A reciprocating-piston internal-combustion engine has a crankshaft 6 driving a balancing shaft 12 at twice crankshaft speed through gears 8, 10. At least one balancing weight 31 performs a substantially linear, oscillating movement, driven by slide member 27 and a throw of the balancing shaft. Compensation weights 20, 26 are similarly driven by slide members 18 and 24 on throws 14 and 16 of the balancing shaft, performing a similar movement to the balancing weights, but out of phase with them, and with each other. The time of minimum speed of the balancing weight coincides with the time of maximum speed of the compensation weight. In the example shown, the compensation weights move laterally on guides 22, while the balancing weight 31 moves vertically. Figures 2 to 4 show arrangements using connecting rods and pivoted mounting levers; figure 5 shows an arrangement for a three cylinder engine. The dual weight system minimizes energy transfer between shafts, thus minimizing noise and vibration.
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