TAPPI TIP 0425-06
Guidelines and Procedures for Inspections and Maintenance of Yankee Dryer Doctor Systems
|Publication Date:||1 January 2019|
This Technical Information Paper (TIP) provides guidelines and procedures for basic creping doctor inspection and maintenance. Proper inspections and maintenance will reduce the risk of Yankee dryer surface damage and inconsistent operations.
Doctor Back - the main structure supporting the doctor blade holder. The doctor back spans the machine width, loads the doctor blade against the dryer, and resists vibration. Doctor backs are typically fabricated from mild steel, mild steel clad in 316 stainless steel or solid stainless steel.
Doctor nose - the machined area of the doctor back to which the blade holder is mounted.
Blade holder - the removable portion of the doctor that attaches to the doctor nose and holds the creping blade.
Blade width - the machine-direction dimension of the doctor blade.
Blade length - the cross-machine dimension of the doctor blade.
Blade thickness - the caliper or gauge dimension of the doctor blade at its base.
Stick-Out - the distance that the doctor blade sticks out past the backing blade. The stick-out can range from 12 mm to over 25 mm (half an inch to over an inch). Changing the blade-stick out will impact blade angle, creping shelf angle, and blade stiffness.
Backing blade - a removable part of the blade holder that provides support to the creping blade while offering some cross-machine flexibility.
Conformatic tube - a polymeric or fluid-filled tube that supports the backing blade (not a loading tube). Force at the creping blade tip transfers to the backing blade which compresses this tube which distributes the force and levels the doctor blade wear profile. Lodding Engineering Corporation registered "Conformatic" as a US trademark in 1971.
Cartridge - a structure that sits in the doctor holder and supports the creping doctor blade. A cartridge has spacers that maintain blade stickout for different width blades (important when blades are reground and reused).
Ladder Style Holder - a holder with a series of machined openings that allow the blade stick-out to be changed via the use of a step-up strip.
Step-Up strip - special spacers used in ladder style holders to change the blade stick-out for different blade widths.
Profiling Screws - a series of screws that allow the adjustment of the doctor holder profile plate to match the cross-machine profile of the Yankee dryer surface.
Edge Fingers - individual plates that are typically 25 mm wide (1") and located on the blade holder outside the sheet edge, allowing greater control of the holder profile. Edge fingers can be rotated towards the Yankee dryer to apply more creping blade pressure, or away from the Yankee dryer to relieve the creping blade pressure.
Doctor Journal - the shafts at each end of the doctor back that are supported by doctor bearing housings. Doctor bearings allow for doctor rotation (to load and unload the blade) and oscillation (cross-machine motion). Doctor journal diameters are sized reduce stresses and vibration.
Mechanical stop - a device that limits the rotation of the Yankee doctor so that only the creping blade will contact the Yankee dryer surface.