Idler Roller Bearings: Living the Good Long Life?

Tag : Shaft Oil Seals

Most bearings are designed for a hard life, expecting to end up ona drive shaft of a high-speed motor with a heavy load. Somebearings, however, end up with a relatively cushy job in therelatively light-duty life as the bearing in an idler roller on acoating line or slitter/rewinder. Note
If you run an extremely high-tension web, like steel or anextra-thick paper product, you likely never think about rollerdrag, so stop here.
People look at me funny when I say you should be able to put in anidler roller and never replace the bearing. To convince yourself ofthis, get out your favorite engineering handbook and calculate thelife expectancy of your bearing based on process rpms and the loadfrom roller weight and web tension.
You likely will find an expected life of 10 or even 100 years.These bearing life expectancies may seem unreasonable, but they areachievable if you consider a few real-world factors. Tend to thesefive factors and you too may see bearings lasting a decade or more. Size
What is the right size idler bearing? The easy answer is thebearing needs to have an inner diameter bigger than your shaft andouter diameter smaller than the roller's shell. On the plus side,larger idler shafts will have less deflection, but larger bearingswill have longer expected life but also more friction, especiallyin greased, contact seal bearings.
Some idlers eliminate the need for large inner diameters byeliminating the through shaft. A small diameter stub shaft canreduce bearing diameter greatly, but the tradeoff is life. I think¾ in. is the place to start on bearing inner diameter unlessyour idlers are running under extremely light loads. Lubricants
From a low-drag point of view, no lubricant is best, but a bearingwith no lubricant will fail quickly. The answer is to use either ofthe following options. Use 5%-25% fill with a light oil lubricant. Avoid petroleum-based lubricant altogether and opt for aPTFE-based, low-viscosity lubricant.
For low-tension web handling, just say no to Zerk fittings on idlerrollers. A Zerk fitting doesn't mean a roller will have too muchthick grease inside, but I've never met a Zerk fitting that doesn'teventually have a date with the pump-pump-pump of a grease gun. Seals
Contact seals add to idler roller friction, yet with no seals, dustand debris will get in and destroy a bearing. Labyrinth seals,where any particle must travel a tortuous route to get to the ballsin the bearing, are the preferred answer. Some low-drag bearingsuse a lubricated felt seal to block contamination. Environment
The top environmental questions for bearings are heat, humidity (orlack thereof), ozone (near corona treaters), and contamination(such as near slitting). Sufficient lubrication, non-petroleumlubricants, and labyrinth seals are the answer. Loads
Bearings are good at supporting loads perpendicular to their axisof rotation, but the real world may exert loads in otherdirections. Undesired side loads may result from an over-muscledroller assembly or from a shell's thermal expansion. Excessiveradial loads may come from thermal expansion differences betweenhigh thermal expansion of aluminum shells andrelative-to-low-expanding steel bearings.
Twisting loads are generated from misalignment of the bearing outerand inner race, usually from overly small shafts. A bearing in aspherical bushing mount can minimize this, or you can use basicengineering to model the shaft and shell deflection and keep thebearing angle mismatch to a minimum (usually less than ¼ deg).