Machine Design
What is Machine Design?
The ratio of the ultimate strength of a material to the actual stress it can withstand in service. It accounts for uncertainties and variations.
Key formula / rule: Factor of Safety
Key points
- Understand the principles of mechanical design.
- Analyze mechanical components under various loading conditions.
- Select appropriate materials based on design requirements.
- Apply theories of failure to predict component lifespan.
Common exam trap
Ignoring dynamic loads and vibrations.
Definitions
- Term
Factor of Safety (FOS)
- Meaning
The ratio of the ultimate strength of a material to the actual stress it can withstand in service. It accounts for uncertainties and variations.
- Term
Stress Concentration
- Meaning
The phenomenon where localized stresses significantly exceed the nominal stress in a component due to geometric discontinuities like holes, notches, or fillets.
- Term
Fatigue
- Meaning
The weakening of a material caused by repeatedly applied loads, which may ultimately lead to fracture even though the stresses are below the yield strength.
- Term
Creep
- Meaning
The tendency of a solid material to move slowly or deform permanently under the influence of persistent mechanical stresses. It is significant at high temperatures.
- Term
Wear
- Meaning
The progressive loss of material from a surface due to mechanical interaction with another surface.
- Term
Lubrication
- Meaning
The process, skill, or function of applying a lubricant (like oil or grease) to reduce friction, heat, and wear between moving surfaces.
- Term
Endurance Limit
- Meaning
The stress level below which a material can theoretically withstand an infinite number of fatigue cycles without failure.
Learning objectives
Understand the principles of mechanical design.
Analyze mechanical components under various loading conditions.
Select appropriate materials based on design requirements.
Apply theories of failure to predict component lifespan.
Design for manufacturability, assembly, and cost-effectiveness.
Utilize design tools and software effectively.
Formulae
- Name
Factor of Safety
- Note
Accounts for uncertainties in material properties, loads, and manufacturing.
- Expression
FOS = Ultimate Strength / Working Stress
- Name
Equivalent Twisting Moment (for shafts)
- Note
Where Tm is steady torque, Ta is alternating torque, Km and Ka are stress concentration factors for mean and alternating stresses.
- Expression
Teq = sqrt((Km * Tm)2 + (Ka * Ta)2)
- Name
Lewis Equation (Gear Bending Stress)
- Note
W=tangential force, v=velocity factor, p=circular pitch, b=face width, Y=formative factor. Assumes cycloidal teeth.
- Expression
σb = (W/v) * (p/b) * (Y/π)
- Name
Archard's Wear Equation
- Note
V=volume of material worn, K=wear coefficient, P=normal load, L=sliding distance, H=hardness of the softer material.
- Expression
V = K * P * L / H
- Name
Bearing Life (L10)
- Note
L10 is the basic rating life for 90% reliability. C is dynamic load rating, P is equivalent dynamic load, p is exponent (3 for ball bearings, 10/3 for roller bearings).
- Expression
L10 = (C / P)p
- Name
Petroff's Equation (Friction in Journal Bearing)
- Note
f=friction coefficient, μ=viscosity, N=speed, L=bearing length, D=journal diameter, C=radial clearance, r=journal radius, c=radial clearance.
- Expression
f = (μ * N * L * D) / C * (r/c)2
Prerequisites
Engineering Mechanics (Statics and Dynamics)
Strength of Materials (Mechanics of Materials)
Thermodynamics
Fluid Mechanics
Manufacturing Processes
Common mistakes
Ignoring dynamic loads and vibrations.
Underestimating stress concentrations.
Incorrect material selection for the operating environment.
Neglecting fatigue and creep effects in long-life applications.
Inadequate consideration of manufacturing tolerances and assembly.
Overlooking lubrication and wear issues.
Insufficient factor of safety.
Keywords
Machine Design
Mechanical Components
Stress Analysis
Fatigue Failure
Creep
Wear
Lubrication
Material Selection
Factor of Safety
Shaft Design
Gear Design
Bearing Design
CAD/FEA
Practice preview
Which of the following statements about stress concentration is TRUE?…
easy
A circular rod of diameter 20 mm is subjected to an axial tensile load of 31.4 kN. What is the tensile stress developed in the rod?…
easy
A solid circular shaft is subjected to a pure torsional moment. If the diameter of the shaft is doubled, how does the maximum shear stress change, assuming the torsional moment remains constant?…
medium