Annex A (informative)

Transcription

1 Annex A (informative) A.1 Introduction While not part of this Standard, the following relates a number of factors which contribute to the safe operation of abrasive wheels. Safety is everyone s business. A.2 General requirements Abrasive wheels should be used with operator safety in mind and in conjunction with the type of protective devices most effective for each application. A.3 Classes of protection devices There are two main classes of protection devices: those that protect the operator, and those that provide protection to others in the work area. A.3.1 Operator protection devices A Safety guards The most positive way to protect the operator as well as others in the immediate area is by guarding the wheel effectively. Section 4 (Safety Guards), page 50, explains the provisions necessary for adequate guarding. A guard protects the operator either by containing or deflecting the pieces of an accidentally broken wheel. Guards also control sparks and swarf. A Safety goggles, spectacles and face shields Because the grinding operation produces sparks and swarf, eye protection is of the utmost importance. Safety goggles or safety spectacles must be worn at all times in the grinding area. ust masks and face shields may be required as well. (See ANSI Z87.1 Practice for Occupational and Educational Eye and Face Protection.) A Protective clothing Safety aprons, gloves, safety shoes, and other protective clothing should be worn as required by the nature of the grinding operation. Well designed protective clothing will allow the operator to work more efficiently and safely. A ust protection ust masks may be required in an extremely dusty or contaminated environment. A.3.2 Work area protection A Barriers In some operations, partitions, walls, or separate grinding booths will provide protection to other people nearby. A Exhaust systems Adequate ventilation and swarf removal should be provided in the grinding area. 138

2 Annex B (informative) ANSI B A guide to grinding wheel guard design B.1 The method used to calculate guard dimensions Force expected to be withstood by guard is: F F = WrC = Centrifugal force in pounds Where: W = Weight of wheel in pounds r = Radius of wheel in inches C = Constant = N 2 N = Speed of wheel in revolutions per minute Assumptions 1. ole size as indicated in table 2. alf of wheel hits guard 3. Impact occurs at a point 4. Wheel density =.1 lb/in 3 5. Guard is stressed across diameter 6. Guard is same diameter as wheel Safety factors Safety factor (SF) is SF = a x b x c Where: a b c = the ratio of ultimate strength to elastic limit For steel and cast iron or cast steel, a = 2. = the character of the stress For a load varying from zero to maximum with impact, b varies from 1 1 /2 to 2. = the material constant For steel, c = 1; for cast materials, c = 2. Therefore, for cast materials the acceptable safety factor varies from 6 to 8. For steel it varies from 3 to 4. The higher safety factor is used for wheels 8" or less in thickness because the wheel is most likely to break as in the assumptions. Thicker wheels are less likely to fly apart in complete sections and are more able to withstand side pressure. A lower safety factor is therefore applied. The force of impact, F, the safety factor, SF, and cross-sectional area, A, of the side member are used to calculate the stress in the side member and compare this to the tensile strength of the material. h = diameter of side member, inches (same as wheel per assumption 6 above) = hole diameter in guard to accommodate arbor, inches 139

3 t A S = thickness of side member, inches = area of side member(s), square inches = stress in side member due to impact. This is equivalent to the minimum tensile strength of the material. Example: What is the minimum thickness of the fixed side member for a 30 x 2 x 12 wheel operating at 14,000 SFPM. Assume all stress is taken by the fixed side member, and it is stressed across its diameter per assumption 5, and has a hole for the arbor of 5 inches. Assume it is made of steel with a tensile strength of 55,000 psi. 14,000 SFPM is equivalent to 1782 rpm for a 30" diameter wheel. The weight of this size wheel at the stated density is 119 pounds. Safety factors: a = 2 b = 1.5 c = 1 SF = a x b x c SF = 2 x 1.5 x 1 = 3 W = 119 pounds r = 30/2 = 15 C = x N 2 C = x = F = WrC F = 119 x 15 x = 160,971 First trial: Assume a thickness for the side member. t = 1/4 inch Then the cross-sectional area of the fixed side member is: A = ( h) t A = ( 30 5) 1/ 4 = in 2 (If the removable side member shares some of the load then appropriate adjustments can be made in the area.) The stress, S, in the side member due to impact from half the wheel is then: SF F S = ( ) 3 = 160, 971 = 77, 266 A 625. psi Since this value is higher than the stated tensile strength of the steel, choose a larger thickness of side member and rerun the calculation. 140

