- December 01, 2007, By Timothy J. Walker Contributing Editor
Differential shafts are great tools, but like any tool, they have their limitations. Last month I covered the complexity of a differential shaft's actual applied torque. This month we cover more differential shaft limitations. My goal isn't to put down differential shafts but to avoid the frustration that occurs when delivered results don't meet expectations.
Axial-loading differential shafts stack alternating cores (or cores with special inserts) and keyed spacers on slotted shafts. The well-known limitation of axial-loading is that each core receives nominally the same torque, which isn't good when winding different widths on the same shaft. The second, less obvious limitation of core-spacer stacks is their sensitivity to core width variations, especially at narrow slit widths. If core width is slightly off, the stacking error creates a slit-to-core misalignment that's nearly impossible to compensate for.
On the plus side, the side contact between the cores (or core inserts) and spacers reduces lateral wander and core twisting. Since the shaft is both solid and the maximum possible diameter to fit within the core's inner diameter, these systems will have the least shaft deflection, especially important for heavier rolls or high winding nip loads.
Radially Loading Systems
In radially loaded shafts, the torque applied to a core is proportional (or nearly so) to the number of elements that grab the core, and each core receives torque as a function of width. Most radially loading differential shafts are slip-core or core-locking.
Both designs adjust air pressure within the shaft to radially pushing out on slipping elements to transmit torque to the winding core. The slip-core designs have small, nonrotating elements, often less than 18 in. wide, that push against the core. The core-locking system uses a series of disks 0.5-1.0 in. wide mounted on a smaller internal shaft. Each individual disk or “doughnut” has a mechanism, usually cam-lock buttons or bearings, to grab the core. A radially loading system should be used on slit rolls so narrow that two rolls would be locked on one element.
All radially loading shafts will have easy core positioning and adjustment, but how the core stays in its position varies by design or supplier. The slip-core systems may contain a core lateral motion with the torque elements outside the core's width; they may have adjustable pins or bearings; or some operations may default back to using a spacer or core between winding rolls, such as axial loaded systems. The cam-lock systems don't have a problem with lateral position, since the locking mechanism restrains the core laterally.
Regarding deflection, since a core-lock cross-shaft may be as much as 78 in. smaller than the core's inner diameter, they will see more deflection-related defects such as dishing, collapsed rolls, or shifted layers with heavy rolls or high nip loads.
Dust can be a problem for any differential shaft; some will create core dust, and others will fail if core or product dust gets inside them. Differential shafts should have good overspeed control to reduce heat and wear. All differential shafts work better when cores are neither too large nor too small. Core-lock shafts may fail to lock onto hard plastic or metal cores.
Differential winding after a well-wrapped surface drum or several idler rollers will have limited ability to pull out anything, but they have the most extreme bagginess at slitting. If differential shafts are turned up to a torque beyond the capacity of the driving motor or clutch, they won't slip differentially and you'll be winding on the world's most expensive lock shaft.
I'd like to thank three differential winding experts for valuable discussions that helped in preparing this two-part column: John Pretto (Goldenrod), Sean Craig (Tidland), and Dan Cain (Tekkote).
Web handling expert Tim Walker, president of TJWalker+Assoc., has 20+ years of experience in web processes, education, development, and production problem solving. Contact him at 651-686-5400; firstname.lastname@example.org; www.webhandling.com.