A hydraulic auto-tensioner includes a cylinder, a valve sleeve, a rod, a spring seat, and a plunger slidably fitted on the outer diameter surface of the rod. A first leakage gap is defined between the fitting surfaces of the plunger and the rod. A second leakage gap is defined between the fitting surfaces of the plunger and the valve sleeve such that the resistance to oil flowing in the second leakage gap is larger than the resistance to oil flowing in the first leakage gap. With the engine running normally, when the tension of the belt increases, and thus a pushing force is applied to the hydraulic auto-tensioner such that the cylinder and the spring seat are pushed toward each other, oil in a pressure chamber leaks through the first leakage gap into a reservoir chamber.

A fixing tool of an auto-tensioner includes inwardly inclined engaging pieces inclined inwardly so as to be opposed to respective tapered surfaces of a spring seat and a cylinder of the auto-tensioner. The engaging pieces have concave surfaces that contact the tapered surfaces over a circumferentially extending area. This minimizes the deformation of the engaging pieces such that the diametrical distances therebetween increase due to contact with the tapered surfaces when sandwiching the spring seat and the cylinder with the engaging pieces from the axially opposite directions, thereby preventing the fixing tool from coming off.

In an aspect, a tensioner includes a pulley, a main piston chamber, a reservoir, and first and second fluid passageways connecting the main piston chamber and reservoir, a main piston that is movable in the main piston chamber, a check valve, a control valve, and a movable reservoir member. The check valve prevents flow in one direction through the second passageway and permits flow in an opposing direction through the second passageway. The control valve is movable between to provide a first flow resistance, or a second flow resistance. The movable reservoir member is movable based on the amount of fluid in the reservoir in such a way as to change the volume of the reservoir, such that the reservoir, the main piston chamber and the first and second passageways together are included in a fluid circuit that is substantially free of any compressible fluids.

The derailleur has a damper that breaks the angular movements of a chain guide. Variable volume chambers in the damper are filled with a damping fluid and connected through a controlled fluid passage system so that fluid flows between chambers in response to angular movement of the chain guide. The damper comprises a dividing body in a mobile body coaxial to the axis X, mechanically constrained so that each rotation of the dividing body about the axis X is accompanied by a translation of the dividing body along the axis X. The dividing body is mechanically constrained to the chain guide and moves with it in the angular direction about the axis X. The chambers are formed in the mobile body on opposite sides of the dividing body and change volume in opposite directions when the dividing body moves along the axis X.

An autonomous hydraulic expansion and contraction apparatus comprises a fixing unit, a cylinder unit, a support unit, a piston unit, a valve unit and oil supply parts. By controlling the expansion and contraction length of the support part by a simple manipulation of the valve part by moving the lever, the autonomous hydraulic expansion and contraction apparatus can be easily expanded and contracted to the combined length of the expansion and contraction equipment, and additionally, the convenience of use can be enhanced.

A work machine having a belt tensioning system is described. A belt is engaged with an engine drive sheave and a driven sheave of a rotor having a plurality of cutting tools configured for reducing wood infeed material. An idler pulley is mounted to an idler arm that is hinged to the machine frame. An actuator is hinged to the idler arm and the frame. The actuator may be extended and retracted in order to press the idler pulley into stronger and weaker engagement with the belt in order to set the desired amount of tension in the belt. One or more sensors may be used to sense the amount of tension in the belt, and the one or more sensors may be in communication with a control system that controls the actuator to monitor and adjust the belt tension so that it is maintained within desired limits.

A hydraulic auto-tensioner includes a sleeve having its bottom end press-fitted in a sleeve fitting hole in the bottom surface of a cylinder, a rod having its bottom end slidably fitted in the sleeve, and a return spring mounted between a spring support at the upper portion of the rod and the bottom surface of the cylinder. A pressure chamber communicates with a reservoir chamber through a passage formed between fitting surfaces of the sleeve fitting hole and the sleeve. The passage has a check valve. The rod has a valve fitting hole and a communication passage through which the valve fitting hole communicates with the reservoir chamber. The valve fitting hole has a relief valve. An orifice is formed between a valve hole formed in a valve seat of the relief valve and a valve body to prevent a sharp drop in pressure in the pressure chamber.

A hydraulic auto-tensioner is provided which includes a cylinder, a valve sleeve, a rod, a spring seat, and a plunger slidably fitted on the outer diameter surface of the rod. A first leakage gap is defined between the fitting surfaces of the plunger and the rod. A second leakage gap is defined between the fitting surfaces of the plunger and the valve sleeve such that the resistance to oil flowing in the second leakage gap is larger than the resistance to oil flowing in the first leakage gap. With the engine running normally, when the tension of the belt increases, and thus a pushing force is applied to the hydraulic auto-tensioner such that the cylinder and the spring seat are pushed toward each other, oil in a pressure chamber leaks through the first leakage gap into a reservoir chamber.

A hydraulic auto-tensioner includes a cylinder with a sleeve fitting hole, and a sleeve press-fitted therein. A rod has its bottom end slidable in the sleeve. A return spring is mounted between a spring seat at an upper portion of the rod and a bottom surface of the cylinder. An oil passage, with a check valve, is formed between the fitting surfaces of the sleeve fitting hole and the sleeve and communicates the pressure chamber with the reservoir chamber. The rod has a valve fitting hole open to its bottom surface, and an oil passage communicating the valve fitting hole with the reservoir chamber. A relief valve, mounted in the valve fitting hole, includes a valve seat with a valve hole having an orifice for restricting the flow rate of oil released from the pressure chamber into the reservoir chamber through the oil passage when the relief valve opens.

A chain tensioner includes an oil supply passage for supplying hydraulic oil into a pressure chamber defined by a cylinder and a plunger, and a check valve provided at the end of the oil supply passage. A leak gap is defined between the sliding surfaces of the cylinder and the plunger. The plunger is biased by a return spring in the direction in which the plunger protrudes out of the cylinder. A plug is press-fitted in a cylindrical inner periphery of a through hole formed in the plunger to extend from a protruding end surface of the plunger to the pressure chamber to define an air vent passage. It is possible to change the hydraulic damper force simply by replacing the plug with another different kind of plug to define a different air vent passage.