The present invention relates to copolymers, in particular poly copolymers, which have advantageous mechanical properties, and also to polymeric compositions comprising them. The disc has a root portion formed at the other end, and an arm portion formed from the root portion to one end. The pitching machine may include a shield to prevent the hitter, or fielder or tennis player from observing the settings of the pitching machine and preparing for the next pitch prior to pitch delivery. The first and the second lips are both in sealed contact, on either side of the separate part, with a substantially radial surface portion of the upper cover or of the bearing seat.