Catenary sag or take-up unit for a plastic modular belt?

Catenary sag or take-up unit for a plastic modular belt?

In an earlier article we looked at the basic drive configurations for plastic modular belts and explained that “one or more catenary sags are used to provide optimal belt-to-sprocket engagement”. These catenary sags not only provide back tension but also compensate for belt elongation caused by the load, wear between module and rod, and the effects of temperature. So there is no doubt that catenary sags are a vital component of conveyor systems.

But there are situations where catenary sags may not be desirable. For example, a catenary sag can be a potential danger to workers’ safety, or the return belt may be at a level that does not permit catenary sags

Vertical or horizontal belt take-up unit

The solution in these cases is to design the conveyor with a vertical or horizontal belt take-up unit. The vertical version consists of a gravity roller that creates a certain tension through its roller weight. The mostly horizontal tension roller version pushes on the belt in a specific area. These units permit adjustment of the belt path on the conveyor to the actual belt length. Both units ensure a small amount of back-tension where the belt leaves the drive unit. The dynamically installed rollers are able to compensate for belt elongation.

If safety is a concern, the take-up unit can be covered and belt elongation can be monitored in this specific area as well. The gravity take-up unit is placed right after the drive, while the tension roller usually replaces a fixed idle shaft.

From my experience, the travel distance of the roller in the unit is often designed too short and the belt needs to be shortened quite soon after installation. As this is not very desirable, we recommend designing the roller travel distance taking into consideration an elongation of about 2% to 3% of the total belt length.

Do you require additional information? You can download the HabasitLINK Plastic Modular Belts Engineering Guidelines or contact me through

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2018 September 20  |  Posted by

Ulrich Hänggi

Ulrich Hänggi is an Application Engineer for the R&D Plastics department and has been working for Habasit since December 2000. He has extensive experience in various applications where plastic modular belts and chains are used. He enjoys hiking in the mountains. Ulrich speaks native German, as well as fluent English.

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