An autotensioner includes a cylinder having its bottom closed and filled with a hydraulic oil and air, an oil seal mounted in the cylinder to seal the hydraulic oil and air, a rod slidably extending through the oil seal, and a piston slidably mounted in the cylinder with its top end abutting the bottom of the rod. The interior of the cylinder is separated into an upper reservoir chamber and a lower pressure chamber by the piston. Both chambers communicate with each other through a channel formed in the piston. The rod is biased by a spring mounted in the reservoir chamber in such a direction as to protrude from the cylinder. A check valve is provided in the pressure chamber near the bottom of the piston to open the channel in the piston only when the pressure in the pressure chamber is lower than the pressure in the reservoir chamber. The cylinder may have an outer casing and an inner sleeve so that the piston can slide on the inner surface of the sleeve.

BACKGROUND OF THE INVENTION 
The present invention relates to an autotensioner for keeping the tension 
of a power transmission member such as a toothed belt or a chain to a 
predetermined level. 
Since a power transmission member in the form of a belt mounted on an 
internal combustion engine is subject to change of tension with the 
fluctuation of the center-to-center distance between pulleys owing to a 
thermal expansion of the engine during operation or with the fluctuation 
of torque, it is a common practice to provide an autotensioner to apply 
tension to the belt through a tension roller and thus to keep constant the 
tension of the belt. 
One such prior art autotensioner comprises a cylinder filled with a 
hydraulic oil, a piston slidably mounted in the cylinder and a pressure 
control spring for biasing a rod of the piston in such a direction as to 
protrude from the cylinder (Japanese Unexamined Utility Model Publication 
62-40355). 
With this type of prior art autotensioner, it is necessary to introduce 
hydraulic oil into the cylinder under vacuum so that it can replace the 
air in the cylinder. Thus it is extremely troublesome and time-consuming 
to assemble such an autotensioner. 
Further it is necessary to seal the hydraulic oil in the cylinder by use of 
a diaphragm seal having a complicated configuration to absorb fluctuations 
in the amount of oil resulting from relative movements of the cylinder and 
the piston and fluctuations in the volume of oil owing to heat. Such a 
seal tends to be costly. 
An autotensioner of an oil-sealed type can be theoretically oriented in any 
desired direction. For all practical purpose though, it is mounted in such 
a direction that its rod extends vertically in most cases in view of the 
layout of engine and the like. Thus, the advantage of the oil-sealed type 
is not fully utilized. 
It is an object of the present invention to provide an autotensioner which 
obviates the abovesaid shortcomings and which is reliable and inexpensive. 
SUMMARY OF THE INVENTION 
In accordance with the present invention, there is provided an 
autotensioner comprising a cylinder having its bottom closed and filled 
with a hydraulic oil and air, an oil seal mounted in the cylinder to 
hermetically seal the hydraulic oil and air, a rod slidably extending 
through the oil seal, a piston slidably mounted in the cylinder so as to 
leave a small gap therebetween and having its top in abutment with the 
bottom of the rod, the interior of the cylinder being partitioned into a 
lower pressure chamber and an upper reservoir chamber by the piston, the 
piston being formed with a channel interconnecting the pressure chamber 
and the reservoir chamber, a check valve provided in the pressure chamber 
to open said channel in the piston only when the pressure in the pressure 
chamber is lower than the pressure in the reservoir chamber, and a spring 
mounted in the reservoir chamber to bias the rod in such a direction as to 
protrude from the cylinder. 
According to the present invention, hydraulic oil is introduced into the 
cylinder before mounting the oil seal and the rod is axially reciprocated 
several times to allow part of the hydraulic oil to flow into the pressure 
chamber. This allows the assembly of the autotensioner under normal 
pressure.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
As shown in FIG. 1, the autotensioner in the first embodiment includes a 
cylinder 1 having a closed end 2. An oil seal 3 is fitted in the cylinder 
1 near its top end and prevented from coming off the cylinder by means of 
a stopper ring 4. The oil seal 3 serves to prevent leakage of the 
hydraulic oil and the air in the cylinder. The hydraulic oil should 
preferably be silicone oil in view of its low variation of viscosity but 
may be an engine oil. 
A rod 6 slidably extends through a hole 5 formed in the center of the oil 
seal 3 and has its bottom end received in a hole 8 formed in the top of a 
piston 7 slidably mounted in the cylinder 1. 
A channel 11 extends through the rod 6 and the piston 7 so that a pressure 
chamber 9 and a reservoir chamber 10 defined immediately under and over 
the piston 7 will communicate with each other through the channel 11. A 
check valve 12 is provided at the bottom end of piston 7 to open the 
channel 11 when the pressure in the pressure chamber 9 sinks below the 
pressure in the reservoir chamber 10. 
