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

Roll surface, circumferential bands, symetrically different

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"

Roll surface, cross machine chop, local

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

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

Roll surface, circumferential bands, local

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

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"

Roll surface, cross machine chop, full width

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

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

Granite roll CD corrugations, varying

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

Granite roll CD corrugations, local

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.

Troubleshooting doctors: loading, wear and roll surface marking

Regular doctors (externally loaded)

DST doctors

Blade wear

Uneven wear

Center wear

Bow wear pattern

One end wear

Rough blade edge

Wear within trim

Wear outside trim

Localized pitting

Horns both ends

Score on blade edge

Feathered blade edge

Bird's mouth

Roll surface

Circumferential bands

Cross machine "chops"

Circumferential bands

Cross machine "chops"

Granite roll problems

CD corrugations

CD corrugations

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