Maintaining a slitting line

Modern slitting lines are high-performance machines that run at speeds up to 2,000 feet per minute and are intended to produce 24 hours a day, 365 days a year. For these reasons, coil processors must be sure all line elements are properly maintained, which represents a titanic effort if they don't understand what components are critical.

A typical slitting line has the following components:

• Static or dynamic coil storage
• Coil car
• Uncoiler
• Peeler
• Centering device
• Pinch rolls
• Straightener
• Scrap shear and reject collector
• Guides
• Breaking rolls
• Slitter
• Knives
• Movable tensioning device with felt pad, pinch rolls, dividing shear, and onboard straightener
• Recoiler
• Exit coil car
• Exit turnstile

These devices act as the link of a chain. If one of them fails, the complete chain fails. Users must be sure that any element in the line is ready to perform properly. And that depends to a great extent on machine maintenance.

Recommended Routine Maintenance

Coil processors need to develop routine maintenance procedures for these critical areas of a slitting line.

Oil. Oil flows at approximately 16 to 32 gallons per minute (60 to 120 liters per minute) at a pressure of 1,600 pounds per square inch (120 kilograms per square centimeter). Over time small dirt particles turn oil into a dangerous agent that can scratch pipes and cylinder inner walls and block solenoid valves and joints. Dirty oil also can damage seals. Another potential contaminant is a dirty filter. Filters, which can be mechanical, electrostatic, or cloth, should be checked every 600 hours.

If oil level depletes too much, it's probable that dirt from the bottom is mixing with flowing oil. A rule of thumb is to check oil levels every 600 hours. When it's time to add oil, make sure it's the same type of oil used previously, because mixing different oils can ruin the working or lubricating characteristics of the fluid.

Hydraulic oil must be prefiltered to eliminate the microparticles that fluids always have and that can be deposited on the tube's walls. Sometimes coil processors use additives, but these additives must be compatible with the line mechanisms. For example, in some cases, stampers might have to add some fluid to the strip surface to facilitate the drawing of the part inside the die. This fluid should not damage other mechanisms or be harmful to the belts or plastic rollers in the line.

Figure 1 All slitters should have a catch pan to prevent leaks from hitting the floor.

Lines. Sooner or later all hydraulic circuits leak. To maintain a safe shop, coil processors must never allow oil droplets from leaky lines to fall on the floor, so all slitters should have a catch pan. Oil that flies off the coil strip under the pressure of the rolls should be collected by a reservoir. These reservoirs should have a low point where a pump can be installed to remove the collected oil (see Figure 1).

In some lines, reservoirs are joined together, and fluid is carried to the lowest possible location, such as a central tank. This tank usually is located at the bottom of a loop's pit and can be emptied with a pump or drained.

Periodical checks—every 600 to 1,000 hours—should be made of the cylinder seals and rotary joints of the recoiler, uncoiler, and slitting arbors. All tubing and flexible hoses should be inspected every 2,000 hours.

Figure 2 Many coil processors find it's cost-effective to use a single hydraulic group with two pumps—one works while the other is on standby.

Pumps. Slitting line devices are powered by pressurized oil generated by a dedicated hydraulic group (see Figure 2). Lines have multiple hydraulic groups so that if one fails, the other group can run the line. However, this can be expensive, so many coil processors find it's more cost-effective to use a single hydraulic group with two pumps—one works while the other is on standby.

Pumps should be checked every 2,000 hours to ensure that the right pressure and flow are achieved and bearings lubricated and cleaned thoroughly. The hydraulic tank should be cleaned every 1,500 hours. All sludge must be removed from the bottom of the tank with a cleaning and washing agent. Line pressure in the hydraulic circuit must be released before maintenance to prevent a sudden oil shower.

Solenoid Valves. Solenoid valves, which act as the nerves of a system, connecting power to all elements, control and distribute clean oil. It's important to check all seals every 1,000 hours. If new seals are needed, they should be coated with grease before installation to prevent them from being scratched because of friction. All pressure should be released, including any buildup inside the solenoid valve chambers. If pressure is not released, the remaining pressurized oil can produce an unexpected shower, or cause erratic and sudden whiplike motions of the flexible pipes.

