Tips for troubleshooting your roll grinding application

May 9, 2022

Belt grinding background

Historically, the majority of paper mill rolls being ground were roughed and finished by using grinding wheels. With the introduction of coated abrasive belts and machine conversions, those who were willing to give it a try found that many of the problems that they had been living with for years were unnecessary.

Belt grinding In the infancy of belt grinding, splices in the belts were a common problem, but this was overcome and many paper mills are using belt grinders to help expedite their grinding requirements. The belt method allows them to finish their rolls in less time and eliminates listening to the cut and making constant machine adjustments.

The belt grinder consists of a rubber covered driven contact wheel and a idler or stretcher wheel. These wheels vary in width from 2-1/2 to 4 inches. Four inch wide belts are most commonly used. Lengths of belts are also variable, some of which are 66 to 132 inches.

The more common types of grinding belts are silicone carbide and aluminum oxide and can be obtained in various grit sizes, some ranging from 36 to 400. Silicone carbide is generally used when a coolant or wet grinding is
done.

Mills that are using belt grinders, generally use them in grinding rubber covered, brass, stainless steel, cast iron, etc. rolls.

Chilled iron calender and granite rolls have been found to be considerably more difficult to achieve the necessary accuracy with belt grinding.

In belt grinding the most accurate and successful grinding is achieved when the serrated contact wheel with the belt, grinds on the horizontal centerline of the work roll.

NOTE: There are significan differences between metal and rubber roll grinding.

Common grinding problems and causes

The following is a list of some of the common problems incurred in roll grinding applications and the possible causes of each. Often, several conditions accumulate to create the problem. So these recommendations are only a guide to troubleshooting.

Wheel

Common grinding faults and how to correct them:

To eliminate chatter:

Try a softer wheel.
Rebalance the wheel.
Tighten the wheel spindle bearings.
Check for external vibrations from other machinery.
Move the journal along the roll neck just a little.
Tighten up the journal bolts.
Make sure the journal bearing is not cocked.
Put a dab of heavy grease on the journal bearings.
Vary speeds of the roll wheel traverse.
Dress the wheel.

To eliminate feed lines:

Use a freer cutting wheel.
Determine if the table is worn low, (make sure the roll is level).
Make sure the head and footstock are in alignment.
Round the wheel corners.
Undercut the wheel to reduce the contact area.

To eliminate scratches on the roll:

Brush the wheel with a wire brush.
Air blast the wheel after dressing.
Use a harder wheel.
Make sure the wheel guards are clean.
Clean the coolant tank.
Round off the wheel edges.
Increase the wheel speed to make it act harder.
Check the coolant to see if it is free of grain. If found, remove the grain by placing cheese cloth over the coolant nozzle.

Shellac wheels have been historically bad actors in scratching rolls; improperly held grains frequently fall out and roll between the wheel and the work. Newer shellac bonds practically eliminate this problem.

To eliminate diamond marks:

Reduce the traverse of diamond when dressing.
Take lighter passes when trueing.
Try a softer wheel.
Use a dull diamond.
Try a larger diamond.
Make sure the diamond nib is tight in the holder.

To eliminate patterns in the roll:

Check the action of the equalizer drive.
Check the spindle bearings.
Make sure the wheel is not oil soaked on one side by standing on the floor.

Belt accuracy unobtainable

1. Crowning mechanism is not engaged or disengaged.
2. Roll is being deflected.
3. Roll is sagging.
4. Machine ways are worn or dirty.
5. Excessive infeed on the roll ends.
6. Belt is worn beyond its useful life.
7. Roll is misaligned with the grinding head.
8. Bearings, journals or centers are worn or sloppy.
9. There is insufficient coolant.
10. Belt is traveling off roll ends too far.
11. Spindle bearings are loose.
12. Roll is excessively polished.
12. Roll is not level or correct in polished horizontal plane.

Belt chatter

Excessive roll speed.
Spindle bearings are loose.
Bearings, journals, centers are improperly lubricated.
Excessive infeed or pressure.
Contact wheel is out of round or balance.
Vibration in machine or belt unit.
Belt joint is improperly constructed.
Idler pulley is loose.
There is insufficient belt tension.
There is hesitation or lag in the roll drive mechanism.
Roll drive connection is too tight.

Barberpole or striping

1. There is excessive infeed or pressure.
2. Idler pulley face is worn.
3. Contact wheel face is tapered or worn.
4. Belt is wandering off the contact wheel face.
5. Belt edges are ragged.
6. Belt cupping sandside concave on contact wheel.
7. Traverse speed is too fast.
8. Contact wheel heel is traveling off the roll ends too far.
9. There is insufficient belt tension.
10. Belt joint is improperly constructed.

Scratches on roll

1. Previous grit marks were not completely removed.
2. Contact wheel is dressed with too coarse an abrasive.
3. Belt edges are ragged.
4. Reverse belt cupping.
5. Belt is worn beyond useful life.
6. There is excessive infeed or pressure.
7. Dirty or contaminated coolant recirculating swarf and abrasive grain particles.

Belt glazing

1. There is insufficient infeed.
2. Coolant is faulty or insufficient.
3. Contact wheel is not aggressive enough.
4. Belt speed is excessive.
5. Roll speed is excessive.
6. C/A belt type is improper.

Belt breakage

1. Belt tension is excessive.
2. Infeed or pressure is excessive.
3. Pounding or chatter is excessive.
4. Belt joint is the improper type.
5. C/A belt type is improper.
6. Idler pulley crown is excessive.
7. Joint or web nicked while “killing” edges.

Belt tracking - loss of ability

1. Belt tension is insufficient.
2. Contact wheel face is tapered or worn, especially concave.
3. There is insufficient belt take up capability.
4. C/A belt type is improper.
5. There is misalignment between the idler and contact wheels.
6. Idler is deflecting due to light construction or wear.
7. Idler pulley face is worn - loss of knurls or helical grooves.

Idler pulley

The idler pulley or wheel can periodically cause problems that are often blamed elsewhere. This area should be one of the check points in your preventative maintenance program.

Idler pulley and contact wheel

A worn or misshapen pulley face will allow the abrasive belt to wander on the contact wheel, leading to barber pole and marking.

If the wear is to the extent that the pulley is smooth, i.e., the knurls or helical grooves have been lost, belt slippage can occur and the useful working life of the belt can be greatly reduced. In severe cases, it is virtually impossible to track a belt on the machine, and premature rupture or breakage is encountered.

If the idler pulley is designed to have a crown for tracking, the same problems as mentioned above will occur if the idler pulley becomes flat. When we talk of belt tension, we mean the actual pounds of force that are being applied to the coated abrasive belt to keep it taut between the contact wheel and the idler pulley. Too little tension or excessive tension reduces the effective life of the belt. It can also lead to premature wear of the spindle, pulley bearings and wheel faces.

Belt tensions in use for roll grinding applications have been found to range from a low of 12 pounds per inch of belt width, to a high of 50 pounds per inch of width. A good starting recommendation would be 20 pounds to 25 pounds per inch, unless specific conditions warrant a higher or lower force.

It is hoped that with the information and data outlined in this guide, the success of your roll grinding applications will be achieved and the coated abrasive grinding method will prove to be a rewarding experience.

For more assistance with roll grinding, contact your Valmet representative.