Design of Piston and Piston Head

Piston rod is made of high tensile Stainless steel materials with chromium plating to avoid corrosion and wear. The selection of materials depends on the buckling of the piston rod.

Material selection;

Grade ASTM-A36
Yield strength σ_y = 250 Mpa
Shear strength τ = 145 Mpa
Modulus of elasticity = 200Gpa

• We have a reason to select this material

• High compressive strength to support the load
• Due to its appreciable hardness
• Give good surface finish
• Relatively low cost to other grade of steel

Objectives

To check the safety of the piston by comparison of critical load with applied load.

When      F < F_cr the design is safe

Design procedure

The piston rod may fail in two ways:

• Fail due to compressive stress (crushing)
• Failure due to instability (buckling)

The cross section of the road critical can be calculated on the following criteria:

• If the length of the rod to least cross-sectional dimension ratio is less than or equal to 11. Then the piston rod is considered as stressed; otherwise it is considered as column.  L/d ≤ 11 Short column
    L/d ≥ 11 Long column

Also, the piston is round shaped d can be substituted b;

d = k√12 Where k is slenderness ratio

• If the length of the slenderness ratio is less than 40 so it is short

But length of the piston is L=22cm

Assuming the diameter of piston is:

So, the piston is short and will fail due to compressive stress

• Next, we have to check the buckling of the piston
• Crushing of the piston

The maximum load F = 5KN

σ_all = σ_y/n Where n is factor of safety

Now considering the crushing strength of the piston

125 Mpa = F_cr/((πdp²)/4) Where dpispiston diameter F_cr =critical load

Now we can conclude from this,

Now this implies that 8 cm piston diameter can sustain up to 6.28KN load. This piston can sustain the 5KN load to lift. Now it is possible to take the 8cm piston diameter; now we can design the cross section of the piston to sustain the 5KN maximum load.

So, we can take the new diameter of the piston to all other parameters found during the geometry analysis with depend on the geometry of the piston rod.

Design analysis for buckling

The length of the piston rod for buckling design.

As we have seen before, the piston is considered as a short column. Now Johnson equation

C=2 for one end fixed and other pivoted since the piston is kept over the fluid in one end which is move free and the other end is mounted fixed

This is the critical load for buckling so the piston rod will never fail due to buckling for the given F = 5KN load