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Where are my dogs?

Roger Lee

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-What are Dogs?

The Rotax gearbox design is a model of simplicity and ruggedness. A geared piston aircraft engine must deal with an incredible amount of “argument” between the propeller and the pistons. The cure for this argument is “Torsional Damping”. The propeller is a huge rotating force that does not want to move (inertia) and the pistons are constantly producing “pulses” as they go through the firing order. Something has to absorb this “argument” or parts fail. This is why so many automotive engine conversions have gearbox problems; they do not have an adequate torsional dampener.

The Rotax torsional dampener is large triangular metal dogs that mesh with each other. The dogs are angled in the contact area so they can “ramp-up” when they are twisted. This ramping-up allows the movements necessary to prevent the piston pulses from reaching the propeller. The dogs are forced together with “disc springs” that further dampen out pulses. The dogs are large and strong and have lots of wear allowance. This gives the gearbox its legendary longevity. Friction torque is important for the maintenance of the Rotax engine as it measures how “tight” the gearbox is. A loose gearbox allows excessive movement between the parts and this leads to worn out parts. A loose gearbox has a distinctive “rattle” at low RPM, if allowed to continue, the rattle will get worse and the internal parts will wear progressively faster. Fortunately the Rotax gearbox is easily removed from the engine if any repairs are needed.


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