CONSIDERATIONS PARTICULAR TO COIL STOCK (Part Two)

STOP MARKS
When material is stopped between rolls – particularly staggered rolls in a straightener – a series of straight lines can be “imprinted” across the surface. These are called “stop marks”, “set marks”, or “witness marks” and often show up after a subsequent process, such as painting or plating, makes them noticeable.

If a pull-through straightener is used, material will stop in the straightener with every stroke, causing marks throughout the run, which directly correspond to roll spacing and feed length. Power straighteners can be adjusted to minimize stopping or be equipped with a “creep” speed for nearly continuous running if stop marks are a critical factor.

TOUGH MATERIALS
Almost all coil handling equipment is rated in terms of its capacity to process mild steel. So, calculations must be made to determine equipment capacity for materials tougher than mild steel. This is particularly critical to the straightening function where the rating of mild steel is considered under 50,000 psi in yield strength.

High carbon steel, stainless steel, and special alloys are among the materials for which caution must be exercised. Knowing material yield strength is necessary for proper application of coil processing equipment, as well as the knowing material shear strength for any cut-off process.

PRODUCTION RATES
It’s important to not interpret coil feeding equipment speed rates as production rates of a press or cutoff process. Catalog listings are charted in general terms to cover a wide range of applications. Coil handling and feeding is an intermittent operation employing several coordinated machines. Material is fed to length, the feed waits for the press or shear to cycle, the feeding area is cleared, and then the feed delivers the next length.

During the feed cycle there is acceleration, feed time, deceleration, stop time and often dwell times involved. Production rates depend on many variables beside the top speed of the coil feeding equipment. The cycle time of the press or shear needs to be considered, along with cycle time that is available for feeding. Individual feed lengths, the cross-section of material, the amount of work being done, the amount of slack material, and the capabilities of all the related machinery have to be known to establish true production rates.

SLACK MATERIAL
Coil handling and feeding consists of a group of functions which range from intermittent to continuous or relatively continuous operation. To synchronize these functions it is typically necessary to accumulate slack material at some point in the process, and there are various methods to accomplish this. This concern is one of the most critical, and it must be addressed to properly specify the coil processing equipment for a given application.

Variables such as material type, material thickness, yield strength, passline above floor, depth of looping pit, distance between machines, and targeted production rates must all be defined to assure the correct equipment is specified for the line.