771,440. Dashpots. MINISTER OF SUPPLY. May 5, 1954 [Feb. 6, 1953], No. 3476/53. Class 108(3). [Also in Groups XVII and XXIX] A device adapted to form a coupling between two members relatively displaceable under load, is capable of effecting a degree of control over the relative displacement of the members dependant upon the nature of the loading, the device having incorporated therein means effective on the one hand to allow the device to extend freely, and contract in length under light loading, and, on the other hand, to lock the device rigid and render it substantially inextensible under heavy compressive loading, in such a manner that on release of the heavy compressive loading the response by free extension and contraction of the device to light loadings is restored. The device comprises a piston C, Fig. 9, having an eye C1 for connection to one of the two members, and slidable in a liquid filled cylinder B having an eye B for connection to the other of the two members. The cylinder B is maintained full of liquid at a predetermined low pressure by the spring-loaded piston F in a replenishment cylinder E. A valve assembly D is provided, and includes a selector valve G, a manually operated control valve K, an automatic control valve H, (.see Group XXIX) and a release valve L, comprising a sleeve L3. The selector valve G may be set to one of two alternative positions by means of a key. When in the first position (shown) and the valve K is screwed in to its innermost extreme position, the end B4 of the cylinder B is connected by duct A4, duct G9 in the periphery of the valve G, duct A7 and the spring housing of the release valve L, to the replenishment chamber E. The other end B3 of the cylinder is connected by duct A2, duct A6, a U-shaped duct G11 in the valve G3, and duct A9, to one end of the housing of the automatic control valve H. Under light loads, the valve H provides restricted flow communication through a fixed orifice H3, and an open mushroomshaped valve H15 to connect with the chamber E. The latter is connected with the duct A2 leading to the end B3 of the cylinder B, by a spring-loaded non-return valve M mounted adjacent the release valve L. Thus under light tension load, liquid passes via the valve M to the end B3 of the cylinder B, and liquid is replenished from the cylinder E, to make up for the movement of the piston rod C2 out of the cylinder B, by also flowing through the valve M. On compression, the flow takes place as shown by the stippled shading in Fig. 9, but is able to pass freely through the valve H15 provided the load does not exceed a predetermined amount. The liquid then joins replenishment liquid from the chamber E to pass via the spring chamber of valve L, peripheral duct G9, valve G3, and duct A4 to the end B4 of the cylinder B. Under a heavy compressive loading, pressure develops in a chamber H at the left hand end of the housing of the valve H, drives a sleeve H2 against a spring H17 to cause its end to engage in closing relation with the valve H15. This results in a hydraulic locking of the piston C. Under a predetermined extreme load, however, liquid pressure in the housing M1 of the non- return valve M, acts on the left-hand annular end-surface of the sleeve L3, to displace it against the action of its spring L4. This provides a release flow back to the end B4 of the cylinder B by the path described above, leading from the spring chamber of the valve L. When the valve G3 is turned to its alternative position, Fig. 10, the duct A2 from the end B3 of the cylinder B, and the housing M1, are connected to a duct A10. The duct A4 from the end B4 of the cylinder B, is connected to a duct A11. The ducts A10 and All are closed by the valve K in its present position, and the piston C is thus locked against tension movement. A plate G5 may be annularly displaced so that a hole in the plate lies in register with the squared end spindle of the valve K to enable the latter to be outwardly unscrewed by means of a socket provided on a key, Fig. 3 (not shown) also used to manipulate the valve G3. The valve K may then be unscrewed until it reaches the position shown in Fig. 11, in which it interconnects the ducts A10 and All, and connects them both to a duct A12 connected via a peripheral groove in the valve G3 to the spring housing of the valve L, and thence to the chamber E. The ports in the valve K which connect with the ducts A10 and A11 are of tapered configuration, so as to provide a progressive action. This manipulation of the valve K thus provides a progressive manual release of the hydraulic lock produced by setting of the valve G3 to the position shown in Figs. 10 and 11. The piston C has a degree of lost motion relative to the piston rod C2, and the latter has a lug C6 which may be rotated into engagement with a lug B5 on the cylinder B. This retains the piston in its retracted position, when the device is not intended for use. The invention is described as applied to the connection of a cantilever bridge section to a pontoon (see Group XVII).