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Bike rim with welded metal piece |
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Bike rim with welded metal piece |
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After our initial trial, we brainstormed further ways to mount a bicycle wheel to the grinding stone. We decided to try to mount directly to the key itself by welding a small piece of square metal to the top. We then used a square pipe that fit exactly on the piece welded to the key. This pipe extended out of the grinding stone. To the bike wheel, we welded a solid square shaft which fit into the square pipe coming out of the stone. This approach requires the ghattera to do a one-time modification to his ghatta key. This did not strike us as a strenuous requirement, as the key wears down through use (it scrapes against the bottom stone during grinding), and is thus regularly replaced with a new one from a local blacksmith.
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Key with welded attachment |
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The square pipe connecting the key to the bike wheel is easily removable (simply pull it out) and can be placed aside when the ghatta is to be used for grinding instead of battery charging. The metal piece welded to the key should be closed at the top (through a slight miscommunication, ours was initially made to be a larger female square pipe rather than male shaft) so as not to fill up with grain during grinding operation. Also, since the turbine shaft of the ghatta sometimes extends beyond the top of the key, it is necessary to allow for this protrusion when welding on the male section to the key.
The journey to our last test site south of Banepa was difficult, and we wanted a closer location to conduct our tests. We used two days to search for an operating ghatta within a closer vicinity to Kathmandu. Finally (after mispronouncing the word "ghatta" countless times), we found two operating ghattas some 45 minutes outside the city on the road to Dakshenkali. We chose to use the more traditional of the two (the second was housed in a giant concrete house and seemed capable of higher-than-normal rpm). We bought a key from the owner.
We returned to Dohmodar Karki at CRT and asked about workshops able to reliably manufacture our design. Dohmodar introduced us to a welding professor at the local school of engineering, Chuda Mani Upadhyaya, who CRT employs for a great deal of their manufacturing. Chuda was able to make us a very good system for testing in a remarkably short period of time.
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Radiator fan motor with mounting |
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We decided to use this test to power a light from a bicycle-dynamo kit we had in the "lab" (the front living room of our apartment). We used a small second-hand18W radiator fan motor (from the local Kathmandu taxis) as our dc generator. Converting the motor for our own purposes was a little tricky. Although the motor itself does not need modification (a dc generator is just a dc motor run in reverse), it is meant to operate a small plastic fan, and not a car belt. We screwed a small piece of metal bar (actually from the bike dynamo) onto the motor's tiny threaded shaft. We then mounted a pulley onto the piece of metal. The pulley had a 1.5" diameter. With the 26" bike rim, we had about a 17:1 wheel-to-pulley ratio.
The bicycle wheel and car belt are aligned parallel to the ground. Thus, the dc fan motor needed to be mounted perpendicular. We were unsure of the height we would need to place the motor at, so we mounted the motor between two long threaded bolts and brought small blocks of wood to place underneath in the event we needed more height. Unfortunately, we miscalculated the height we needed and we were forced to put rocks and stones underneath the motor as well to raise it up. The motor mount rocked back and forth a bit (the stones underneath were irregular), but we seemed to be able to get the tension we needed.
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Fan motor setup |
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This test - and subsequent tests as well - was complicated by the sheer number of people present. The new ghatta we were using, off the road to Dakshenkali, was located unfortunately not 100 meters from a public school. This made testing a bit difficult as, whenever there was any kind of break in teaching, the small hut would overflow with children. We had to keep one eye on our equipment at all times, as it tended to get "examined" by a large number of youngsters.
We hooked a 6V light to the motor and had the ghattera open the water chute. Almost immediately, the light became rather bright (the locals seemed impressed by this). During operation, we measured between 5.3V and 6.2V across the light, with a corresponding current of 0.35A to 0.42A. The change in voltage seemed to correspond to the wobble in the car belt, which was presumably produced by an irregular motion of the bike wheel.
The mechanical load on the ghatta was actually quite high during this test (the ghatta stone slowed considerably from its estimated unloaded speed of 60-90 rpm) - much more than we expected for such a small power output. Closer observation revealed that most of the mechanical load was coming from the bearings in the radiator fan motor. The motor was never designed to turn a pulley under tension; its axle was significantly bent when attached to the car belt, and was very difficult to turn under these circumstances.
We also had difficulties keeping the pulley connected to the fan motor. The small nut connecting our pulley to the threaded shaft worked itself off several times, causing the pulley to fly off the motor. We were unable to run our test for more than about 5 continuous minutes before the whole thing fell apart.
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Light from ghatta power |
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Key Lessons from this Test:
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