User's Manual for the Scratch Length Calculator

Application

To identify possible rollers responsible for web scratching observed during conveyance in a web line.

How To

Choose the units of measure as either Metric or US. As you enter the key values you get instant feedback on the key outputs in the blue boxes. Some users prefer to use Text entry (it's more precise). Others tend to prefer Slider entry (especially on smaller devices). Feel free to choose whichever is most useful for any occasion.

While you can choose either Metric or US units, you cannot specify which type of units within those systems is used. The most common usage is fixed. For example, speed will be given in meters per min and feet per min. Pay special attention to make sure that your input/output values are in the units specified or convert to/from as needed.

Which is the Bad Roller?

You know your web speed (assumed constant in this app) and you can measure your scratch pitch (distance between repeat scratches) and scratch length. Now assume that the scratch is caused by a roller moving at a different speed relative to the web. In order to determine possible candidates, you guess at a roller diameter. Displaying erudition, you probably select a diameter corresponding to a roller you think is the problem. The calculation gives the speed of the roller, V_r, and the wrap angle in degrees required to yield the measured scratch and pitch. Does the roller diameter you selected have a corresponding wrap as computed by the program? If not, perhaps there are other rollers with the same diameter that do have the computed wrap angle. If so, then any of these matching rollers are possible scratch generators. If the calculated values do not correspond to actual rollers, move on by selecting another roller diameter.

To further aid in diagnosing which roller is causing the scratch, the analysis indicates which direction the scratch is forming. If the roller is slower than the web then the scratch will be plowing back upstream, if it's faster the scratch will be plowing downstream. Of course, this frame of reference assumes that the surface of the roller is harder than the surface of the web; otherwise, the frame of reference reverses. The direction should be clear from microscopic examination and gives another clue of which roller is faulty; especially since the user will know which rollers are idlers (likely to cause drag leading to scratches plowing upstream) and which rollers are driven (can cause scratches in either direction depending on whether the driven roller is motoring (leading to scratches plowing downstream) or braking (leading to scratches plowing upstream).

That's all there is to it. Within a few minutes you should have tracked down the problem, provided that your conveyance line does not have dozens of rollers of the same exact size! At the very least, you should be able to significantly reduce the number of possible rollers that might be causing the problem.

Here are the formulae used in the app:

V_r = π x Diameter x Speed / Scratch Pitch

and

Wrap = 360 x Scratch Length / (π x Diameter - Scratch Pitch)

Credits

The SLC is based on the ideas of Dr Kevin Cole of the Media Conveyance Facility at Optimation. He can be contacted for further discussion or questions at kevin.cole@optimation.us.