What the helix does is convert the the belt pull to side pressure on the sheaves so the belt does not slip. They used to use springs for this .Due to the nature of springs using only a spring without a helix created unneccesary side force on the belt at High speeds creating heat and belt failures.
A helix provides a more linear side force and can be adjusted by the helix angle.
In a perfect world you want just enough side force to to prevent belt slip. Too much will cause too much heat - hence inefficiency.
Secondary springs now are mainly used to provide the initial side force and allow the sheaves to back shift.
Go and get Olav Aaens clutching book. It is a great read on how these mechanisms work.
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