This month we continue our diagnosis of differential rewinding. The last two columns reviewed the whys and hows of differential rewinding, but before filling your prescription for two new differential shafts, let's review potential detrimental side effects.

Unknown Tension
Differential rewinding is a torque-based system, changing current, overspeed, or air pressure to vary the torque and — through the roll's radius — winding tension. One repeated theme in this column is measure your tension. A tension-sensing roller, whether used for control or measurement only, will ensure you get the right tension or know when you don't. If you can't afford to measure tension, at least know the tension you can get by calibrating your system with a spring scale.

Roll-to-Roll Torque Variations
Differential shafts eliminate tension variations from differing diameters or incoming web length, but they introduce a new source of tension variability: clutch performance. Differential shaft clutching mechanisms, especially systems dependent on consistent core-to-spacer friction, commonly have 2:1 cross-shaft torque variation. More advanced designs still will have torque variations caused by nonuniform machining, assembly, lubrication, or wear. I strongly recommend measuring your differential shaft's variability using a spring scale. Differential shaft suppliers should be able to answer questions about torque variation.

Too Mmuch or Too Little Torque
All torque-based systems, whether pneumatic, electronic, or magnetic, will be hard-pressed to operate over more than a 40:1 ratio. If you design for high torque requirement, you give up the low end, and vice versa. Absolute high-end torque, like any clutching system, is limited by heat dissipation. A differential shaft's clutch is trapped inside the core, making heat dissipation difficult. Controlling at extreme low torques is limited by rolling resistance and breakaway friction.

Core Gripper Torque Limit
Differential shafts with core grippers are less prone to core slip and associated heat or debris generation, but they are not immune. Many core grippers are designed to sink their teeth into paper cores but may fail if asked to grip plastic or metal cores, or even paper cores, under high-torque winding.

Pack-Differential Incompatibility
Differential shafts and pack rollers don't go together well. The combination of close proximity, differing diameters, and quick width changes makes pack-winding many rolls on a differential shaft a daunting task. Using a short-entry web span ahead of differential rewinding will reduce wrinkling and tracking problems but won't squeegee out entrained air. Lack of a pack roller limits differential wind quality, especially with increasing speeds, widths, diameters, and decreasing tensions.

Slipping in the Wrong Place
The simplest slitter/rewinders drive their transport rollers and, via a clutch, the rewind shafts from a single motor. The rewind clutch controls torque when full width or lock bar rewinding. If a differential shaft is used with this type of slitter, it puts two clutches in series. With two clutches in series, only one will slip. If a differential shaft breakaway torque is higher than the drive train clutch torque, the differential bar will fail to slip, creating the world's most expensive lock bar. To ensure the correct clutch slips, turn up the torque of the drive train clutch when differential rewinding.

Difficult Integration
Differential shafts are most prevalent on slitter/rewinders and rarely are seen at the end of extruding or coating lines. Why? Coaters and extruders must run continuously for product uniformity. Maintaining quality of edge trim and single-roll transfers with minimum downtime is hard enough. Differentially rewinding, with multiple knives and core starts, increases the downtime risk exponentially, a risk too great to put in-line with highly valued continuous processes.

That completes our differential rewinding diagnosis. Take two differential shafts and we'll see you in another month for your next appointment.

Timothy J. Walker has 20+ years of experience in web handling processes. He specializes in web handling education, process development, and production problem solving. Contact him at 404/373-3771; tjwalker@tjwa.com; tjwa.com.