The purpose of a winding guide is to permit winding to proceed safely at relatively high speeds by preventing collisions between the cages and between cages and the side of, or fittings in, a shaft. It must be (1) rigid enough to prevent material deviation of the cages or skips from the vertical; (2) strong, since a broken guide causes danger from damage; (3) smooth, so as to offer as little resistance to the movement of a cage as possible; and (4) firmly supported and maintained vertical. Guides may be of two types, rigid or flexible. The former may be of timber, and in new shafts have generally been replaced by steel channels, steel rails or angles; the latter are steel ropes of round or semilocked section steel rods. Rigid guides are adopted in shafts of rectangular cross section; these and the shaft sides and fittings are small. Rope guides are used in circular and elliptical shafts where adequate clearances can be provided. Rope guides maintain the vertical 3607 automatically, and expand and contract with temperature variation without complication.