Hydraulic Power Unit Calibration

Hydraulic Power Units (HPUs) are portable aircraft ground support equipment that provide pressurized hydraulic energy to hydraulic motors, cylinders, and other aircraft components for maintenance and ensure leak-proof and well-functioning aircraft systems. Unlike conventional pumps, which transfer hydraulic fluid, HPUs generate hydraulic energy (flow and pressure) using multi-stage pumps and transfer the power to one or more actuators (motors or cylinders) using hoses or pipes.

HPU Working Principle:

Regardless of the type, HPUs consist of the following basic components: pump, reservoir, filter, directional valve, check valve, pressure relief valve, selector valve, heat exchanger, and an actuator. Once the HPU is started, the motor (or engine) drives the pump to transfer hydraulic oil from the reservoir through a filter (to clean the oil of particulate contamination) and then into the actuator. Flow and direction control valves control the flow parameters and direction to guide the hydraulic energy to the desired location. The pressure relief valve integrated into the HPU ensures that the working pressure is under design limits. Modern HPUs are combined with efficient coolers (heat exchangers) to control fluids’ working temperature, avoid oil oxidation, and ensure safe working conditions.

Selection and Types of HPUs:

HPUs are classified into three different categories based on their:

Size: The size of HPU is selected based on the pressure requirements and transfer flow rate capability. Broadly, HPUs are available in three different sizes:

• Micro hydraulic power units: Maximum flow rate of 5 L/min and a maximum pressure of 250 Bar. These are compact units used for small motorized machines and consume about 800W electric power.
• Mini hydraulic power units: With a Maximum flow rate of 30 L/min and a maximum pressure of 350 bar, these HPUs are used as stationary equipment or mounted on frames for portability. Generally, these HPUs are driven by motors rated less than 5.5 kW.
• Standard hydraulic units: With a Maximum flow rate of 100 L/min and a maximum pressure of 4500 Bar, these HPUs are portable systems mounted on chassis with wheels and are used for large machinery like aircraft.

Driving mechanism: Based on the Pump driver (prime mover), HPUs are classified into three types:

• Electric motor Driven: These HPU pumps are driven by AC or DC motors. AC Motors are used for fixed HPUs (workshops or industries) and DC driven Motors for HPUs mounted on vehicles (i.e., commercial vehicles or forklifts) and powered by a battery source.
• Internal Combustion (IC) engine: In applications where a continuous AC power source is not available, HPUs powered by a gasoline, diesel, or gas combustion engine are selected. IC engines range from a single-cylinder model generating 4 HP power to multi-cylinder engines, generating 100 HP power.
• Air-driven HPUs: These are common at Explosion-proof zones (ATEX), where electrical and IC engine-driven systems are restricted. Air-driven HPUs require a pneumatic circuit to operate and can generate pressure ratios up to 400:1.

Frequency of operation: According to DIN VDE 0530, HPUs are classified into three types based on their operational frequency:

• S 1: Continuous operation
• S 2: Short-term operation cycles generating little or no heat
• S 3: Intermittent operation cycles to control heating (running time as a percentage of a 10-minute operation cycle).

Need for Aircraft HPUs Inspection and Calibration:

Hydraulic Power Units are considered critical operational equipment for aircraft maintenance shops and commercial, business, and general aviation aircraft. Because of continuous aircraft schedules, HPUs are targeted to operate at zero downtime, and hence, maintenance and calibration schedules for HPUs are to be strictly followed. Society of Automotive Engineers (SAE) laid out SAE ARP1280 standards for Aerospace Hydraulic Power systems operation and maintenance guidelines, according to which the following components of the HPUs are treated critical and should be monitored and calibrated at defined intervals:

1. Pump and Motor (or diesel) assembly: Structural deficiencies of the bearings, coupling, and shaft lines should be monitored with linear vibration analysis according to ISO 10816 standards. Pumps should be periodically pressure tested along with motor windings and electrical tests.
2. Filter Maintenance: Hydraulic filters should be monitored using backpressure readings on the pressure gauge. Filter clogging due to particle contamination causes choking of the hydraulic system. Depending on the operational hours, filter elements or inserts should be replaced. Replacement filter selection should be according to the OEM standards. For achieving higher cleanliness levels, external or by-pass filters can be used periodically.
3. Hydraulic Fluid Cleanliness: According to ISO 4406 or NAS 1638 Standards, Hydraulic fluids should be maintained at NAS 5-6 cleanliness level. Higher NAS values imply heavy particulate contamination and oil oxidation, resulting in oil additive depletion and poor fluid quality. Monitoring fluid cleanliness standards using regular lab analysis or real-time oil quality monitoring sensors is essential and can avoid catastrophic damage to the HPU. Although water or moisture inclusion is rare, maintaining moisture levels less than 50 PPM is ideal.
4. Hose and Electrical Circuit Inspection: Fluid transfer hoses, connections, and power cords should be inspected for any damage, cuts, leaks, or loose fittings. At high operating pressures, minute errors can accumulate and cause catastrophic injuries.
5. Pressure Gauge and Pressure Switch Calibration: Periodic testing of pressure gauges with calibrated lab gauges ensures that the targeted pressure readings are achieved. Errors in the pressure reading can damage the aircraft equipment by directing higher pressured fluid into the aircraft components.
6. Flow meters and Flow transducer Maintenance and Calibration: Flowmeters and transducers indicate the fluid transfer volume in real time. Adjusting the flow parameters regularly to suit various aircraft hydro equipment can induce functional errors and should be calibrated to meet design accuracy.

Cumulative defects in HPUs can often result in operational uncertainties leading to Catastrophic failures, downtime, and maintenance losses. Preventive maintenance and Calibration avoid these errors, ensure components’ accuracy for sustained and repeated, and enhance the service life of HPUs. Following a regular and timely calibration schedule ensures accuracy of measurement and enhances the service efficiency and accuracy.

e2b calibration offers industry-leading ISO-certified HPU maintenance and calibration services. Our labs are ISO/IEC 17025 accredited and operated by a team of qualified calibration experts to test and calibrate your Pressure Gauges. Our verifiable services are unmatched in the industry. We are registered with ANAB. We are also ANSI/NCSL Z540-1-1994 certified. We have the NIST Traceable Wide scope of ISO/IEC 17025 accreditation. Contact e2b calibration for all your equipment calibration needs.