Changing these seals is a precise task, so it should be performed by skilled personnel. Every pipe in the solenoid valve manifold should be labeled for easy recognition. Only authorized personnel should be near the line during maintenance, because manipulating solenoid valves can unexpectedly activate the line.

Cylinders. The hydraulic cylinders combine signals from solenoid valves and oil pressure to activate the various components, including the mandrels, straightener opening, felt pad opening, coil cars, and pinch rolls. Cylinders must be checked every 1,000 hours if they are activated frequently or 2,000 hours if they work intermittently.

Figure 3 Neat, organized tubing ensures proper accessibility for maintenance and inspection procedures.

Other Hydraulic, Pneumatic Components. Pressure monitors, electrical components, flow sensors, and lubricating units should be checked every 1,800 to 2,000 hours. Neat, organized tubing ensures proper accessibility for maintenance and inspection procedures (see Figure 3).

Tooling. Separators and knives must be checked from time to time. During cutting, some knife scratches can be produced that increase the burr on the cuts and lowers the edge quality. At the least, a visual inspection should be done every 1,000 hours. Slitters equipped with automatic hydraulic clamping must be verified every 2,000 hours to guarantee the clamping force. The eccentric gap-adjusting device also should be checked every 2,000 hours

Electric Motors. Many operators assume that electric motors do not require maintenance, especially AC motors. Not true. All motors require some sort of periodic maintenance.

Figure 4 If clean air can't flow freely through the motor, its temperature increases and performance decreases.

For example, keeping dirty filters clean extends the life of a motor (see Figure 4). If clean air can't flow freely through the motor, its temperature increases and performance decreases. Lubricating main shaft bearings also extends a motor's life. Shaft bearings should be greased every 2,000 hours. During this operation, all dirt inside the motor should be removed and damaged seals replaced. All power must be switched off before motor maintenance is performed.

Electric Cabinets. Every 2,000 hours electric cabinet filters must be changed. Also, every 2,000 hours safety switches need to be verified as well as cleaned of all the dust deposited, which is attracted by the electromagnetic fields of the electrical elements.

All cables should be labeled so that all maintenance procedures can be performed easily. Internal cabinet organization also is important; areas for the programmable logic controller, CNC, the power section, and safety devices should be separated. Again, all power should be off before maintenance.

Clutch and Brake. Manipulation of a clutch and brake can unexpectedly activate some mechanisms, for instance, the shear, so extreme care must be taken to prevent injury. The clutch and brake should be checked every 1,000 hours. Maintenance tasks include controlling the motions of the friction pads, springs, and tie rods.

It's also important to check the rotary joint to be sure that air or oil flows properly into the clutch and brake—the performance of the entire unit depends on this flow. At the least, this task should be done every 1,000 hours.

Ball Screws. In machines with a high degree of vibration, such as tensioning devices and slitters, screws must be checked every 1,000 hours and lubricated every 600 hours. They also should be secured with epoxy or wire.

Gearboxes. If an anomalous noise is heard, the whole device must be inspected and tested. Additionally, temperature levels should be measured every 1,000 hours, as well as oil levels, to lessen internal wear. An infrared thermometer is ideal for checking temperature.

Guides. Many mechanisms are guided in one place or another. The clearance between these guides must be checked every 1,000 hours. An equipment supplier should provide a manual that includes all recommended clearances. Components or guides that aren't automatically lubricated require manual greasing.

Sensors. Magnetic sensors or switches should be calibrated every 2,000 hours. It's critical to maintain the correct distance between the sensor and the element to be detected.

Safety Fences. According to many safety standards, a line's perimeter must be fenced. For example, gates can be controlled electronically so that the line stops automatically when one of the gates is opened. These electric gates must be checked every 500 hours. Photoelectric curtains also must be inspected every 500 hours.

An Ounce of Prevention

If properly maintained, a slitting line can operate efficiently for 20 or more years. Establishing and following these routine procedures can help coil processors safely maintain their slitting lines without compromising ergonomics or operator safety.

Flix Remrez is steel division manager with Fagor Arrasate USA Inc., 205 W. Grand Ave., Suite 113, Bensenville, IL 60106, 630-595-3780, fax 630-595-6172, fagorusa@fagorarrasate.com, www.fagorarrasate.com.