Walk-behind working machine

摘要

A walk-behind working machine (10) includes an engine (14), a pair of rear wheels (13, 13), and a traveling clutch (30) interposed between the engine and the rear wheels. The clutch includes a case (24) containing lubricating oil (Ju), a driving member (22b) having a female portion (38), and a driven member (32) having a male portion (39). The driving and driven members are submerged in the oil. The clutch provides a characteristic curve shown in a coordinate system where a vertical axis of the system shows a coefficient of friction ( mu ) between the female and male portions and a horizontal axis of the system shows a parameter obtained by multiplying a viscosity ( eta ) of the oil by the number of times (N) the male portion rotates relative to the female portion per second, and then dividing the resulting product by a surface pressure (P) between the female and male portions. The characterristic curve includes first and second straight lines (C1, C2). The first and second straight lines meet each other at a bent point (Cp). The bent point corresponds to a switch point when the working machine is switched from a high load running state to a low load running state or vice versa. The female portion may have a helical groove (38b) and plural straight grooves (38a') formed on a surface thereof. The straight grooves extend perpendicular to the helical groove. <IMAGE>