US Patent 6848523 Walk-behind working machine

A walk-behind working machine has a drive source, wheels rotationally driven by the drive source, and a traveling clutch for transmitting driving power from the drive source to the wheels. A driving member is disposed in the case and has a first friction surface. A driven member is mounted in the case for undergoing rotation relative to the driving member and has a second friction surface positioned to be brought into surface contact with the first friction surface at a preselected surface pressure. A characteristic curve of the traveling clutch plotted in a coordinate system including a vertical axis representing a coefficient of friction between the first and second friction surfaces and a horizontal axis representing a parameter obtained by dividing by the preselected surface pressure a product of a viscosity of the lubricating oil and a number of times the driven member rotates relative to the driving member per second has a first straight line and a second straight line connected to the first straight line via a bent point. The preselected surface pressure is determined so that the bent point corresponds to a switch point when the working machine is switched from a first state in which the walk-behind working machine operates at a high load to a second state in which the walk-behind working machine operates at a low load.