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===Bicycle pedals===
===[[Pedal]]e===
[[Bicycle pedal]]s are left-threaded on the left-hand [[Crankset#Cranks|crank]] so that precession tightens the pedal rather than loosening it. This may seem counter-intuitive, but the torque exerted due to the precession is several orders of magnitude greater than that caused by a jammed pedal bearing.
[[Bicycle pedal]]s are left-threaded on the left-hand [[Crankset#Cranks|crank]] so that precession tightens the pedal rather than loosening it. This may seem counter-intuitive, but the torque exerted due to the precession is several orders of magnitude greater than that caused by a jammed pedal bearing.


Shimano SPD axle units, which can be unscrewed from the pedal body for servicing, have a left-hand thread where the axle unit screws into the right-hand pedal; the opposite case to the pedal-crank interface. Otherwise precession of the pedal body around the axle would tend to unscrew one from the other.
Shimano SPD axle units, which can be unscrewed from the pedal body for servicing, have a left-hand thread where the axle unit screws into the right-hand pedal; the opposite case to the pedal-crank interface. Otherwise precession of the pedal body around the axle would tend to unscrew one from the other.


===Bicycle bottom brackets===
===[[Innenlager]]===
English threaded bicycle [[bottom bracket]]s are left-threaded on the right-hand (usually drive) side into the [[Bicycle frame#Bottom bracket shell|bottom bracket shell]]. This is the opposite of pedals into cranks because the sense of the relative motion between the parts is opposite.  (Italian and French threaded bottom brackets have right-hand threading on both sides.)
English threaded bicycle [[bottom bracket]]s are left-threaded on the right-hand (usually drive) side into the [[Bicycle frame#Bottom bracket shell|bottom bracket shell]]. This is the opposite of pedals into cranks because the sense of the relative motion between the parts is opposite.  (Italian and French threaded bottom brackets have right-hand threading on both sides.)


===Bicycle sprockets===
===[[Ritzel]]===
Splined sprockets precess against any lockring which is screwed into the freehub.  [[Shimano]] uses a lockring with detents to hold cassette sprockets in place, and this resists precession.  [[Sturmey-Archer]] once used 12-splined sprockets for 2- and 3-speed racing hubs, and these were secured with a left-threaded lockring for the same reason.  ([[Fixed gear bicycles]] also use a left-threaded lockring but this is not because of precession; it is merely to ensure that the lockring tends to tighten, should the sprocket begin to unscrew.)
Splined sprockets precess against any lockring which is screwed into the freehub.  [[Shimano]] uses a lockring with detents to hold cassette sprockets in place, and this resists precession.  [[Sturmey-Archer]] once used 12-splined sprockets for 2- and 3-speed racing hubs, and these were secured with a left-threaded lockring for the same reason.  ([[Fixed gear bicycles]] also use a left-threaded lockring but this is not because of precession; it is merely to ensure that the lockring tends to tighten, should the sprocket begin to unscrew.)


===Bearings in manual transmissions===
===Lager in [[Nabenschaltung]]en===
A bearing supported gear in a [[Manual transmission#Maintenance|manual transmission]] rotates synchronously with its shaft due to the dog-gear engagement. In this case, the small diametrical clearance in the bearing will induce precession of the roller group relative to the gear mitigating any fretting that occurs if the same bearing rollers always push against the same spot on the gear. Typically the 4th and 5th gears will have precession inducing features, while 1st through 3rd gears might not since cars spend less time in those gears. Transmission failure due to lack of precession is possible in gear boxes when low gears are engaged for long periods of time.{{cn|date=January 2016}}
A bearing supported gear in a [[Manual transmission#Maintenance|manual transmission]] rotates synchronously with its shaft due to the dog-gear engagement. In this case, the small diametrical clearance in the bearing will induce precession of the roller group relative to the gear mitigating any fretting that occurs if the same bearing rollers always push against the same spot on the gear. Typically the 4th and 5th gears will have precession inducing features, while 1st through 3rd gears might not since cars spend less time in those gears. Transmission failure due to lack of precession is possible in gear boxes when low gears are engaged for long periods of time.


== Siehe auch ==
== Siehe auch ==

Version vom 17. Juni 2020, 13:20 Uhr

Mechanical precession is the process of a round part (in blue) in a round hole (in red) rolling in the direction opposite to the rotational direction of the applied radial force. (The applied radial force is depicted by the green arrow. The arrow's counterclockwise rotation depicts the direction of precession, while the direction of rotation is shown by the clockwise rotation of the blue square. The center of the blue square is traversing counterclockwise along a small circle, the orbit, of diameter equal to the difference of the diameters of the red circle and the blue circle, even though the blue square rotates clockwise). If the blue circle has a diameter and the red circle a diameter d + δ.  The instant the green arrow is pointing downwards, the blue circle is pressed against the red circle at the bottom (point A on the blue circle). The force rotating counterclockwise causes the blue circle to roll around the red circle clockwise. When it has rolled a distance πd, the circumference of the blue circle, point A again touches the red circle. Since the circumference of the red circle is π(d + δ), point A touches the red circle a distance πδ  clockwise from the bottom.

