Strength of Bolt Material - Essay Example

Strength of Bolt Material The strength of the bolt which is ed to direct tension is unexpected. This tension is function of amount of initial tension which is induced during tightening down the nuts by hand with common tools. A series of experimentation was conducted by R.H. Harry Stanger on different sizes of bolts for the evaluation of the tensile stresses. Diameter of Bolt Stress (tons per sq in) Yield Point of Thread based on Area at Bottom of Thread Yield Point of Shank based on Area of Shank Failure of Bolt based on Area at Bottom of Thread 3/8” After first tightening down by hand with standard length spanner 14.73 19.97 34.08 ½” 15.36 16.85 27.68 5/8” 9.73 16.89 28.51 ¾” 10.00 15.90 28.20 7/8” 17.99 18.34 31.69 3/8” Without previous tightening down 27.46 20.97 34.28 ½” 20.09 16.82 27.87 5/8” 20.24 17.66 27.48 ¾” 19.23 17.03 28.25 7/8” 19.67 18.00 27.58 TURNED AND FITTED BOLTS 3/8” After first tightening down by hand with standard length spanner 18 NA 31.24 ½” 22.45 21.2 34.9 5/8” 20.86 19.18 32.46 ¾” 15.2 20.1 33.73 7/8” 19.1 20.7 34.63 3/8” Without previous tightening down 25.59 NA 32.9 ½” 25.01 NA 35.01 5/8” 20.86 18.67 30.2 ¾” 15.19 21,42 34.1 7/8” 19.13 21.03 33.99 It is important to note that when the nuts are simply screwed by hand, without excessive tightening through tools, “the stresses at the yield point of thread, based on area at bottom of thread, were fairly consistent”. It was further observed that, “after tightening the nuts down by hand with a standard length spanner, the average stress declined, and a reduction of practically 30 percent”. Similarly, “the stresses at the yield point of the thread, based on the area at bottom of thread, when nuts were screwed on without tightening down by spanner against the bolts tightened by spanner, had reduction of 20 percent”. It is important to understand that the failure of bolt is the characteristics of the minimum residual stresses of the bolt material. During an exercise, consistent stresses were derived for ordinary black, and turned and fitted bolts upon their respective failure. As per Peter (1950) experimentation, “the yield point of thread, however in the case of the black bolts was approximately 30 percent, which is lower than yield point obtained for turned and fitted bolts”. It is important to specify that in all the failures the damage initiated from the root of the thread, however there were instances when the threads stripped. It has been experimental proven by Peter (1950) that the “strength of a bolt in direct tension is greatly affected by the amount of initial tension induced when the nuts were made spanner tight; this is a factor which is uncontrollable from design perspective. The strength is dependent upon resistance of the threads against stripping”. It is practically concluded that, “most of the stress on the threads will be shear as the tendency in screwing up will be to force the threads off the bolt or nut in a direction parallel to the axis i.e. circumferential due to the friction between the adjacent threads”. Peter (1950) has noted that “when considering the allowable unit stress which may safely be adopted the probability that some of the stock bolts may be wrought iron, instead of mild steel” must not be ignored. The bolts which assist in the hanging of the runways are exposed to direct tension. It is therefore recommended that conservative approach shall be adopted, with specific reference to the determination of the safe loads for these bolts. Types of Bolts Black Bolts We shall start with experiment, considering ¾” diameter bolt having shank of ¾”, “it is practically impossible to roll black bars exactly to the nominal diameter it follows that there exists an allowance in the size of the shank of the finished bolt, which is usually allowance of 1/32” over the nominal diameter. It is known that the nominal diameter of the bolt is greater than tops of threads, “this amount varies in some vases it will be about 1/64”, but good bolt makers work to be very much slower figure. With reference to Black Bolts, “these bolts are never expected to enter hole o the same diameter, and it is usual to drill or punch the holes which is greater than 1/16” over the nominal diameter. Bolt Material Strength The table identifies the strength of the material against their sizes. The acting stresses on the bolt due to external forces shall never exceed the tabulated values of stresses to avoid damage. ASTM Grade Bolt Size Tensile Strength (ksi) Yield Strength (ksi) Proof Strength (ksi) A307 ¼” - 4” 60 N/A N/A A325 ¼” - 2 ½” 120 92 85 A354 ¼” - 2 ½” 