Ever wondered why wound roll edges are rough? Ever wondered why bearings on nip rollers don’t seem to last as long as others do? The answer to both may be friction limited thrusting. Few people know that a mutual thrust load is developed when two cylinders are in nip. This could be in the roller-roller nip of a calender, laminator or printer or the roll-roller of a winder. This thrust load is caused because the world is not perfect. Specifically, the diametral profile of the rollers (alignment etc) is not perfect. This causes the two rollers to act like a cone drive which mechanical designers know causes thrust.
The thrust developed is not very dependent on the size of the error; the tiniest of errors can cause similar thrusting as the largest. The thrust is in fact friction limited to the normal (nip) load times the coefficient of friction between the two rolls. Thus, if you had a modest 1 PLI on a 50” wide film winder lay-on roller with a COF of 0.5, you would develop 25# of side load. On a wide paper winder, thrust loads have been measured at #10,000 lbs. Thrust loads on calenders can even be higher because the nip loads are higher on calenders than on almost any other nip in the web industry.
The problem is first with the bearings and frameworks. Every designer knows that almost all bearings (except thrust bearings) used on rollers do not take side loads very well at all. One (new) calender builder did not realize this and sized loads only for the obvious radial loads. The result was a 24 hour bearing life with a bearings that cost several thousand dollars each. Also this side load tends to spread framework that can cause a wide variety of problems.
On winders we have a different issue. Here the side loads can cause the roll edges to dish. This looks something like an offset or telescope. It happens right near the core when side loads first develop and then push the (wound) roll one way and the (nip) roller the other way. Curiously, the more level you get the web profile and machine alignment the worse the problem becomes. If things are way out of whack, the winding set will immediately thrust to one side and stop against the bending frame. If, however, things are nearly balanced, the set may toggle from one side of the play/flexibility band to another causing offsets throughout the wind.
The moral: keep the frame stiff and ball (roller) bearings strong when engaging nips.
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