Document pejkpY28wK2MMo1bMM6ZoK2jE

thought and it is flimsily made and tacked on to the machine as cheaply as possible Such guards may even aggravate noise from a ma chine This is one reason why manufacturerbuilt guards are recommended when possible over the "home made" kind However, for noise reduction, many manufacturer-built guards are no better than "home made" guards Noise travels primarily by conduction and vibration through air It can be attenuated by barriers which effectively stop or restrict further air activity either by absorption or by reflection and conflnement of the sound waves Since guards are usually positioned at either the point of operation or power transmission (whence noise originates), they can be de signed as a barrier for noise as well as a barrier against personal injury A guard can be designed to be either absorbent or reflective of sound waves A common way to absorb sound is to either line or cover the surrounding frame with a soft cellular material which soaks up the sound with dead-air spaces Sometimes a thin layer of lead is sandwiched between two layers of such soft material to further reduce sound transmission However, if oil is used in the process, this could create a senous fire hazard if the area has a heat buildup, for some of these soft materials tend to soak up oil It has also been found that if the guard is made of heavier metal stock (16 gage or better), it provides a dense, vibrationless surface when it is properly mounted Proper mount ing is essential for the success of either method, because both methods (absorpbve and reflective barriers) require that the guard have an identical configuration with the surface on which it is attached Some sort of gasket material should be used around the edges and any other place that metal-to-metal contact can occur, and the whole guard se cured with shakeproof fittings In effect, this unitizes the guard with the machine and con tains the sound withm, thus reducing the con ducted sound More details are in Chap ter 40, "Noise and Hearing Conservation " Summary In summary, a complete guarding program is essential for any company It prevents ac cidents, whether to people or machines, and thus facilitates production goals It can be achieved only by studying and understanding the relationships between people and the machines they operate Machines can pro duce only if they are used correctly--and they are used correctly only when all guards and safeguards are in place References American Mutual Insurance Alliance, 20 N Wacker Drive, Chicago, 111 60606 Safe Open ings for Some Point of Operation Guards, Technical Guide No 2, 3rd ed , 1966 American National Standards Institute, 1430 Broadway, New York, N Y 10018 "How To Operate a Power Press Safely," 1975 Safety Requirements for Floor and Wall Openings, Railings, and Toe Boards, A12 1 Safety Specifications for Mills and Calenders in the Rubber and Plastics Industries, B28 1 Safety Standard for Construction, Care and Use of Mechanical Power Presses, Bll 1 Safety Standard for Forging, B24 1 Safety Standard for Mechanical Power-Transmission Apparatus, B15 1 National Fire Protection Association, 470 Atlantic Ave, Boston, Mass 02210 Electrical Metalworking Machme Tools, Standard No 79 National Electrical Code, Standard No 70 National Safety Council, 444 N Michigan Ave, Chicago, 111 60611 Guards Illustrated U S Department of Labor, Occupational Safety and Health Administration, Washington, DC 20210 Principles and Techniques of Mechanical Guarding, Bulletin 2057, 1972 809