Scrap Steel Winder Problem Slowing Down
Workers at a coil steel-processing mill received rolls of stainless steel from its parent company to slit in half and trim the edges approximately 1″ to 2″. They recoil the sheets and ship them to their OEM customer that fabricates stainless steel refrigerators.
They have a hydraulic motor-driven unit that coils the scrap edges of the coils onto drums. This operation needs to keep up with the speed of the splitter, or the scrap starts to “bunch up,” which requires the workers to stop the line and remove the excess manually. The hydraulic unit was over 20 years old and needed updating. A local hydraulic distributor built them a new unit, and the mill did the installation.
During commissioning, they found that the new unit could not keep up with the slitter line speed as the scrap coil increased in diameter.
The unit was upgraded from a fixed-volume open-circuit design to a closed-circuit pressure-compensated pump design to improve energy efficiency. The only other difference was the use of a flow-control module under the pilot-operated directional valve. The old unit had line-mounted flow controls.
The builder convinced the mill that it must be the mill’s existing motor leaking more flow to the case, as the pressure increased when the scrap steel diameter increased, causing the motor to slow down. They then replaced the hydraulic motor with a new one, but they still had the same problem.
Any idea what the problem could be?
See the Solution
The reason the hydraulic motor on the slitter slowed down was the use of non-pressure-compensated flow controls in the module mounted under the directional valve. They missed the fact that the original flow controls were pressure compensated. With non-pressure-compensated flow controls, the flow they pass decreases as the pressure drop across them decreases. The pump compensator setting stayed consistent, while the motor pressure increased as the scrap coil diameter increased, reducing the flow. Replacing them with pressure-compensated flow controls solved the problem.
By Robert Sheaf, CFPAI/AJPP, CFPE, CFPS, CFPECS, CFPMT, CFPMIP, CFPMMH, CFPMIH, CFPMM, CFC Industrial Training