Previously, I have tried to find “theoretically” the tooth counts of the Nexus-7 internal gear hub. Some nice numbers were obtained, but there were some suspects (you may think yourself what can be wrong with the numbers given in the link). Soon after, I disassembled the real hub and counted the teeth of all rings. Here are the real numbers. This is how the hub works:
- There are two planet carrierrs. Call “first” the one which is closer to the sprocket.
- Both rings have two suns, two gears on the planets, and only one ring gear. The ring gear meshes with the smaller gear of the planets.
- In lower gears 1 and 2, power input applies to the ring of the first planet carrier. This carrier lowers down the transmission ratio, depending on which sun is locked to the axle by a pawl. The second carrier is not involved.
- In higher gears 6 and 7, the first carrier is not involved. Power is applied to the planet ring of the second carrier. Depending on which sun is locked to the axle by a pawl, two step-up ratios are obtained.
- In middle gears 3, 4 and 5, both planet carriers are involved.
The table explains which exactly rings are engaged, and gives tooth numbers:
| Gear | Power transmission |
Sun | Planet meshed with sun |
Planet meshed with ring |
Ring | Exact ratio |
Documented ratio |
| 1 | Ring1->Planets1 | 42 | 14 | 72 | 12/19 | 0.632 | |
| 2 | Ring1->Planets1 | 36 | 20 | 20/27 | 0.741 | ||
| 3 | Gears 1 and 6 together | 0.843 | |||||
| 4 | Gears 2 and 6 together | 0.989 | |||||
| 5 | Gears 2 and 7 together | 1.145 | |||||
| 6 | Planets2->Ring2 | 30 | 19 | 14 | 66 | 279/209 | 1.335 |
| 7 | Planets2->Ring2 | 36 | 17/11 | 1.545 | |||
Note that in the step-up ring (Ring2), 30+19+14=63, while the ring has 66 teeth. Also, 36+14+14=64. I saw one tooth differences between sums of sun and planets and the ring, but at the moment I do not understand how so big difference (3 teeth) can make working combination.
Post a Comment