UAVE 120 cc four-stroke | Engine dossier to keep it ‘neutral’ and free from any inclination to rotate in an unbalanced fashion,” Slater says. “We reported all our numbers and findings to AMB Engineering, and the core matter of their response was that we needed to put inserts of a special tungsten alloy into the crankshaft counterbalance, and they gave exact dimensions for the inserts we needed too. “But we’d also made our own calculations, so we made two crankshafts for balance testing – one with their inserts and one with ours. Unsurprisingly, they were right and we were wrong.” Three years later, UAVE continues to use the inserts that AMB specified, including the exact proportions of the alloy, Mallory metal. This highly dense and heavy tungsten alloy is developed and specialised for crankshaft counterbalance inserts, especially in single-cylinder engines, which can require compensation for the single reciprocating mass. JS Engineering, a specialist engineering company located within one mile of UAVE, mills out a hole in the DS120’s crankshaft counterbalance, and inserts and finishes the Mallory metal insert with an interference fit, in a manner such that the engine’s vibration is heavily minimised. “Vibration becomes especially critical when you’re trying to optimise your engine’s performance up to its very top echelons, and thanks to this whole shaft-balancing journey, we now have a phenomenally lower-vibration engine than the older version we flew in the Arctic and in India,” Slater says. Aside from the Mallory metal inserts, the crankshaft is largely milled from a single piece of EN36 steel with a very light case-hardening of roughly 0.009 cm in thickness, including the shaft, counterbalance and the crank pin for the con-rod big end; the latter being press-fitted into a crank arm opposing the counterbalance (though not before the single-piece con rod is fitted onto the pin). The crank-journal surfaces are ground to precision-fit the bearings and the entire shaft is then sent to a gear-cutting specialist, which cuts the helical teeth for driving the cam gear. The bearings are pre-greased, oil-sealed bearings, similar in geometric design, although the bearing closest to the combustion chamber has a steel seal (to withstand the heat it must endure), while the two nearer the propeller run much cooler and thus have plastic seals. The con rod is of milled steel, with a needle roller bearing in its big end and a bushing in its small end. UAVE is looking to replace the big end bearing with a new, graphite-impregnated bronze bushing. Such bushings have been used previously in very high-rpm electric motors; rather than featuring an oiling hole, such a bushing would gradually release atomised graphite over its lifetime, lubricating effectively. Piston The DS120’s aluminium piston is supplied by Vision Spares (Trading as DIY Spare Parts). Early in the engine’s development, while frustrated with wear and overheating from DGS’s billet-cut pistons, Short provided Slater with a test engine unit fitted with a COTS piston he had found (for its “skinnier” piston rings and the prospect of reduced friction) and subsequently modified to optimise weight and other parameters. 75 Uncrewed Systems Technology | December/January 2025 Mallory metal inserts, designed and specified by AMB Engineering, balanced the crankshaft and minimised vibration in the DS120 Vibration becomes especially critical when you’re trying to optimise your engine’s performance up to its very top echelons
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