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.

The corresponding International System of Units (SI) unit would be the hertz and we have:

3600 r/min = 60 revolutions per second = 60 Hz

In the SI one often uses the unit for angular velocity which is radians per second (rad·s⁻¹):

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

To convert revolutions per minute to revolutions per second (hertz), simply divide by 60.

[edit] Examples

Main article: Orders of magnitude (angular velocity)

• 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, 33⅓, 45 or 78 r/min (⁴⁄₁₅, ⁵⁄₉, ³⁄₄, or 1.3 Hz).
• Modern dental drills can rotate at up to 500,000 r/min (8 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 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.
• An automobile's engine typically varies between 700 and 7000 r/min (12–120 Hz) though some cars’ engines can spin as quickly as 11,000 r/min (180 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