# A flat steel strip is used as a spring to maintain a force against part of a cabinet latch in a commercial printer as shown in figure (5). When the cabinet door is open, the spring is deflected by the latch pin by an amount y1 = 0.25 mm. The pin causes the deflection to increase to y2 = 0.40 mm when the door is closed. Specify a suitable material for the spring if it is made to the dimensions shown in the figure

Question-AnswerCategory: Machine DesignA flat steel strip is used as a spring to maintain a force against part of a cabinet latch in a commercial printer as shown in figure (5). When the cabinet door is open, the spring is deflected by the latch pin by an amount y1 = 0.25 mm. The pin causes the deflection to increase to y2 = 0.40 mm when the door is closed. Specify a suitable material for the spring if it is made to the dimensions shown in the figure A flat steel strip is used as a spring to maintain a force against part of a cabinet latch in a commercial printer as shown in figure (5). When the cabinet door is open, the spring is deflected by the latch pin by an amount y1 = 0.25 mm. The pin causes the deflection to increase to y2 = 0.40 mm when the door is closed. Specify a suitable material for the spring if it is made to the dimensions shown in the figure

This is a problem related to Beam-deflection for statically indeterminate beams.

Here we have a force ‘P’ acting variable distance from both sides of a spring which is a supported cantilever

Assuming that

Deflection is proportional and the bending moment are proportional to the force P

We have

L = 40 mm

b = 5 mm

d = 0.6 mm

y1= 0.25 mm

y2 = 0.40 mm

Factor of safety, n = 3

Modulus of Elasticity of spring, E = 209 GPa = 209 x 103 N/mm2

General representation of these conditions is shown in Figure.1 Deflection at point B due to force P is given by We have

■ Moment of Inertia, I is  ■ Section modulus, Z is  Solving for P Substituting values  For yb = y1 = 0.25 mm, the force P acting is  For yb = y2 = 0.40 mm, the force P acting is  ■ Calculating moments at A and B at this condition

Moment at A is   Moment at B is   Therefore at P1 = 11.80 N, the moment at A is given by  ■ Stress at this point σA is   At P2 = 18.89 N, the moment at A is given by  ■ Stress at this point σA is   ■ The mean average stress σm is given by   ■ The alternating stress σa is given by   The Stress ratio R is given by   From the σm and σa values the suitable material for the spring is SAE 4140 OQT 400

Tensile strength = 2000 MPa

Yield strength = 1730 MPa

It has the highest ultimate strength with > 10% elongation for good ductility.

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