Stationary Hydraulic Power

Introduction

A stationary hydraulic power installation is similar to a "shop air" compressed air system. Typically a central power unit (pump) provides pressurized hydraulic fluid to a plumbing system, which distributes it to multiple points of access around a building. While this type of installation is relatively uncommon in current practice, it has potential advantages in the GVCS ecosystem. This page is an exploration of stationary hydraulic power implementation issues.

Benefits

• The same hydraulic-powered tool with quick-connect hose fittings can be used
  • in the shop,
  • in a remote location with a Powercube, or
  • as an accessory to the LifeTrac.
• Shop tools benefit from the high power density of hydraulic actuators

Design Principles

Constant Pressure

In contrast to simple mobile hydraulics (like the LifeTrac design), a stationary hydraulic power system must support a number of independent tools which can be running simultaneously (e.g. used by different people) without interfering with each other. This requires a constant-pressure, variable flow system, rather than the constant flow approach which is common in small mobile machinery.

Constant-Pressure pump systems

The most common industrial approach to making a constant-pressure hydraulic power unit is to employ a variable-displacement pump driven by a constant-speed electric motor. Vane pumps, and axial piston pumps (with adjustable swash plates), are popular designs. However, alternative approaches are possible, including

• fixed-displacement gear pump on a variable-speed motor
• cyclic (on/off) pump action charging a pressure reservoir ("hydraulic accumulator")
  • this is similar to an air compressor cycling on and off to keep a shop air tank up to pressure
  • a continuously-running gear pump may be intermittently connected to the accumulator, through an automatic valve under pressure-sensor control (this is similar to an "unloader" valve commonly used on large shop air compressors). A standard Powercube can be connected in this mode.
• A steam engine with variable valve timing can adapt its output speed and power dynamically to a varying hydraulic system load.

It should be noted that a hydraulic accumulator operates at a constant high pressure (e.g. 2500 psi) and is therefore a heavier and more dangerous tank than a compressed-air tank.

When the system is in "standby" (nobody is using hydraulic power at the moment), the lines remain pressurized. As long as the total system leakage is small, the main pump can be turned off and the accumulator will maintain pressure. When a tool is connected and fluid begins to flow, the pressure will drop slightly and the pump will turn on.

Constant-Pressure Tools

For use on a constant-pressure shop supply, a tool's flow path should be blocked (no flow) when it is not in use. In contrast, on a constant-flow mobile system, a tool's flow path is open, through the "power beyond" valve gallery to tank return, when it is not in use. This is a concern for tools which incorporate their own valving; for example, it may be necessary to make a porting change in order to connect a CEB to the constant-pressure shop supply line. On the other hand, a simple actuator which connects to the LifeTrac hydraulic PTO (and expects to be controlled by the LifeTrac PTO control valve) will operate equally well from an appropriate "shop" control valve connected for constant-pressure supply.