Patent ID: 6848523
Filing Date: 2005-02-01
Classification: F16D

Abstract:
1. A walk-behind working machine comprising: a drive source; a plurality of wheels rotationally driven by the drive source; a traveling clutch disposed between the drive source and the wheels for transmitting driving power from the drive source to the wheels to cause the walk-behind working machine to travel along a surface, the traveling clutch having a case containing a lubricating oil; a driving member submerged in the lubricating oil contained in the case of the traveling clutch and having a friction surface; a driven member having a friction surface and being submerged in the lubricating oil contained in the case of the traveling clutch for undergoing rotation relative to the driving member while a film of the lubricating oil is always disposed between the friction surfaces of the driven and driving members, the friction surface of the driven member being positioned to be brought into surface contact with the friction surface of the driving member at a preselected surface pressure to place the traveling clutch in a slipping state during which the driven member slips on the driving member; and a clutch lever manually operable to switch the traveling clutch from a disengaged state in which the traveling clutch does not transmit drive power from the drive source to the wheels, through the slipping state, to an engaged state in which the traveling clutch transmits drive power from the drive source to the wheels, the clutch lever being connected to the traveling clutch so that the surface pressure between the respective friction surfaces of the driving and driven members varies in direct proportion to a pushing force applied on the clutch lever and so that the pushing force remains substantially constant regardless of an increase in a running resistance to the walk-behind working machine while traveling along the surface; wherein a characteristic curve of the traveling clutch plotted in a coordinate system including a vertical axis representing a coefficient of friction between the friction surfaces of the driving and driven members and a horizontal axis representing a parameter (η×N)/P, which is obtained by dividing by the preselected surface pressure P a product of a viscosity η of the lubricating oil contained in the case and the number of times N the driven member rotates relative to the driving member per second, has a first downward-sloping curved portion having a first slope and a second downward-sloping curved portion having a second slope smaller than the first solve and being connected to the first downward-sloping curve portion via a bent point, the preselected surface pressure being determined so that the bent point corresponds to a switch point when the walk-behind 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, the first downward-sloping curve portion representing the relationship between the coefficient of friction η and the surface pressure P between the respective friction surfaces of the driving and driven members when the parameter (η×N)/P is smaller than a parameter value of 5×10