125 130 105 A449 ¼” – 1” 120 92 120 A574 5/8” – 4” 180 N/A 140 Grade Bolt Size Tensile Strength (MPa) Yield Strength (MPa) Proof Strength (MPa) 4.6 M5-M36 400 240 210 4.8 M1.6-M16 450 360 330 5.8 M5-M24 500 440 390 8.8 M17-M36 520 480 410 9.8 M1.6-M16 600 580 490 10.9 M6-M36 670 620 550 12.9 M1.6-M36 1000 890 790 Turned Bolts These bolts are provided with specified diameter, within acceptable limits. As per research, “the diameters of the threaded portion should be exactly the same as the shank, it being possible to work very much closer than with ordinary black holes”. These bolts are composed of black rolled bars, “the head is upset whilst hot in a forging machine, and having an oversize shank of sufficient diameter for turning down to the required diameter”. Bright Bolts The bolts are manufactured under Newall Standard of Tolerance of 0.0005”. Peter (1950) suggested that these bolts are “machined all over, including the sides and tops of the heads and for push fit”. It is noted that these bolts “are turned down from hexagon bar and sometimes in the manner described for turned bolts, the method chiefly depending upon the quantity and length of the bolts”. The average axial stress acting on the fastener is function of tensile stress area. James (1993) derived that the average axial stress on the bolt is the function of axial force, root diameter, pitch diameter and tensile stress area. It has been observed through testing of the threaded rods that "unthreaded rod have diameter equal to the mean of the pitch diameter and the minor diameter will have the same tensile strength as the threaded rod". The contact between the mating threads "establish the shear area of an external thread". The shear area of the external thread is the fucntion of the threads per inch, thickness of external thread at critical shear plane, and maximum minor diameter of internal thread. As per the recommended practices of ANSI, "standard bolts and nuts of equal grades are designed to have the bolt fail before the threads in the nut are stripped". Case Study Considering a special gantry bracket case, where the bolts secure the brackets to stanchion, the bolts are required to carry the load of 7tons (acting along the plane), responsible for shear. This will also resist the rotating tendency which is caused by load. James (1993) assumed that this point that “these two actions will produce resultant stress on each bolt which must not exceed the allowable strength of the bolts most distant from the centre of rotation or centre of gravity of the group of bolts”. The shear stress of the bolt can be adjusted to safer margin through varying the depth of the bracket. In this specific case let us assume two different bolts material i.e. Carbon Steel and Stainless Steel. The sizes of the bolts are approximated at ¾” diameter. The bolts are arranged in such a manner that it experience tensile and shear stresses at specific locations, however it is assumed that there is neither compressive nor tensile stresses due to the rotation. The bolts are considered to be turned and fitted. The area at the bottom of the thread of ¾” diameter bolt of both the materials is approximately 0.304 square inch. The permissible tensile stress for carbon steel material is 7 tons per square inch, and for stainless steel it is 12 tons per square inch. From above data, the allowable tension on the bolts can be calculated by multiplication of area, permissible tensile stress with factor of two. The derived result is then divided by the applied force which 4 tons, this value is based upon assumption. It is important to calculate the moment at specific location where the bolts are mounted. It is known that the shear force shall be different for each bolt, and is function of the distance from the centre location of rotation. After calculating the moment, the arm of couple is determined; this is the product of moment and reciprocal of allowable tension in bolts. The shear acting on these bolts of different material can be calculated by relating the load and distance from the moment arm. This shall be different value for bolts at random locations, however shall be similar for the every bolts irrespective of their composition or material. However this shear shall be always less than the allowable stress value for every material as listed in ASME Section-II. It is however important to learn that this is not accurate approach for the estimation of shear forces on the bolts. As per existing practices, Peter (1950) assumed that the “centre of rotation is at the foot of the bracket, and that all the bolts are in tension”, the influence of