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Revolutions per minute

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Template:Short description {{#invoke:other uses|otheruses}} Template:Redirect Template:More citations needed Template:Infobox unit

Revolutions per minute (abbreviated rpm, RPM, rev/min, r/min, or r⋅min−1) is a unit of rotational speed (or rotational frequency) for rotating machines. One revolution per minute is equivalent to Template:Sfrac hertz.

StandardsEdit

ISO 80000-3:2019 defines a physical quantity called rotation (or number of revolutions), dimensionless, whose instantaneous rate of change is called rotational frequency (or rate of rotation), with units of reciprocal seconds (s−1).<ref>ISO 80000-3:2019.</ref>

A related but distinct quantity for describing rotation is angular frequency (or angular speed, the magnitude of angular velocity), for which the SI unit is the radian per second (rad/s).

Although they have the same dimensions (reciprocal time) and base unit (s−1), the hertz (Hz) and radians per second (rad/s) are special names used to express two different but proportional ISQ quantities: frequency and angular frequency, respectively. The conversions between a frequency Template:Mvar and an angular frequency Template:Mvar are

<math>\omega = 2 \pi f, \quad f = \frac{\omega}{2 \pi}.</math>

Thus a disc rotating at 60 rpm is said to have an angular speed of 2π rad/s and a rotation frequency of 1 Hz.

The International System of Units (SI) does not recognize rpm as a unit. It defines units of angular frequency and angular velocity as rad s−1, and units of frequency as Hz, equal to s−1.

<math>\begin{array}{rcrcr}
1~\dfrac{\text{rad}}{\text{s}}                &=& \dfrac{1}{2\pi}~\text{Hz} &=& \dfrac{60}{2\pi}~\text{rpm} \\
2\pi~\dfrac{\text{rad}}{\text{s}}             &=& 1~\text{Hz}               &=& 60~\text{rpm} \\
\dfrac{2\pi}{60}~\dfrac{\text{rad}}{\text{s}} &=& \dfrac{1}{60}~\text{Hz}   &=& 1~\text{rpm}

\end{array}</math>

ExamplesEdit

  • For a wheel, a pump, or a crank shaft, the number of times that it completes one full cycle in one minute is given the unit revolution per minute. A revolution is one complete period of motion, whether this be circular, reciprocating or some other periodic motion.
  • On many kinds of disc recording media, the rotational speed of the medium under the read head is a standard given in rpm. Phonograph (gramophone) records, for example, typically rotate steadily at Template:Frac, Template:Frac, 45 rpm or 78 rpm (0.28, 0.55, 0.75, or 1.3, respectively, in Hz).
  • Air turbine rotating up to Template:Val (25 kHz)
  • Modern air turbine dental drills can rotate at over Template:Val (13.3 kHz).
  • The second hand of a conventional analog clock rotates at 1 rpm.
  • Audio CD players read their discs at a precise, constant rate (4.3218 Mbit/s of raw physical data for 1.4112 Mbit/s (176.4 KB/s) of usable audio data) and thus must vary the disc's rotational speed from 8 Hz (480 rpm) when reading at the innermost edge to 3.5 Hz (210 rpm) at the outer edge.<ref name="MPEG DVD Spec">{{#invoke:citation/CS1|citation

|CitationClass=web }}</ref>

|CitationClass=web }}</ref> The exhaust note of V8, V10, and V12 F1 cars has a much higher pitch than an I4 engine, because each of the cylinders of a four-stroke engine fires once for every two revolutions of the crankshaft. Thus an eight-cylinder engine turning 300 times per second will have an exhaust note of Template:Val.

  • A piston aircraft engine typically rotates at a rate between Template:Val and Template:Val (42 Hz – 166 Hz).
  • Computer hard drives typically rotate at Template:Val – Template:Val (125 Hz – 166 Hz), the most common speeds for the ATA or SATA-based drives in consumer models. High-performance drives (used in fileservers and enthusiast-gaming PCs) rotate at Template:Val – Template:Val (160 Hz – 250 Hz), usually with higher-level SATA, SCSI or Fibre Channel interfaces and smaller platters to allow these higher speeds, the reduction in storage capacity and ultimate outer-edge speed paying off in much quicker access time and average transfer speed thanks to the high spin rate. Until recently, lower-end and power-efficient laptop drives could be found with Template:Val or even Template:Val spindle speeds (70 Hz or 60 Hz), but these have fallen out of favour due to their lower performance, improvements in energy efficiency in faster models and the takeup of solid-state drives for use in slimline and ultraportable laptops. Similar to CD and DVD media, the amount of data that can be stored or read for each turn of the disc is greater at the outer edge than near the spindle; however, hard drives keep a constant rotational speed so the effective data rate is faster at the edge (conventionally, the "start" of the disc, opposite to a CD or DVD).
  • Floppy disc drives typically ran at a constant 300 rpm or occasionally 360 rpm (a relatively slow 5 Hz or 6 Hz) with a constant per-revolution data density, which was simple and inexpensive to implement, though inefficient. Some designs such as those used with older Apple computers (Lisa, early Macintosh, later II's) were more complex and used variable rotational speeds and per-track storage density (at a constant read/record rate) to store more data per disc; for example, between 394 rpm (with 12 sectors per track) and 590 rpm (8 sectors) with Mac's 800 kB double-density drive at a constant 39.4 kB/s (max) – versus 300 rpm, 720 kB and 23 kB/s (max) for double-density drives in other machines.<ref>{{#invoke:citation/CS1|citation

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See alsoEdit

NotesEdit

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ReferencesEdit

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