Sciencemadness Discussion Board

How do the PTFE "Chem-Vac Stirrer Bearings" work?

SuperOxide - 6-3-2022 at 10:52

I recently purchased a PTFE stir bearing, thinking that it would have some sort of cylinder inside that would spin and be able to sinch down onto a stir shaft, but it turns out that's not at all the case.
This is basically the same thing.


I did Google it, and I see videos of people showing how to put them together, but none that show how to put it into use properly with a vacuum tight seal. But maybe my Google-fu is lacking.

macckone - 6-3-2022 at 23:48

The orange piece provides the seal but requires something to fit almost exactly to begin with.
You would put vacuum grease on the fitting and then tighten down the top so the shaft turns but not so tight that it creates a lot of friction.

The stirrer bearing I have just uses an o-ring which is less than ideal for high vacuum and leaks a good bit.

With any stirrer bearing I would expect to need continuous vacuum. The one exception is mercury stirrer bearings, those will hold a really good vacuum.

I would expect this will not hold 5 torr but will achieve it with a running vacuum pump.

SuperOxide - 7-3-2022 at 06:27

Quote: Originally posted by macckone  
The orange piece provides the seal but requires something to fit almost exactly to begin with.
You would put vacuum grease on the fitting and then tighten down the top so the shaft turns but not so tight that it creates a lot of friction.

The stirrer bearing I have just uses an o-ring which is less than ideal for high vacuum and leaks a good bit.

With any stirrer bearing I would expect to need continuous vacuum. The one exception is mercury stirrer bearings, those will hold a really good vacuum.

I would expect this will not hold 5 torr but will achieve it with a running vacuum pump.

Thanks! I actually noticed the little gap at the bottom that would seem to accommodate a seal like that.

How about the larger ring that screws on from the bottom? What is that used for?

monolithic - 8-3-2022 at 07:34

Quote: Originally posted by macckone  
The stirrer bearing I have just uses an o-ring which is less than ideal for high vacuum and leaks a good bit.


What kind of stir shafts/paddles do you use? I also have one of these shitty Chinese o-ring style stir bearings. Even with mineral oil lubricant, it burned a groove into my PTFE stir shaft within a few hours.

SuperOxide - 8-3-2022 at 10:03

Quote: Originally posted by monolithic  
Quote: Originally posted by macckone  
The stirrer bearing I have just uses an o-ring which is less than ideal for high vacuum and leaks a good bit.


What kind of stir shafts/paddles do you use? I also have one of these shitty Chinese o-ring style stir bearings. Even with mineral oil lubricant, it burned a groove into my PTFE stir shaft within a few hours.

None yet. What would you recommend I use? I've only ever used Magnetic stirrers before.

macckone - 10-3-2022 at 08:31

Quote: Originally posted by monolithic  
Quote: Originally posted by macckone  
The stirrer bearing I have just uses an o-ring which is less than ideal for high vacuum and leaks a good bit.


What kind of stir shafts/paddles do you use? I also have one of these shitty Chinese o-ring style stir bearings. Even with mineral oil lubricant, it burned a groove into my PTFE stir shaft within a few hours.


I use a chinese ptfe stir shaft/paddle and stirrer bearing. The first thing I did was replace the rubber o-ring with a viton o-ring. Viton o-rings are pretty cheap at auto supply places. This give a better seal but I still use some oil, this seems to prevent a lot of erosion.

macckone - 10-3-2022 at 08:47

Quote: Originally posted by SuperOxide  
Quote: Originally posted by macckone  
The orange piece provides the seal but requires something to fit almost exactly to begin with.
You would put vacuum grease on the fitting and then tighten down the top so the shaft turns but not so tight that it creates a lot of friction.

The stirrer bearing I have just uses an o-ring which is less than ideal for high vacuum and leaks a good bit.

With any stirrer bearing I would expect to need continuous vacuum. The one exception is mercury stirrer bearings, those will hold a really good vacuum.

I would expect this will not hold 5 torr but will achieve it with a running vacuum pump.

Thanks! I actually noticed the little gap at the bottom that would seem to accommodate a seal like that.

How about the larger ring that screws on from the bottom? What is that used for?


The larger ring is for removing the bearing. You screw it down and it forces the bearing out of the neck. If you have a good grease seal and a high vacuum it can get wedged pretty good.

SuperOxide - 11-3-2022 at 17:20

Quote: Originally posted by macckone  
The larger ring is for removing the bearing. You screw it down and it forces the bearing out of the neck. If you have a good grease seal and a high vacuum it can get wedged pretty good.