A spring 13 is mounted under the piston 7 to bias the piston 7 upwardly and 
hold it in engagement with the rod 6. 
A bearing 14 is slidably mounted in the reservoir chamber 10 to support the 
rod 6 at its upper portion and thus to keep it in alignment with the 
cylinder 1. The bearing 14 is made of a porous sintered alloy and 
impregnated with hydraulic oil. Also it is formed with holes 15 for 
passing air and hydraulic oil. 
A spring seat 17 is mounted between the bearing 14 and the piston 7 with 
its outer peripheral portion resting upon a shoulder portion 16 formed on 
the inner periphery of the cylinder 1. A gap is defined between the inner 
periphery of the spring seat 17 and the rod 6. The spring seat 17 serves 
to prevent the entry of air into the pressure chamber 9 owing to 
fluctuations of oil level resulting from change of mounting position of 
the autotensior or its vibration. 
A pressure control spring 18 is mounted between the spring seat 17 and the 
bearing 14 to bias the rod 6 in such a direction as to protrude from the 
cylinder 1. It has a biasing force larger than that of the spring 13 
mounted under the piston 7 so that it can receive most of the load on the 
rod 6. 
The ratio of hydraulic oil to air sealed in the cylinder 1 has to be 
determined so that the air can absorb by changing its own volume any 
fluctuation of volume of the hydraulic oil resulting from fluctuation of 
internal volume of the cylinder owing to the axial movement of the rod 6 
or from temperature fluctuation of hydraulic oil and that the air is 
prevented from getting into the pressure chamber 9 even if the mounting 
position of the autotensioner changes or the degree of vibration 
increases. The optimum ratio of air was found out by experiment to be 
(amount of air/amount of oil+amount of air).times.100=10-40 per cent. 
The autotensioner according to the present invention is mounted with the 
end of the rod 6 protruding from the cylinder 1 upward. If the tension of 
a belt 23 is controlled by means of a tension roller 22 mounted on one end 
of a roller arm 21 having its other end pivotally mounted on a pin 20, the 
cylinder 1 is mounted on a suitable fixing member with the protruding end 
of the rod 6 in abutment with the underside of the roller arm 21. 
If in this state the center-to-center distance between pulleys increases 
owing to a rise in temperature, thus increasing the tension of the belt 23 
above a predetermined level, a downward force will be transmitted to the 
rod 6 through the roller arm 21 to push down the rod 6 as well as the 
piston 7, while compressing the pressure control spring 18. Thus the 
tension of the belt 23 will reduce gradually. On the other hand, hydraulic 
oil in the pressure chamber 9 is compressed by the descending piston 7 and 
gradually flows through a small gap formed between the inner periphery of 
the cylinder 1 and the outer periphery of piston 7 into the reservoir 
chamber 10, thus dampening the impulsive load acting on the belt 23. 
When the belt 23 slackens, the rod 6 will move upwardly urged by the 
pressure control spring 18 to push up the roller arm 21 and thus to move 
the tension roller 22 in such a direction as to stretch the belt 23 taut. 
Thus the tension of the belt 23 is kept constant. Since the piston 7 rises 
together with the rod 6, the pressure in the pressure chamber 9 will fall 
below the pressure in the reservoir chamber 10, allowing the check valve 
12 to move to a position to open the channel 11. The hydraulic oil in the 
reservoir chamber 10 will now begin to flow fairly smoothly into the 
pressure chamber 9 through the channel 11, allowing the rod 6 and piston 7 
to move smoothly following the slackening of the belt 23. 
In assembling the autotensioner, the spring 13, piston 7, spring seat 17, 
pressure control spring 18, bearing 14 and rod 6 are set in the cylinder 1 
one after another. Before fitting the oil seal 3, the cylinder is filled 
with a predetermined amount of hydraulic oil and the rod 6 is axially 
reciprocated several times until the pressure chamber 9 is filled up with 
hydraulic oil. Thus the autotensioner according to the present invention 
can be assembled under atmospheric pressure. 
Further, since hydraulic oil and air can be sealed in the cylinder by means 
of an ordinary oil seal instead of a diaphragm seal having a complicated 
configuration, the reliability of the entire device increases and the 
production cost decreases. 
Air will not get into the pressure chamber because the autotensioner is 
mounted with its rod upward and the pressure chamber at the bottom. This 
will also lead to an improvement in reliability. 
