Question 4.9: The frame of a hydraulic press consisting of two identical s...

Given  P=20 kN \quad S_{y t}=380 N / mm ^{2}

(fs) = 2.5.

Step I Calculation of permissible tensile stress for the plates

\sigma_{t}=\frac{S_{y t}}{(f s)}=\frac{380}{2.5}=152 N / mm ^{2}          (i).

Step II Calculation of direct tensile and bending stresses in plates
Since the plates are identical, the force acting on each plate is (20/2) or 10 kN. The plates are subjected to direct tensile stress and bending stresses. The stresses are maximum at the inner fibre.
At the inner fibre,

\sigma_{t}=\frac{P}{A}+\frac{M_{b} y}{I} .

=\frac{10000}{(150 t)}+\frac{10000(200+75)(75)}{\left[\frac{1}{12} t(150)^{3}\right]} .

or      \sigma_{t}=\frac{800}{t}           (ii).

Step III Calculation of plate thickness
From (i) and (ii),

\frac{800}{t}=152 \quad \text { or } \quad t=5.26 mm .