4 Second trial: Increase the side member thickness to 3/8 inch. ANSI B A = ( 30 5) 3 / 8 = in 2 3 S = 160, 971 = 51, psi Since this value of stress is lower than the stated tensile strength of the steel, this thickness would be adequate to withstand the forces as calculated. B.2 Safety guard systems for abrasive wheels The current edition of ANSI B7.1 includes specifications for safety guard design as preceding editions have done. These specifications are based on careful calculation of the forces that the safety guard must withstand when an abrasive wheel is accidentally broken. A test method for guards used on portable electric grinders is outlined in Section As grinding technology has progressed, special abrasive wheel speeds have become more common. Guard design based on calculations alone may be less reliable when the increased energies of special speed are involved. Therefore, development of more efficient grinding processes has been accompanied by improvements in safety guard design and testing. The safety guard developments include mechanisms which react very quickly to an accident and the guard becomes a total enclosure in time to contain all significant debris. Among other developments are energy absorption devices combined with proper enclosure design to contain broken wheel fragments. Guard test methods generally involve breaking a rotating abrasive wheel within the guard by either impact of a bullet or detonation of an explosive at the wheel bore. Several safety guard designs and test methods have been reported in both European and American literature. Since grinding technology is nearing the point where it will not be feasible to include guard design standards in the Safety Requirements for the Use, Care and Protection of Abrasive Wheels, it is considered necessary for future editions to include performance criteria for safety guards. The performance criteria will be applicable to all grinding conditions, and specific design standards will not be necessary. The following is an example of the characteristics which should be considered in defining safety guard performance criteria. It is expected that the performance criteria in future safety standards will be similar to this example. In the meantime, machine builders may find the example, with properly selected safety factors and other values, a useful guide to safety guard design. The example may be particularly valuable in the special conditions where current safety standard design specifications are not applicable. It may also be helpful in situations when it is desirable to deviate from Safety standard specifications and proof of the efficacy of the modified design is necessary. B.3 An example of performance characteristics of a guarding system Outlined below is an example of performance characteristics and test method for guarding systems used on fixed base grinding machines utilizing abrasive wheels which are 12" and greater in diameter and 1" and greater in width. This is presented only as an example which might be used in design of a guarding system and is not to be interpreted as the only suitable criteria 141

5 for the design of such guarding systems. For guards used on portable electric grinders, see Section A. Test Method (1) The kinetic energy of a full size wheel rotating at maximum approved operating speed for the machine shall be multiplied by the selected safety factor to determine the kinetic energy of the wheel at burst. (E = 1 /2Iw 2, where E is kinetic energy, I is mass moment of inertia, and w is angular velocity.) (2) Wheel head position, truing device position, and workpiece choice (or no workpiece) shall be chosen to represent the conditions in which the guarding system is expected to be least effective during normal machine use. (3) Performance characteristics of guard system shall be determined by deliberately bursting a wheel which is rotating at the speed which provides the energy level required to meet the selected safety factor. B. Performance Characteristics (1) The maximum kinetic energy of any single escaping fragment shall not exceed a preselected limit. (2) The maximum kinetic energy of any single fragment entering the zone normally occupied by operator s head shall not exceed the impact strength of safety glasses conforming to ANSI Z87.1 standard. (3) All machine components shall remain attached to the machine and shall not move or rotate across any zone normally occupied by an operator. Much of the initial kinetic energy of fragments is dissipated through impacts and ricochet before escape from some types of guarding systems. Therefore, the example performance characteristics are excessively stringent if theoretical kinetic energies of escape fragments are calculated from the peripheral speed of the grinding wheel before breakage. One way of determining actual kinetic energy of escape fragments is to determine trajectories from high speed moving pictures of the test and calculate the energies from weights, final locations and trajectories of individual particles. 142

