plastic gears for autoclave
plastic gears for autoclave
(OP)
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!
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!





RE: plastic gears for autoclave
RE: plastic gears for autoclave
H
www.tynevalleyplastics.co.uk
It's ok to soar like an eagle, but weasels don't get sucked into jet engines.
RE: plastic gears for autoclave
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).
RE: plastic gears for autoclave
management is used to ultem prices, they may balk at peek but if performance is improved, the beancounters can suck it!
RE: plastic gears for autoclave
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.
RE: plastic gears for autoclave
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!
RE: plastic gears for autoclave
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.
RE: plastic gears for autoclave
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.
RE: plastic gears for autoclave
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.
RE: plastic gears for autoclave
RE: plastic gears for autoclave
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.
RE: plastic gears for autoclave
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...