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Posted by: sleepdoc Jul 27 2004, 02:57 PM

Hi All,

I am working on replacing my rear wheel bearings and am about to put the new ones in. I want to shrink the bearing so it will slip in easily and have seen the great advice printed here on the board about freezing the parts.

http://www.914world.com/bbs2/index.php?act=ST&f=5&t=12197

My idea up for discussion is what about using lower temperatures to get a greater shrinking effect. I have access to both liquid nitrogen and dry ice. Both have long been used in this context:




(quote ripped off a gas producer site)

" Shrink fitting
Cryogens such as Dry Ice, Liquid Carbon Dioxide, and Liquid Nitrogen can be used as low temperature refrigerants in shrink fitting of metal components. The process is rapid, effective and simple, and alleviates the possibility of damage of distortion through more traditional methods of force fitting, and metallurgical change caused by uneven or excessive local heating processes.

By using cryogens it is possible to reduce the dimension of one component from that of another by thermal contraction. When immediately offered up and fitted to the other component, and left to warm up and expand again to room temperature, a tight interference precision fit will be generated. Cryogenic shrink fitting can also be used to dismantle assemblies.

Shrink fitting can extend the life of roller bearings and other shaft fitted parts, as it maintains the interference fit for which the components were designed. When new bearings fail relatively quickly after installation, the chances are that although the bearing itself was sound, the improper installation cut short its life expectancy."





Does anyone have any concern about negatively effecting the metals in this process? For instance most are familiar with freezing a super strong bike lock in liquid nitrogen and smashing the now brittle lock with a hammer. I am certainly not going to freeze my bearing in liguid nitrogen and smack it into the hub with a hammer!

But lets say the thing slides right in and warms up in place, do you think the grease in the assembled bearing will be degraded by these very low temperatures? Or the plastic in the bearing cages changed, damaged? Or the metal does it become permanantly embrittled?

Any machinists or metalurgists out there? What do you guys think?

best Mark

Posted by: ArtechnikA Jul 27 2004, 03:07 PM

0F to -10F (freezer range) is enough.
dry ice if you must; i wouldn't need nitrogen.
there's no need, and it's an unpleasant enough job without having to do it more often than necessary.

'course it's your car, do what you want ...

Posted by: balljoint Jul 27 2004, 04:33 PM

I just used the chest freezer in my garage. Worked fine. That cryo stuff sounds fun but it has to be expensive and may be better used to keep baseball heroes in stasis while their children fight over their money and DNA.

Posted by: ! Jul 27 2004, 04:43 PM

Move Jimmy Hoffa over in the deeep freeze and plant the bearing on his chest....why complicate the issue.

Posted by: jkeyzer Jul 27 2004, 05:19 PM

Sounds like a cool idea to me. I wonder if the liquid nitrogen would actually alter the material properties of the bearing (like those cryogenic rotor guys claim). I wonder if there is any danger of creating cracks if you just dump the bearing in the LN2. I bet it would slip right in when you're done though!

Posted by: Mueller Jul 27 2004, 05:40 PM

You could always contact the bearing manufacture

http://www.fag.com

Posted by: aircooledboy Jul 27 2004, 08:54 PM

Posted by: jkeyzer Jul 27 2004, 08:57 PM

Hey, if it's free, might as well give it a try. I have access to "free" LN2 as well.

Posted by: Verruckt Jul 27 2004, 09:14 PM

I used to work as a toolmaker. We used liquid nitrogen to shrink-fit parts all the time. The limited amount of time will have very little, to no effect on your parts. Other than shrinking them a few thousandths of an inch, just enough (hopefully) to get them in place. Most of the parts and fixtures we made were for the aircraft industry, and they have some bad@ss specifications. I would get any grease out of the way and make absolutely sure that there are no "foreign bits" in this space. Finding out that your new bearing is crooked while its warming up would not be 'fun'...

Posted by: scotty Jul 27 2004, 10:20 PM

I quite agree...although I enjoy playing with liquid nitrogen every year at school

Posted by: morphenspectra Jul 28 2004, 12:16 AM

i just got my rear wheel bearings out of my freezer and tapped them in.love the basic laws of physics .my wife hates parts in the freezer,drives her nuts if she sees them in there might need to get one of those little beer fridges just for me and my parts
lol

Posted by: ablose58 Jul 28 2004, 12:37 AM

Beer fridges are COOL

Posted by: Otmar Jul 28 2004, 12:44 AM

AFAIK liquid nitrogen is best used for making ice cream at parties.
Good visuals and you get ice cream in about a minute.

Posted by: morphenspectra Jul 28 2004, 01:06 AM

alittle bit of trivia folks what two materials do the exsact opposite of all other materials when heated or cooled. there are only two.

Posted by: crash914 Jul 28 2004, 05:32 AM

Hmmmmmm....I would say Ice(water), expands when frozen, and ?????Mercury?

Posted by: crash914 Jul 28 2004, 05:33 AM

Oh yea!


Ice cream expands when frozen too.....

Posted by: ArtechnikA Jul 28 2004, 05:38 AM

rubber has a negative coefficient of expansion, and contracts when heated ...
water is most dense at 39F and expands when heated above or cooled below that.

NiTinol does weird stuff with change of temperature, but that's an artificial effect...

Posted by: andys Jul 28 2004, 10:04 AM

Huh? Can you post some evidence or a reference for this?

Andy

Posted by: ArtechnikA Jul 28 2004, 10:16 AM

i wouldn't even begin to know where i packed my Chemical Rubber properties of materials handbook ...

take a rubber band and suspend horizontally. between pushpins, for example.
heat gently, as with a butane lighter; observe the phenomonon ...

Posted by: scotty Jul 28 2004, 10:25 AM

Wait'll you see her reaction when you put parts in the oven

Posted by: andys Jul 28 2004, 10:41 AM

As with NiTinol, your example is a special case where the manufacturing process (extrusion) governs this phenomenon. Here's what a a quick search brought up: http://www.efunda.com/materials/common_matl/Common_Matl.cfm?MatlPhase=Solid&MatlProp=Thermal

Andy

Posted by: morphenspectra Jul 28 2004, 10:44 AM

water and rubber we have winner kinda good awnser artechnika.no prize on this round but the question only get harder.join us next time on james worthless questions!