Re: Re: Re: Re: Rapid Passages on Rotaries


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Posted by Rick Denney on March 19, 2003 at 13:30:10:

In Reply to: Re: Re: Re: Rapid Passages on Rotaries posted by Anthony on March 19, 2003 at 08:16:41:

I gotta chime in on this one, Anthony.

Let's remove the variables and assume there is no friction. We'd be left with what is really the primary influence on valve speed: Inertia.

In short, heavier valves require more force to move at the same acceleration as lighter valve. Force = mass X acceleration. If acceleration is constant (meaning equal technical facility), then greater mass requires more force.

Now, which valves present the greatest mass to the mechanism? In raw mass, a piston is perhaps half the mass of a rotor, or a bit less, according to my back-of-envelope calculations. Because it spins, though, the inertial mass is less, being perhaps a quarter of the piston. So, if you applied the force at the outside edge of the rotor, the force application to get a particular acceleration might be less than half that of the piston.

But the linkage actually serves to increase the force required to move the rotor, because the movement required at the edge of the rotor might be a quarter of its circumference, which is something in the range of 1.2". That motion at the rotor is achieved by a movement of perhaps half an inch at the paddle, which multiplies the force by two or three times. Thus, you still end up with a higher force, even assuming no friction. String linkages are worst, unless the pulley is really large; something like the diameter of the rotor.

Increasing the travel requires longer spatulas and shorter levers between the spatula's pivot and the rod going to the rotor. Some of the old rotary tubas (e.g. the King that Bell played) had very long spatulas and low mechanical advantage. If the finger had to move a long enough distance, the force required to move the rotor might be less than what the piston requires, even though the rotor itself is heavier.

To go back to main point, acceleration is purely a function of mass and force. To turn it around from my earlier approach, if the force is the same, but the mass of the rotor is higher, the acceleration will be less. Thus, the only way to get a solid rotor to be as fast as a piston is to increase the travel of the finger or have much greater explosive strength in your fingers.

Again, this assumes zero friction and no springs. If the spring stiffness is equal and both mechanisms are in perfect working condition, then this is a reasonable assumption for the sake of discussion.

I think the best technical facility will result from the lightest action, because that's what provides the highest acceleration for a given force. The distance of travel is not important--you still have to completely realign the ports for the valve to work--but it has to be matched to what works on the finger. A middle range of finger travel is probably physically easiest, because it is the best compromise between distance of motion and requirement for fine motor control.

So, you can indeed design rotary valves to be as fast as piston valves, but the travel of the finger has to be proportionately longer based on the difference in mass, so that the force is the same.

The issue of reliability is a separate issue altogether, but most folks who play piston instruments manage to deal with that somehow. It definitely requires a more meticulous approach.

My own experience bears out these conclusions: My technique is better on my piston instruments, but they require much more care and feeding.

Rick "whose tonguing isn't nearly as fast as his fingers in any case" Denney


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