One of the main challenges facing engineers is to take a process which is very difficult and make its application appear very simple. The use of proprietary hydraulic equipment is no different in this respect and a closer look at the workings within the strut shows that a simple lever movement on a motorised pump, or the working of a manual hand pump, triggers a series of intricate mechanical movements worthy of further investigation.

The hydraulic ram

Commonly referred to as ‘stroke’, most Groundforce hydraulic struts have one metre of adjustment. The 3D image shows that the hydraulic ram itself is housed within the inner sleeve section of the strut casing, while the piston rod is connected to the outer sleeve: this is free to slide along the inner sleeve as the ram is pumped in or out. In order to pump out, or extend the length of the strut, hydraulic fluid is injected into the full bore side of the piston (shown in red). This causes the piston to move from left to right, which in turn causes fluid to be displaced from the area behind the piston or annulus side (shown in blue) back into the pump reservoir. Since this is connected to the outer sleeve, the strut increases in length. To allow the strut to retract, the flow on the pump is simply reversed. To demonstrate this, two schematic circuit diagrams are shown below: one for extension and one for retraction. The operator must connect both hoses between the pump and the strut to complete the hydraulic circuit. A single hose connection would prevent the flow of fluid and would result in a build-up of pressure isolated within the pump system. With a typical ratio between full bore and annulus of 2:1, this means that twice as much fluid is required to expand the strut as is required to retract the same distance. It is therefore possible to cause the pump reservoir to overflow if fully retracting several struts. Twice as much force is generated while expanding the strut when compared with retracting using the same pump pressure.

A working example

Consider, for example, that a strut is installed between two fixed parallel walls. When the strut is pumped out to the point where it makes contact with both walls, there is no room for further expansion of the hydraulic unit. With the pump lever still set to the ‘expand’ position, the fluid continues to be compressed into the full bore area of the hydraulic ram, although this time there is no increase in volume. The pressure in the strut then rises to a maximum predetermined value which is also dependent on the capacity of the pump. This is referred to as ‘installation pressure’ and is recommended to be maintained at 1000 psi (69 bar) for most ground-shoring applications. Visit the Groundforce website to find out more.