A stainless-steel plate mill receives raw stainless ingots from the company’s overseas parent company for processing. The ingots usually are about 4 ft. wide, 12 to 18 in. thick, and approximate 12 to 16 ft. long. They use a furnace to heat the ingots to a cherry-red state and then use a rolling mill to flatten them down to ½ to about 1½ in. thick.
Once the plate reaches the desired thickness, a straightening machine grabs both ends and pulls hard enough to eliminate all the waves produced by the rolling process until the plate lays perfectly flat. The last operation is to load the large sheet in a shear and trim it into 8-ft. by 20-ft. sheets.
The shear looks like a super-large vertical knife blade with the cutting edge held at a 15- to 30-deg. angle. The shear blade is driven down by two large hydraulic cylinders to cut the plates to the needed sizes. This blade is 22 ft. wide and held tight against a back stop by two pull-back cylinders. The circuit shown maintains 1200 psi on the cylinder’s rod side when the blade is extended to shear the plate. When the rod retracts, accumulators are supposed to maintain pressure at 400 psi or greater to keep the blade tight against the back stop. The pressure switch alerts operators if the pullback pressure drops below 400 psi.
A problem caused the blade pull away from the backstop when shearing, causing the cut to stray off line and chip the cutting edges. Technicians found wiring to the pressure switch had broken, which explained why the alarm did not activate when pressure dropped below 400 psi.
Getting back to the problem, the technicians checked both cylinders, and they were not leaking by the piston. They installed a gauge and found pressure would drop to 0 psi when the blade retracted.
What do you think was preventing the pull-back cylinders from holding the blade against its backstop?
Find the Solution
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Solution to Last Month’s Problem:
Directional Valve Slams When It Shifts
A pilot-operated directional valve can be externally or internally piloted. When these types of valves are ordered with an internal pilot connection, most manufacturers use a plug with an orifice to control the spool speed. It slows the valve down a few milliseconds and prevents the spool from slamming into the forged steel endcap, which would make a thumping sound. Installing the orifice plug from the old valve solved the noise problem.
