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Simple yet memorable lessons learned – how to break a locked air pocket

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I’m in a remote part of rural Odisha, with green hills in all directions and amazing people who know so much about their ecosystem, their land, their traditions, their fellow community members and very likely themselves. The initiative that has been bringing me here every few months for the past 3 years is trying to even the balance as these communities interact more and more closely with markets and commerce oriented individuals and institutions. One part of that initiative is a rotary cold press oil mill and another is a set of machines to process millets such that their nutritional value is retained. This visit was planned to be a refresher course for local youth who have been trained to run these machines. But the rotary mill had a breakdown a week ago. With more than 40 sacks of mahua seeds waiting to be crushed, the primary objective was now to fix the rotary mill.

A rotary mill is essentially a mortar and pestle with the pestle held stationary while the mortar is driven by a motor. A gear box is used to achieve 20 rpm rotational speed of the mortar (from the motor’s 960/1440 rpm). The shaft of this gear box on which the mortar sits had broken and needed to be replaced. Even with a ratchet and winch, it took some effort to get the heavy mortar and pestle off – each of them weighs about 100 kgs. After being instructed about a couple of neat tricks, the broken shaft was exposed, available to be hauled out of the gear box.

The gear shaft sits on 2 bearings at either end with a coupling gear in between. The only points of contact of the shaft with the body are through these components. When rotated, the shaft was rotating without any problem – so all the components were fine, though the shaft itself had broken just above the gear box. The friend who made this rotary mill insisted that we just need to lift it and the shaft assembly would come off and out of the gear box. But the thing refused to budge. We tied a bamboo over it and tried to lift it, no movement. We hitched it onto the winch and the whole milling machine frame with motor started to come off the ground !

Our man on the phone insisted there isn’t anything else to it and that we should just take a time out and figure out how to break the locked air pocket that would have formed at the bottom bearing – the whole assembly is drowned in gear oil. I couldn’t see what else we could do, so we stepped back and took a breather, mulling over the fact that what we needed to do was lift the gear shaft assembly out of the gear box. We tired shaking the gear shaft, hammering it with a mallet, and yanking at it at different angles using some ropes tired around it – but the locked air pocket just did not break. Time to step back again.

The next thing I could think of was using was the principle of a lever. I picked up a 2′ long piece of bamboo, with the body of the gear box as fulcrum, I set the far end under the top bearing and the stated ‘levering’ away while rotating around the shaft and increasing the force as I went around. After a good 2 minutes of doing this and just as I was starting to go beyond my arm and use my body weight on the piece of bamboo, I hear the ‘pssschuk’ !! The locked air pocket was broken. And we were able to lift the gear assembly as if it was just sitting there!

Lessons learned:

  1. Locked air pockets can lift 300+ kgs.
  2. An off axis load that is a small fraction of the on axis load can break the locked air pocket.
  3. Step back and re strategize.

Ah, the joys of learning while doing !!

Written by Dwiji

Wednesday, July 24th, 2019 at 06:49

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