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FLEXO Magazine : August 2009
TECHNOLOGIES & TECHNIQUES PRESS BUYER’S GUIDE The Basics of Specifying Core Shafts By Don Chapin A ir shafts have been used in converting operations for the last 40 or more years. Yet, many users know little about the design variables that affect shaft integrity and performance when ordering a shaft. Some think shaft diameter and price are all that matter. Air shaft suppliers, however, know that more details are needed before quoting or producing a product properly suited to the customer’s application. The performance of the air shaft—and the safety of the operator—depend upon the application data gathered by the manufacturer’s sales engineer. THE BASIC DESIGN There are several types of air shafts, all of which share a basic design principle—a tubular body designed to carry the load with expanding gripping elements that hold the material or core in place during winding. Historically, the shaft has been made of steel or aluminum. Carbon fi ber tubing has been used more recently for its superior strength-to-weight properties. Lighter core shafts reduce the risk of worker injury. One of the most popular air shaft designs has machined holes in the body fi tted with buttons, lugs or leaves. When an internal air bladder is infl ated the gripping elements are forced outward to securely grip the core. Conversely, they release their grip and retract when the air pressure is reduced. Another common shaft design is the external bladder, with easy-to-maintain multiple air bladders and the ability to replace parts without removing journals. NOT JUST A SHAFT Each application places a unique set of demands on an air shaft. The customer may also have unique parameters that need to be met, including handling weight and cost. Today, more and more converters are seeking the lightest shaft possible to minimize risk of worker injury from physically handling heavy shafts. Consequently, there is no such thing as “just a shaft.” Each air shaft must be designed to meet the customer’s specifi c applications by: 1. Supporting the weight of the roll. 2. Handling slit loads and tension requirements. 3. Providing proper grip with the core(s) without slippage or damage. 4. Meeting the customer’s objective regarding weight and cost. For example, loading forces on a shaft change with differences in shaft length, roll width, maximum roll weight, slit patterns and a number of other variables. Safe loading forces 82 FLEXO AUGUST change as roll widths and shaft materials change. To further complicate shaft specifying, other factors also affect shaft stressing, including: Body Material. Steel and aluminum tubing is available in varying alloys. Carbon fi ber is available in three basic fi ber grades (33M, 50M, and 90M). These variables will directly affect the stiffness and the price of the shaft. Wall Thickness. Standard duty, heavy duty and extra heavy duty are all options for wall thickness. These variables will affect shaft stiffness, weight and price. Strength of Shaft. The strength of each shaft is at its highest when the tube is at its thickest and when there are fewer lugs, buttons, leaves or other interruptions to the integrity of the shaft exterior. GATHER THE DATA Suppliers often use data sheets to capture all the various operating parameters and shaft mounting details, even before a quote can be generated. As a general rule, the less standard the application, the more information gathered the better—especially if your goal is an ergonomically correct shaft designed to carry the maximum load and priced to meet your budget. Usually data sheets include a drawing of a simple shaft to depict support or bearing centerline distances. If the supplier does not provide such a drawing, create your own. A hand sketch is better than nothing. CONCLUSION In order to receive prompt and accurate quotes from a shaft vendor, be prepared with a thoroughly fi lled out datasheet. Use the aforementioned parameters as a guide, but don’t stop there. Include type of winding (center, surface, two drum, differential, etc.), core parameters (thin wall, conventional or coreless), winding speeds, chucking/mounting requirements, and the like. A good shaft vendor will seek to have this knowledge in order to provide you with the right shaft design. Be diligent in fi lling out data sheets and/or answering questions. The information you supply will have a direct bearing on the performance of your shaft, maybe even the safety of those who work around it. ■ ABOUT THE AUTHOR; Don Chapin is national sales manager for NimCor Inc., and has more than 25 years of experience in serving the converting industry, including the specifi cation of airshafts, bowed rolls and chucks. 2009 www.flexography.org