- December 01, 2002, Timothy J. Walker, TJWalker & Assoc. Inc.
Last month we made the case for using differential rewinding shafts. This month, let's talk about what differentiates one differential shaft from another.
Differential shafts can be divided into two categories, axially or radially loaded, depending on the direction of the clutching action.
Axially loaded shafts, the granddaddy of differential winding, load a stack of cores and spacers laterally against a fixed collar, creating friction at every core-spacer interface. The cores are free to rotate, but the spacers are keyed to turn with the shaft. As the shaft turns, the spacers are driven at an rpm greater than the web-restrained cores.
The friction created at the core-spacer interface, two sides per core, limits the torque that creates winding tension for each roll. As with all differential shafts, each core rotates independently to compensate for strand-to-strand variations. The torque transferred is roughly proportional to the axial load, usually set by air pressure to a pneumatic cylinder.
Axially loaded shafts are an inexpensive, mechanically simple design used by most differential slitter/rewinders. They can differentially wind slit strands less than ¼ in. wide successfully.
Where axially loaded shafts have an advantage in handling narrow widths, inherently they are weak at handling differing widths on a common shaft. Since torque is created at the core-spacer interfaces, the torque and winding tension are equal for wide and narrow rolls within a loaded set. This disadvantage doesn't discourage many converters, since many slitting processes run uniform slit width. Some axially loaded systems have two zones per shaft by using a central locked collar and loading from both sides.
Other potential concerns of axially loaded shafts include core dust, heat dissipation, and frictional torque variations. At high torques and speeds, both dust and heat are generated and can damage the core or product. Due to the frictional variations of paper cores and plastic spacers, torque variations as high as 2:1 are common. Radially loaded differential shafts attempt to address these concerns, plus provide torque proportional to core width.
Radially loaded shafts inherently create torque proportional to core width. An internal bladder pushes multiple elements out radially against the core's inner surface, creating friction. Wider cores engage more frictional elements and receive more torque than narrow cores. Radially loaded shafts still may use the core as a frictional element but use the core's inner surface rather than its sides. This greater frictional surface area reduces dust, heat, and torque variations.
More advanced differential shafts take the core out of the clutching mechanism by using a core-gripping ring. In these designs, a core gripper locks onto the core, moving the clutch slip point to the core gripper-shaft interface. The core-independent clutch design uses engineering materials and lubricants, resulting in a dust-free, heat-tolerant, and consistent torque generator. The frictional core gripper shafts cost 3x-5x more than core-dependent alternatives.
Core-based shafts' frictional torque is created by axial or radial load and roll weight. Differential shafts with core grippers have an internal bearing to reduce influence of roll weight on torque generation. The ultimate differential shafts use magnetic hysteresis in place of frictional clutching. This improves rewind torque accuracy and repeatability greatly. You pay for this performance, with custom magnetic hysteresis shafts over 10x more expensive than entry-level alternatives.
Both of these core-independent shaft designs have a minimum element width (usually around ½ in.). Therefore, these advanced shafts cannot handle extremely narrow widths or tight roll-to-roll spacing.
Understanding the advantages of climbing the differential shaft evolutionary designs will help you find the right design for your product.
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; firstname.lastname@example.org; tjwa.com.