On a typical conveyor, the belt is troughed for the carrying portion of its journey and returned to a flat configuration for the return run. Consequently, at a terminal (head or tail) pulley, the belt must be converted from flat to troughed shape or from troughed back to flat. This changing of the belt’s profile is commonly called the transition. Transitions exist at the tail (loading) and head (discharge) pulley locations of a troughed conveyor and can occur in other areas of the conveyor, such as at a tripper head.
The distance from the centerline of the terminal pulley to the first fully-troughed idler is called the transition distance. This area poses more risk to the belt than any other area of the conveyor. In changing from a flat belt to a fully-troughed profile, tension at the sides of the belt is greater than at the center, the outer one-third of the belt must stretch and travel further than the center one-third. This can cause the splice, either mechanical or vulcanized, to fail at the belt edges. In addition, the plies of the belt can separate due to this stress.
The transition distance, the spacing allowed for this change in the belt’s contour, must be sufficient at each terminal pulley. Otherwise, the belt will experience extreme stress in the idler junctions (the points on a troughed idler set where the horizontal roller meets the inclined rollers). Because the outer third of the belt is stretched farther, the result may be a crease in the idler junction that may eventually tear along the entire length of belt. Also, if the elasticity of the carcass is slightly exceeded, the belt may not tear but rather stretch beyond its limits, leading to belt-tracking problems. If the transition distance is too short, an excessive difference between edge and center tensions can overcome the belt’s lateral stiffness. This can force the belt down into the trough, so it buckles through the center or catches in the idler junctions where the rollers of the idler join. The first sign of idler-junction failure will be noticed as a “W” or “M” fold or shape in the belt’s return side. The increased edge tension seen in the belt from having too short a transition area will place an increased load on the outer bearings of troughing idlers and could lead to premature idler failure.
Belt tension can be kept within safe limits by maintaining the proper transition distance between the pulley and the first fully-troughed idler, thus minimizing the stress induced in the belt.
To properly support the belt at these transitions, idlers with intermediate angles should be used between the terminal pulley and the first fully-troughed idler. These transition idlers will allow the belt to gently change its profile to the proper trough angle. Strain on the belt at the idler junction is then minimized since it has been spread over several idlers and a greater distance.
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