon, XT for the gears would be like using a cheesecake as a bullet-proof vest. I'm quite sure I need metal but I think there are carbon gears since recently.

What filament can I use to print these gears with please?

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I don't know if there are any materials that would hold up to that, but, what I'm thinking is that there's a reason the slightly more expensive servos don't use plastic. And that's purpose made plastic/nylon that has been injection moulded.

But then again I guess it depends heavily on how much use the gears are going to see.

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3 N m.

At 11.5 mm OD that's 289.8 N of force on each tooth (not really, as the OD isn't the pitch diameter, but we'll go with it). That's respectable, but still less than you'd see on 12V power tools.

How thick is this metal gear? I'd give ProtoPasta HT CF PLA a try. I'll have to dig into my reference material to give you a GOOD answer, but here's the relevant data for ColorFabb's XT20: http://colorfabb.com/files/TDS-carbon-en.pdf

Have you considered printing both gears and switching to a double helical profile? You would be spreading the force over multiple teeth. A single helix gear would introduce axial forces so you'd want a double helical gear.

Edited by Guest

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PEEK is very strong but you need a printer that can go up to around 330C. PEEK would probably actually work for this but I doubt anything else will - not PLA, ABS, NYLON, PET, XT, etc.

There are CF filaments (carbon fill) but none of them are stronger or stiffer than regular PLA because the CF is cut up too small to help. But they look nice!

I've tested many materials - they are all about the same ultimate strength as PLA - the main difference is in flexibility. The more flexible the more durable but that doesn't help with gears - you don't want it flexible because then it will last forever but it will also slip gears often. I mean nylon will last forever - no human can break a nylon gear without tools but it will slip due to it's flexibility.

If you are a mechanical engineer and have solid works or other modeling program you can model the gear strength. The young's modulus, ultimate strength numbers are posted for many types of filaments plus I've tested these for a few filaments.

Stick with gears from china.

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Out of curiosity I looked at the numbers for ColorFabb XT to compare to ColorFabb XT CF20, and they use different test standards.

http://colorfabb.com/files/TDS-carbon-en.pdf

http://colorfabb.com/files/amphora_pds_en.pdf

I'm not sure if the methods are comparable, as one is reported as ISO and the other is ASTM. I am, unfortunately, only familiar with ASTM test methods. I should really get more familiar with ISO methods.

IF the methods are comparable then it looks like the XT CF20 performs better across the board.

I'd need to get out my machine design books and go through the gear calculations to be any more confident than just saying "try it".

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Nice!  Well comparing XTCF20 to their regular PLA/PHA product here:

http://colorfabb.com/files/FKUR/TD_BIO-FLEX_V_135001_en.pdf

It's about the same strength (e.g. tensile strength) (comparing XT and PLA) but much stiffer - twice as stiff.  Usually this means it will shatter but this XTCF20 has a decent elongation at break so instead I would assume it would kind of have a small permanent dent if you dropped something made from this material onto a hard floor.  Which is still better than shattering.

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Nice specifications!  About twice as strong as PLA and twice as stiff.  Still a LONG way from  brass which is 1000X stiffer!  Making tiny gears will be strong enough but they aren't stiff enough and will just warp enough that they will slip and then when they slip they will self destroy.

It will obviously depend on what the servo is actually being used for... many examples of consumer robotics use plastic gears and cogs. Brass often being far too heavy... all of the cogs in a Hasbro robotic cat are nothing more than cheap plastic... as are many other consumer type robots with most little servo's only driving part of an arm or leg etc... rigid plastics are far and away the most used materials in my experience.

A good rigid nylon will be fine as would the Nylforce for most small consumer style robtics... however if your robot is some kind of large metal orientated creation then perhaps 'plastic' shouldn't even have been a consideration from the outset.

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Taulman Alloy 910 seems a good option...

4027139.jpg?844

Edited by Guest

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no no no!  Not good for tiny gear teeth.  taulman is WEAKER (or about the same) tensile strength than PLA and more flexible by about 6X.  Would be worse than PLA for tiny gears.

Nylon is INCREDIBLY STRONG because it is so flexible.  So for many applications like large gears you can put MUCH MORE FORCE on it than on PLA because due to the flexibility it will warp enough such that the forces are distributed all over the part.  But the tensile strength is about the same as PLA.  So for example if you made a short rope out of PLA and NYLON 910 they would each hold about the same weight but if you made a cube and drove a car over them the nylon would be fine but not the pla.  The nylon would squish like a pancake and pop back up after the car passed over.  Or if you made a stick and told someone to break it over their knee the PLA would break easily but not the nylon.

