A valve device is provided for use in a pressurized hydraulic fluid path, for example the hydraulic path of a hydraulic leg of a mine roof support which is required to yield under two different sets of conditions. The valve device has a port 15 for connection to the pressurized hydraulic fluid path, a bleed valve 25 being arranged to permit hydraulic fluid to bleed from the path at a first rate when the pressure in the path exceeds the first limit, for example to permit the hydraulic leg to coll...

A positive displacement pump system provides first and second delivery 11, 12 passages of which the first passage 11 communicates with a main discharge passage 15 containing a discharge orifice 17. The second delivery passage 12 opens to a valve bore and is in permanently open communication with the first delivery passage through an axial passage 29 in a valve member 20 slidably mounted in the valve bore. Lifting of the valve member with increasing pressure drop across the orifice 17 places the ...

A valve assembly is provided for use with a hydraulic mine roof support, the assembly having uni-directional control means 49 interconnecting first and second pressure sectors of the valve assembly, and further control means 20 embodied in the first pressure sector which further control means is arranged for activation by fluid pressure in the first pressure sector to allow pressure fluid to flow from the second pressure sector to the first pressure sector until the fluid in the first pressure s...

A positive displacement pump system has two delivery passages communicating under the control of a valve with a main discharge passage and/or an overspill duct. The valve is constituted by a valve member slidably mounted in a valve bore and the valve member has lands and intermediate recesses controlling communication between the delivery passages and the overspill duct. The pressures upstream and downstream of a discharge orifice in the discharge passage are applied to the valve member in oppos...

A positive displacement pump system has first and second delivery passages for the pumped fluid and a main discharge passage. A control valve operating in dependence on the pressure drop across an orifice in the discharge passage diverts through overspill porting a proportion of the flow through the second delivery passage, the remainder added to the flow from the first delivery passage and passed to the discharge passage. The control valve also regulates the by-passing of a proportion of the fl...