Die mechanische Präzession ist ein Begriff aus der Kreieltheorie. Hier soll es ausschlißelich um die Brgriffsklärung für unser Thema Fahrrad gehen. Die meachnaische Präzession bezieht sich auf ein rundes Bauteil, dass sich inmerhalb eines Runden Lochs dreht, weil die Fertigungstoleranzen zwischen den Bauteilen und ausgeübte Kräfte zu einer Richtungsänderung führen. Die Drehrichtung des innenliegenden Bauteils läuft gegensätzlich zur Drehrichtung der Rotationskraft. Fraßkorrosion zwischen dem Bauteil und dem Loch ist oft das Eregbnis dieser Bewegung. Diese minimale Bewegung zwischen zwei auf den ersten Blick fest verbundenen Bauteilen zueinander erzeugt dieses Schadensbild.

Bei einem Fahrradpedal wird eine Rotationskraft durch das abwärts pedalieren auf die Welle, die mit der Kurbel rotiert, ausgeübt. Die vorherrschende Abwärtskraft rotert effektiv um die Pedalwelle. es ist nicht direkt offensichtlich, aber sogar fest verbunden engpassende Bauteile hebn immer noch minimales Spiel wegen ihrer Elastizität. Metalle sind keine starren Materialen, wie man beispilsweise bei Stahlfedern sehen kann. Unter Last werden in solchen Verbindungen Mikrodeformationen erzeugt, die zu minimalen Bewegungen führen. Das sieht man auch an den Verschleißerscheinungen, wo die Pedalachesn in den Kurbelaufnahmen sitzen.

Diese mechanische Präzession ist ein rein auf Kontaktkräfte basierender Prozess und hängt nicht von Masseträgheit ab oder ist invers proportional zur Bewegungsgeschwindigkeit. Sie ist vollständig unabhängig von drehmomentfreier und drehmomentinitierter Präzession.

Beispiele aus dem Fahrradbereich

Präzession, die durch Fraßkorrosion unter hohen Drehmomenten ausgelöst wird, kann Schrauben dazu bringen, sich selbst zu lösen.


Pedale

Bicycle pedals are left-threaded on the left-hand crank so that precession tightens the pedal rather than loosening it. This may seem counter-intuitive, but the torque exerted due to the precession is several orders of magnitude greater than that caused by a jammed pedal bearing.

Shimano SPD axle units, which can be unscrewed from the pedal body for servicing, have a left-hand thread where the axle unit screws into the right-hand pedal; the opposite case to the pedal-crank interface. Otherwise precession of the pedal body around the axle would tend to unscrew one from the other.

Innenlager

English threaded bicycle bottom brackets are left-threaded on the right-hand (usually drive) side into the bottom bracket shell. This is the opposite of pedals into cranks because the sense of the relative motion between the parts is opposite. (Italian and French threaded bottom brackets have right-hand threading on both sides.)

Ritzel

Splined sprockets precess against any lockring which is screwed into the freehub. Shimano uses a lockring with detents to hold cassette sprockets in place, and this resists precession. Sturmey-Archer once used 12-splined sprockets for 2- and 3-speed racing hubs, and these were secured with a left-threaded lockring for the same reason. (Fixed gear bicycles also use a left-threaded lockring but this is not because of precession; it is merely to ensure that the lockring tends to tighten, should the sprocket begin to unscrew.)

Lager in Nabenschaltungen

A bearing supported gear in a manual transmission rotates synchronously with its shaft due to the dog-gear engagement. In this case, the small diametrical clearance in the bearing will induce precession of the roller group relative to the gear mitigating any fretting that occurs if the same bearing rollers always push against the same spot on the gear. Typically the 4th and 5th gears will have precession inducing features, while 1st through 3rd gears might not since cars spend less time in those gears. Transmission failure due to lack of precession is possible in gear boxes when low gears are engaged for long periods of time.

Siehe auch

Quelle

Dieser Artikel basiert auf dem Artikel Prescession (mechanical) aus der englischen Version der freien Enzyklopädie Wikipedia und steht unter der GNU-Lizenz für freie Dokumentation. In der Wikipedia ist eine Liste der Autoren verfügbar. Deutsche Übersetzung des Artikels durch bikegeissel.