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Mythbusting Belt Drives

Dan Horton, EAA 443866 This is an edited version, by the author, of the original article

Sport Aviation

Dan Horton’s Grand Champion Light Plane award winner (AirVenture 1998) made the cover of EAA Sport Aviation in September 2004 and was featured in Experimenter (October 1998). Although he did a superb job building this plane, what he really brings to the table is his understanding of propeller speed reduction units (PSRU) and torsional vibration. The following is an article he submitted to CONTACT! Magazine in the hopes of dispelling certain myths and old wives’ tales.

Dan Horton
Dan Horton, Sun ’n Fun 2004

“A belt-driven PSRU drive isolates the torsional vibrations coming from the propeller.” I contend that the statement is based on myth, printed and reprinted until accepted as fact. Belts have no magic properties, regardless of what you read on the Internet or hear at the vendor’s booth.

System frequencies are a function of the system’s inertias coupled by its stiffnesses. A simple torsional model of a belt drive with a two-plate frame would have at least four inertias (crank, flywheel and lower sprocket, upper sprocket, and prop), coupled by three stiffnesses (crank stub, belt, and prop shaft). In truth, the dynamic system has many more elements. For every element (a stiffness and an inertia) there is a natural frequency.

The point is that the belt serves as a connecting stiffness, nothing more. It makes a contribution to the system’s fundamental frequency as well as the natural frequency of the adjacent inertia. The overall system or the individual element can be driven into resonance by a matching exciting frequency. Exciting frequencies not matched can perhaps be considered as “isolated,” but in practice, piston engine output includes a whole range of exciting frequencies. If you have a lot of elements, something resonates to some degree almost everywhere in the rpm range. As a practical matter we concern ourselves with the worst resonant frequencies by shifting an inertia or a stiffness. To do that you must know what they are.

It isn’t difficult to measure or estimate an inertia. The same is true for the torsional stiffness of a simple shaft. A connecting member like a belt or chain has a torsional stiffness equivalent derived from its “stretch” and the arm of its sprocket.

The torsional spring rate equivalent of the “average” belt (there are considerable variations between brands) is a non-linear curve, soft under initial loading and far stiffer as the tensile members in the belt backing are tensioned. The shape of the curve is a function of both tooth shore hardness and the tensile material’s properties. The wide variation in equivalent torsional spring rates means you can’t make a general statement about all belts. You must know the spring rate equivalent of the belt you’re using to draw any conclusions about its stiffness contribution to frequency or isolation. For the record, also note vibratory effects and frequencies brought to the design by the inclusion of a belt rather than gears or chains. An example might be a flap or standing wave in the belt spans.

Now let’s look at what we’re isolating. What torsional vibrations come from the propeller? In a simple torsional model, the propeller is an inertia with no inherent capability to excite the system. A more complex model does treat the prop as a series of inertias and stiffnesses, and indeed, blades do vibrate.The blades can be driven into resonance from a number of sources, but they are usually not the source of a significant exciting torsional frequency. There are a few exceptions, but we’re usually trying to protect the prop from the engine, not the other way around.

There is no shortage of textbook and SAE material on the subject of torsional vibration. Component data is available from the manufacturer. There are established techniques to allow actual measurement of torsional behavior. Isn’t it about time we gave up trial and error (and old wives’ tales) to design our auto conversions?

Dan's engine

Dan's engine

Dan's engine

Dan's engine
Chevy Sprint/Suzuki three-cylinder engine. The tight confines of the engine compartment made it difficult to photograph Dan’s PSRU.

 
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