The Anatomy of a Loop Slitting Line Purchase

Introduction

This will be a practical operations-level approach to the issues and decisions inherent in the selection and purchase of a slitting line. It is primarily aimed at the line supervisors and plant engineering personnel who would be most likely engaged by management to identify the available equipment and suppliers, match up internal considerations, specifications, and performance criteria with the competing suppliers, and to report and recommend based on their investigations.

There are many things to consider after you have justified the feasibility of a new slitting line. The most obvious questions are sometimes the most difficult to answer concisely:

The weighty issues of your customers' requirements, the trends in the markets for your customers' products, what you are capable of processing, what your suppliers can produce, and what the local infrastructure can support with its roads, bridges, and rail service, must be addressed through market studies and customer feedback.

We would like you to come away from this presentation equipped with tools to help you make the right choices when specifying and recommending the selection of a slitting line. We have prepared a Decision Kit for you to take with you to facilitate your selection process


Where will I put it?

Open floor space should not be a compelling reason for slitting line placement. To achieve maximum productivity and non-stop processing, it makes sense to plan its placement very carefully and not try to shoehorn it into a "convenient" spot just because you see open floor. Why stunt the productivity of the line by not thinking through the material flow and crane service? I know one installation where the line was placed in a "convenient" spot and the incoming coils had to be towmotored three feet from the end of the crane coverage to the entry turnstile.

In many cases you will need to do extensive foundation design and construction, including digging a looping pit. Make sure you only have to do that once. Take adequate core samples exceeding the maximum depth of any of your pit work.

Make certain you have good up-to-date layouts, foundation drawings, and elevations to work with your machinery builder. It would be tragic if you paced off 20 ft. column spacing when it was actually 18 and that third column from the end is exactly where your recoiler drive stand belongs.

Consider placement in relation to driveways. Account for special considerations required by the trucks that bring your metal and take it away. I attended an open house in a brand new slitter bay at a mid-sized service center. Everything looked great inside and out. There was a beautiful dusting of snow on the new building and the 20¹ grade leading up to the truckway was completely impassable. Even if the delivery semis could get into the building, the 20¹ downgrade out the exit side would have been really treacherous.


How Much Line?

Common folk wisdom dictates that, when you are going to buy a line get it bigger and faster than you need so you'll be ready for any eventuality. Or as the Pontiac pitchman tells us, "Wider is Better," or, perhaps, "Heavier is Better."

This is only true if you will really use the extra capacity. Important elements of this decision are found in the market study I alluded to at the beginning, i. e. customer demand for a range of sizes of product and capabilities of the supplying mills to produce master coils.

There are some rules of thumb that you may want to pay attention to before you talk yourself into more machine than you need. Based on a .250" gage line:


How much line is too much?

Should you go with Coil Car to Coil Car or Turnstile to Turnstile? The safest way to put a coil up on an uncoiler is with a coil car. The safest way to put a coil on a coil car is with a turnstile, which can be safely loaded with an overhead crane and a C-hook.

Turnstiles:
Most lines need entry turnstiles. Apart from the important safety considerations, An entry 4-arm gives your slitter operator a certain degree of independence from the overhead crane by having the ability to stage up to four coils at one time [five, if you count the coil car] giving an hour or more of staged supply to the slitter.

An exit turnstile is the safest and least expensive [not necessarily most efficient] place to complete the packaging of slit mults. After the tails have been taped, welded, or banded once to prevent "clock-springing" on the recoiler they are lifted onto the exit turnstile where they can be radial-banded and sorted to order or transferred to a packaging line for banding, sorting, skidding, and wrapping. Obviously if you are a processor or service center, this packaging function is a critically important potential bottleneck. However, if you are slitting to process further in the same location, such as in a tube mill or roll forming operation, the packaging could be kept simple.

Because of the concentration of weight and frequent off-center dynamic load conditions, the turnstiles require special foundation engineering. This is not an item you can purchase as an afterthought and just bolt to your 4" floor.

Coil Cars:
Coil cars are essential to the safe handling of coils moving to and from uncoilers and recoilers. If you have never witnessed it, maybe you can imagine the damage a 50,000 pound coil can do when it is being slung onto an uncoiler and misses the mandrel by a bit. On the other end of the line, it's very tricky to unload a gripper mandrel full of 2 inch mults between the forks of a towmotor without pulling out the middles of the coils. You could also have a pusher on your uncoiler, pushing directly onto a two or three-arm turnstile. However this arrangement is also a potential for pulling the IDs out of the slit mults. A further expense with the recoiler pusher approach is the requirement for hardened drum segments and a pusher on the turnstile. Coil cars are designed to move and manipulate coils and mults with stability, smoothness, speed and pinpoint accuracy.

There are three basic types of coil cars:

The Scissors Car operates on a flat floor. It has a hydraulic cylinder that lifts and lowers a scissors jack connected to the coil saddle. The lift height is limited to the stroke of the cylinder which is limited by the wheel-base of the car.

The Pit Car rides along the top rims of a trench deep enough to accommodate a lift cylinder sized to accomplish the required lift.

