Essco highly recommends positive loading control of all doctors. While numerous gravity load only doctors have been installed worldwide, many paper mills can attest to the fact that gravity load doctor performance deteriorates with incremental speed increases and higher amounts of recycled furnish.
Typical problems from improper loading include doctors hydroplaning in the forming section, which can leave stock ringing the roll and cause premature roll and forming fabric wear; build-up on dryer cans, resulting in reduced heat transfer, dirt contamination, sheet defects, and dry end sheet breaks; and roll wraps on sheet shedding doctors.
Pressure Rigs
Spring-loaded pressure rigs are normally supplied with doctors up to about 160" wide. Pressure rigs offer limited ability to adjust blade-loading pressure. Pressure rigs link to the doctor via a lever mounted on the tending side doctor journal. Depending on natural doctor design balance and lever length, maximum blade load pressure may reach about 1pli.
The feasibility of pressure rig loading is determined by machine width, doctor weight, machine speed, and doctor position on the machine.
Air Cylinders
Air cylinder loading becomes necessary as roll width and machine speed increases. Air cylinders may also be necessary for sheet shedding positions and on applications that require the doctor to pivot farther away from the roll. All Essco-supplied air cylinders are dual acting, to control both load and unload functions. Air cylinders also provide safer operation as controls are located farther from moving machinery.
Essco offers a variety of air cylinder options, from basic throwaway type cylinders for low initial cost, to high-end, heavy-duty stainless steel cylinders. Air cylinder loaded doctors are supplied with cross-machine stainless steel tubing for simple installation, and a psi/pli calibrated NEMA 4 fiberglass control panel. Stainless steel and custom panels are also available.
Gravity load doctor performance can be improved by retrofitting an air-loading package for a modest investment and relatively rapid payback.
Torque Arms
Torque arms (tumbuckles) are normally supplied with ETUniform doctors. ETUniform blade holders use air or liquid pressure to load the blade against the roll. The doctor back must be held in place by torque arms to prevent the air pressure from rotating the doctor away from the roll.
Essco offers several models of torque arms, suitable for all doctor sizes. All torque arms are made from 316L stainless steel.