Materials of construction.

(a) The general configuration of the wooden protective jacket must be a hollow cylindrical shell constructed of one-piece discs and rings of plywood or solid hardwood reinforced with steel rods.

(1) The specification 20WC-2 must be additionally completely encased, snugly fit, within an 18-gauge steel shell. The steel shell must be provided with at least four 6 millimeter (0.25-inch) diameter vent holes. Each hole must be covered with durable weatherproof tape, or equivalent device.

(2) The specification 20WC-6 jacket must be additionally completely encased, snugly-fit, within a 12-gauge steel shell. The steel shell must be provided with at least twelve 1.2 cm (0.5-inch) diameter vent holes, located in 3 rows of 4 holes each, spaced at 90 degree intervals near the top, middle, and bottom of the drum. Each hole must be covered with durable weatherproof tape, or equivalent device.

(b) Plywood must be exterior-grade, void-free, Douglas fir (or equivalent) not more than 2.5 cm (1 inch) thick. Solid hardwood is authorized for specification 20WC-2 only.

(c) Discs and rings must be glued together with a strong, shock-resistant adhesive, such as either of the following:

(1) A resorcinol-formaldehyde adhesive, which has been bonded under both heat and pressure; or

(2) A polyvinyl-acetate emulsion, which has been reinforced with cement-coated nails. The nails must be randomly spaced and must be at least 2.5 times as long as the minimum thickness of the plywood discs or rings.

(d) Full-length steel rods are required for reinforcement and lid closure.

(1) The minimum number of rods and the minimum rod diameter are as shown in the following table:

Specification Minimum number or rods Minimum rod diameter Inches Mm 20WC-1 6 0.25 6.0 20WC-2 6.25 6.0 20WC-3 12.375 9.5 20WC-4 16.375 9.5 20WC-5 16.50 12.0 20WC-6 16.50 12.0

(2) For specifications 20WC-1 and 20WC-2, steel rods must be equally spaced around the circumference to the rings and discs, midway between the O.D. and I.D. of the rings. For specifications 20WC-3 and 20WC-4, bolts may be staggered alternately in two rows, at ±1.2 cm (0.5-inch) from the line midway between the O.D. and I.D. of the rings. For specifications 20WC-5 and 20WC-6, bolts may be staggered alternately in two rows at ±2.5 cm (1 inch) from the line midway between the O.D. and I.D. of the rings.

(3) Rod ends must be threaded and secured with lock nuts and steel washers, or equivalent device, to provide at least a 2.5 cm (1 inch) diameter bearing surface on each end. Ends of the rods must terminate 1.4 cm (0.75-inch) below the surface of the plywood for specifications 20WC-1 and 20WC-2. For specifications 20WC-3, 20WC-4, 20WC-5 and 20WC-6, the ends of the rods must terminate 3.7 cm (1.5 inches) below the surface of the plywood, and that portion of each end disc which extends beyond the rod ends must be further held in place with lag screws at least 10 cm (4 inches) long.

(e) Thickness of wooden shell:

(1) Specification 20WC-1: At least 10 cm (4 inches) thick.

(2) Specification 20WC-2: At least 7.5 cm (3 inches) thick.

(3) Specification 20WC-3: At least 13 cm (5 inches) thick for the jacket wall, and at least 15 cm (6 inches) thick for the end discs. In addition, at least 3 plywood chines, 5 cm (2 inches) wide and protruding 5 cm (2 inches) beyond the outer surfaces, must be located at each end and midway along the length of the jacket.

(4) Specification 20WC-4: At least 15 cm (6 inches) thick for the jacket wall, and at least 15 cm (6 inches) thick for the end discs. In addition, at least 3 plywood chines, 5 cm (2 inches) wide and protruding 5 cm (2 inches) beyond the outer surfaces, must be located at each end and midway along the length of the jacket.

(5) Specifications 20WC-5 and 20WC-6: At least 15 cm (6 inches) thick for the jacket wall, and at least 20 cm (8 inches) thick for the end discs. In addition, at least 5 plywood chimes, 5 cm (2 inches) wide and protruding 5 cm (2 inches) beyond the outer surfaces, must be located at each end and equally spaced along the length of the jacket.

[Amdt. 178-35, 39 FR 45252, Dec. 31, 1974. Redesignated by Amdt. 178-97, 55 FR 52716, Dec. 21, 1990, and amended by Amdt. 178-99, 58 FR 51534, Oct. 1, 1993; 66 FR 45387, Aug. 28, 2001]