The Side Mast Car carries its lifting cylinder on the side outboard of the wheel track . This arrangement gives the car high lift and stability. It is the most popular type for light to medium duty slitting. It offers optimum smoothness of acceleration, deceleration and high lift capabilities for loading and unloading a large range of coil IDs and ODs as well as the ability to install and remove mandrel drums and risers. All of these capabilities are available without special foundation work.

Uncoilers:
Uncoilers are either tension braked or driven. If you are paying off loosely wound surface critical, very thin or soft material, or you are processing grain-oriented silicone electrical steel, you will want a driven uncoiler paying off into a shallow loop before the slitter. Otherwise you will only need a tension brake uncoiler providing a tight passline to the slitter head. These types of uncoilers can use water-cooled brakes or hydrostatic braking similar to the tensioning system in the roll tensioning device we will discuss a little later.

The advantage of the hydrostatic braking is commonality of services and the elimination of an additional utility required at the line and its attendant water cooler.

This type of uncoiler is provided with a sliding sub-base, which reacts to a positioning signal before the slitter head. This signal tells the uncoiler to move in or out to maintain edge or center registration of the strip relative to the slitter head. Uncoilers can be single or double-turret types.

An uncoiler will have a driven hold down roll to safely control the first wraps from clock springing when the bands are cut and to aid in driving the threading process. Some rolls are simply a two or three-foot wide polyurethane "snubber" roll. Other uncoilers are fitted with a full width polished chrome roll with banding slots that aid not only in threading but also make retracting and re-banding a partially run coil very easy.

Depending on coil weight, width and pay off speed, an outboard bearing support may be necessary. Uncoilers are massive, dynamic machine elements and require proper foundation engineering.

Crop shears:
Most slitting lines have at least one crop shear; usually at the entry end, just upstream of the slitter head. This shear crops the nose of the coil and, possibly, the first damaged wrap or two. It also can cut of the tail and the last two or three wraps.

The Exit Crop Shear is used to trim the mults so that all the tails are in line on the recoiler. The Exit Crop Shear is essential if you split coils, i.e. run partway through a coil, crop, package, then rethread to run another batch of mults from the same master coil, through the same or different set-ups.

Slitter Heads:
The slitter head is the heart of the system. We will discuss the Slitter Head in terms of:

Drive

In loop slitting, the head is driven. The drive method can be Hydrostatic, DC electric or AC vector electric. There seems to be a trend toward electric drives.

Hydrostatic drives are very well suited to light duty applications. Maintenance is simple but every processor may not have a hydraulics expert on the maintenance staff. A hydraulic drive is quiet and compact and offers good sightlines. However, the cost of replacement pumps, seals and motors is relatively high.

The DC drives offer ease of operation and much improved housekeeping over hydrostatic. They are universally acceptable as an industry standard. Maintenance is readily available almost anywhere.

AC Vector drives are gaining acceptability and their prices are coming down. They are still 15%-20% higher in purchase price than DC. However they are more efficient than DC and since they have no commutator, there are no commutator brushes to replace [a frequent service part in a DC motor]

Adjustment

A slitter head is chosen according to the following Parameters:

All of these factors dictate the arbor diameter and horsepower. During slitting, the metal being slit is trying to push the arbors apart. The arbors need to be stout enough to remain parallel and maintain the knife gap adjustments to maintain an acceptable cut edge.

The distance of the arbors is adjustable to allow for the variation of thickness and shear strength as well as the grind-down of the slitter knives. This adjustment is either by jackscrews adjusting the upper arbor chocks or eccentric bearing housings adjusting either both or the upper slitter arbor.

While eccentric adjustment of the slitter arbors maintain pass line through adjustment either for metal parameters or knife grind-down, it tends not to have the resistance to separating forces and amount of usable grind-down that the jack-screw slitter head with the same arbor diameter has.

Arbors

Slitter arbors and tooling are either keyed or keyless. In most applications of 3/16" or thinner cold-rolled material, Arbor keys are not needed. Without arbor keys and keyways in the tooling, you have lighter tooling that goes on the smooth arbor much easier for faster setups.

Lock-up

Jet-nuts are commonly used to lock the slitter tooling on the slitter arbors. A jet-nut is a grease-packed nut with a zirk fitting. After the nut is spun on and snugged up to the end of the set-up arbor, a grease gun is plugged into the zirk and the grease is pressurized forcing an annular piston tightly against the set-up.

An alternative to this is lock-up integral to the slitter head, such as hydraulically deployed dogs which, at the push of a button, emerge from the slitter arbors and bear against the tooling stack. This method depends on the use of shimless precision tooling and can save 10-20 minutes on every set-up

Set-up

Tooling set-up changes can be made in numerous ways:

The most cost-effective style of slitter set-up change is head indexing; one head on-line while one head is off-line being set up. This arrangement takes up less floor space and leaves good sightlines for the operator from one end of the line to the other.

Outboards

The Outboard bearing support is the bearing set and structure supporting the slitter arbors on the opposite end from the shoulder or datum face of the slitter

Scrap Management

Disposing of edge trim is a fact of life in the slitting operation. The slitter operator seeks to dispose of this scrap with as little added labor and downtime as possible. The denser the scrap is "packaged" the better. High density scrap management saves labor, space and, very often, commands a higher scrap price. There are three main ways to handle scrap, each has its advantages and limitations, they are listed in order of their ability to produce a dense by product.

