Patent ID: 7650791

Claim:
A method for automated execution of bolted joints using a power wrench provided with real-time sensing means that continuously measure torque and turn angle values, a computer connected by communication means to said power wrench, said computer being provided with memory means wherein said method comprises sequential tightening steps which apply increasing stress forces to said fasteners without exceeding the elastic limits of the fastener material, the data relating to torque and turn angle collected in a given step being used to compute the torque or turn angle applied by said power wrench in the following step: providing an algorithm, which comprises of a sequence of computing operations and a sequence of checking operations, said algorithm performing the following steps: initial tightening to a pre-established torque which is a fraction of the final torque value, storing continuously the data collected by sensing means provided in the wrench to generate a curve relating torque and turn angle; determination of the best fitting straight line passing through two predefined points in the straight portion of said curve; determination of the slope of said straight line which relates the values of turning moment ΔM A and the angular displacement θ; computation of the K factor using the formula (1) below: K = 360 · ( δ s + δ p ) p · d · Δ ⁢ ⁢ M A θ ( 1 ) computation of the torque to be applied in the final tightening step using the target value of the clamping force (preload) according to formula (2): M Ads=F M · d−′K (2) checking whether the relation M Amin <M Ads <M AMax holds true, where M Amin e M AMax are previously defined, rejecting the junctions that do not fit this criterion; applying torque M Ads to the fastener; plotting a new substitutive straight line and corresponding angular coefficients ΔM A ′e θ′; using formula (1) recalculation of valor de K′ using the new angular coefficients; checking whether the relation D min <(K/K′)<D max holds true, rejecting the junctions that do not fit this criterion; recalculation of the clamping force variation according to the expression F M ′=M Ads /(K′·d); checking whether the relation Dmin<(F M /F M )<D max holds true, rejecting the junctions that do not fit this criterion; computing the friction coefficient μ ges by means of the formula μ ges = K - ( 0 , 16 · p / d ) ( 0 . 58 · d 2 + 0 . 5 · d kM ) / d where d kM = C 4 · dw 3 - dh 3 dw 2 - dh 2 checking whether the relation μ gesMin ≦μ ges ≦μ gesMax holds true rejecting the junctions that do not fit this criterion; determining the angle θ t between the horizontal axis and a straight line passing through the points of the curve corresponding to M Ads and M A =0 calculating the value θ t1 =(M Ads /ΔM A )·θ′ checking the whether the relation D min <(θ t /θ t1 )<D max holds true, rejecting the junctions that do not fit this criterion; computing F M ″ = θ t · p 360 · ( δ ⁢ ⁢ s + δ ⁢ ⁢ p ) checking whether the relation D min <(F M″ /F M )<D max holds true, rejecting the junctions that do not fit this criterion.