shear factor is minimal. It is therefore important with reference to the installation of the structures that the bottom of it shall be rigid. It is common understanding that, “rotation as being about the centre of the bolt group and only the bolts above location is in tension, which is incorrect”, because “the whole of the contact area below this point would then be in compression and naturally would be of much greater value than the area in tension”. Peter (1950) observed that “the correct centre of rotation will lie somewhere between these two extremes, but the labour involved in trying to calculate a more accurate position in not justified and in the end would only be approximate”. Peter (1950) therefore recommended that, “centre of rotation shall be concentric with the centre of gravity of the bolt group”. With reference to the stresses acting on the bolts, it is recommended that the bolt shall be preloaded to a specific value which shall be safer than bolt which has been tightened to random value; this shall protect the bolt against unaccounted stresses and forces. As per John (1998) the recommended practice “preload of about 80 percent of the proof strength of the bolt material is normally used”, this signifies that the principle is applicable on all bolts irrespective of their material. In this specific case, the bolts of carbon steel and stainless steel were considered; therefore with specific reference to the stainless steel bolt material, the preload on this bolt shall be greater than that of carbon steel, for the bolts of equal sizes. Considering a specific case, where bolted arrangement has been used for the clamping of joint. An external force is applied on this arrangement for disengagement, the applied load shall cause elongation or extension of the bolts, and this shall be due to the generation of the shear stresses on the bolt. This external force shall be responsible for the “increase of the joint thickness reducing of the compressive load on the joint”, Peter (1950) explained that “application of the longer bolts of similar diameter, irrespective of material, shall be responsible for the uniform bolt loading under varying external forces”, however these external forces shall not induce stresses greater than the recommended stresses of the bolt material as prescribed in the standards. It is important to specify that the proportion of the load undertaken by the bolt and by the joint is the function of component stiffness values, therefore instead of the bolt material the material being clamped also influences the generation of stresses. Peter (1950) has verified that the torque acting on the bolt shall be “summation of thread torques and collar torques”. The significant amount of shear stresses are induced on bolt during tightening, “majority of the torque is required to overcome the thread and the collar friction forces, therefore any error in the value of the friction coefficient will have a large variation on the bolt tensile load”. As specified before, the shear stress on Bolt A (Carbon Steel) and Bolt B (Stainless Steel) is function of bolt tension and bolt torque. As per Alan (2001), this shear stress is based upon “thread torque, bolt tension, thread root area and root diameter”, thus shear stress is geometrical characteristics and is independent of the nature of material. Tolerance Limits As the standard practice, the recommended bolt load tolerance of +5% is safely suitable for the bolted joints. It is important that the bolt is protected against overstretching, unknown relaxation and flange surface galling. The tolerance limits for the bolts have been defined after bolt load accuracy test, explosion test, heat corrosion test, bolt relaxation test were successfully conduced. Conclusion From the above discussion we conclude that Bolt A and B shall sustain equal shear stresses for the given arrangement, however the magnitude of the shear stress acting the specific bolt material shall be less than the allowable shear stress for the material, as per the recommended of ASTM and ASME Section-II. References 1. Peter Russell, George Dowell. 1950. Competitive Design of Steel Structure. Chapman and Halll Publication. pp. 173, 250-287. 2. James Carvill. 1993. Mechanical Engineers Data Handbook. Butterworth-Heinemann. pp. 156-178. 3. Alan Williams. 2001. Structural Steel Design: ASD. Kaplan AEC Engineering. pp. 89-102. 4. James E. Ambrose. 1993. Building Structures. John Wiley and Sons. pp. 56-67. 5. John H. Bickford, Sayed Nassar. 1998. Handbook of Bolts and Bolted Joints. CRC Press. pp. 567-578. 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