Troubleshooting doctors: loading, wear and roll surface marking
Apr 27, 2021
Some doctor problems are rather straight forward, readily recognized, and corrected. However, occasionally one will crop up that seemingly has no solution.
The usual operator solution to these problems is to crank up the blade pressure. This, of course, results in increased maintenance due to worn rolls and blades as well as increased drive loads.
Many doctoring problems are the result of load means not being transmitted to the blade. Some possible causes:
Regular doctors (externally loaded)
air cylinder(s) bottom out
bearing hang up
interference
loose load arms
blown air cylinders or airides
disconnected or failed air hoses
failed linkage
DST doctors
debris in-holder
leaking load tube
failed air hose
failed finger rivets
tubes that have extended in use
failed or loose pivot brackets
Usually a persistent doctor problem is due to a combination of things. It may not be a problem with the doctor at all, only manifested in doctoring. In tackling such a job, one should learn to “read” blade wear and roll surface conditions.
Blade wear
Always inspect your worn blades, they will give you valuable indications of any doctoring problems. The table below shows some typical wear patterns and probable causes.
Wear type
Cause
Remedy
Uneven wear
Holder not profiled to roll
Poor roll surface
Damaged or dirty blade holder
Deposits
Profile holder to roll
Use more flexible blade
Clean, repair or replace holder
Use abrasive blade
Center wear
Doctor not machined to match roll crown
Doctor back (beam) sagging (deflecting) into roll
Refit and shim holder to doctor back as necessary. Return new doctor for replaning if mismatch is excessive.
Replace old doctor.
Bow wear pattern
Doctor not machined to match roll crown.
Doctor back (beam) sagging away from roll (beam bent).
Refit and shim holder to doctor back as necessary. Return new doctor for replaning if mismatch is excessive.
Replace old doctor.
One end wear
Doctor misaligned (not parallel to roll axis).
Shim or move bearings and brackets as needed to align doctor parallel to axis.
Rough blade edge
Deposits on roll.
Excessive pressure on blade.
Blade material too hard.
Use abrasive blade as required.
Reduce pressure.
Use softer blade.
Wear within trim
Deposits left on roll (abrasive furnish).
Use abrasive blade as required.
Trim blade edges frequently.
Use end-slotted blades to prevent build-up of excessive pressure on roll ends.
Apply water lubrication to face of roll.
Wear outside trim
Sheet lubricates roll.
Slot blade ends to relieve pressure.
Apply water lubrication to face of roll.
Localized pitting
Blade attacked by:
Electrostatic discharge
Electrolysis
Heat.
Insulate doctor from machine frame or use non-metallic blade.
Horns both ends
Oscillator not working.
Blade too long.
Fix oscillator.
Trim length of blade to match roll face.
Score on blade edge
Damaged roll face.
Repair roll face.
Feathered blade edge
Blade material too soft.
Excessive pressure.
Blade angle too flat.
Use harder blades.
Reduce pressure.
Move bearings or brackets to increase angle. Check for correct angle with gauge.
Bird's mouth
Blade load increased after start-up, may result in sheet pass.
Never increase load on worn blade. Load decrease is OK.
Roll surface
Roll surface conditions can also tell a lot about doctoring. Some of these conditions can be characterized. The attached charts classify some roll surface conditions as they relate to doctors. They also show roll surface patterns and some possible causes.
Circumferential bands
Pattern
Causes
Ends symmetrically different from center
Misalignment of doctor - skewed (scissored) across roll
Doctor not properly fit
Heat distortion of doctor back or blade
Loss of crown on roll
Severe sheet moisture profile
Roll running deflected (eg. O.S. felt roll)
Doctor back too weak
Doctor not machined for location where used
Cross machine "chops"
Pattern
Causes
Local (short) cross machine chops center
Blade vibration due to:
Local hard spot on roll surface
Scab on roll
Dent in roll surface
Circumferential bands
Pattern
Causes
Local.
High local wear or scouring:
Misalignment
Improper fit
Rough roll surface
Local hot spot on roll
Debris on blade
Embedded grit in blade
High blade angle
Sheet moisture streaks
Blade not free to expand - against stop
No oscillation
Roll not doctoring consistently, change in position (guide rolls, etc.)
Doctor mounting or loading design problem
Cross machine "chops"
Pattern
Causes
Full cross machine chatter all the way around
Doctor back vibration:
Due to design of doctor back
External excitation of mountings or load means
Doctor blade vibration:
Inadequate loading
Blade too short
Binding load mechanism
Improper set stops
Load interference
Angle too steep
Full cross machine at certain spot(s) on roll circumference
Doctor excited by roll vibration or bounce
Hard spots in roll cover (lap joints)
Roll not running true
Scored roll surface
Projection(s) on roll surface
Influence of dryer dippers (Yankees)
Loose roll cover
Lubricating shower turned off
Granite roll problems
A common problem with doctoring granite rolls with DST or Equalizer doctors is that a series of corrugated bands may appear on the roll surface. This problem can occur regardless of the granite source, machine builder, doctor supplier, or doctor design. The problem is very random; with two identical machines, one may experience the problem while the other machine will have no problem. The only common element in all cases of corrugation is the use of a DST or Equalizer type doctor.
Typical granite roll corrugations
CD corrugations
Pattern
Causes
Varying intensity and frequency. Generally, worse at machine centerline.
Doctor back too weak in section
Yankee dryers - circumferential vibration of dryer shell.
CD corrugations
Pattern
Causes
Local
Mechanical vibrations being transmitted to the end of the roll or doctor mounting. Bad bearing, couplings or gearing.
For more assistance with doctor related problems, download our white papers in the Related Links section and contact your Valmet representative.