Acetal gear failure (temperature 212F 100C)

[mailtrams]

Hello.

I have been given a task to examine why a gear (around 12" diameter and 1" thick) have failed.
The application is in a steamer so it's getting hot.
The material is acetal but I am not sure which grade but I will get the information later today. There are speculations if it is the high temperature the wrong oil or the washing water. The gear is brown, probably from the oil, on the outside and brittle on the inside and there are crumbs of acetal. I think it's from the temperature due to that I have read that acetal isn’t suitable for temperatures over 194F (90C). And the steam, water is 212F (100C). What do you think?

Thanks Thomas.

 

[patprimmer]

Acetal is not suitable for contact with water over 80 deg C as it starts to hydrolise rather quickly at about 80 deg C.

Polypropylene will be excellent in that regard at least, but may lack the mechanical strength required.

Nylons will still degrade, but at a much slower rate than acetal. That is why some special grades are used for automotive radiator header tanks.

Polyesters and polycarbonate will be worse than acetal.

PPO is also very good with hot water, but has some serious restrictions for use as a gear that will be lubricated with grease or oil.

Regards

eng-tips, by professional engineers for professional engineers
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[mailtrams]

Thanks for the post, what does acetal looks like when it has been hydrolysed?
More info about the acetal is that it is unfilled copolymer.

 

[patprimmer]

It does not necessarily look a lot different until it starts to get micro cracks form as it is about to fall to bits, but it takes on colour from impurities in the water, so it depends on what water you use.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[Pud]

I assume from the diameter and thickness this is a "one off" application and you are going to have to get another machined up?

If so, you could look at polyphenylene sulphide (PPS) as a mid-range cost. It is used for domestic hot water boiler heat exchangers.

Top end cost would be polyetheretherketone (PEEK), which will be fine for years at 100 deg C in Water etc.

As Pat said, you might try the special nylons as used for rad header tanks, The problem might be getting it in a stock form from which you can machine your gear (also may apply to PPS and PEEK).

You will probably find that stainless steel with maybe a PEEK bearing bush will be the cheapest option if you can use it....


Cheers

Harry

 

[mailtrams]

Thank you Pud.
I am not a native English speaker so I don't understand what "one off" means but if I assume right and it's only one gear it's not so. It's made extruded and maybe a couple of hundreds a year, we also use them in room temperature applications. Now we changed to PVDF but it's much more expensive. And it was stainless steel to begin with but it wore too much on the chains.
Thanks for your recommendation on the other materials.

 

[MikeHalloran]

If it mates with a chain, it's properly called a sprocket, not a gear.



Mike Halloran
Pembroke Pines, FL, USA