The invention relates to an accumulator comprising:

- a main cylindrical housing having a closed first axial end and a second axial end;

- a piston arranged in the main cylindrical housing and sealed to the inner surface of the main cylindrical housing, which piston is movable in axial direction and provides with the main cylindrical housing and the closed first axial end of the main cylindrical housing a variable accumulating space;

- urging means arranged between the piston and the second axial end of the main cylindrical housing for urging the piston towards the first axial end of the main cylindrical housing;

- a fluid supply opening arranged in the first axial end of the main cylindrical housing, which fluid supply opening is in fluid connection with the variable accumulating space. Such accumulators are used in a number of applications to maintain pressure in a hydraulic system. In particular in an aircraft a number of accumulators are used to ensure proper functioning of the different hydraulic systems.

Typically, the urging means are embodied as a pressurized gas present on the opposite side of the piston, but could also be a coil spring or the like.

When such a prior art accumulator is subjected to heat, for example during operation of the airplane or in case of fire, the gas and hydraulic fluid inside of the accumulator is heated quickly via the main cylindrical housing wall. This impairs functioning of the accumulator.

It could also occur that the accumulator has to take an impact, which results in a dent or even a crack in the wall of the cylindrical housing. A dent will prevent the piston from proper functioning, as the dent will prevent full axial movement of the piston and could also cause the typical seal on the piston not to function properly, such that hydraulic fluid can flow past the piston.

Furthermore, it could occur that due to stresses in the accumulator, a weld malfunctions or the wall of the cylindrical housing cracks and pressurized gas leaks into the airplane.

One could chose to strengthen the prior art accumulators by increasing the wall thickness, however, this would also increase the weight of the accumulator, which is typically disadvantageous for an airplane.

Therefore, it is an object of the invention to reduce or even remove the above mentioned disadvantages.

This object is achieved according to the invention with an accumulator according to the preamble, which is characterized by a protective cylindrical housing having a first and second axial end, wherein the protective cylindrical housing is arranged concentrically with and spaced apart from the main cylindrical housing, wherein the protective cylindrical housing is arranged with the first axial end to the first axial end of the main cylindrical housing and wherein the second axial end of the protective cylindrical housing is closed and spaced apart from the second axial end of the main cylindrical housing.

With the protective cylindrical housing a lightweight solution is provided which protects the accumulator from heat, impacts and prevents that in case of leakage of the accumulator the fluids flow into the aircraft or other space, where the accumulator is arranged.

The protective cylindrical housing shields the main cylindrical housing from heat, because the space between both housings provides a thermal barrier.

When the accumulator according to the invention suffers an impact, the dent and possible crack will be formed in the protective cylindrical housing, while the main cylindrical housing remains unaffected due to the spacing between the protective housing and the main housing.

Furthermore, when a crack occurs due to stresses in the main cylindrical housing and the gas or hydraulic fluid would leak, then these fluids will be contained by the protective cylindrical housing.

In an embodiment of the accumulator according to the invention a hole is arranged in the protective cylindrical housing, preferably in the closed second axial end.

The hole in the protective cylindrical housing ensures that no pressure difference can occur between the ambient pressure and the pressure in the space between the protective cylindrical housing and the main cylindrical housing.

In another embodiment of the accumulator according to the invention a one way valve is arranged in the hole. The one way allows for the space between the protective cylindrical housing and the main cylindrical housing to be pressurized and be filed with a desired type of fluid.

Preferably, the space between the protective cylindrical housing and the main cylindrical housing is vacuumized. When a leak would occur in the main cylindrical housing, the vacuumized space would absorb the leaked fluid.

In yet another embodiment of the accumulator according to the invention, a fire extinguishing substance, such as an inert gas or an expansive foam, is arranged in the space between the protective cylindrical housing and the main cylindrical housing.

In case of fire the inert gas can flow out of a crack in the protective cylindrical housing to reduce the fire, while an expansive foam can provide an additional protection of the main cylindrical housing. In yet another embodiment of the accumulator according to the invention the second axial end of the main cylindrical housing is closed to provide with the main cylindrical housing and the piston pressure chamber and wherein the urging means comprise a pressurized gas arranged in the pressure chamber.

Using a pressurized gas allows for a accumulator according to the invention to be easily set to a desired counter pressure. If a spring is used, then the spring has to be exchanged when a different counter pressure is desired.

Preferably, the accumulator according to the invention comprises a fluid port extending through the closed second axial end of the protective cylindrical housing and the closed second axial end of the main cylindrical housing, which fluid port is for filling and pressurizing the pressure chamber.

These and other features of the invention will be elucidated in conjunction with the accompanying drawings.

The figure shows a cross-sectional view of an embodiment of an accumulator 1 according to the invention. The accumulator 1 has a main cylindrical housing 2, which is closed on the first axial end by an adapter plate 3, which is provided with an adapter 4 with a passage 5 for connection with a hydraulic system.

The second axial end 6 of the main cylindrical housing 2 is provided with a fluid port 7 which is sealed by a plug 8.

A piston 9 is axially movable arranged in the main cylindrical housing 2 and is provided with a seal 10 for sealing against the inner wall of the housing 2. The piston 9 divides the inner space of the main cylindrical housing 2 into an accumulating space 1 1 and a pressure chamber 12.

The pressure chamber 12 is preferably filled with a pressurized gas via the fluid port 7 and ensures that the piston 9 is urged towards the adapter 4. When the adapter 4 is connected to a hydraulic system, fluid will flow via the passage 5 into the accumulating space 1 1 and push the piston 9 towards the second end 6. This allows for the hydraulic system to maintain a certain pressure.

A protective cylindrical housing 13 is arranged concentrically to the main cylindrical housing 2. A space 14 is provided between both protective cylindrical housing 13 and the main cylindrical housing 2. The first axial end of the protective cylindrical housing 13 is sealed to the adapter plate 3, while the second axial end 15 of the protective cylindrical housing 13 is closed and has the fluid port 7 extending there through.

The closed second axial end 15 of the protective cylindrical housing 13 is provided with a hole 16, which is in fluid connection with the space 14 and allows for pressure equalization. This hole 16 could be provided with a one-way valve to vacuumize the space 14 or to provide a pressurized gas or an expandable foam in the space 14.