Scrap winder
Usually two [left and right under the line or diagonally off to the side of the line] are needed. The winder accumulates scrap under tension from the edge trimming knife sets. Usually a level wind device is included. If trim breaks, the line must stop for re-threading. A winder is good for thin gage and low to middle yield strength material on coils with good edges. Higher strength and thick materials tend to reduce density of full wind-ups.

Scrap Baller
Usually mounted off-line and fed through a scrap trench from a shallow scrap pit in the floor. The baller takes in either strands [2] or a tangle of scrap. The baller winds scrap into an open-faced reel which is mounted under a heavy pressure drum which compacts the scrap as it builds upon the reel. Very often the scrap is left to accumulate through the beginning part of the slitting run. This causes a tangle to develop. The baller then is started and pulls a strand from both sides of the line into itself. If trim should break during the run, it is very likely that it will accumulate in the pit only until it becomes tangled with the trim being pulled into the baller, thus re-establishing continuity almost immediately.

Scrap Chopper
Usually mounted in pairs under the line immediately below the trim knife sets where infeed guides are mounted. The scrap moves into the chopper by the process of the driven slitter's normal operation. Trim break is thus not a concern at all as the trim has no where else to go but into the chopper knives. The scrap is chopped into relatively consistent lengths as the chopper speed is controlled by a signal from the slitting line. It is then discharged on a conveyor and dumped into a container. In many areas of the country, chopped scrap commands a price premium over balled, bailed, or wound scrap.


Strip Tension

The key to tight-wound, straight-walled mults is the tension stand. It is essential to provide consistent tension across all of the slit strips. This tension causes the thinner strips to accumulate in a loop in the pit, as they are being wound more slowly than the thicker strands. There are a number of choices of tensioning methods:

Drag Board
Tension can be provided by a device as simple as a drag-board, a set of wooden boards, covered in carpet, and pressurized together to provide pinch pressure across all of the strips. The advantage of this system is simplicity and low capital cost. The disadvantages are high maintenance costs and the high potential for surface damage.

Drag/Roll
Pre-tensioning is provided by a drag board, immediately followed by a set of nip-rolls. The drag boards and rolls can also be used independently.

Roll only
This type of tensioning is especially critical for surface sensitive applications. Eliminating the drag board eliminates one potential for surface damage and simplifies the procedures for the operator. The rolls are mounted in anti-backlash precision screw jacks for precise side-to-side nip pressure and hydrostatically synchronized to the recoiler to provide the correct back tension. Tension rolls are available in polyurethane covers for clean, dry applications and compressed non-woven fabric covers for oily and dirty surfaces. The roll sets can be changed out in less than a half-hour using a crane and sling or, in minutes, with a tension stand index system.

The tension stand can be on rails to traverse the pit, grab the strips from the slitter and transport them to the gripper in the recoiler drum.


Deflector Rolls
The deflector roll manages the strips exiting the tension device as the strips build up on the recoiler. If the strip is of low yield strength or the roll is not of sufficient diameter, cross braking can occur as the strip is forced into a backbend around the deflector roll. This won't happen if the back-bend is limited to the as-rolled condition of the incoming coil; that is sizing the roll's diameter to equal the master coil's OD or having an arrangement of three or more rolls arranged in a cascade of this diameter.

Recoiler

The last processing step in the slitting line is the recoiler. The recoiler is usually sized to the uncoiler. The assumptions being that if you are going to slit a master coil, you will slit all of it. However, if you always slit to a weight significantly less than your incoming coil because of limitations of down-stream equipment, it may be economical to consider sizing the recoiler accordingly.

If you are a service center or a processor you will be asked to provide coils in various IDs. ID changes are accomplished in different ways:

Risers
Risers are bolt-on plastic segments which add dimension to the basic recoiler diameter; for example raising the ID from 18" to 20" or 24". Risers are inexpensive but create a very long ID tail from the gripper to the first wrap.

Mandrels
Some recoilers have removable, interchangeable mandrels which pull out of the gearbox. This is usually accomplished by a crane and can take 40 minutes to an hour for a

Drums
A very popular alternative is the interchangeable recoiler drum system. Each needed diameter is represented by a separate drum that slides over the recoiler shaft that stays attached in the gearbox. One major advantage to this type of system is the smoothness of the drum surface. The interface between segments is virtually seamless and there are no bolt holes for risers. This translates into smoother wraps immediately next to the drum surface and minimized transmission of irregularities from one wrap to the next. Each drum has a dedicated gripper to minimize tail length, a drum can be changed as easily as removing and replacing a coil on the recoiler-about 10 minutes. The drum costs much less than an interchangeable mandrel.

Now you have a coil on the recoiler. Hopefully it's tightly wound and ready to satisfy the customer, whether that is within or outside your organization.

With this clearer understanding of how all the various parameters and components interact. You now have the tools to consult intelligently with your management to select the best combination of components to meet our slitting needs.