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Sturmey-Archer SW Drei-Gang-Naben: Unterschied zwischen den Versionen
Sturmey-Archer SW Drei-Gang-Naben (Quelltext anzeigen)
Version vom 20. Mai 2021, 11:13 Uhr
, 20. Mai 2021→Sperrklinken: Bild hinzugefügt
(→Was ging schief?: Deutsche Überschriften) |
(→Sperrklinken: Bild hinzugefügt) |
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===Sperrklinken=== | ===Sperrklinken=== | ||
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? | 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?[[Datei:SW-Pawls-issues.png|right|Typische Probleme bei Sperrklingen der SW Nabe]] | ||
This question may never be fully answered, but I'll offer some speculation: The development testing most probably involved some initial hand-built prototypes that did not use production tooling, and as such, were probably machined rather precisely by the best machinists Sturmey-Archer had. Under these conditions, the SW design probably was proven handily, and authorization to go into full-scale production ensued. With production tooling, however, the SW design really fell apart due to the sloppy techniques common to manufacturing plants of the time. Remember, these hubs had to be relatively inexpensive to prove economically viable. Evidence of this abounds when one examines the SW parts. Very rough machining and sloppy fit of most moving parts is usual. (An engineering colleague of mine in our Gear Technology group described the gear machining as "horrible"). This is not meant as a slam against the quality of Sturmey-Archer workmanship: most of the parts are hardened, accentuating the machining marks more than normal, and again, these hubs had to be economical. Also, powder metallurgy was not available as a viable option in 1955. As the old saying goes, "You can have it good, fast, or cheap — pick any two." | This question may never be fully answered, but I'll offer some speculation: The development testing most probably involved some initial hand-built prototypes that did not use production tooling, and as such, were probably machined rather precisely by the best machinists Sturmey-Archer had. Under these conditions, the SW design probably was proven handily, and authorization to go into full-scale production ensued. With production tooling, however, the SW design really fell apart due to the sloppy techniques common to manufacturing plants of the time. Remember, these hubs had to be relatively inexpensive to prove economically viable. Evidence of this abounds when one examines the SW parts. Very rough machining and sloppy fit of most moving parts is usual. (An engineering colleague of mine in our Gear Technology group described the gear machining as "horrible"). This is not meant as a slam against the quality of Sturmey-Archer workmanship: most of the parts are hardened, accentuating the machining marks more than normal, and again, these hubs had to be economical. Also, powder metallurgy was not available as a viable option in 1955. As the old saying goes, "You can have it good, fast, or cheap — pick any two." | ||
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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. | 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. | ||
===Verschleiß=== | ===Verschleiß=== | ||