hydraulic power unit

Understanding Hydraulic Power Units (HPUs)

Hydraulic Power Units, also known as hydraulic power packs, are self-contained systems comprising a motor, fluid reservoir, and a pump. Their primary function is to apply the hydraulic pressure required to drive motors, cylinders, and other components within a hydraulic system.

Operation of Hydraulic Power Units

In a hydraulic system, enclosed fluid is employed to transfer energy, generating rotary motion, linear motion, or force. Hydraulic power units facilitate this fluid transfer by providing the necessary power. Unlike conventional pumps, hydraulic power units utilize multi-stage pressurization networks and often integrate temperature control devices. The performance of a hydraulic power unit is influenced by factors such as pressure limits, power capacity, reservoir volume, size, power supply, and pumping strength.

Components of Hydraulic Power Units

  1. Accumulators: These containers, attached to hydraulic actuators, collect fluid from the pumping mechanism, aiming to build and maintain fluid pressure to supplement the motor pumping system.
  2. Motor Pumps: Hydraulic power units can be equipped with a single motor pump or multiple devices, each with its accumulator valve. In a multiple pump system, usually, only one operates at a time.
  3. Tanks: Designed as storage units with sufficient volume, tanks allow fluid drainage from pipes, and actuator fluid may sometimes need draining into the tank.
  4. Filters: Positioned along the top of the tank, filters are self-contained bypass units with their motor, pump, and filtering apparatus. They can be used to fill or empty the tank by activating a multi-directional valve, and often replaced while the power unit is functioning.
  5. Coolers and Heaters: As part of the temperature regulation process, an air cooler can be installed near or behind the filter unit to prevent temperatures from exceeding operational parameters. Additionally, a heating system, such as an oil-based heater, can be used to elevate temperatures when necessary.
  6. Power Unit Controllers: The hydraulic controller unit serves as the operator interface, containing power switches, displays, and monitoring features. It is crucial for installing and integrating a power unit into hydraulic systems and is typically wired into the power unit.

Design Characteristics of Hydraulic Power Units

A robust hydraulic power unit designed to function under diverse environmental conditions incorporates several design features distinct from a typical pumping system:

  1. Accumulators: Essential containers attached to hydraulic actuators, collecting fluid to build and maintain supplemental pressure.
  2. Motor Pumps: Hydraulic power units may feature either a single motor pump or multiple devices, each with its accumulator valve.
  3. Tanks: Serving as storage units with adequate volume for fluid drainage from pipes, including actuator fluid.
  4. Filters: Typically installed atop the tank, self-contained bypass units with motor, pump, and filtering apparatus.
  5. Coolers and Heaters: Temperature regulation includes air coolers and heaters to prevent overheating or elevate temperatures when necessary.
  6. Power Unit Controllers: Operator interface containing switches, displays, and monitoring features, crucial for installation and integration.

Understanding these components and design characteristics provides insight into the operating principles and effectiveness of hydraulic power units in industrial hydraulic systems.

Choosing the Right Hydraulic Power Motor

The motor, often referred to as the prime mover, plays a critical role in hydraulic power units. Its selection is based on considerations of speed, torque level, and power capacity. An appropriately sized motor, aligned with the hydraulic power unit’s specifications, not only minimizes energy wastage but also enhances long-term cost-efficiency.

Motor Selection Criteria

The criteria for motor selection depend on the type of power source utilized. For instance:

  • Electric Motor: An electric motor exhibits initial torque significantly greater than its operating torque.
  • Diesel and Gasoline Motors: These motors offer a more consistent torque-to-speed curve, delivering a steady amount of torque across both high and low running speeds.

It’s crucial to match an internal combustion engine properly with the hydraulic power unit to ensure it can initiate a loaded pump and provide sufficient power to reach operating speed.

Motor Size

When selecting a motor size, certain parameters need consideration:

  • Horsepower: A critical factor in determining the motor’s power.
  • Gallons per Minute: Reflects the liquid flow rate.
  • Pressure (psi): Represents the pumping pressure.
  • Mechanical Pumping Efficiency: Indicates the motor’s efficiency in converting power to pump fluid.

As a general rule, the power rating for a diesel or gasoline motor used with a hydraulic power unit should be at least double that of an electric motor for the same system. While the cost of electricity consumed by an electric motor over its lifespan might exceed the motor’s initial cost, finding an appropriately sized unit is crucial to avoid energy wastage.

In cases where the hydraulic system requires varying pressure levels during different pumping stages, horsepower can be calculated using the root mean square (rms), potentially allowing for a smaller motor. However, this smaller motor must still meet the torque requirements for the highest pressure level in the cycle. Once the rms and maximum torque are determined, including initial and operational levels, they can be cross-referenced with a motor manufacturer’s performance charts to verify if the motor is the right size for the application.

Electric and Internal Combustion Motor Power in Hydraulic Systems

Electric Motor Power: Electric motors and internal combustion engines, such as diesel or gasoline engines, display distinct torque characteristics that influence their power capacities. In a typical three-phase electric motor, the operating sequence begins with the rotation of the rotor. As the rotor accelerates, the torque level experiences a slight drop before increasing again at a specific RPM rate. This temporary drop is referred to as “pull-up torque,” while the maximum value is known as “breakdown torque.” Once the rotor speed exceeds the breakdown level, torque decreases steeply. The torque-to-speed curve of an electric motor remains relatively consistent regardless of power capacity. It is typically run at full-load speed but below the breakdown point to minimize the risk of stalling.

Gasoline and Diesel Motor Power: Internal combustion motors exhibit a notably different torque-to-speed curve with fewer fluctuations. Generally, diesel and gasoline motors need to operate at higher speeds to achieve the necessary torque for powering a pump. An internal combustion engine typically requires a horsepower rating approximately two and a half times greater than that of an electric motor counterpart to reach the torque levels necessary for a hydraulic power unit. Manufacturers often recommend continuous operation at only a portion of the maximum rated power to prolong the motor’s lifespan, and maintaining torque below the maximum level can enhance fuel efficiency.

Operating Process of Hydraulic Power Units: When a hydraulic power unit initiates its operation, the gear pump draws hydraulic fluid from the tank and transfers it to an accumulator. This process continues until the pressure within the accumulator reaches a predetermined level. At this point, a charging valve switches the pumping action to start circulating fluid. The pump releases fluid through a charging valve back into the tank at minimal pressure. A one-way valve prevents fluid from flowing out of the accumulator, but if the pressure drops significantly, the charging valve reactivates, refilling the accumulator with fluid. Further along the line, a reduced-pressure valve regulates the flow of oil to the actuators.

If the accumulator features a fast-stroking device, it can be connected to other accumulators to allow them to charge pressure simultaneously. An automatic thermostat or fan is often included to alleviate rising temperatures. In case of overheating, a temperature switch can shut off the motor-pump, aiding in refilling the tank if the fluid level is too low. Hydraulic power units with multiple motor pumps may use a flow switch to alternate pumps in case of reduced fluid supply. Pressure switches regulate accumulator pressure, and a monitoring system alerts operators when pressure drops too low, indicating an elevated risk of power unit failure.

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