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

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

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

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

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

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

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

Wear outside trim
  • Sheet lubricates roll.
  • Slot blade ends to relieve pressure.
  • Apply water lubrication to face of roll.

Localized pitting

Localized pitting

 Blade attacked by:
  • Electrostatic discharge
  • Electrolysis
  • Heat.
  • Insulate doctor from machine frame or use non-metallic blade.

 

Horns both ends

Horns both ends
  • Oscillator not working.
  • Blade too long.
  • Fix oscillator.
  • Trim length of blade to match roll face.

Score on blade edge

Score on blade edge
  • Damaged roll face.
  • Repair roll face.

Feathered blade edge

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

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

 

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

 

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

 

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

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.

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