Saturday, May 23, 2009

Large Pulley

Ever wonder what is up with the large pulleys recently showing up on the inexpensive shimano derailluers? Decent engineering. Fairly common knowledge since the 18th century days of mechanical clock making*, to minimize friction loss, the ratio of axle diameter to wheel diameter should be minimized. ( e.g. the axle/bearing diameter as small as possible, the wheel/pulley as big as possilble).

For rotational systems, power (or power loss, in this case) = torque * rotational speed. Torque = force times a lever arm. If the lever arm, or radius of axle approaches zero, well of course so does the torque and so does the power loss. Or for identical axle diameters, if the Pulley diameter increases, the rotational speed decreases, and thus also power loss.

10 tooth ~ 32 mm dia
11 tooth ~ 36 mm dia
13 tooth ~ 44 mm dia
15 tooth ~ 52 mm dia

So by going from 10 tooth pulley to 15 tooth pulley the friction loss is nearly halved (to 60% 32/52)
Another advantage is the chain isn't flexed as much, albeit not saving probably a huge amount of energy, but some nonetheless.

Obviously if you have frictionless pulley bearings (i.e. standard deep row ball roller bearings), then this is not as big of deal. Yet it still applies since many bearings have tight rubber seals. I have seen where this seals have way more friction than standard bushing pulley, and regardless of what is claimed, they don't "loosen" up all that quickly (these are not electric motors spinning at 6000 rpm).

So if you are going to use sealed pulleys, pick those with smaller bearings, where the seal friction acts at smaller diameter. I personally use shielded bearings, but for a lot of wet weather or washing (..ccx..), it might pay to use simple bushing'ed large diameter pulleys.

On subject, how about Ceramic bearings for Pulleys? Not logical, since the way ceramic bearings save energy, is that they don't deform as much under load (hertzian contact...). And if the load is next to nothing to begin with, such as in a Derailluer pulley, then the relative energy saving will also be nothing. If they seem like less friction, it is probably only because the seals are really light. Don't waste your money!

*Book about John Harrison: Longitude: The True Story of a Lone Genius Who Solved the Greatest Scientific Problem of His Time).

2014 comments.  Friction Facts now has tested pulleys (among other things) and actually measured this.  Go to their site and support them !