6 Annex C (informative) ANSI B , Safety Code for Portable Air Tools, lists several tables of wheel types and dimensions appropriate for use on portable air driven grinders. While not part of ANSI B , these tables have been reproduced below for the convenience of the reader. (See Section 1.5 in ANSI B7.1 and Section in ANSI B186.1.) Table C-1 Type 1 Abrasive Wheels for Portable Air Grinders GRINING T T Vitrified Bonds 6500 SFPM or less Organic Bonds 9500 SFPM or less 2 or less (50) 1/2 (13) 1/4 (6) 2 or less (50) 1/2 (13) 3/8 (10) Organic Bonds 9500 SFPM Or Less 3 or less (75) 1/2 (13) 3/4 (10) 3 (75) 1/2 (13) 1/2 (13) 4 (100) 3/4 (20) 1/2 (13) 4 (100) 1 (25) 5/8 (16) 6 (150) 1 (25) 5/8 (16) 8 (200) 1 (25) 5/8 (16) Organic Bonds Over 9500 SFPM 2 or less (50) 1/2 (13) 1/4 (6) 2 or less (50) 1/2 (13) 3/8 (10) 3 (75) 1/2 (13) 3/8 (10) 4 (100) 1 (25) 1 (25) 6 (150) 1 (25) 1 (25) 8 (200) 1 (25) 1 (25) imensions are Inches. Approximate Millimeter Equivalent Shown in ( ). 143

7 Table C-2 Type 6 and Type 11 Abrasive Wheels for Portable Air Grinders GRINING W 3/4" MIN. GRINING W 3/4" MIN. T T E E J T W E J Organic Bonds 9500 SFPM or less (type 6 and type 11) 4 (100) 5/8-11 UNC-2B 2 (50) 3/4 (20) 3/4 (20) 3 (75) 5 (125) 5/8-11 UNC-2B 2 (50) 1 (25) 3/4 (20) 3 3/4 (95) 6 (150) 5/8-11 UNC-2B 2 (50) 1 1/2 (40) 3/4 (20) 4 3/4 (120) Organic Bonds 12,500 SFPM (type 11 only) 4 (100) 5/8-18 UNF-2B 2 (50) 3/4 (20) 3/4 (20) 3 (75) 5 (125) 3/4-16 UNF-2B 2 (50) 1 (25) 3/4 (20) 3 3/4 (95) 6 (150) 7/8-14 UNF-2B 2 (50) 1 1/2 (40) 3/4 (20) 4 3/4 (120) imensions are Inches. Approximate Millimeter Equivalent Shown in ( ). 144

8 Table C-3 Type 27 epressed Center Abrasive Wheels for Portable Air Grinders CUTTING K U U = E Y E O U & E O K Y Organic Bonds Only 14,200 SFPM 3 (80) 1/8 (3.2) 1/4 (6) 3/8 (9.5) 5/32 (4) 3/4 (20) 1 5/16 (34) 4 (100) 1/8 (3.2) 1/4 (6) 3/8 (9.5) 5/32 (4) 3/4 (20) 1 5/16 (34) 4 (100) 1/8 (3.2) 1/4 (6) 5/8 (15.9) 11/64 (4.6) 1 3/32 (28) 2 3/64 (52) 7 (180) 1/8 (3.2) 1/4 (6) 7/8 (22.2) 3/16 (5) 1 5/8 (42) 2 5/8 (68) 9 (230) 1/8 (3.2) 1/4 (6) 7/8 (22.2) 3/16 (5) 1 5/8 (42) 2 5/8 (68) imensions are Inches. Approximate Millimeter Equivalent Shown in ( ). Table C-4 Type 28 epressed Center Abrasive Wheels for Portable Air Grinders U GRINING K E O Y J U = E V 1 U & E O K Y P V 1 Organic Bonds Only 14,200 SFPM 7 (180) 1/4 (6) 7/8 (22.2) 3/16 (5) 1 5/8 (42) 2 5/8 (68) 4 (100) 15 9 (230) 1/4 (6) 7/8 (22.2) 3/16 (5) 1 5/8 (42) 2 5/8 (68) 4 (100) 15 imensions are Inches. Approximate Millimeter Equivalent Shown in ( ). 145