Your bouncing belt is often called "belt flap" and is caused by poor engineering/design.

Three major causes are:

1. Belt's natural or fundamental vibration frequency modes, for a given or selected idler spacing, are coupling with the return idler roll spin frequency. That is, when their ratio approaches a fundamental vibration mode (1, 2, 3 or 4)

2. Belt's natural frequency is resonating near the same critical mode of the idler supporting transom and stringer.

3. Elevated steel structural modes of vibration resonate near the one of the belt's critical frequency modes. Sometimes "white noise" can set the structure into modal resonance.

There are many types of belt resonance which are too pedantic to detail here.

Overland conveyor designers must be very cognizant of these problems and take care not to allow coupling of these various vibration effects.

First, I think I have commented on the "volcano" effect, in the forum before, which causes the transported product to coalesce into mounds. This event has even caused transverse waves, preserved through transfer stations, to ultimately spill product over the belt edge. This event is when the carry roll spin frequency couples to belt and supporting structures.

Second, designers need to minimize noise due to belt flap. This is symptomatic of the belt beating the surrounding air we often see and hear (wha, wha, wha, wha ….).

Third, idler life is shortened due to the belt beating and product coalescence into individual mounds which can result in high impact to idler shells, end disks, bearings and support structures.

Get a good designer. ■

Dear Mr. Hilmi,

Please first check whether the belt level reference line at tail pulley is same as the reference level of the idlers. Now a days, it is quite common that tail pulley diameter is less than the distance between carrhying run and return run as fixed by idlers arrangement and stringer size. In this situation when the tension in the belt happens to be more at that location, such as starting condition or fully loaded condition etc. then the belt will show the lift up symptom. If this is in order then you may investigate about the vibrations etc. as stated by earlier respondents. The belt profile in a conveyor is very important and it should be carefully done while preparing the arrangement of the conveyor, so that belt remains at the drawn profile in all situations.

The usual idler pitch for in-plant conveyors can be seen from various literature. It depends upon belt width, material bulk density, etc. Such usual spacing is to have economical conveyor design as concluded by the long time observation in the industry. Such spacing are not holistic and one can adopt more or less spacing depending upon the specific situation. For example, if somebody takes longer spacing, then minimum tension to limit the sag would become more and thereby certain cost difference. However, if conveyor is very long with a very high tension already in the belt, then choosing larger spacing does not change belt, and could be economical. There are other fine aspects like belt ability to support load etc. and all these cannot be narrated here as design related decisions / instructions.

The use of snub pulley is to bring the return belt from drive pulley exit point to the reference level of the return belt (when drive pulley or tail pulley is large). The other purpose is to increase the wrap angle so that tension in the belt becomes less and thereby economy in conveyor.

Regards,

Ishwar G Mulani.

Author of Book : Engineering Science and Application Design for Belt Conveyors.

Author of Book : Belt Feeder Design and Hopper Bin Silo

Advisor / Consultant for Bulk Material Handling System & Issues.

Email : parimul@pn2.vsnl.net.in

Tel.: 0091 (0)20 25871916 ■