How Much Torque Can a Bolt Accept?
High-strength bolts are high-strength bolts and belong to a standard component. In general, the load that high-strength bolts can bear is greater than that of ordinary bolts of the same specification.
- The shape and connection structure of high-strength bolts are basically the same as ordinary bolts. The main difference between the two is that ordinary bolt connections rely on shaft pressure and shear to transmit shear force. When the nut is tightened, the pretension force generated by the bolt is very small. Its impact is not considered; the working principle of high-strength bolt connection is to intentionally apply a large pre-tension to the bolt, which causes a squeezing force between the contact surfaces of the connected parts, so there is a large friction force perpendicular to the direction of the screw. Friction to transmit connection shear. The pre-tension of high-strength bolts is achieved by tightening the nuts. Ordinary high-strength bolts generally adopt the torque method and the angle method. Torsion-shear type high-strength bolts use twisted bolts to control the pretension.
- The bolts for high-strength bolt connection are made of 10.9S or 8.8S high-quality alloy structural steel and heat treated. High-strength bolt holes should be drilled. The bore diameter of high-strength bolts for friction type connections is 1.5 to 2.0 mm larger than the nominal diameter d of the bolts; the bore diameter of high-strength bolts for pressure-type connections is 1.0 to 1.5 mm larger than the nominal diameter d of bolts. [1]
- High-strength bolts are mainly used in steel structure engineering. A very important feature of high-strength bolts is that they are limited to a single use, and are generally used for permanent connections. Repeated use is strictly prohibited!
- Divided according to the state of stress: friction type and pressure type:
- In general, the load that high-strength bolts can bear is greater than that of ordinary bolts of the same specification.
- ordinary
- The wire used for high-strength bolts is 45-gauge steel, etc. When the screw is heat-treated, C1035 is used for hardening. This hardening method can generally achieve high strength of 22-32HRC.
For this type of high-strength bolt, level 8.8 corresponds to level 8 nuts and level 10.9 corresponds to level 10 nuts. Many people will ask why the use of nuts is less rigid than screws? This is for a reason. In order to protect the high-strength bolts and extend the service life during use, there must be some wear during the continuous disassembly. When the hardness of the 8-level nut is relatively low, it can be effective. In the cost of using this type of fastener, this combination is more reasonable, just like the wrench has the highest hardness when compared to the hardness of the wrench.
- It can be said that
- High-strength bolts exert pretension and transmit external forces by friction. The ordinary bolt connection relies on the shear strength of the bolt and the bearing wall pressure to transmit the shear force. When the nut is tightened, the pre-tension force is very small, and its effect can be ignored. In addition to the high strength of the material, the high-strength bolt also exerts a lot on the bolt The pre-tensioning force causes a squeezing force between the connecting members, so that there is a large frictional force perpendicular to the screw direction, and the pre-tensioning force, the anti-slip coefficient and the type of steel all directly affect the bearing capacity of the high-strength bolt.
- There are pressure-bearing and friction-bearing types according to the characteristics of the force. The two calculation methods are different. The minimum specification of high-strength bolts is M12, and M16 ~ M30 are commonly used. The performance of oversized bolts is unstable, and should be used with caution in design.
- High-strength bolts are high-strength bolts, such as bolts with a hard bottom. Generally refers to the level of 8.8 or higher. For example, 10.9 level bolts, 12.9 level bolts are very strong hardness performance. Strong torsional performance.
- Calculation of high-strength bolt length
- High-strength bolted connections must strictly control the length of the bolts. The length of the torsional shear-type high-strength bolt is the length from the lower bearing surface of the screw head to the end of the screw tail; for the high-strength hexagon head bolt, a thickness of a washer should be added. Is the bolt length).
- The general formula for high-strength bolt length is as follows:
- L = L '' + L
- Where L = M + NS + 3P
- Where L is the length of the high-strength bolt;
- L '' total thickness of the connecting layer;
- Ladditional length, that is, the length of the tightening length;
- MNominal thickness of high-strength nut;
- Nnumber of washers, 1 for torsional shear high-strength bolts, 2 for high-strength hexagon head bolts
- SNominal thickness of high strength washer
- PPitch of the thread.
- Lengthening value of high-strength bolts = Bolt length-Plate thickness. Generally according to the thickness of the connecting plate
- Long value, and take an integral multiple of 5mm.
- Before construction of high-strength bolts, the torque coefficients of high-strength bolted joints should be inspected according to the factory batch. Eight sets of each batch are re-examined. The average value of the torque coefficients of the eight sets should be within the range of 0.110 to 0.150. . The re-inspection method of its torque coefficient is carried out in accordance with GB50205 "Code for Acceptance of Construction Quality of Steel Structure Engineering". After the test, the installation of high-strength bolts should be performed within a short period of time.
