Document 9JwzZQ9mYLj3Jard0pRZb6OLq

680 JEROME R. COX, JR. The stiff bottom layer prevents bending of the compliant layer and forces i be compressed between the top and bottom layers. Effective damping of^ movement of a piece of heavy sheet metal is accomplished by this techn"_ whereas even thick layers of undercoat applied to a single piece of sheet rgi't of equal weight produce an almost negligible improvement. -sj. Plant installations of the "sandwich" kind of treatment produce noise* ductions of about 10 db providing other noise sources are less important. Ef 27 shows one such installation. Figure 27. Shells (30 caliber) drop several inches onto the chute and then slideiu incline to be picked up by the upward slanting conveyor. Rubber has been placed ati' points along the sides of the chute. The bottom of the chute is lined with a metal-J$ metffil""sBHiawiT!h:"'(GDurtesy-Renrington--A-rms-0orrIncr)-------- ------- ---------------V- i)''..; e. Sand Chutes. The movement of sand through ducts and chutes$ a source of noise in foundries. The installation of heavy gauge ducts-,isiu impractical, but the application of a thin layer of abrasion-resistantfe' lined duct usually outlasts a metal duct since the particles of sand bouneefin of cutting into the rubber. /. Punch Presses. The principal sources of noise in the common! press are: (1) the impact of the die striking the work, (2) the iinpuci.T' NOISE AND THE CONSERVATION OP HEARING 681 gBthe crank and ram against one another as the slack is taken up at the die meets the work, and (S) the noise of the air blast used to eject |feted part. (inoise of the die striking the work may sometimes be reduced by |prepared dies. These dies should be ground to cut by a gradual shearing fjgf&ftkrather than by punching through the work at all points at the same jfftgood "shear-ground" die is difficult to make, but will last longer trground" die. A "multiple-stage" die should be stepped so that the lar.-through the work one after another instead of all at once, pessary noise is produced if the work is positioned well above the bottom Ip? The minimum amount of noise is generated if the die begins to shear Ijhesiwork .very near the bottom of a stroke and. completes the punching "jjajust as the crank passes dead center. The velocity of the impact is li and, therefore, a machine of larger capacity may be required, tlbing of an operation on a small, high-speed punch press may sometimes by transferring the job to a press with a greater capacity, but with !|peed. Equal production can be regained by punching more than one ich stroke of the larger press. felubrication of many punch-press operations, particularly drawing, cjf|work Of better quality, more quietly. An overhaul is needed ; if the f|d by a punch press emanates;'from the vicinity of the,;crank, bear- ||k;irather than from the dies. Itt: such cases ,the noise at the.instant of kilts from thq sliminatjnhtAf t.he slanlf in t.Kiy'. menhflnicft.U?hlnii.iii. con- pmotor to the die. A maintenance program that includes : regular overhaul is, therefore, an important part of a hearing-conservation fplants using punch presses. air blast used to eject parts from a punch.press can be eliminated iition of mechanical ejection. This substitution* will not always- be Jmother alternatives are discussed below (page 684). ftbrials Handling. An important source of noise :-in steel-fabricating fehandling and straightening of metal plates": aid beams. Improved ...... Handling procedures may pay dividends of reduced noise and less-dami^iS{>iicrialssiIn"Pr6duction*0perations the handling'equipment can 'bd'ctistom^jt^o~clfeaatecthevdrop.pin'gaotemateriadBSSg52I^gl'.l:^6S^ul^^S'i'AL-.... Ijfi^pligninent'of'behE'aridt dainaged plates is often accomplished with a and careful handling may also help to eliminate this source of ^rom "^l-to-metal contact can be minimized byKthe- use - of Pallets, or shims. The principal requirements .of^such m npise- ^^Mi,rdgmrPaTe^resight-a-nd-a-witltrigiTes5~tiT_t'ry new techniques. fs/Mrg7J(wsfegronveyors. The conveyor lines that carry cans are usually con- wo stationary guide rails and a moving wire rope that travels under-' pprede, "Control of Impact Noise," The Acoustical Spectrum. TJniv. of Mich.' 1952, p.117. NfiKF . fcwi3'aAai\JK.. "v fsl$4siLIj