Sturmey-Archer SW Drei-Gang-Naben: Unterschied zwischen den Versionen

Die Planetenräder laufen auf dicken Schäften, die ein integraler Bestandteil des [[Planetenkäfig]]s sind. das ist anders als bei der AW Nabe, bei der die Schäfte abgestuft sind. Die Schäfte in der SW werden nicht von beiden Seiten des Getriebes getragen (auskragende Anordnung) und benötigen mehr Durchmesser, um Bewegungen in höheren Gängen zu widerstehen. Dicke Schäfte erhöhen den Widerstand im Betrieb. das geht möglicherweise so weit, dass es den Vorteil, dass es nur drei Planetenräder gibt, ausgleicht.

Despite the theoretical advantages of the 3-planet design, the SW hubs were rather disappointing in practice, particularly in the operation of their pawls and gear rings. The most common symptom was skipping under load in normal or high gear. Interestingly enough, low gear (the most highly stressed condition for these pawls) seldom produces this problem. The reason for skipping in normal and high is twofold — in one case it's due to worn clutch dogs, to be described later, and in the other case, it's due to problems with the pawls. Under even non-abusive service, the pawls can develop chips and indentations indentations and can wear to the point of not being picked up by the ratchets. The ultimate cause of this premature wear would surely have been evident in the development testing of this hub design, thus preventing the eventual embarrassments in actual service. So what went wrong?

Tony Hadland reports that:

He concluded that the SW would have worked fine if Sturmey-Archer had a) slightly increased the depth of the well for the pawls and b) fitted each pawl with a spring and modified the profile of the ratchet teeth in the ball cups."4

A stress analysis of a single pawl operating under a 200-pound pedal force is shown in the stress on single pawl animation accompanying animation. The stress levels are extremely high and demonstrate why the pawls typically chip and/or indent. If all three pawls would engage at once, the stress would be reduced by a factor of 3 and the pawls would then probably survive quite well. Under the "overload" conditions, the von Mises stress at the "indent" portion of the pawls is approximately 300ksi, and 800ksi at the tip, if anyone is interested.

One area of wild speculation as to the contributing factors of hub demise is that of lubrication and wear. I have found pawls that were not chipped and not dented, but were rather simply worn down on the tips of the crescent, rendering them inoperative (see picture at right). worn pawls It is unclear whether such a condition is caused by partial engagement in the ratchets or merely from the millions of oscillations imposed by coasting. The latter appears a bit more likely since the wear seems to occur equally on either end. It may be due to the presence of grit in the lubricating oil, or the forces required to rock the pawls back and forth, or a lack of lubrication, or a combination of these. Lack of lubrication could be due to bicycles' having kickstands on the left side and therefore leaning to the left — any oil in the hub will tend to migrate to the left side, thus starving the right side pawls. Lack of lubrication may also be due to overly heavy lubricant — grease in the pawl sockets is recommended for ease of assembly, and perhaps this grease contributed to excessive pawl wear due to its high viscosity. Who knows? I hope I am not provoking a "holy war" on the selection of oil, but my recommendation is for a fairly-light oil such as ATF or the like, preferably synthetic. Cold-weather operation may prove troublesome with too heavy an oil.

Other high-wear areas include the planet cage-to-axle pivot, the gear spacer ring, and the gear ring-to-ball ring interface on the right side. I have seen a lot of sludge build-up around the planet cage pivot, and this may be the reason for a design change from the original conical fixing plate to a nut-and-spacer arrangement.

These hubs often run rough in low and high gear (normal gear does not load the planetary gears, and is always smooth). This roughness, a result of misalignment of the gear ring and planet cage, can be felt easily by the rider as and is due mostly to loose cone adjustment, sloppy thread clearance from the axle to cones, bent axles, or possibly single-pawl engagement. Also, with this and some other Sturmey Archer 3-planet designs, the pinions have a modified tooth form to allow the "wrong" number of teeth to be used on the pinions (the SW's pinions have 15 teeth, but would normally be designed with 16 to match the 52-tooth ring and 20-tooth sun). This design choice allowed the pinion teeth to be stronger, but also inevitably meant that the smoothness suffered. Furthermore, the gear teeth are very crude, possibly also contributing to the roughness. A test hub with the gear-ring teeth ground down about .005" and their edges radius-ground has improved, but not eliminated the roughness. However, another test in which tight-fitting cones were chosen from the parts bin virtually eliminated the perceived roughness. Chainstay-dropout alignment has also been shown to affect the smoothness; make sure your frame is properly aligned.

My original 1958 hub's internals had worn, sloping gear-ring and pawl-ring dogs which permitted the pawl ring to slide or "cam" out of engagement (against the spring force) in normal or high gear. If normal gear was selected, this motion slammed the hub into low gear, surely taking a great toll on the gear-ring internal splines (mine were really chewed up) and may be the main cause for low gear pawl chipping. This "camming out" effect may be what Tony Hadland is identifying as the primary cause of trouble with this hub4. Camming out is more prevalent in high gear than normal because a rider typically transmits more torque in a slower/higher gear. As mentioned above in the section on pawls, the slipping of these dogs can be related to problems with incomplete pawl engagement, although even with perfect pawl operation, clumsy shifting can result in partial engagement of these dogs, again leading to slippage.