- The construction torque of high-strength bolts is determined by the following formula:
- Tc = 1.05k · Pc · d
- Tcconstruction torque (N · m);
- kaverage torque coefficient of high strength bolted joints;
- Pcpre-tension for high strength bolt construction (kN), see Table 1;
- dhigh-strength bolt screw diameter (mm);
- Table 1 Pre-tension Pc (kN) of high-strength bolt construction
- The torque wrench used before the construction of high-strength bolts must be corrected before use, and its torque error must not be greater than ± 5%. The torque error of the calibration torque wrench must not be greater than ± 3%.
- The steel structure manufacturing and installation unit shall perform the anti-slip coefficient test and re-inspection of the friction surface of the high-strength bolt connection according to the provisions of Appendix B of the code GB50205-2001. The result should meet the design requirements.
Basic requirements for high-strength bolts
- The manufacturer and installation unit should conduct the anti-slip coefficient inspection in units of steel structure manufacturing batches. The manufacturing batch can be divided into batches of 2000t according to the division (sub-segment) project division, and less than 2000t can be regarded as a batch. When two or more surface treatment processes are selected, each treatment process shall be inspected separately. Three sets of test pieces per batch.
- The anti-slip coefficient test should use the two-bolt spliced tensile test piece with double friction surfaces (Figure B.0.5)
- Appendix B of Specification GB50205-2001 Figure B.0.5 [2]
- The test specimens for the anti-slip coefficient inspection shall be processed by the manufacturer. The test specimens and the steel structural members shall be of the same material, manufactured in the same batch, used the same friction surface treatment process and have the same surface state, and shall be applied in the same batch. High-strength bolted joints of the same performance level are stored under the same environmental conditions.
- The thickness t1 and t2 of the steel plate of the test piece should be determined according to the representative plate thickness in the steel structure engineering. At the same time, the net load surface of the steel plate of the test piece is always in the elastic state before the friction surface slips; the width b can refer to the table. The value specified in B.0.5. L1 shall be determined according to the requirements of the fixture of the testing machine.
- Table B.0.5 Width of test plate (mm)
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- The surface of the test piece should be flat, free of oil, and free of burrs and burrs on the edges of the holes and the plate.
Test method for high-strength bolts
- The error of the testing machine used for the test shall be within 1%.
The high-strength bolts, pressure sensors and resistance strain gauges with resistance plates attached to the test should be calibrated by a test machine before the test, and the error should be within 2%.
The assembly sequence of test pieces shall meet the following requirements:
First punch the nails into the holes of the test piece, then replace them one by one with high-strength bolts equipped with pressure sensors or resistor strips, or the same batch of torsional shear-type high-strength bolts that have been pre-tensioned and retested.
Tighten the high-strength bolts separately and tighten them. The initial tightening should reach about 50% of the standard value of bolt pretension. After final tightening, the bolt pretension should meet the following requirements:
1) For high-strength bolts equipped with a pressure sensor or an electric paste, the pre-tension value of each bolt of the control specimen is measured with a resistance strain gauge between 0.95P-1.05 P (P is the pre-tension value of the high-strength bolt design) ;
2) When the actual measurement is not performed, the pre-tension (tightening axial force) of the torsional shear-type high-strength bolt can be taken as the average value of the pre-tension of the same batch.
The specimen shall be drawn on its side with a straight line to observe the slippage.
Put the assembled test piece on the tensile tester, and the axis of the test piece should be strictly centered with the center of the tester fixture.
When loading, the anti-slip design load value of 10% should be added first, and the load should be smoothly applied after stopping for 1 minute. The loading speed is 3-5kN / s. Straighten until the sliding failure, and measure the sliding load.
When one of the following conditions occurs during the test, the corresponding load can be determined as the slip load of the component:
1) Needle back phenomenon occurs in the testing machine;
2) The side drawing of the test piece is misaligned;
3) The deformation curve on the XY recorder changes abruptly;
4) There is a sudden squeaking noise in the test piece.
The anti-slip coefficient should be calculated according to the measured slip load and the pre-tension of the bolt pre-tension P in the following formula, and two significant figures with a decimal point should be taken.
- In the formula: Nvslip load (KN) measured by test;
- nf-the number of friction surfaces is nf = 2;
- PiThe sum of the measured values of the pre-tension of the high-strength bolts on the sliding side of the test piece (or the average value of the pre-tension of the bolts in the same batch)
- (Take three significant figures) (KN);
- mthe number of bolts on one side of the test piece, take m = 2.