First Prototype

prototype-view-1

Overview

The first prototype has four speeds and an overall ratio range of 327%. The configuration of the hub follows the principles of the second embodiment presented in the patent. This means that the bevel planet gears are arranged around the circumference of the planet carrier in opposite pairs at different angles. There are three pairs of bevel gears for a total of six. Each pair gives a specific ratio, so, along with the 1:1 direct ratio, this gives the four gears. The rollers are mounted to the ring and output elements, and the sprocket teeth are on the planet elements. Gear change is accomplished by holding individual ring gears stationary, using a sliding clutch stopper mechanism. Both the axle and hub shell are cantilever-mounted. The axle is supported by the ring gear carrier and the hub shell supported on the axle. Thus the hub is configured for monoblade mounting on the ring gear carrier. However, it can also be dual-blade mounted by fitting an external bearing on the side of the shaft opposite the ring gear carrier.

Gear ratios

  • First:       1:1.00
  • Second:  1:1.89
  • Third:     1:2.57
  • Fourth:   1:3.27

For direct-drive with a 27 inch front wheel (700C – 622-32) this gives in gear inches and meters development [in square brackets]:

  • First:       27  [2.15]
  • Second:  51  [4
  • Third:     69  [5.54]
  • Fourth:  88  [7.05]

Current status

The first prototype is complete except for the freewheels, hub shell, crank mounts, and finalized gear shifter. Static testing indicates that the hub rotates freely in all gears. The first prototype was displayed at the Recumbent Cycle-Con show in Cincinnati in October, 2016.

Lessons learned

Two design issues have emerged from the first prototype so far:

- Outer ring gear bearings: The two outer ring gears (bevel gears with rollers) are mounted on ball bearings running directly in V-shaped grooves cut the aluminum hub and ring gear. This was done to avoid the high cost of commercially available large thin bearings. The aluminum ball race design is unusual, but it was judged acceptable because of the high number of ball bearings present, which theoretically should reduce the load per ball bearing, eliminating the chance of permanent indentation of the bearing race under high load. However, in practice, there is insufficient rigidity in the ring gear housing to distribute the load over many ball bearings. In other words, the support is highly localized and the hard material of a commercial ball bearing race would have been a better choice. Since there is no room to retrofit commercial bearings, another solution is needed. To address this issue, it has been decided to add a second stopping pawl to the gear shift mechanism, located 180 degrees opposed to the existing pawl. This reduces the reaction force on the ring gear bearings considerably, which should help to extend the life of these bearings.

- Sprocket tooth profile anomaly: Regular gears in a planetary gear train require a different tooth profile for the ring gear than for the sun gear. This allows both sun gear and ring gear to mesh correctly with the same planet gear. However, when both the sun gear and ring gear have roller teeth, the tooth “profile” is fixed – it is the roller in both cases. So no change of tooth profile is possible. The profile therefore cannot be customized for correct meshing with the same planet gear. Of the four possible iterations of hub outlined in the patent, only one has this problem. If the rollers are mounted on the planet gears, there is no problem, and if the planet gears are perpendicular to the sun and roller gears, there is no problem either. It is only when the planet gears do not have the rollers and are not perpendicularly that the problem arises - and this was the design chosen for the first prototype! This was only discovered after construction of the first prototype was well underway. However, the variation in tooth profile required to meet the theory is very small – reaching a maximum of about three thousands of an inch on each side of the tooth tip. So, to address this problem, the tooth profile of the planet gears were built to the smallest required dimension – that required for correct meshing of the smallest sun gear. This eliminates jamming at the tooth’s initial mesh, but initial meshing at the large ring gears is slightly delayed from the theoretical best.

Neither of these two design issues should prevent the final completion and testing of the first prototype, but they have motivated an early start on the preliminary design of a second prototype (see second prototype page). This design addresses both these concerns, and is more compact, with a slightly larger ratio range.