Hello, I have an application wi

[reyemka]

Hello,

I have an application with a simple journal bearing and I'm using Acetal for the shaft and Noryl for the bearing. For the shaft I have parts in both Celcon M90 and a Lubricomp grade that uses a silicone additive. There is only a small torque applied to the shaft so minimizing friction is important. Imagine trying to turn a large water wheel using only a garden hose, only on a much smaller scale and using air.

My design works initially, but after several cleaning cycles the performance drops. To clean the device, we use two methods: 15min soak in soap & water, or 5 min soak in a 1:50 bleach/water solution. The device is then rinsed in clean water and left to air dry. I should note that the two parts mentioned above remain assembled during cleaning.

I'm thinking the cleaning is changing the coefficient of friction between the two materials, or the acetal is absorbing water and creating more friction. The testing we've done has conflicting results. We've tried to measure the coefficient of friction between the two parts and noticed an increase after cleaning. On the other hand, parts that failed after a few cleaning cycles start to work again after drying for a few days.

Does anyone have any insight into what failure mode is occurring here?

Thanks in advance.

 

[Demon3]

You are probably washing off the silicone. Try using a solid lubricant like PTFE or ultrahighmolecular weight silicone (from Wacker or Dow Corning Multibase) because that won't wash off.

Chris DeArmitt - PhD FRSC CChem
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[btrueblood]

Um, so wet parts have more friction? Why the cleaning cycle? If running on air, there shouldn't be much to contaminate the parts - add an air filter to the supply? Do the parts have to get wet (like, they operate underwater)? Water in the tiny gap between shaft and bearing is likely staying there, and adding some drag. You could try a vacuum drying step, or multiple vacuum cycles to help pull the moisture out of the crevices. Though Chris' answer is probably true too. How hot is the wash/rinse water - above 180F and you can cause some degradation of the acetal (I doubt this is the problem, since the degradation will be irreversible). Acetal might swell a little bit due to water absorption, but not much (tenths of a percent?), and Noryl should be even less. How much gap is in the bearing? Maybe try opening up the gap a bit, or adding some grooves - this could help if the problem is the wet film in the bearing groove, or if slight swelling is occurring.