Dorr-Oliver® and EIMCO® Traction Thickeners have a stationary center pier, which partially supports the rake mechanism and serves as a pivot about which the rakes rotate. The rakes are powered by one or two drive units that run steel traction wheels round a steel rail at the periphery of the tank. The units are normally installed in concrete tanks and are available in diameters up to 650 feet (198m) utilizing friction and positive drives. Under the proper climatic conditions, traction thickeners are a very efficient, economical design.