SHOCK TRANSMISSION UNIT

Abstract: A shock transmission unit has a cylinder, a piston (13, 2) within the cylinder, dilatant material (47) within the cylinder on either side of the head of the piston, and a valve (44, 13) operable to control the passage of the dilatant material (47) within the cylinder from one side of the piston head (13, 2) to the other. The valve (44, 13) may be responsive to temperature, in order to compensate for the change in viscosity of the dilatant material with temperature. The valve may be responsive to the pressure differential across the piston head, allowing passage of the dilatent material from one side of the head to the other in response to a pressure differential below a given threshold.

Description: SHOCK TRANSMISSION UNIT A shock transmission unit (STU) acting between two force transmission locations comprises a piston and cylinder unit, with space on either side of the piston being for a viscous material. A special form of viscous material is a dilatant material which is non-Newtonian i.e., viscosity is not independent of shear rate. Its viscosity rises with increasing rate of shear - the harder it is stirred the more viscous it gets. At a certain shear rate, the dilatant material becomes effectively solid. This should not be confused with thixotropic materials (where viscosity decreases with the time for which shearing forces are applied) or rheopectic materials (viscosity increases with the time for which shearing forces are applied) . On the piston and cylinder unit, there is one force transmission location at the closed end of the cylinder and another at the end of the piston extending from the cylinder. If the viscous material cannot escape past the piston then the unit is locked solid and forces are transmitted without loss between the two ends of the unit. If the material can escape from one side of the piston to the other within the cylinder there will be some loss of transmission between a force applied at one force transmission location and a force transmitted from the other since movement of viscous material will allow movement of the piston within the cylinder. Movement of the viscous material past the piston takes place at a rate increasing with applied load. A sustained load thus results in the piston travelling through the material until it reaches the limit of its available travel. The present invention is concerned with the problem of ensuring that the piston only travels through the viscous material under sustained load conditions less than a given threshold. This threshold might be set at a load imposed by normal thermal movements of the structure the STU is protecting. As presently envisaged the invention provides a shock transmission unit comprising a cylinder, a piston within the cylinder, dilatant material within the cylinder on either side of the head of the piston, and a valve operable to control the passage of the dilatant material within the cylinder from one side of the piston head to the other. The valve may comprise a hollow cylindrical valve member movable into and out of a cylindrical bore extending through the piston, the valve member having its end directed away from the piston closed and a passage through the cylindrical wall adjacent the closed end which allows viscous material to pass through the passage and the valve bore when the closed end is spaced from the wall of the piston, the valve member being biased towards its open position. The valve will close when the passage of viscous material into the bore through the valve reaches a given maximum rate since then the differential pressure on the closed end of the valve exceeds the bias keeping it open. A said hollow cylindrical valve member may be placed in each end of a cylindrical bore through the piston, for two- way operation. A plurality of cylindrical bores may be provided in the piston, each with a cylindrical valve member. The rate of movement of dilatant material past the piston head also depends on its viscosity, which varies with temperature. In order to provide a STU which performs as uniformly as possible as the temperature changes, the valve may be adjustable in response to temperature. The valve may be responsive to the expansion of a volume of dilatant material, preferably isolated from the main body of dilatant material. The valve may be mounted on the periphery of the piston and movable radially relative to the periphery of the piston so as to control the cross- section of the passage for the viscous material. The valve preferably comprises piston-ring segments. Alternatively the passage may extend through the cylinder wall. Examples of the invention will now be described with reference to the accompanying drawings in which Figure 1 is a diagrammatic view of a STU, Figure 2 is a plan of the piston of the STU of Figure 1, Figure 3 is a section on line A-A of Figure 2, Figure 4 is a plan of an alternative piston for use in the general arrangement of Figure 1, Figure 5 is a section on lines A-A of Figure 4, and Figure 6 is a longitudinal section through another STU embodying the invention. In Figure 1, a cylinder 1 comprises a main chamber within which a piston 2 on a shaft 3 can move axially. The chamber is closed at one end by the wall 4 of the cylinder and at the other end is closed by a seal 5 between the bore of an aperture in the cylinder and the shaft 3. A first force connection lug 6 is provided on the exterior of wall 4 and a second such lug 7 is provided on the free end of the shaft 3. Within the chamber is dilatant material on both sides of the piston which divides the chamber into two