Walk-behind working machine

A walk-behind working machine comprising:

• a drive source;
• a plurality of wheels rotationally driven by the drive source;
• a traveling clutch 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 having a first friction surface and being disposed in the case of the traveling clutch;
• a driven member for undergoing rotation relative to the driving member and having a second friction surface positioned to be brought into surface contact with the first friction surface at a preselected surface pressure, the driven member being disposed in the case of the traveling clutch so that a film of the lubricating oil is always disposed between the first and second friction surfaces;
• 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 first and second 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 first and second friction surfaces 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 and a number of times N the driven member rotates relative to the driving member per second, has a first downward-sloping curve portion having a first slope and a second downward-sloping curve portion having a second slope smaller than the first slope 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 5×10−8 at the bent point, and the second 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 greater than the parameter value at the bent point.

A walk-behind working machine comprising:

• a drive source;
• a plurality of wheels rotationally driven by the drive source;
• a traveling clutch 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 having a first friction surface and being disposed in the case of the traveling clutch;
• a driven member for undergoing rotation relative to the driving member and having a second friction surface positioned to be brought into surface contact with the first friction surface at a preselected surface pressure, the driven member beina disposed in the case of the traveling clutch so that a film of the lubricating oil is always disposed between the first and second friction surfaces;
• 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 first and second 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 first and second friction surfaces 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 and a number of times N the driven member rotates relative to the driving member per second, defines first, second and third lubrication states, the first lubrication state corresponding to a state in which a film of the lubricating oil exists between the first and second friction surfaces while the first and second friction surfaces are in surface-to-surface contact, the second lubrication state corresponding to a combination of a hydrodynamic pressure and a contact pressure between the first and second friction surfaces, and the third lubrication state corresponding to a state in which the first and second friction surfaces are completely separated by a film of the lubricating oil.