This invention relates to a hydraulic or pneumatic brake of the kind comprising two chambers which are filled with a brake fluid and are separated from each other by a movable wall while communicating with each other through, on the one hand, a comparative¬ ly wide passage having a non-return valve allowing a substantially unimpeded fluid flow in one direction and, on the other hand, a throttled passage allowing fluid flow in the opposite direction under considerable resistance.

Brakes or dampers of this kind are known in numerous designs, in which the throttled passage is constituted by an ordinary straight and smooth bore, the through-flow capacity of which may possibly be reduced by means of a stationary or movable needle that narrows the free area of flow. Said flow area may also be variable by means of a slender, conical needle that is adjustable in its longitudinal direc¬ tion. Unlike these known devices, the brake according to the invention is characterized in that the throttled passage is constituted by a helical groove formed between a threaded surface and a cylindrical or conical surface in close contact with the tops of the threads. In this case, for a given extent in the axial direction the throttle passage may have an extremely great length whereby the throttle resistance is corres¬ pondingly increased when the same sectional area is concerned. This means that the accurate size of said area has not the same critical significance as in the case of the known designs.

According to circumstances, a single or a multiple thread may be used, and this thread may either

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be an internal one cooperating with a plug having a smooth, circumferential surface, or an external thread on a plug inserted in a plain hole or bore. Moreover, the fineness of the thread may obviously be chosen according to wishes.

It is a particular advantage that the effective length of the throttle passage can easily be reduced by putting one or more thread windings out of operation, for instance by turning off or filing off at least parts of the threads at one end or by short-circuiting one or more windings between the ends.

An embodiment of a hydraulic brake according to the invention, and in which the two chambers are provided in a cylindrical housing and in a piston-like tubular slide therein, resp. , is more specifically explained in the following with reference to the accompanying drawing that illustrates an axial section through the brake concerned in its neutral position or position of rest. The illustrated brake comprises a stationary housing 1 forming a cylindrical chamber 2 receiving a piston-like, tubular slide 3, the bore of which is divided into two chambers 5 and 6 by means of a plunger 4. 7 designates O-rings to provide tightness between the housing 1 and the outer surface of the slide 3 and between the inner surface of said slide 3 and the plunger 14, respectively. Through a side port 8 there is free passage to the atmosphere from the chamber 6 which houses a possibly adjustable spring 9 that urges the plunger 4 in the direction to the left.

A plug 11 with an external thread 12 is pressed into a circumferential recess 10 in the left end of the slide 3 with the tops of the thread in contact with the smooth wall of the recess 10. Said plug 11 thus separates the two chambers 2 and 5 that, are supposed to

be filled with a brake fluid, such as oil, and between which the groove of the thread 12 forms a long throttle passage which is triangular in cross-section. The two chambers 2 and 5 further communicate with each other through a rather wide channel 13 bored through the plug 11 and provided with a non-return valve 14 allow¬ ing a substantially unimpeded fluid flow in the direc¬ tion from chamber 5 to chamber 2. In the shown embodi¬ ment the communication from the thread groove 12 to the chamber 5 is ensured by one or more slots 15 at the edge of the inner end of the plug 11.

When the slide 3 is urged in the direction indicat¬ ed by the arrow, then oil from the chamber 2 will flow through the throttle passage formed by thread groove 12— o- the chamber 5 so as to displace the slide 3 slow¬ ly to the left, and plunger 4 will simultaneously retreat, i.e. in the direction to the right, in relation to the slide 3. This movement continues till the slide 3 has reached the bottom of the chamber 2 or till equilibrium has arisen between the pressure exerted by spring 9 on the plunger 4 and the oil pressure on the left end surface of said plunger. When the force acting upon the slide 3 ceases, the spring load on the plunger 4 will force the oil to return from the chamber 5 to the chamber 2, namely through the channel 13 with the non-return valve 14 allowing a quick return move¬ ment of the slide 3.

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