- December 01, 2010, By Pete Eggen, Webex Inc.
When idler rolls are used in elevated temperatures, there are many concerns. One is the expansion of roller metals — especially the different rates of expansion for dissimilar metals.
Modern roller designs use press-fitted bearings. Here each end of the roll is bored to a dimension just smaller than the bearing housing. Then heat is applied to the roll body, expanding the opening enough to accept the bearing. When the body cools and shrinks in diameter, the bearing is locked in tight.
The bearing bore must be very precise. It must have just enough press fit to hold the bearing securely but not so much as to compress the bearing's internal clearances.
In a web line, rollers can become reheated from surrounding processes (like dryers) or from continual contact with heated webs. As an aluminum roll fitted with a steel bearing heats up, the roll body will expand at twice the rate of the steel bearing. At some point, the bearing bore can enlarge enough that the press fit is gone, and the bearing may fall out of the roll. This can happen at temperatures as low as 125 deg F. (See the Table I.)
When rollers might be affected by elevated temperatures, steel bearing inserts should be specified or used (see Figure 1). Steel sleeves are installed in roll ends using a heavy shrink fit (expanding the roll with heat and shrinking the bearing with liquid nitrogen prior to installation). The sleeve now acts as an interference layer, expanding at the same rate as the bearing housing, maintaining a secure fit, even with elevated temperatures.
Steel inserts also are used in aluminum rolls when a more robust design is desired. For example, if routine bearing replacement is a possibility, steel inserts would be recommended.
When ordering or specifying any roll, not just idlers, it's always a good idea to recognize if the roller can become heated from the web or environment. If you can measure the approximate temperature, you'll have even more specifics to share with your roller original equipment manufacturer (OEM). The OEM then can tell you if special bearings, bearing inserts, lubricants, and/or roll material are needed to provide optimum roll performance over the long haul.
Pete Eggen has 20+ years of experience within the converting industry in process analysis, product design, development, and applications engineering for web rollers. He is currently product manager for rolls and customer support manager at Webex Inc., Neenah, WI. Eggen earned his bachelor's degree from the Univ. of Northern Iowa and is a published author of several technical papers on related subjects. Contact him at firstname.lastname@example.org; 920-729-6666.
|100 deg F||25 deg F||0.0005 in.||Bearing fit is reduced by half|
|125 deg F||50 deg F||0.0009 in.||Bearing press fit is virtually gone. Bearings likely will fall out.|
|150 deg F||75 deg F||0.0014 in.||Bearing is a clearance fit. Severe damage will result.|
|175 deg F||100 deg F||0.0019 in.||Bearing is a clearance fit. Severe damage will result.|
|200 deg F||125 deg F||0.0023 in.||Bearing is a clearance fit. Severe damage will result.|
*Given ambient temperature of 75 deg F, **Aluminum bore will expand this much more than the steel bearing at the given Delta-T
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