Before the early 1980’s, most chain and flight systems were furnished with heavy cast iron or steel sprockets. Over time, these cogs became rusted and corroded, causing them to seize up on the shafting and wear out rather quickly. Nonmetallic sprockets were subsequently introduced to the market along with lightweight plastic chains, flights, and accessories that improved system longevity and efficiency. There are currently three major nonmetallic sprocket materials used in water and wastewater clarifiers: UHMW-PE (Ultra-High-Molecular-Weight Polyethylene), Polyurethane, and Cast Nylon 6.
• UHMW-PE is an inexpensive option and offers some benefits as a bearing material with its low coefficient of friction and minimum water absorption. UHMW-PE, however, is the softest of the three plastics and has the lowest yield strength. The major issue with UHMW-PE sprockets is their tendency to cold flow and creep. Over time, UHMW-PE will begin to permanently deform under the influence of persistent mechanical stresses. This can quickly lead to overworn teeth, splitting, and keyways smearing.
• While Polyurethane is harder than UHMW-PE, there are numerous variants of the material and ranges of hardness. It is important that a proper hardness be specified when selecting the material. Polyurethane can also be attacked by ferric chloride, a common coagulant used in water treatment.
• The ideal material for chain and flight collector sprockets is Cast Nylon 6. While more expensive than the above materials, Cast Nylon is twice as hard as UHMW-PE and has tensile strength almost double that of Polyurethane. Cast Nylon wears longer, is more resistant to corrosion, and has a proven track record. In some cases, Polychem’s Cast Nylon sprockets have been used in the same tank for over thirty years with no visible wear.