The shelf of the Los Angeles machine is subject to
severe surface wear and impact. With use, the working surface
of the shelf is peened by the balls and tends to develop a ridge
of metal parallel to and about 11⁄4 in. (32 mm) from the junction
of the shelf and the inner surface of the cylinder. If the shelf is
made from a section of rolled angle, not only may this ridge
develop but the shelf itself may be bent longitudinally or
transversely from its proper position.