While much machine information today is displayed with digital meters, analog meters still have their place in the industrial world. One such application is the display of tension in a running web.
The tension on the web is affected by many mechanical factors in the process. When unwinding a roll of material, the quality of the roll itself is a major factor. If this roll is not perfectly concentric, which is usually the case, this will cause speed differences which will result in tension differences. The same applies to worn air shaft journals, safety chuck journal seats, as well as worn core chucks.
Other factors can be worn bearings, the web slipping on rollers, vibration due to non balanced rolls, worn gears or unstable drives. Some of these problems are easy to correct but many are not. Many do not have a major impact on product quality.
Since the tension is measured by highly sensitive and accurate load cells, these mechanical issues will be displayed as fluctuations in tension. In high speed applications, the refresh rate of a digital display or the inherent dampening of an analog meter will smooth out these fluctuations and give a good indication of the “average” tension.
In slower speed applications, these fluctuations will be much more pronounced and it can be very difficult to determine what the “average” tension is. One advantage of the analog meter in this situation is that it can be used to troubleshoot the problem. For instance, if the tension fluctuation is in sync with each revolution of the unwind roll, this will obviously indicate where the problem lies. It will then be easy to determine if it is a worn component that can be corrected or a non concentric roll which you may have to live with. Digital meters, on the other hand, will not show an easy to read “trend”.
Another advantage of the analog meter is that it is very easy to dampen. Simply connect an electrolytic capacitor to the connector of the meter. Any value between 100 and 300 microfarad should work. Just be sure to connect the positive of the capacitor to the positive of the meter. See drawing below.