An oil seal allows incoming air to easily pass therethrough but not the 
passage of outgoing air. Thus the oil seal serves to keep the pressure in 
the reservoir chamber and the pressure chamber always higher than the 
atmospheric pressure. Just before the ball of the check valve comes off 
the plunger and the oil in the reservoir chamber begins to flow into the 
pressure chamber, the pressure chamber will be under negative pressure. 
But because the pressure in the entire cylinder is kept high, the pressure 
in the pressure chamber will be prevented from dropping so far below the 
atmospheric pressure as to cause cavitation. 
The rod of the autotensioner is required to protrude quickly when the 
engine is started in a cold state. As the rod protrudes, the volume of the 
reservoir chamber reduces and the pressure increases, allowing smooth and 
rapid flow of oil into the pressure chamber. This means a good 
startability from a cold state. In contrast, with the prior art 
autotensioner with a diaphragm in the reservoir chamber (U.S. Pat. No. 
4,708,696), when the rod is moving in the protruding direction, the 
pressure in the reservoir chamber will be prevented from rising owing to 
the action of the diaphragm seal, thus hampering a smooth and rapid flow 
of oil into the pressure chamber. This may result in a sharp drop in the 
pressure in the pressure chamber and cause cavitation especially in case 
the engine is started in a cold state. Because of the low pressure in the 
pressure chamber, the rod cannot rise quickly. This may cause the skipping 
of the belt. 
Since the oil in the cylinder is positively prevented from leakage, there 
is no fear of the deterioration of belt resulting from oil adhesion. 
Since the large-sized pressure control spring is provided in the reservoir 
chamber while the spring in the pressure chamber is much smaller in size, 
any air bubbles mixed into the pressure chamber can be driven out easily. 
EMBODIMENT 2 
Since the autotensioner in the second embodiment has substantially the same 
construction as the one in the first embodiment, like parts are 
represented by like numerals. 
This embodiment differs from the first embodiment in that the cylinder 1 is 
made of an aluminum alloy, that a bottom-closed cylindrical sleeve 30 is 
disposed in the cylinder 1 at the bottom end thereof to define a chamber 
31, and that a gap (not shown) is formed between the outer periphery of 
the piston 7 and the inner periphery of the sleeve 30. 
In general, the sleeve 30 is made of steel or cast iron, whereas the piston 
7 is made of steel or copper alloy but they may be made of any other 
material provided it has good leak down properties and good resistance to 
friction. 
Between the cylinder 1 and the sleeve 30, a gap is defined which is adapted 
to remain present over the temperature range from approximately 
-30.degree. C. to 120.degree. C. This gap serves to prevent the inner 
diameter of the sleeve 30 from being influenced by the expansion and 
contraction of the inner diameter of the cylinder 1. 
The spring seat 17 has its outer periphery supported on the top end of the 
sleeve 30. The sleeve is formed on its inner periphery at its lower part 
with a shoulder 32 protruding radially and inwardly to restrict the 
downward stroke of the piston 7. The coil spring 13 is received in a 
recess defined inside the shoulder 32 to abut the bottom of the sleeve 30. 
FIG. 5 shows a modified sleeve 30 which comprises a cylindrical body 33 and 
a cap 34 press-fitted in the cylindrical body from its one end. The top 
edge of the cap 34 serves as the shoulder 32 for restricting the downward 
stroke of the piston. 
The sleeve 30 do not necessarily have to be of the bottom-closed type as 
described above but may be in the form of a mere cylinder provided a seal 
member 35 is disposed between the cylinder 1 and the sleeve 30. 
The second embodiment is substantially the same in operation as the first 
embodiment. 
Though in the preferred embodiments the pressure control spring 18 is 
mounted in the cylinder 1, it may be mounted outside the cylinder e.g. 
between the arm 21 for the tension roller 22 and a fixed member. 
The second embodiment will offer the following effects. 
By the provision of sleeve between the cylinder and the piston, the 
cylinder can be made of aluminum alloy. This will make it possible not 
only to reduce its weight to a minimum but to make it by die casting. 
The leak down characteristics are determined by the relationship between 
the materials for the piston and the sleeve, not between the materials for 
the piston and the cylinder. Thus the piston does not have to be made of 
an aluminum alloy but of any other desired material even though the 
cylinder is made of an aluminum alloy. 
Machining of the frictional surface is easier with the sleeve than with the 
cylinder because the former is shallower than the latter. 
By the provision of the gap between the sleeve and the cylinder, the leak 
down characteristics will not be influenced by change of inner diameter of 
the cylinder owing to temperature fluctuation during use.