Shrinking bearings with cold stuff Options

[sleepdoc]

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