a modular plastic conveyor belt , formed of successive rows of assembled injection molded modules connected by laterally - extending pins , has arched tops in each module row and increased depth in the laterally - extending spine of the belt . the belt structure enables the modular plastic belt to be retrofitted in a spiral system normally carrying a steel conveyor belt . in one embodiment the arched - top module rows , having a greater depth than normal , are accommodated on a small - diameter roller , by forming a generally cylindrical undercut at the bottom of each module row , reducing the distance of protrusion outwardly from the roller . the arched - top configuration lends itself not only to transfer of articles as the conveyor passes over the sprocket for a roller , but also the provision of increased beam depth at the center of the module , with lower profiles at the extremities of the link ends .