Dancer roller assemblies are an excellent option for stable, closed loop automatic tension control. In addition to their ability to accurately control a power transmission device (brake, clutch or drive), they provide very stable web tension. Dancer roller assemblies provide "web storage" within their travel. When a dancer roller moves through it’s travel, not only does it tell a power transmission device to make an adjustment, it also acts as sort of a web shock absorber, so web tension fluctuations are kept to a minimum. Other closed loop tension control devices, such as loadcell type, certainly will control a power transmission device but lack web storage to absorb fluctuations.
When considering the application of a dancer roller assembly for any web tension control application or when trouble shooting an existing dancer roller assembly, here are some things to consider:
- Minimize the effects of gravity. Dancer roller assemblies should be designed so that the loading on the web is not affected by the weight of the moving rollers within the dancer travel. This weight is not variable or controllable, so the effects of gravity should be avoided or minimized. Rotational type dancers minimize gravity’s effect on the dancer roller control system.
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Momentum and inertia is always a concern for any dancer design. A body in motion wants to stay in motion and dancers are always moving. Try to keep components as light weight as possible to lessen tension fluctuations due to momentum and inertia.
- Stay away from weight loaded dancer designs. They are very susceptible to tension inaccuracies caused by momentum and inertia. A better choice is to pneumatically load the dancer roller assembly to minimize the mass of moving parts. A precision air regulator or even an electronically controlled E to P converter can be used to accurately control the dancer loading.
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Rotational type dancer design incorporates a non-linear, rotational motion. This has a “push – pull†action loading the web with tension. This design greatly reduces momentum and inertia concerns and provides stable dancer web tension control.
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Frictional losses in cylinders and bearings are a detriment any dancer design. Use free spinning bearings in all idler rollers. Use low friction ball bearings and bushings for moving frames. Load dancers with glass lined cylinders (no seals or o-rings) or rolling diaphragm cylinders to cut friction down to a minimum.
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Obviously, the dancer controller system that processes the dancer position and controls the output to adjust speed or torque of the power transmission equipment must be properly designed. High performance, turnable and programmable PID feedback loop is generally required to stabilize any dancer roller automatic tension control system.
Dancer rolls can be an excellent method to controlling your web tension. They are extremely useful for controlling out of round unwind rolls, as well as other tension effecting defects. Ultimately, their success or failure in your application depends on how they are designed.
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