Amana Tool uses only the finest industrial quality precision ball bearings available. Bearings are double steel-shielded for extra rigidity and to inhibit dust. A special high-performance, high-temperature grease is used for maximum life. Ultra-Glide High Performance Ball Bearing Guide Assemblies Ultra-Glide bearing assemblies feature satin-smooth Delrin sleeves. Delrin is an industrial synthetic plastic material similar to nylon, and will leave no marks on the material being cut. Applications include solid surface materials such as Gibraltor, SSV, Corian, Surell, and Fountainhead, etc. Plexiglas & other clear acrylics; and plastic laminates. Not recommended for regular wood applications due to a higher wear factor. Bearing # 47709, 47725, 47726, 47731, 47733 include Teflon dust shields. These bearings provide a better seal to increase bearing life while routing materials that generate very fine dust particles (such as solid surface). Note: Solvents should not be used to clean ball bearings, as this will deteriorate the special grease. 'Frozen' bearings (ones that do not rotate freely), should be replaced immediately.