Abstract: Spirally-tending ribbing on the medial and angular idler rollers of a troughing conveyer including waveform ribbing on the medial roller and normal ribbing on the inner halves of the angular idler rollers for spreading the belting outwardly from center and oppositely-tending spiral ribbing on the outer halves of the angular idler rollers due to lateral shifting of the center of gravity of the material on the belting toward one or the other sides of the trough-formed belting. The oppositely-tending spiral ribbing being of a scalloped type affording more positive traction against the belting especially upon shifting of the load of material on the belt toward an edge thereof over one or the other of the angular idler roller. For extreme cases of such outward climbing of the belting, a feather touch idler roller is set wedge-like relative to the outer halves of the angular idler rollers for effecting greater traction of the belt edge against the latter.