Detailed explanation of test tower cranes bolted preload and methods !

Stress analysis of the  Steel crane  bolted , drawn preload installed in the tower 's importance to derive preload application methods . This article focuses on the tower crane bolt connection. Keywords: tower crane bolt preload application .

( A ) by the bolt preload and the relationship between force

 If fully tighten the nut , then the nut in the axial direction of the bolt forward h, is bound by the bolt preload leaving the board under pressure , assuming that the bolts between the plates cut along with the P stands for preload , T represents cubicle the extrusion pressure , obtained by the equilibrium condition P = T, and the extrusion pressure is equal to the preload , when the elongation of the bolt shaft Δl1, deformation of the pressing plate , terms of E1 and A1 represents the elastic rod bolt modulus and cross-sectional area , E2 and A2 represents plate modulus and cross-sectional area , according to Hooke's law , as P = T.

N role when the external force , because the force is not directly applied to the bolts , but added to the board are connected , thus tends to be even parts separate , deformation decreases, reducing the pressing force between the plates , the by the extrusion pressure is reduced to this case by the equilibrium conditions between the plates when the increase to the original compression completely disappeared, the pressing force between the plates disappeared , with a balanced preload disappeared. But it does not mean that the bolt is not exposed to tension, on the contrary , when the deformation between the plates is completely disappeared , corresponding to an elongated bolt , the bolt force increment elongated , by the force equilibrium conditions at this time . From the above analysis , tensile bolt connection must preload , the connected parts being squeezed, when subjected to external forces , connections between the extrusion pressure decreases with force , preload also decreases. When the external force so that the two connections increased to just separated from the preload is reduced to zero , will be greater than the tension bolt suffered original static preload applied when connecting .

      ( Two ) the characteristics of high strength bolts

The shaft bolt 45 heat-treated steel or alloy steel, its performance rating of 8.8 , 10.9 or higher. Nuts and washers for the corresponding performance level made ​​of steel , the installation requires very tight bolts , made ​​great preload, the component contact surfaces pressed . Friction -type high strength bolts completely rely on the friction between the contact surface to transfer shear , and slippage as the carrying capacity limit state . Pressure type high strength bolts to connect the failure of bearing capacity limit state , and slippage limit the normal operating state . Preload applied generally bolt material yield limit of the net cross-sectional area multiplied 0.7 times , the choice should be less than the maximum working tension preload to the contact surface between the member is still residual preload, to ensure the normal work reliability.

      ( Three ) standard section of tower crane tower casing bolts

Standard tower sections for the main force component , working under axial force , bending moment and torque. Connection between standard section , I plant a tube connecting bolts , tower main limb composed by a single angle , the main casing head and extremities flat upper and lower ends connected to the friction between the plane can withstand the torque caused by the work shear , therefore , can be regarded as the connecting bolt friction type , but with the bolt connection , so the preload is one not to be neglected . We found that a considerable number of tower crane standard section in connection with the bolt preload specified values ​​are not reached , when working with the tower overhead gantry crane  , tower main limb repeated tension and compression alternating , nuts relaxation, the main limb the gap between the end faces increases, the boom and the tower at the junction of horizontal displacement increases, causing the tower widening secondary stress , so that security reduced, and some even lead to an accident down the tower , so the pre- clamping force implementation methods and numerical control is an important part of the tower crane installation .

      ( Four ) tower crane slewing ring bolts

According to the work force characteristics , slewing ring friction-type high strength bolt should belong , slewing ring and the upper and lower bearing seat connection work is generally done on the ground , tighten the bolts , the available torque angle method or the method of symmetrical uniform in the circumferential direction several times tighten to achieve the required preload.

      According to Hooke's law for more than bolted stress analysis , export preload on the tower crane on the application method , in the absence of a torque wrench in the case , the use of angle method can effectively achieve and guarantees required when installing preload value. Stress analysis for bolted connections , draw preload installed in the tower 's importance to derive preload application methods . This article focuses on the tower  jib crane  bolt connection. Keywords: tower crane bolt preload application .