The problem for these gears is that nylon is too flexible - the gears would slip because the entire body of the gear would compress away from the teeth of the other gear.  Once it starts slipping you get grinding and the nylon gears are soft and will rip to shreds.  If you could add lots of pressure between the gears somehow (push the axis together with 10kg of force) then it could easily hold the 60 kg rotating force at the gear teeth.  But adding 10kg of force between the gears will cause too much friction!  Plus it might cause the metal gears to eventually wear the nylon.

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no no no!  Not good for tiny gear teeth.  taulman is WEAKER (or about the same) tensile strength than PLA and more flexible by about 6X.  Would be worse than PLA for tiny gears.

Nylon is INCREDIBLY STRONG because it is so flexible.  So for many applications like large gears you can put MUCH MORE FORCE on it than on PLA because due to the flexibility it will warp enough such that the forces are distributed all over the part.  But the tensile strength is about the same as PLA.  So for example if you made a short rope out of PLA and NYLON 910 they would each hold about the same weight but if you made a cube and drove a car over them the nylon would be fine but not the pla.  The nylon would squish like a pancake and pop back up after the car passed over.  Or if you made a stick and told someone to break it over their knee the PLA would break easily but not the nylon.

The problem for these gears is that nylon is too flexible - the gears would slip because the entire body of the gear would compress away from the teeth of the other gear.  Once it starts slipping you get grinding and the nylon gears are soft and will rip to shreds.  If you could add lots of pressure between the gears somehow (push the axis together with 10kg of force) then it could easily hold the 60 kg rotating force at the gear teeth.  But adding 10kg of force between the gears will cause too much friction!  Plus it might cause the metal gears to eventually wear the nylon.

 

You Yanks always make I laugh... if you're not an expert your Dad is...

You are quite probably correct Sherlock but telling him what he can or cannot do was not what the OP was asking us was it?

Burdickjp's alternative suggestion of using the same materials for both cogs was also a good a good idea rather than having metal on plastic.

I'll recap... he said that he is "quite sure" that they will need to be metal but then asked if there was any filament available that COULD replace it.

The cog is around 50mm long with an 11.5 diameter and fits into a 'rotating adaptor' which presumably is fitted too or joined to the moving part.

He did not mention that this cog was moving 10kg, 60kg or any other power/weight ratio... we simply do not know so your assumptions may be way off the mark... besides as you well know what some 3D printed parts are actually capable of is still very much part of the unknown... how durable they are even more arbitrary.

Let the OP make his/her own mind up whether to take note or not of suggestions and stop telling other contributors just how wrong they are and just how right you are... not required... no matter how much of an 'expert' you think you are... a forum is where contributors make suggestions which are either followed up or not... leave it at that thanks!

P.S. The final answer to the OP's question: Is there a filament strong enough to work for high torque gears?... erm... in this instance... No! :D

 

Edited by Guest

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I agree that I know of no 3d printable melted filaments that could do this other than peek. And the UM printers can't do PEEK without major modifications.

I just like to educate. I can't help teach. modulus of elasticity aka plain old modulus is a measure of flexibility where the higher the number, the stiffer. and the lower the number, the more rubbery.

tensile strength is how much pulling tension it can take before it is damaged or breaks (some people call it yield strenth where it's just enough that if you stop pulling it will recover). It's in psi (pounds pulled per square inch) or in pascals. Larger numbers are stronger.

Another measurement is elongation at fracture in percent which is interesting because if it's small like 1% then it's brittle like glass no matter how strong it is. And if it's larger then it is more likely to dent or stretch than shatter.

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I agree that I know of no 3d printable melted filaments that could do this other than peek.  And the UM printers can't do PEEK without major modifications.

I just like to educate.  I can't help teach.  modulus of elasticity aka plain old modulus is a measure of flexibility where the higher the number, the stiffer.  and the lower the number, the more rubbery.

tensile strength is how much pulling tension it can take before it is damaged or breaks (some people call it yield strenth where it's just enough that if you stop pulling it will recover).  It's in psi (pounds pulled per square inch) or in pascals.  Larger numbers are stronger.

Another measurement is elongation at fracture in percent which is interesting because if it's small like 1% then it's brittle like glass no matter how strong it is.  And if it's larger then it is more likely to dent or stretch than shatter.

 

Bless... his hearts in the right place though innit... :D

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