Subject: Fretting damage in Bicycle Mechanics
From: Jobst Brandt
Date: May 11, 2001
Fretting or to fret: to eat or gnaw away, to erode.
In machinery, fretting is the micro-motion of tightly fitting parts that superficially appear immobile with respect to each other. Classically, transmission shafts and gears or axles with a press fit show evidence of motion on disassembly by the presence of rouge, rouge being iron oxide particles that are generated in such interfaces by micro-motions far smaller than conventional measuring equipment can resolve.
On bicycles such an interface occurs between the square taper on the pedal crank and its spindle, where rouge is evident on the face of the steel spindle regardless of whether it was assembled with grease or not. That fretting occurs is also evident by the need for a retaining bolt to prevent crank disengagement from its spindle and of pedals from their crank. Removing a crank requires substantial force with an extractor, yet continual fretting will disengage the crank in the absence of a retaining bolt. Likewise pedals are not easily removed, but without a left hand thread on left pedals, they will unscrew.
In addition to disengaging the press fit of a crank, fretting moves cranks up the taper until the preload of the retaining and installation bolt matches the press times the slope of the taper. That is to say, fretting relaxes surface friction loads in the interface. Additionally, load distortion of a crank causes it to move away from the face of the retaining bolt, up the taper of the spindle.
Pedals have similar relative motions in the attachment thread and pressure face on the shoulder of the spindle. This is also a dynamic joint that appears to be static. In the case of the pedal, fretting motion is directional and can cause precession by the "wandering" load whose center of pressure rotates in the crank thread opposite to the rotation of the crank. Even without clearance, elastic deformation of the crank and pedal spindle cause micro motions that, if not countered by an appropriate thread direction, will unscrew the pedal. The presence of a left hand thread on the left pedal and on many bottom bracket right side bearing cups is proof that fretting occurs.
If these motions did not occur, then bolt locking devices, such as cotter pins, lock nuts and lock washers would not be necessary. Most nuts and bolts so secured do not come loose in service and therefore should not rotate. Presence of locking means gives evidence that fretting is more ubiquitous than most people (mechanics and engineers included) believe.
Fretting in bearings is a different but similar effect, that is the bane of steering gears and other mechanical devices that are intended to rotate but are primarily used in a fixed position (straight ahead). Automotive patents for anti-fretting steering gears abound. Saginaw, Gemmer, and Ross steering gears come to mind. In bicycles this effect is seen in the bearings of the fork, or head bearings, that are meant to rotate but often experience straight ahead, non rotating use.
Because fretting involves invisibly small motion, it remains difficult to understand and hard to convey to the user who suffers fretting symptoms on a piece of machinery. It was long believed that impact cause Brinelling of bicycle head bearings even though mechanics who installed cottered cranks should have noticed an inconsistency in that pounding in cotters with a large hammer with all the shock taken up by one 1/4" ball under the crank spindle never caused a dent, yet 20 balls loaded by a much smaller force through a rubber tire was believed to cause dimpled head bearings. Beyond that, the top bearing that carries practically no load and receives no impact, also became dimpled and, like the bottom load bearing one, did so in the fore and aft quadrant. These dimples were not shiny as Brinell indentations are, but are milky finish typical of tear-outs from asperity welding.
Ball bearings operate in two modes that became apparent in the computer disc business because their data actuators often move step by step from track to track, with a radial arm about 1" long, there being more than 20,000 tracks per inch. Servo control engineers must analyze bearing drag to be overcome for this purpose. In such small motions, ball bearings are essentially locked solid with their lubricant film, the bearing appearing as welded balls acting as springs. This "pre-roll" stage of motion is the one that causes the dimples in the bicycle head bearings because they, unlike the disc bearings, have been lubricant depleted from fretting, not having made a larger motion for a longer time, motion that would replenish lubrication between ball and race.
Ball bearings roll on a film of oil that is so thin that it does not present liquid properties, being several mono-molecular layers thick as it adheres to ball and race. If it weren't for this behavior, oil would not remain in the interface. However, with fretting, oil is displaced and pin point welding takes place. Bicycle head bearing fretting is caused by fore and aft rocking of the fork crown, a motion that lies below visible resolution, and is small enough to not replenish lubricant. Bearing damage appears as dimples from myriad asperity contacts that welded and broke loose as the ball fretted in place, leaving a milky finish.
Road bicycles are more subject to this damage than off road bicycles because they spend more time traveling straight ahead, especially when coasting downhill. Fretting damage occurs during these times, because lubrication is not replenished by steering motions. The compound bearings offered by Shimano seem to have greatly reduced the problem by taking up fork crown rocking motion in a plain steel on aluminum spherical cup that is not prone to metal to metal contact, while steering rotations are borne in a pre-loaded full complement angular contact ball bearing supported by this plain bearing.
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