Revolutions per minute

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Revolutions per minute (abbreviated rpm, RPM, r/min, or r·min⁻¹) is a unit of frequency: the number of full rotations completed in one minute around a fixed axis. It is most commonly used as a measure of rotational speed or angular velocity of some mechanical component.

Standards organizations generally recommend the symbol r/min, which is more consistent with the general use of unit symbols. This is not enforced as an international standard. In French for example, tr/mn (tours par minute) is commonly used, and the German unit reads U/min (Umdrehungen pro Minute) or 1/min (= 1 min^-1).

The corresponding International System of Units (SI) unit is hertz. To convert revolutions per minute to revolutions per second (hertz), divide by 60. To convert from hertz to RPM, multiply by 60.

1/min = 1 rpm= 1/(60s) = 16.667 milliseconds = 1/60 Hertz.

Another common SI unit for angular velocity is radians per second (rad·s⁻¹):

1 r/min = 2π rad·min⁻¹ = 2π/60 rad·s⁻¹ ≈ 0.10471976 rad·s⁻¹

[edit] Examples

• On some kinds of disc or tape-like recording media, the rotational speed of the medium under the read head is a standard given in r/min. Gramophone (phonograph) records, for example, typically rotate steadily at 16+²⁄₃, 33+¹⁄₃, 45 or 78 r/min (⁵⁄₁₈, ⁵⁄₉, ³⁄₄, or 1.3 Hz respectively).
• Modern ultrasonic dental drills can rotate at up to 800,000 r/min (10 kHz).
• The "second" hand of a conventional analogue clock rotates at 1 r/min.
• Audio CD players read their discs at a constant 150 kB/s and thus must vary the disc's rotational speed from around 500 r/min (actually 8 Hz), when reading at the innermost edge, to 200 r/min (actually 3.5 Hz) at the outer edge.^[1] CD-ROM drives’ maximum rotational speeds are rated in multiples of this figure, even though they do not hold to constant read speeds when reading from most disc formats.
• DVD players also usually read discs at a constant linear rate. The disc's rotational speed varies from 1530 r/min (actually 25.5 Hz), when reading at the innermost edge, and 630 r/min (actually 10.5 Hz) at the outer edge.^[1] DVD drives’ speeds are usually given in multiples of this figure.
• A washing machine's drum may rotate at 500 to 2000 r/min (8–33 Hz) during the spin cycles.
• A power generation turbine rotates at 3000 r/min (50 hz) or 3600 r/min (60 Hz), depending on country - see AC power plugs and sockets.
• Automobile engines are usually operated at 2500 r/min (41 Hz), with the minimum speed usually around 1000 r/min (16 Hz), and the redline at 6000-10,000 r/min (100-166 Hz).
• A piston aircraft engine typically rotates at a rate between 2000 and 3000 r/min (30–50 Hz).
• Computers’ hard drives typically rotate at 5400 or 7200 r/min (90 or 120 Hz)—most commonly with ATA or SATA interfaces—and some high-performance drives rotate at 10,000 or 15,000 r/min (160 or 250 Hz)—usually with SATA, SCSI or Fibre Channel interfaces.
• The engine of a Formula One racing car can reach 19,000 r/min (320 Hz) under some circumstances.^[2]
• A Zippe-type centrifuge for enriching uranium spins at 90,000 r/min (1,500 Hz) or faster.^[3]
• Gas turbine engines rotate at tens of thousands of r/min. JetCat model aircraft turbines are capable of over 100,000 r/min (1,700 Hz) with the fastest reaching 165,000 r/min (2,750 Hz).^[4]
• An electromechanical battery (EMB) works at 60,000–200,000 r/min (1–3 kHz) range using a passively magnetic levitated flywheel in vacuum.^[5] The choice of the flywheel material is not the most dense, but the one that pulverises the most safely, at surface speeds about 7 times the speed of sound.
• A turbocharger can reach 290,000 r/min (4,800 Hz), while 80,000–200,000 r/min (1–3 kHz) is common.

[edit] See also

• Orders of magnitude (angular velocity)
• Constant linear velocity, or CLV, used when referring to the speed of audio CDs
• Constant angular velocity, or CAV, used when referring to the speed of gramophone (phonograph) records
• Turn (geometry)

[edit] References