RPM

For other uses, see RPM (disambiguation).

"RPM" redirects here. For other uses, see RPM (disambiguation).

Revolutions per minute (abbreviated rpm, RPM, r/min, or r·min⁻¹) is a measure of the frequency of a rotation. It annotates the number of full rotations completed in one minute around a fixed axis. It is used as a measure of rotational speed of a 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 equivalent reads U/min (Umdrehungen pro Minute).

International System of Units

According to the International System of Units (SI), rpm is not a unit. This is because Revolutions is a semantic annotation rather than a unit. The annotation is instead done in the subscript of the formula sign if needed. Because of the measured physical quantity, the formula sign has to be f for (rotational) frequency and ω or Ω for angular velocity. The corresponding basic SI unit is s⁻¹ or Hz. When measuring angular speed, rad·s⁻¹ can also be used as unit.

Even though angular velocity, angular frequency and hertz all have the dimensions of 1/s, angular velocity and angular frequency are not expressed in hertz, but rather in an appropriate angular unit such as radians per second. Thus a disc rotating at 60 revolutions per minute (rpm) is said to be rotating at either 2π rad/s or 1 Hz, where the former measures the angular velocity and latter reflects the number of complete revolutions per second. The conversion between a frequency f measured in hertz and an angular velocity ω measured in radians per second are:
ω=2*π*f and f=ω/(2*π)
rpm to Hz
2400*2*3.14=15072 rad/min
15072/60=251.2 rad/s
(1800*2*3.14)/60=188.4 rad/s
(3300*2*3.14)/60=345.4 rad/s

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 rpm. Gramophone (phonograph) records, for example, typically rotate steadily at 16 ²⁄₃, 33 ¹⁄₃, 45 or 78 rpm (⁵⁄₁₈, ⁵⁄₉, ³⁄₄, or 1.3 Hz respectively).
• Modern ultrasonic dental drills can rotate at up to 800,000 rpm (13.3 kHz).
• The "second" hand of a conventional analogue clock rotates at 1 rpm.
• Audio CD players read their discs at a constant 150 kB/s and thus must vary the disc's rotational speed from around 500 rpm (actually 8 Hz), when reading at the innermost edge, to 200 rpm (actually 3.5 Hz) at the outer edge. 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 rpm (actually 25.5 Hz), when reading at the innermost edge, and 630 rpm (actually 10.5 Hz) at the outer edge. DVD drives’ speeds are usually given in multiples of this figure.
• A washing machine's drum may rotate at 500 to 2000 rpm (8–33 Hz) during the spin cycles.
• A power generation turbine (with a 2 pole alternator) rotates at 3000 rpm (50 Hz) or 3600 rpm (60 Hz), depending on country - see AC power plugs and sockets.
• Automobile engines are usually operated at around 2500 rpm (41 Hz), with the minimum speed usually around 1000 rpm (16 Hz), and the redline at 6000-10,000 rpm (100–166 Hz).
• A piston aircraft engine typically rotates at a rate between 2000 and 3000 rpm (30–50 Hz).
• Computers’ hard drives typically rotate at 5400 or 7200 rpm (90 or 120 Hz)—most commonly with ATA or SATA interfaces—and some high-performance drives rotate at 10,000 or 15,000 rpm (160 or 250 Hz)—usually with SATA, SCSI or Fibre Channel interfaces.
• The engine of a Formula One racing car can reach 18,000 rpm (300 Hz) under some circumstances. The exhaust note of the car has a much higher pitch, 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 1200Hz.
• A Zippe-type centrifuge for enriching uranium spins at 90,000 rpm (1,500 Hz) or faster.
• Gas turbine engines rotate at tens of thousands of rpm. JetCat model aircraft turbines are capable of over 100,000 rpm (1,700 Hz) with the fastest reaching 165,000 rpm (2,750 Hz).
• An electromechanical battery (EMB) works at 60,000–200,000 rpm (1–3 kHz) range using a passively magnetic levitated flywheel in vacuum. 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 rpm (4,800 Hz), while 80,000–200,000 rpm (1–3 kHz) is common.

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)