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Belt drive with elliptical cog wheel?

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floridapilot1

Active Member
Joined
Nov 14, 2013
Messages
30
Location
Cologne Region Europe
As we know most of the time a 4 stroke engine is driven by the flywheel. In a 4 cylinder there are 2 power strokes per revolution which keep the thing running – and give rise to the omnipresent torsional vibrations.

They might be greatly reduced in a belt drive by using an elliptical (e.g. primary) cogwheel at no expense except for the production process being more sophisticated. The trick is to compensate the higher rotational speed during the power stroke by a (very slightly) smaller leverage arm aka wheel radius at that phase of rotation. This will smooth the velocity of the belt and the prop shaft.

Consider a setting with an additional fixed deflection pulley on the tension side of the drive and a springy belt tension pulley on the return side to compensate minor variations of carbon backed belt length (see Honda B20B Belt PSRU)

Now think of a flat belt and do a first gedankenexperiment:
Imagine that on the elliptical cogwheel the arriving and departing belt fully encloses the cogwheel, 360°. When you now rotate the cogwheel all the elliptical circumference is in use, the total circumference of the belt is constant at all times (even no need for compensation by springy tension pulley).

Now the real situation:
With the two idle pulleys on the backside guide the belt in such a way that arrival and departure leg run parallel. For symmetry reasons that means that only half of the cogwheel's circumference is in use while the other half is free – all the time, almost constantly. Again “constant” belt circumference.

“Almost” means that there are minimal variations given by the fact that during revolution the effective diameter of the ellipse varies for some per mill – therefore the springy second idle pulley.

By the way, it need not be a mathematical ellipse: the shape could be adapted to the nature of the power pulse and the inertia of engine and prop side. All what is needed is 2fold symmetry.
For timing belt drives this could be a viable way to reduce driving forces for torsional vibrations using appropriate mean values for the power range in use. Production of the grooves might be more complicated, but viable.

I am aware that I am surrounded by most competent specialists here. Critics, ideas and comments are welcome.



Richard
 
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