Sorta' follow that....but way out of my league I reckon Bruce.
At first I thought of turning a cone and an inverted cone like below, but anything like that is going to be have to be turned by the engine as well and will eat up engine energy (and there's little already with most of our engines).
So far I'm thinking the idler wheel as suggested may be the way to go, but not sure how to do that either.
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