Degrees of freedom and Gruebler's equation
For a planar mechanism, DOF = 3(n - 1) - 2j₁ - j₂, where n is the number of links (including the fixed frame), j₁ is the number of lower pairs (revolute, prismatic), and j₂ is the number of higher pairs. A four-bar linkage has n = 4, j₁ = 4, j₂ = 0, giving DOF = 3(3) - 2(4) = 1.
Check yourself: Have you counted the fixed link and identified all kinematic pairs correctly?
Velocity analysis using instant centers
The instant center of two links is the point at which they have the same velocity. For a four-bar linkage, Kennedy's theorem locates instant centers at the intersection of lines connecting known pairs. The velocity of any point on a link equals ω x (distance to the fixed pivot's instant center).
Check yourself: Are you measuring the distance from the correct instant center?
Gear trains and speed ratios
For a simple gear train, the speed ratio equals the product of driving teeth divided by driven teeth. An odd number of external meshes reverses direction. In an epicyclic (planetary) gear train, use the tabular method: fix the arm, apply relative rotations, then superpose arm rotation to match boundary conditions.
Check yourself: Have you correctly fixed the arm and applied tooth ratios in the relative-motion step?
Free vibration of a single-DOF system
A mass-spring system has natural frequency ωₙ = √(k/m) rad/s. The general undamped solution is x(t) = A sin(ωₙt + φ). With viscous damping c, the damping ratio ζ = c/(2√(km)). Underdamped (ζ < 1) response oscillates with exponentially decaying amplitude at damped frequency ωd = ωₙ√(1 - ζ²).
Check yourself: Is ζ less than 1, confirming underdamped oscillatory motion?
Forced vibration and resonance
Harmonic forcing F₀ sin(ωt) on an undamped system gives steady-state amplitude X = F₀/k / |1 - (ω/ωₙ)²|. Resonance occurs when ω = ωₙ, causing unbounded amplitude in the undamped model. With damping, peak amplitude is finite and occurs near ω = ωₙ. Transmissibility measures force transmitted to the foundation.
Check yourself: Are you using the forcing frequency ω or the natural frequency ωₙ in the amplitude formula?