portions 8, 9. The central region of the piston 10 is formed with an aperture 11 within which is received the shaft 3 of the unit, the piston being fixedly mounted on the shaft. The piston is of a size such that the clearance between the piston and cylinder is only that necessary for the two parts to be free from one another. The piston contains paths for passage of dilatant material from one portion of the chamber to the other each path comprising a cylindrical bore. If all such paths are closed off, then any forces applied to the input terminal are transmitted directly to the output terminal at the other end of the unit with no movement of the piston since the dilatant material cannot flow past the piston and the material is substantially incompre-ssible . When the cross- section of the paths is non-zero then dilatant material can travel between the two portions of the cylinder allowing motion of the piston as a result of the applied load. The response of the unit depends on the presence or otherwise of the paths. As shown in Figures 2 and 3, paths in the form of a plurality of cylindrical bores 12 extend through the piston 2 one plane face to the other. Each bore has a valve member movable into and out of the bore, the member having a hollow cylinder with one or more apertures in its cylindrical wall near the end which can protrude from the end of the bore, this end being closed by a conical cover. An axial threaded rod 18 is mounted in the bore 15 on a perforated cross-wall 17, carrying a spring 16 to bias the member 13 outwards from the piston and a nut 19 to limit the outward travel of the valve member 13. The Lock- Up-Device is in a quasi-static condition when the load applied to the device is very low (this condition arises when the structure into which the LUD is built is subject to thermal movement) ; in this condition the spring 16 maintains the valve 13 in the open position. This allows the dilatant material to flow through the apertures 15, down the hollow cylindrical portion of the valve and through the bore 12 and the perforated wall 17 and thus to the other side of the piston. In this condition the piston 2 is able to move down the cylinder to accommodate the thermal movement of the parent structure. When a force external to the structure is applied greater than forces generated by thermal movements, for example as a result of an earthquake acting on a building structure, the bias of the spring 16 is overcome due to the pressure increase in the dilatant material which applies a force to the valve member greater than that of the spring and the valve slides down towards perforated wall 17 and the apertures 15 are closed off. In this condition the dilatant material is prevented from passing from one side of the piston to the other thus the piston is constrained within the cylinder. Removal of the externally applied load results in the release of pressure in the cylinder with the consequential opening of the valve and the quasi-static condition is restored. Suitable choice of components 13-19 enables the sensitivity of the arrangement to be adjusted. The effect of the device so far described is to allow small relative movements between two bodies across which the lugs 6 and 7 are connected, but when one body tends to move relative to the other body with a greater amplitude than a given threshold, the STU locks up and ensures that the whole amplitude of movement of one body is transmitted to the other body. Considered in terms of a building structure such as a bridge across a valley, an STU connected between two panels of the bridge deck allows small thermal movements between those panels but if an earthquake for example tends to move one bridge deck panel by a large amount, the whole deck will lock up as one integral panel and the foundations of the whole structure can be relied upon to hold the bridge deck in place. The differential pressure on the closed end of the valve in the STU at which the valve closes to passage of dilatant material therethrough is in practice much lower than the pressure which would have to be reached before the rate of shear of the dilatant material passing through the bore 12 would reach the value at which the dilatant material becomes effectively solid, at which point the dilatant material becomes its own valve. Because these pressures are different, the separate valve is required. The above-described apparatus has been tested with viscous materials which are not dilatant, for example silicone oil, and the tests have proved satisfactory. In the alternative piston of Figure 4, the piston head 2 is formed with an aperture 23 within which is received the shaft 3 of the unit, the piston head being fixedly mounted on the shaft. An annular groove 34 extends around the periphery of the piston within which are mounted two semicircular piston-ring segments 35. The segments are hinged to the piston at one end 36 to move radially into and out of the groove 34. A plurality of radial passages 37 are drilled in the piston ring segment in order to allow passage of any dilatant material trapped between the piston-ring and the base of the groove to escape when the piston-ring is hinged towards the base of the groove. One such passage is illustrated in Figure 5. Two cavities 24, 25 forming approximately quarter toroids concentric with the shaft 3 are formed within the piston head, closed at one end and extending through a cylindrical chamber 26 to the periphery of the piston head at the other. Within the cylindrical chamber 26 is mounted a piston 27 forming a close seal with the bore of the cylinder. The pistons 27 are