Abstract: The invention relates to a water-based biodegradable and non-toxic functional fluid for mechanical drives, preferably for use in steam engines. The object of providing such functional fluids for use in mechanical drives, particularly for the hydrodynamic lubrication of plain bearings, preferably for crankshafts of superheated steam engines, having customary bearing play which is not overly precise, is achieved. It should be capable of being used in closed power plant systems also at sump temperatures higher than 120° C., e.g. in the crankcase under a crank shaft, and should be water-based or water-dilutable. According to the invention, this is achieved by addition of a low percentage of polyaspartic acid to water or to water/glycol mixtures, preferably polyaspartic acid having a molar mass of from greater than 1 000 to 10 000 g/mol being used.