2.1: Operation of a Hydraulic Power Unit

Electric Motor

Most industrial hydraulic systems use a constant-speed electric motor to drive the pump. The motor's shaft is connected to the pump shaft through a coupling, allowing efficient transfer of mechanical power.

Hydraulic Pump

The hydraulic pump generates fluid flow. As the electric motor drives the pump, it, in a way, pulls oil from the reservoir through its inlet and forces it into the system via its outlet.

Reservoir

The reservoir serves multiple functions:

• It stores hydraulic fluid when not in use.
• It allows dirt and contaminants to settle before re-entering the system.
• It helps dissipate heat, keeping the fluid at an appropriate temperature.

Suction Line

The suction line connects the pump inlet to the reservoir, allowing the pump to draw in oil.

Supply Line (Pressure Line)

The supply line connects to the pump outlet and delivers pressurized oil to the hydraulic system. This line runs through the relief valve manifold block and exits through a port usually labeled “P”.

Sometimes, a supply line will distribute oil to a supply manifold, a machined block with multiple ports that provide hydraulic fluid to various system components.

Return Line

The return line is responsible for directing oil from the hydraulic system back to the reservoir. This ensures continuous circulation of hydraulic fluid, allowing it to be reused in the system.

Relief Valve

The relief valve is a crucial safety component that prevents excessive pressure from building up in the system. It is designed to open when system pressure exceeds a preset limit, allowing excess fluid to return to the reservoir and maintain safe operating conditions. The pressure setting can be adjusted using a control knob located on the valve body.

Suction Strainer

Located within the suction line at the pump inlet and submerged in hydraulic fluid, the suction strainer plays a critical role in protecting the pump from contaminants. It acts as a first line of defense, preventing large dirt particles from entering the pump and causing damage.

Filler/Breather

The filler/breather serves two essential functions in a hydraulic power unit:

1. Reservoir Filling – The unit features a removable cap that allows hydraulic oil to be added to the reservoir. A protective screen is often included to prevent large contaminants from entering during filling.
2. Airflow Regulation – Since the fluid level fluctuates during system operation, the filler/breather ensures that air can enter or exit the reservoir, maintaining a pressure balance close to atmospheric levels. Air passes through filtered openings in the breather cap to prevent dust and debris from contaminating the fluid.

Liquid Level Gauge

Often, there is a liquid level gauge that provides a visual indication of the amount of oil in the reservoir. There are two common types:

1. Oil level gauge – This gauge is mounted externally on the reservoir and is connected to the oil inside, providing an accurate reading.
2. Sight glass – A small, circular glass window mounted at the correct oil level on the reservoir, allowing operators to verify the fluid level at a glance.

Return Filter

Sometimes, a return filter is installed in the return line to remove contaminants that may have been picked up as the oil circulated through the system. By filtering the returning fluid before it re-enters the reservoir, it helps maintain hydraulic fluid cleanliness and prolongs the lifespan of system components.