provided with O-ring seals 41 to improve the seal with the cylinders 26 so as to maintain the dilatant material within the cavities 24, 25 isolated from the exterior of the piston 13. The outer end of the piston is formed with a clevis 31 to which a free end 32 of a piston-ring segment 35 is attached. At its end facing the cavity, the piston is provided with an eye 28 which is engaged by a coil spring 29 extending to a fixed anchorage 33 within the cavity, biasing the piston 27 towards the closed end of the cavity. The coil springs 29 assist the return of the pistons 27 within the cylinders 26 as the temperature falls and the dilatant material within the cavities contracts. The dilatant material within the cavities 24, 25 expands from a first volume at a lower temperature to a second volume at a higher temperature and its expansion drives the piston 27 within the cylinder 26 towards the periphery of the main piston 2, thus hinging the piston- ring segments 35 outwards from the periphery of the main piston towards the inner wall of the cylinder, thus reducing the cross-section of the passage between the periphery of the piston and the cylinder through which dilatant material can pass from one side of the piston 2 to the other. Stops 38 are provided between the free ends 39 of the piston-ring segments and the hinged end 36 of the adjacent piston-ring segment to prevent passage of dilatant material through the intervening region. As the temperature increases, the cross-section of the passage through which dilatant material can pass from one portion of the chamber to the other is reduced by the outward hinging movement 'of the piston-ring segments. The reduction in cross-section of the passages compensates for the reduction in viscosity with increasing temperature of the material so that any changes in the transmission of forces between the two ends of the unit caused by the decrease in viscosity due to the increase in temperature is compensated for by the reduction in the cross-section of the passageways for movement of the material between the two sides of the piston in response to the same increases in temperature. In this way by a suitable choice of dimensions of components 24-36, it has been found possible to provide a uniform response to transmission of steady loads over a wide range of temperatures, such as from -25° to +40° centigrade. In Figure 6 the passage through which dilatant material can flow from one side of the piston head 13 to the other extends through the cylinder wall 41 and comprises conduits 42, 43 outside the cylinder. These conduits meet at a temperature sensitive valve 44 which controls the cross-section of the passage. The valve can be of any suitable type such as a gate valve or needle valve and Figure 6 illustrates a needle valve. The needle closure member 45 for the valve extends into a chamber 46 filled with dilatant material 47. A return spring 48 tends to return the valve to a mid-point position against the action of the dilatant material as the temperature changes. As in the first embodiment, the purpose of varying the cross-section of the connecting passage is to adjust the total flow path between the two sides of the piston to compensate for viscosity changes to provide a uniform response to transmission of steady loads over a wide range of temperatures, such as from -25° to +40° centigrade.

Claims: CLAIMS 1. A shock transmission unit comprising a cylinder, a piston within the cylinder, dilatant material within the cylinder on either side of the head of the piston, and a valve operable to control the passage of the dilatant material within the cylinder from one side of the piston head to the other. 2. A unit as claimed in claim 1 wherein said valve is adjustable in response to temperature. 3. A unit as claimed in claim 2 wherein said valve is responsive to the expansion of a volume of dilatant material. 4. A unit as claimed in claim 3 wherein said volume of dilatant material is isolated from said dilatant material located on either side of the piston head. 5. A unit as claimed in any one of claims 1 to 4 wherein said valve is mounted on the periphery of the piston and movable radially relative to the periphery of the piston so as to control the cross-section of the passage for the viscous material. 6. A unit as claimed in claim 5 wherein the valve comprises piston ring segments, said passage being located between the piston head and the cylinder. 7. A unit as claimed in any one of claims 1 to 4 wherein the valve comprises a hollow cylindrical valve member movable into and out of a cylindrical bore extending through the piston, the valve member having its end directed away from the piston closed and a passage through the cylindrical wall adjacent the closed end which allows viscous material to pass through the passage and the valve bore when the closed end is spaced from the wall of the piston, the valve member being biased towards its open position. 8. A unit as claimed in claim 7 comprising a said hollow cylindrical valve member in each end of a cylindrical bore through the piston, for two-way operation. 9. A unit as claimed in claim 7 or claim 8 wherein the piston head is formed with a plurality of said cylindrical bores, each with a said cylindrical valve member. 10. A shock transmission unit as claimed in claim 1 substantially as herein described with reference to the accompanying drawings.

Inventors: TOWNSEND NIGEL HUGHES, TOWNSEND, NIGEL, HUGHES
Applicants: COLEBRAND LTD, TOWNSEND NIGEL HUGHES, COLEBRAND LIMITED, TOWNSEND, NIGEL, HUGHES
Application year: 1998
Main classification: F16F9
IPC codes: F16F9