plastic gears for autoclave 3

[subsearobot]

Hi All,
I am beginning a design that will require a planetary based speed increaser used in a small hand held device. As I am starting this design, I haven't really begun to wrap my head completely around the requirements; the user will manually turn a knob connected to the carrier, the sun will be the 4x output which will turn a threaded rod which will adjust the position of an element.

I have spec'd gearboxes for other applications, but never designed one. here are a couple of general questions:

1) anyone familiar with a decent bearing grade thermoplastic that will withstand ~131degree steam cycles? we will injection mold the final product.

2) I suspect that the torque requirements will be low (~3 in.oz input); I have read that plastic tooth profiles are not necessarily a standard involute. Anyone familiar with a good reference for tooth profiles in plastic?

3) I am on the fence whether this application warrants hiring a consultant. please comment. I am not an idiot, but again, no experience (nor organizational experience) in gear profile design. can I expect to pick up the required knowledge in a week or more like months?

thanks!

 

[Compositepro]

My first thought is that it seems silly to design a speed increaser to turn a screw when it would be far simpler to design a ball-screw or similar that does not require a gear box at all.

 

[Pud]

Q1: Poly Ether Ether Ketone (PEEK) will be ok for autoclaveable gears.

H

www.tynevalleyplastics.co.uk

It's ok to soar like an eagle, but weasels don't get sucked into jet engines.

 

[subsearobot]

hey now compositepro, I said I was not an idiot! ;-) that said, I am ignorant of gear design.

Much background work, and many concept mechanisms, have gone into choosing this design path. couple reasons include need of an operator selectable transmission to change between 1:1 and 4:1 gear ratios. Using a high-lead screw had back-drive problems from the output side, plus analog position feedback becomes overly complicated with a leadscrew, which still requires small instrumentation type gearing that will withstand autoclave.

Our typical autoclave stable material is ultem 1000 or 1010, but I have heard anecdotal stories that it has a propensity toward galling, especially when interfacing other moving ultem parts (threads or gears). (we do not use ultem for bearing type applications).

 

[subsearobot]

will look at peek- thanks.

management is used to ultem prices, they may balk at peek but if performance is improved, the beancounters can suck it!

 

[tbuelna]

No offense subsearobot, but as compositepro pointed out, the epicyclic input gear stage configuration you describe seems to act in opposition to the leadscrew output stage.

There are many thermoplastic materials, such as PEEK, Torlon or Vespel, that would be capable of exposure to 130degC steam. But I don't know if they are suitable for a medical device application if that's what you intend. For the epicyclic or leadscrew assemblies, a combination of thermoplastic and corrosion resistant steel would likely be your best bet.

 

[subsearobot]

not sure I follow regarding a gearbox opposing a leadscrew. But I understand your reticence. My first couple concepts revolved around the simplicity of using a high-pitch leadscrew vs using a gearbox and less-steep threads. I did not want to add so much complexity with gears and couplings. Perhaps I have not given enough detail of all the design requirements for a clear understanding. I'd love to give more insight, as the mechanism is quite interesting, but trade secrecy is important in our industry.

suffice it to say, I seek a relatively cost effective injection moldable plastic that has good wear properties, low moisture absorption and high temperature stability. Preferably, a material that is typical for use in gear train systems. PEEK may be the answer; I really like Torlon for other applications though. Has torlon been successfully used for gears? We are looking into Rydell and ultem, as both will be used for other components in the system.

As far as tooth profiles, I think our requirements are low enough that a standard agma profile will be adequate, but would love to hear anyone's experience on that front.

thanks again for your input!

 

[3DDave]

Why aren't the gears entirely external to the autoclave - for certain the knob is.

If I had this to do I would have a fine adjust and coarse adjust with the coarse being direct drive of the shaft and fine being a worm that is pushed into engaging a wheel on the drive shaft. Other than that I'd end up looking for existing mechanisms to do this as you've described.

The only thing I think about plastic gear teeth is that their stiffness is much lower so there needs to be more tip clearance to account for tooth bending, though helical teeth can help out with making smooth transitions. However if the loads are small then the deflection will be small also; you'll need an FEM to get a better idea based on the expected loads.

 

[tbuelna]

Based on the extremely low input torque described (~3 in-oz) and the requirement for no back-driving, I imagine a simple lead screw mechanism should suffice. Make the screw from stainless steel and make the nut from PTFE. Both materials are compatible with autoclave sterilization.

It is not practical to mold very small profile gears in plastic due to factors like shrinkage. And while you can machine plastic gears at smaller profiles than is possible with molding, there are still limits to how small the plastic gears can be produced with reasonable accuracy. With a 4:1 ratio epicyclic having a 3 in-oz input torque, the profile size of the gear teeth would be extremely small. And it would present a bit of a challenge to produce the gears with any accuracy even using metal.

 

[Pud]

Small gears moulded - I have seen this machine in action (although the parts were red when I did ).

Micro gears

There are many others moulding tiny gears. The shrinkage is allowed for in the tool - even the differing shrinkages due to varying tooth thickness.

H

www.tynevalleyplastics.co.uk

It's ok to soar like an eagle, but weasels don't get sucked into jet engines.

 

[gearcutter]

Nice video Pud, thanks for the link.

 

[subsearobot]

Pud- exactly. I purchased a cheap hobby motor with a planetary reducer to see the current state of the art. the planets are about .050" in diameter. the thing cost ~$10. I am going well over that size, so molding will not be a problem. (famous last words!)

3ddave, thanks for the input. creating a higher clearance gear sounds necessary, though torque wise, I believe I will be over-sizing this gearbox by quite a bit due to other design constraints. (primarily, a shaft runs through the sun gear). the mechanism is not part of the autoclave, simply an instrument that will need to withstand typical autoclave conditions (not while in use).

tbuelna. thanks. As I stated before, I looked heavily into lead screws initially. leadscrews did not work with all of the design constraints that we are working with.

We have experienced mold engineers over here, as well as plastic engineer. We routinely design and manufacture injection molded parts with tolerances of better than .0002" (2-tenths) in ultem and .0005" in polypro. shrinkage is (sort-of) predictable, and it's simply an iterative process to deal with it. that said, no one has experience with gears, hence part of my reason for the post.

 

[subsearobot]

for the archives:
this article is a great summary of polymers used in gearing.

"The most often used thermoplastics in gears are acetal, polyester, and nylon. These engineering plastics create strong and precise gears having good fatigue and wear resistance. The range of plastics available for gears helps designers cope with the temperatures gearsets will seein use. Acetal copolymer works well up to 100°C, polybutylene terephthalate can be used to about 150°C, nylon 6/6 to about 175°C, and polyphenylene sulfide (PPS) to 200°C. High temperature nylons and polyphthalamide have somewhat lower thermal limits than PPS, while liquid crystal polymer, imides, and polyetheretherketone have higher limits."

http://www.gearsolutions.com/article/detail/6366/a-plastic-gear-design-update