Here is the equation used to calculate the mechanical time constant for the SM and NeoMetric motors:
Tau(m) = (R*J)/(Ke*Kt)
R: winding resistance
J: rotor inertia
Ke : back EMF constant
Kt : torque constant
(all units metric for this equation)
The mechanical time constant represents the amount of time for an unloaded motor to rise to 63.2% of its final velocity after application of a command voltage.