- February 01, 2002, Mark Wilson, Amacoil Inc.
Note: All figures for this technical report can be viewed in the print edition of PFFC, vol. 76, no.2 (February 2002), p48).
Braking moments of force affect efficiency in converting operations. Which is why... Shaft Collar Decisions Can Be Momentous
Whether the operation is rewinding or unwinding, it is essential to hold cores and rolls firmly in place on shafts during converting applications. A number of shaft collar systems are available to do this. It is extremely important to the success of the application to have the roll move in perfect unison with the shaft. The decision on which type of shaft collar to use depends on how critical it is to transmit braking moments from the shaft to the core.
Some operations, such as low-speed take-up of trimmed foil edge waste, do not require a high degree of synchronization between core and shaft rotation. In fact, many converters don't even wind edge waste onto a core; they simply let it fall to the floor and sweep it out at the end of the day.
However, to maintain efficiency and accuracy in high-speed web feeding, slitting, or winding procedures with frequent sudden stops and starts, it is necessary to transfer braking moments of force precisely from the rotating shaft to the core or roll. A roll of polyester film, for example, can tear or bunch and crimp if rotation is stopped suddenly and the roll doesn't stop in unison with the shaft.
Standard Ring-Set Screw Setup
If “slippage” of the roll on the shaft will not affect production or quality control adversely, a standard ring shaft collar held in place with a set screw generally will meet application requirements (Figure 1).
When the core “slips,” the load or tension on the core overcomes the surface friction between the shaft surface and the inner bore surface of the roll. If the shaft collar set screw isn't tight, or if the collar isn't snug up against the roll flange, slippage will occur, and the roll will not rotate at the same rate as the shaft. In extreme cases the shaft simply will spin inside the roll bore and the roll won't rotate at all, or it will “free spin.”
An option for solving this problem is to use a shaft collar with pins that match up with holes on the core flange. When the shaft collar is installed, the pins leverage the rotation of the roll, matching it to the rotation of the shaft. If the load is too great, however, the pins can snap off.
Another issue when using a simple ring-set screw arrangement is the chance the set screw will loosen from vibration or wear. When this happens the collar can spin off the shaft, creating a potential hazard to the production procedure as well as to workers.
Shaft collar clamp manufacturers have designed new systems that take shaft collar clamping technology up a notch and also enhance production. For example, Figure 2 shows a cutaway view of a ball-bearing, spring-loaded collar. This type of shaft collar secures itself on the shaft using compression. The device slides onto the shaft with one hand. When released, the ball bearings compress against an inner retention ring, effectively locking the collar to the shaft with a force of about 500 lb on threaded or nonthreaded shafts.
For roll changeovers, pushing the collar slightly toward the center of the shaft releases the compression ring, and the collar may be removed quickly (Figure 3). The net result is a more secure collar that lends itself to achieving higher production rates. Furthermore, workers needn't be concerned with interrupting their work to check out tools at tool bins; compression-held shaft collars require no tools for installation or release.
The Role of Tensioning Controls
For applications running at high speeds — such as converting foil prior to packaging — roll and shaft rotation must be synchronized during sudden starts and stops. Neither the roll nor the shaft may move independently of each other.
The shaft collar, therefore, must have a tensioning adjustment control so the braking moments of force are transferred from the shaft to the core without slippage. Shaft collar systems allowing tensioning adjustments generally consist of two parts: a clamping or fixed pintle and a tensioning pintle (Figure 4).
The fixed pintle is affixed to the shaft so it remains stationary. The clamping pintle then is slid onto the shaft and “snugged” up against the roll flange. In many cases, both the clamping and fixed pintle feature tapered ends that insert part way onto the roll core. This enhances the “grip” between shaft and roll.
An adjustable tensioning control on the clamping pintle lets the user adjust tension to suit application requirements, thereby enhancing efficiency in high-speed packaging, stretch wrapping, and numerous other applications. Braking moments are transmitted effectively from the shaft to the core, permitting quick shutdown, if necessary, without rotational play on the core.
Shaft collars aren't complicated to understand or use. Selecting the right system for a specific application is based on knowing how critical it is for the shaft and roll to be perfectly synchronized during rotation. Shaft collars are simple and inexpensive solutions that can enhance efficiency and productivity in many converting operations.
Mark Wilson is an applications engineer at Amacoil Inc., Aston, PA, a manufacturer and value-added distributor of “rolling ring” linear drives. He received his training in power generation and linear motion in the US Navy and has also held positions as technical services manager and sales support engineer. Contact him at 610-485-8300; email@example.com.