Brakes – Internal-resistance motion retarder – With means compensating for change in temperature or viscosity
Reexamination Certificate
1998-12-07
2001-01-30
Graham, Matthew C. (Department: 3613)
Brakes
Internal-resistance motion retarder
With means compensating for change in temperature or viscosity
C188S276000, C188S317000, C236S09300A
Reexamination Certificate
active
06179099
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a piston-cylinder unit suitable for use in, for example, a tailgate of a vehicle to open the tailgate upon operation of the unit through a partial extension stroke of the piston, stop the opening at the end of the partial stroke, and upon application of a manual force to the tailgate open the tailgate beyond the partial stroke.
DE 3,301,544 A1 discloses a piston-cylinder unit in the structural form of a gas spring, which is used for opening gates capable of swinging upward, for example, the tailgates of vehicles. The special feature of the gas spring lies in that two opening angle regions are predetermined. In a first opening angle region, the gas spring, because of the compressive force acting on the cross section of the piston rod, extends automatically until the piston has reached the end of a bypass groove that connects the two working chambers which are separated by the piston.
In the region of the second opening angle region, fluid exchange is influenced by a pressure-control valve that consists of a helical spring and a valve seal. In use, the pressure-control valve can be opened by an additional external force, the manual force of the user, whereby the gate can be moved into the second opening angle region.
As soon as the gate is released, the valve seal, biased by the spring, opens a valve aperture. The gas cushion confined in the working chamber remote from the piston rod holds the gate in the desired position in the second opening angle region. If the gate is to be moved further, manual force is again required.
A basic problem, which is common to all gas springs, is that the extension force is highly temperature-dependent. There are many possible solutions to this problem. One possibility, for example, is to maintain the temperature of the gas in the gas spring at a desired value by electric heating. DE 4,421,773 A1 describes such a system. A fundamental disadvantage is that the expenditure for equipment and input of energy are of course considerable.
U.S. Pat. No. 5,404,972 describes a self-blocking gas spring with a temperature-dependent bypass valve. The temperature-dependent bypass valve contains a bimetallic spring whose elastic force increases with decreasing temperature and, in so doing, prestresses a valve in the direction of closing. Depending upon the temperature, the valve is opened progressively by the operating pressure in the working chamber on the piston-rod side in order to keep the push-out force of the gas spring as constant as possible.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a piston-cylinder unit in the structural form of a gas spring having at least two opening regions in such a way that the holding force of the gas spring is independent of temperature in the region in which the extending motion of the piston rod is released or continued by hand.
According to the present invention, the foregoing object is accomplished in that the spring of the pressure-control valve has a temperature-dependent elastic force which increases with increasing operating temperature, as a result of which the compressive force necessary to open the pressure-control valve increases correspondingly. The higher the temperature, the greater the compressive force in the direction of opening of the pressure valve. But the compressive force acting in the direction of extension of the piston rod also increases to an equal extent. Consequently, active compression and elastic forces act in the same direction, so that any manual force necessary in operation remains substantially constant and, above all, the holding force of the gas spring likewise becomes constant, independent of temperature. In the prior art, it had to be ensured that a sufficient holding force was available in the intermediate stop position even at high operating temperatures, with the result that a large manual force was required for overcoming the intermediate stop position at low temperatures. With the arrangement according to the invention, this problem no longer arises.
With a view to a simple and inexpensive structure, the temperature-dependent spring is a bimetallic spring. External energy or control devices are not needed, since skillful design of the components permits temperature and hence force compensation to be present in the usual temperature range between −40° C. and 80°.
In advantageous embodiments, the piston has a sleeve section with a base, the base having a passage which is capable of being blocked by the valve body of the pressure-control valve. A valve seat is simple to manufacture. The sleeve section in the piston has a sealing piece on which the spring of the pressure-control valve is supported. Thus, the sleeve section forms a housing for the pressure-control valve. The piston can be readily assembled, particularly when the pressure-control valve and the sealing piece are arranged in the axial direction of the piston-cylinder unit.
The spring of the pressure-control valve, like other components of the piston-cylinder unit, is frequently subjected to comparatively strong vibrations, particularly in a motor vehicle. In addition, temperature changes may produce extensions in length, which alter the position of the spring. To secure the assembly position of the spring of the pressure-control valve, the sealing piece has a slot-like guide.
Since the spring has an essentially U-shaped cross section, the sealing piece provides an additional guide surface, in the direction of the arm, for an arm of the spring which bears the valve body. The freedom of motion of the spring is thus limited to one axis.
In addition, the slot-like guide has a stop surface, on which the arm of the spring comes to rest at extremely low temperatures. The U-shaped spring is to be prevented from bending so much that the valve body falls out of its intended position.
In order to obtain as short and direct a fluid flow as possible, the sealing piece has a fluid connection to the working chamber remote from the piston rod.
It is possible also for the sealing piece to have a sealing plate which provides a stop surface for a spring damper that arrests the movement of the piston near the fully retracted position of the gas spring.
To simplify manufacture of the piston, the piston is accepted by a pin of the piston rod and fastened there. The receiving aperture in the piston has a section which provides a valve antechamber for the pressure-control valve. The added expense of a separate valve antechamber is avoided. The connection between the piston and the piston pin need not even be pressure-tight, since a little leakage has no effect on the operating behavior of the piston-cylinder unit.
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Koch Klaus
Sauer Rainer
Sch{umlaut over (u)}ttler Oliver
Baker & Botts L.L.P.
Graham Matthew C.
Stabilus GmbH
Williams Thomas J.
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