BUILDING & MOUNTING THE STRUCTURES 3-11 E UROCARGO Euro 4/5 Size of the subframe In the case of a flexible connection between the chassis frame and subframe, the bending moment M f must be divided between the chassis frame and subframe in proportion to the moments of inertia of the sections:
+ = M M M M c t f I M c t = I c t I M M c f c σ ≤ = σ + x c c c t M = I I W amm c I M M t f t = σ ≤ = σ + x t t c t M I I W amm t M f = static bending moment produced by the body (Nmm) M c = portion of the static bending moment M f applied to the subframe (Nmm) M t = portion of the static bending moment M f applied to the chassis frame (Nmm) I c = moment of inertia of the section of the subframe (mm 4 ) I t = moment of inertia of the section of the chassis frame (mm 4 ) σ c = maximum static stress applied to the subframe (N/mm 2 ) σ t = maximum static stress applied to the chassis frame (N/mm 2 ) W c = modulus of resistance of the subframe section (mm 3 ) W t = modulus of resistance of the chassis frame section (mm 3 ) σ amm = maximum static stress permitted on the chassis frame (N/mm 2 ) Computational example of the stresses in case of flexible connection with the chassis frame We consider two C-shaped sections with the following dimensions 3.
chassis: 250 x 70 x 5mm 4. subframe: 140 x 70 x 7mm and stressed at a given section by the maximum bending moment M f equal to 15000 Nm and applied perpendicularly to the plane containing the rib of the structural member. From the calculation the following values are obtained: I x (cm 4 ) W x (cm 3 ) 1. chassis 1545 123 2. subframe 522 74 Applying the formulae we obtain: 1. M t = M f x [I t / (I c + I t )] = 8500 x [588 / (588 + 183)] = 11200 Nm 2. M c = M f x [I c / (I c + I t )] = 8500 x [183 / (588 + 183)] = 3790 Nm and then: 1. σ t = M t / W t = 91N/mm 2 2. σ c = M c / W c = 51N/mm 2 Connections between frame and counterframe Print 603.93.431 Base - January 2008