Yes, I found this one out the hard way. I got a new vacuum desiccator (from Syn) and used wayyyy too much grease. I damn near broke it trying to slide the two layers apart. I blew in air at a slow rate and eventually got them to unstick enough that I could slide the top off, lol.

But yeah, I had a feeling that's what it was for, thanks!

Dr.Bob - 20-3-2022 at 16:26

I use 10 mm glass stir shafts often for positive pressure inert reactions. You can use them with glass stir bearings and oil, as I have before, but I now use some plastic sleeve bearings, that allow a decent seal, but can run for hours without any real wear, but I do need a small flow of nitrogen to keep the inert atmosphere.

I have not tried the ones shown above, but have never had great luck with any mechanical stirring and a real vacuum. Other than very expensive one, most leak and wear quickly. Also, the glass rod can wear some, and eventually wear down, which makes most seals leak worse, so a new glass rod is likely needed for a vacuum seal. Most of the glass rods I use are well worn but work fine for my uses. Good luck.

[Edited on 21-3-2022 by Dr.Bob]

Dr.Bob - 22-3-2022 at 13:35

Also, if you are in the US, I have PTFE paddles for the normal glass stir shafts in many sizes. I have some other odd stirring things, but for simple glass shafts, the crescent shaped ones are best. I would be curious to know if the vacuum bearings work, as I might be able to use them at work. I keep having to scale things up there, so using a lot of big flasks and stirrers.

[Edited on 22-3-2022 by Dr.Bob]

Organikum - 22-3-2022 at 16:54

Quote: Originally posted by macckone  

........
I use a chinese ptfe stir shaft/paddle and stirrer bearing. The first thing I did was replace the rubber o-ring with a viton o-ring. Viton o-rings are pretty cheap at auto supply places. This give a better seal but I still use some oil, this seems to prevent a lot of erosion.
Actually out of the box and even with a replaced o-ring the glass thermometer adapters work better, they come with a compressible PTFE/rubber seal with flat surface and are not so prone to ruin the shaft.

The Chinese parts though are easily modified to resemble a classic "stuffing box" and will then work very well in the sealing department, not in the bearing department - a 7mm shaft in a 8mm hole is a joke obviously. One has to solve the bearing part separately.
Modification is to drill the already existing 12mm part of the hole deeper by 110 to 15mm and to buy some woven PTFE stuffing for stuffing boxes. looks like a wick and is greased with some PTFE like stuff, the same which is added when sintering PTFE parts. And then you fill the stuffing box with the wick and compress it with the top screw and there it is the surface seal you want. Taking the direction of your stirrer into account one can also lay the wick as spiral into the space (the rod or a placeholder must be in place logically) and then one will get a slight pumping action. Usually one chooses outwards and this will suffice for pretty high vacuum when the stirrer is running.

The PTFE wicks for stuffing show up on Ebay (Germany) from time to time, they should be always on US Ebay and Dr. Bob knows probably even where to buy them otherwise.

SuperOxide - 26-3-2022 at 08:01

The stir bearing I got above is a PTFE bearing for a 29/42 joint. I just got a different type for a 24/40 (this one), and I tested it out under vacuum and it seems to work well. However I noticed there was a certain amount of friction that I wouldn't expect from anything that would be considered a "bearing". So out of curiosity, I took it apart, and I noticed this:


It looks like when it comes down to it... it's literally just a fancy way to cinch down on a greased O-ring to make a seal. And the only thing that rotates is the shaft that's in contact with the O-ring.
Am I understanding that right? Because if that's the case I don't see how a simple thermometer inlet adapter (O-ring based) couldn't do the exact same job.

SuperOxide - 27-3-2022 at 11:27

Quote: Originally posted by Dr.Bob  
Also, if you are in the US, I have PTFE paddles for the normal glass stir shafts in many sizes. I have some other odd stirring things, but for simple glass shafts, the crescent shaped ones are best. I would be curious to know if the vacuum bearings work, as I might be able to use them at work. I keep having to scale things up there, so using a lot of big flasks and stirrers.

[Edited on 22-3-2022 by Dr.Bob]


I have a set of the crescent paddles as well.

(the paddle is obviously upside down, but it was just for the pic).

As for the vacuum distillation thing, while I haven't used it for a full vacuum distillation, it seemed to work ok if the stirrer motor you use can get a good grip on the shaft. I only had a drill chuck on it which I worried would crack the glass if I went too tight on it, and because of that the shaft was getting pushed in from the pressure outside the 1L flask.