A PRIMER ON: Expanding Core Shafts
CORE SHAFTS are actually axles for roll or web products that unwind or rewind on various machines such as slitters, printing presses, laminators, bag machines etc. The core shaft is a bar or tube that will go inside the rolls to allow mounting onto the machine. It can be round, square, hex. etc.
THE BASIC CORE SHAFT will have jam or cone chucks, that are forced into each end of the core. This centralizes the roll allowing it to turn easily and concentrically, making for smooth winding. Sometimes they are used with damaged cores when an air shaft cannot be inserted into the damaged or squashed core. Changing of rolls can be slow and cumbersome.
EXPANDING CORE SHAFTS can be either mechanically or pneumatically expanded, and have various methods of connecting the shaft to the roll core. The expanding elements can be lugs, buttons, strips, or leaves. The main idea is to make roll changing must easier and versatile.
MECHANICALLY EXPANDING CORE SHAFTS are usually reserved for high web speeds where roll concentricity is very important. The mechanical shaft can hold the roll exactly concentric with little effort. It usually is more expensive initially than an air shaft and requires regular maintenance and lubrication in order for the internal screws and slides to work efficiently. The bodies are usually only of steel.
AIR EXPANDING CORE SHAFTS are most popular of the shafts. They are usually made from tubing with ends (journals), with bearings, and some method to hook up to a drive or brake. The tube contains a rubber air bladder which, when inflated with compressed air pushes lugs, buttons or leaves outward to grip the core. The bodies can be of steel, aluminum or composite.
THE LUG TYPE is the most popular and versatile of the gripping types. It is the most economical, easiest and quickest to repair when that time comes. A reputable AIR SHAFT MANUFACTURER will have lugs available of aluminum for light weight, steel for rough jobs, and polyurethane for metal or plastic cores. It will also have a protective rubber sleeve over the bladder to protect it from wear caused by rubbing against the lug or button bases. A properly designed lug requires no springs for retraction.
THE BUTTON TYPE requires button retraction springs that are its weak point. The springs eventually suffer from metal fatigue, and break becoming little" razor blades" that cut the air bladder, causing air leaks. The button type is labor intensive (therefore expensive) to repair, requiring that the buttons be "clipped" or screwed in the up position in order to remove the air bladder. This is especially important when considering the "small" button type, because of the large number of "small" buttons requiring lots of time to clip in the up position. These however do work well for some NARROW THICK WALL CORES. This type of shaft suffers from damage of elongation of the button holes when operated with too low a pressure. This elongation causes failure of the springs, and bladder, and can only be solved by a new shaft body.
THE LEAF TYPE is usually the most expensive, and heaviest type of shaft, recommended for use with THIN WALL CORES. Some people want it for use with NARROW cores, on slitters, which only works if ALL THE CORE INSIDE DIAMETERS ARE IDENTICAL. If different diameters are mixed, some will grip and some will slip, since the leaves cannot conform to the different dimensions. This type of shaft should NOT be considered for use as a "differential type shaft" on slitters.
BODIES:-Steel was the original material used for air shafts. It is the strongest, toughest of the shaft materials. It is of course the heaviest. Lighter shafts were required since most times the core shaft is manipulated by hand, with the resultant back injuries. Heat treated aluminum is used for lightness, but it has only one third the strength of steel so the shaft capacity has to be reduced accordingly. Sometimes, high strength is required along with lightness, and this is where Carbon Fiber comes in. Also known as composite, this type of material is made by encapsulating strong fibers of carbon with an epoxy plastic. This results in a strong lightweight shaft. The downside is, Carbon Fiber composites have to be treated with much respect, they cannot be thrown around as can steel and aluminum. Early shafts broke without warning, being damaged by poor handling, breaking the stiff but brittle carbon fibers. As the fibers break the shaft becomes weaker, until it can no longer hold the weight of the load.
STEEL:The steel shaft has been around the longest, mainly because of its inherent strength. It can take a beating without failure. They have the longest life of all shafts. Large shafts are sometimes too heavy for one person to safely carry and require some sort of handling equipment.
ALUMINUM:aluminum shafts are used for light weight jobs, where strength is a secondary consideration light weight makes lifting the shafts easier, when changing rolls.
CARBON FIBER:This Hi-Tech material provides high strength with light weight, but at a cost. It is by far the most expensive of the shaft materials. It is also made with a strong but very brittle carbon fiber. It cannot take abuse, without breaking the fibers and therefore should be used with caution. It should be remembered that it is plastic reinforced with strands of carbon fiber.