Who's Driving This Nip?

What is the best way to drive a set of nipped rollers? Two rollers and two drive options (driven or not) lead to four options.

To make this discussion a little easier, I will assume the two-roller nipping system includes one hard steel roller and one rubber-covered steel roller.

  • Option 1 | Both Rollers Are Idling

    This option is probably the least common from what I've seen. There are two problems with an idling nip set. First, idling nips steal tension from the web to overcome drag from bearings, inertia, and rubber hysteresis, not to mention whether your nips are used as part of a drag-creating process, such as calendering, embossing, or metering viscous fluids.

    Second, since idling nips steal tension from the web, they should be set at high nipping loads to create enough friction to transfer this tension to the rollers without slippage, though large wrap angles, tensions, and friction coefficients can reduce the friction needed from nipping. Artificially high nip load to meet friction needs are ripe problems, including rubber roller wear and deflection.

  • Option 2 | Drive the Rubber Roller, Idle the Steel Roller

    I won't say that this is a dumb idea, but there are better ideas. The biggest problem with driving a rubber roller is that the rubber's surface speed is a function of nip load and rubber indentation. A first estimate of this effect is that a driven rubber nip will increase web speed equal to about one quarter of the percent maximum indentation of the rubber covering.

    For example, for every 10-mil indentation change you make to a half-inch thick rubber covering, the web speed will change 0.5% (¼ × 10 mils/0.5 in. = 0.5%). For stiff webs, such as many papers and polyester film, this is a massive speed change and potential source of a tension upset or induced web-roller slip. If the driven rubber roller is in closed loop tension control, then this small percent speed change is easily handled within a standard tension trimming. But if this roller is operating in speed ratio control or is the pacer of your process, a half-percent speed change is likely a big deal.

  • Option 3 | Drive the Steel Roller, Idle the Rubber Roller

    This is by far the most common option in rubber-steel nip systems used in tension control, coating, and laminating. The steel roller's indentation is insignificant, so surface speed is independent of nip load. The rubber roller is driven by either the steel roller contact outside the web (for thin webs) or through the product (for thicker webs). Either way, the motor can provide the torque needed to overcome bearing, inertia, rubber hysteresis, and process drags.

  • Option 4 | Drive Both the Steel and Rubber Rollers

    At first, this seems to be a belt-and-suspenders plan, but there are some scenarios in which it makes sense. As mentioned under Option 3, for thicker webs, the rubber roller will be driven by the steel roller, but this force is transmitted through your web. When the rubber-turning resistance is high and your product's sensitivity to shear stress is low (such as with thick adhesives or nonwovens), driving the rubber roller is a good idea.

Once we decide to drive both rollers, we create more questions (and some answers). Should the two rollers be driven by one motor via gears or a timing belt? Yes, this is quite common in printing where the nip is lubricated with ink.

Should the two rollers have independent speed control? Rarely, since it is difficult to figure out how to speed match them, especially through nip load adjustments.

Should one or both rollers be driven in torque mode? Yes, clutching the rubber roller is a good advanced design with great process flexibility.

Driving steel rollers is rarely a problem, but the driving plan, like most rubber roller topics, is complicated, but not illogical.

Web handling expert Tim Walker, president of TJWalker+Assoc., has 25 years of experience in web processes, education, development, and production problem solving. Contact him at 651-686-5400; tjwalker@tjwa.com; www.webhandling.com.


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