Constrained Layer Damper and Noise Suppressor for Rock Drill Steel.
U S Pat 3 842 942 1974 Oct; :
A constrained layer damper and noise suppressor is made an integral part of a rock drill steel shaft. Rearward of the drill bit a viscoelastic layer constituting a damping medium is bonded around the drill steel for a major portion of its length. Coextensive with the outer diameter of this medium a smooth thin-walled, rigid, abrasion-resistant tube is bonded. Usually the diameter of the constraint tube is less than the major diameter of the drill bit so the entire invention can penetrate a drilled hole.
U.S. Pat. 3,842,942, Oct. 22, 1974, Chem. Abstr. Not Found