Flywheel Kinetic Energy and Speed Fluctuation
The rotational kinetic energy stored in a spinning flywheel and how much its speed swings under a pulsing load -- the number a millwright or machine builder uses to size a flywheel for a punch press, shear, or engine. KE = 1/2 I omega^2; in US units I = (weight/32.174) x k^2 (k = radius of gyration: disk r/sqrt(2), rim ~ r -- rim flywheels store far more per pound, so real ones put mass outside), and omega = rpm x pi/30. A 100 lb flywheel with a 1 ft radius of gyration at 1,000 rpm stores ~17,000 ft-lb. When the machine draws an energy pulse per stroke, the flywheel slows: the coefficient of fluctuation (fractional speed swing) = pulse / (I omega^2) = pulse / (2 KE), so a 2,000 ft-lb draw swings this flywheel ~5.9%. Machines target a Cs from ~0.002 (AC generators) to ~0.2 (punches/shears) and size the flywheel until the swing is small enough. A sizing aid; the actual inertia from the geometry, the load's energy profile, and the drive/prime mover govern.
Formula and source
I = (weight_lb / 32.174) x radius_of_gyration_ft^2; omega = rpm x pi/30; kinetic_energy_ftlb = 0.5 x I x omega^2; speed_fluctuation_pct = 100 x energy_fluctuation_ftlb / (I x omega^2) = 100 x pulse / (2 x KE).
Flywheel stored rotational kinetic energy and coefficient of fluctuation (machine-design first principles; Machinery's Handbook flywheel practice), by name; the actual moment of inertia from the flywheel geometry, the load's real energy profile, and the drive and prime mover govern the design.
Audience
This tile is built for mechanics and the adjacent professions in the Mechanic - Auto, Marine, Aviation group. The interactive calculator runs entirely in your browser. No account, no fee, no advertising, no tracking.
Related tools
Posture
Rough Logic answers the math question the working professional asks on the job. The site is a calm, fast, ad-free, account-free, ever-free reference. It does not interpret code. It does not replace the licensed professional. It does not store your inputs. The Authority Having Jurisdiction governs all installations and inspections.