Comprehensive Study Guide: Fundamentals of Power Hydraulics and Systems

Introduction to Hydraulics and Scientific Foundations

Definition: Hydraulics is defined as the science of the practical applications of liquids, specifically focusing on the utilization of their movement and flow.
Academic Context: Hydraulics is closely related to fluid mechanics, which serves as the fundamental theoretical basis for hydraulic applications.
Basic Functionality: The discipline revolves around leveraging liquid flow to transmit power and control movement within various mechanical systems.

Advantages and Disadvantages of Power Hydraulics

Advantages of Hydraulic Systems: - Higher Force Generation: Capacity to produce significantly larger forces compared to other systems. - High Operational Reliability: Potential for high operational certainty and dependability during use. - Movement Precision: Greater precision in motion control due to the lack of compressibility of the working medium (liquid). - Low-Speed Control: Capability to achieve and maintain very small movement speeds. - Silence: Inherent lack of significant noise during operation. - Stepless Regulation: Allows for the infinitely variable (stepless) change of speed and motion. - Ease of Automation: Systems are naturally conducive to automated control integration. - Overload Protection: Simplistic implementation of safety mechanisms to protect the system from structural or functional overloading.
Disadvantages of Hydraulic Systems: - High Component Costs: The initial investment for hydraulic elements is typically expensive. - Sensitivity to Contamination: Extremely vulnerable to dirt or impurities within the working fluid. - Sensitivity to Aeration: Performance is compromised if air enters the system (aeration). - Temperature Requirements: Often requires the maintenance of a constant operating temperature for the medium. - High Manufacturing Precision: Components require exceptionally high manufacturing tolerances and precision. - Inevitable Leakage: The system is prone to unavoidable internal and external leaks. - High Operator Qualifications: Requires personnel with high levels of specialized training and skills. - Health and Safety (BHP) Risks: Presents greater safety hazards compared to some other energy transmission methods. - Power Losses: Experience of power loss during the flow of the working medium throughout the installation.

Practical Applications of Power Hydraulics

Construction: Used for controlling excavator arms, loaders, lifting systems in cranes, bulldozers, and rollers.
Heavy and Manufacturing Industry: Application in hydraulic presses, injection molding machines, and assembly lines.
Agriculture and Forestry: Essential for agricultural machinery, combine harvesters, and forestry machinery (such as harvesters).
Transport and Logistics: Utilized in forklifts, loading tail lifts, and power steering systems.
Aviation and Maritime Economy: Integration in aircraft systems and various ship mechanisms.
Rescue Services: Critical component in hydraulic spreaders and cutters used by emergency responders.

Hydraulic Power Units (HPU): Components and Parameters

Functional Purpose: These devices generate high liquid pressure to obtain large working forces and ensure efficient, smooth control of actuators within a system.
Primary Components of an HPU: - Hydraulic Pump: Acts as the generator of the liquid stream. - Electric or Internal Combustion Engine: Provides the mechanical drive to power the pump. - Filters: Necessary for maintaining the appropriate cleanliness level of the working medium. - Valves and Distributors: Direct the flow of liquid to specific parts of the circuit. - Gauges (Manometers and Flowmeters): Provide information regarding system parameters. - Tank (Reservoir): Serves as the storage location for the hydraulic oil. - Coolers and Heaters: Used to maintain the medium at a constant temperature. - Pressure Limiting Valves: Act as a primary safety and protection element.
Key Parameters of Power Units: - Flow Rate/Efficiency: Measured in $\text{m}^3/\text{s}$ or $\text{L/min}$ . - Maximum Discharge Pressure: Measured in $\text{MPa}$ . - Tank Capacity: Measured in $\text{L}$ . - Engine Power: Measured in Watts (W). - Efficiency: Measured as a percentage ( $\%$ ). - Physical Characteristics: Mass measured in $\text{kg}$ and geometric dimensions in $\text{mm}$ .
Compact vs. Stationary Units: - Principle of Operation: Identical for both types. - Stationary Units: Characterized as large and heavy, permanently mounted on non-mobile machinery. - Compact Units: Optimized for minimal mass and dimensions, used primarily in mobile equipment.

Hydraulic Pumps: Classification and Operating Principles

Core Concept: The pump is the "heart" of the system, responsible for converting mechanical energy (rotary motion) into hydrostatic energy.
Positive Displacement Principle: All hydraulic pumps work via positive displacement, involving sealed and closed chambers within the body. A vacuum is created at the inlet to suck in oil, which is then displaced into the discharge channel.
Classification by Flow Capacity: - Fixed Displacement Pumps: Delivery depends solely on unit volume (oil taken per single revolution) and the engine's rotational speed. - Variable Displacement Pumps: Can continuously change the flow rate during operation; these are complex and expensive.
Hand Pumps: These may be piston, diaphragm, or vane-type depending on construction. They feature suction and discharge valves and are used for powering hand tools (like cutters), in emergencies, or in labs for leak testing.

Specific Pump Architectures and Mechanics

Gear Pumps: - Design: Simplest and most common; consist of two counter-rotating and meshing gears in one casing. - Process: Vacuum at suction draws oil, which is carried by the teeth to the outlet side. - Wear: Major wear occurs through the rubbing/scuffing of teeth and the casing, reducing efficiency. Repair is usually uneconomical due to low initial costs.
Piston Pumps: - Types: Inline (Row), Radial, and Axial. - Mechanics: Use cylinders and pistons as working elements, allowing for better sealing than gear pumps. This leads to higher volumetric efficiency, higher pressures, and greater flow capacities. - Axial Piston Pump: Pistons are pressed by a swash plate (numbered 3 in diagrams) at the end of a shaft (numbered 4). The plate is not perpendicular to the shaft axis but is inclined at an angle $\alpha$ . The piston stroke length is determined by this angle $\alpha$ . The cylinder block contains two suction and two discharge valves per cylinder, connecting to main suction and discharge channels. - Radial Piston Pump: Operation is similar to a vane compressor. The rotor contains pistons working in cylinders. Each has a suction channel from the tank and an outlet channel to the system. The rotor is positioned eccentrically relative to the housing, resulting in high efficiency.

Control Elements and Energy Accumulators

Valves and Distributors: Used to control the direction of liquid flow and protect the system from overload via overflow/relief valves. Distribution styles include slide (spool), seat (poppet), or rotary types.
Hydraulic Accumulators: - Definition: Devices used to store hydraulic energy by converting it into another form, such as the elasticity of a solid, compressed gas, or potential energy of a weight. - Types based on construction: Gas-loaded, Weight-loaded, and Spring-loaded. - Gas Accumulators: Operate on two properties: 1) The low compressibility of liquid (hydraulic oil) allows for high energy storage in small volumes. 2) The high compressibility of gases (usually Nitrogen, $\text{N}_2$ ) compressed by pressure acting on a separator (piston, diaphragm, or bladder). - Applications: - Storing hydraulic energy. - Emergency power supply. - Assisting during periods of increased system demand. - Damping pulsations and hydraulic shocks (water hammer). - Functioning as a "hydraulic spring." - Separator function: Transferring energy from pneumatic to hydraulic forms and vice-versa. - Accumulator Parameters: - Maximum oil-side volume. - Maximum stored pressure. - Geometric dimensions. - Description of oil-side and gas-side connections. - Type of hydraulic oil and working gas used. - Operating temperature range. - Volumetric coefficient.

Hydraulic Distributors and Port Designations

Function: Distributors act as intermediate elements between actuators and the power unit, directing the liquid stream to specific locations and then back to the reservoir.
Design Types: Slide (spool), seat, and rotary. Slide distributors are most common due to simple construction and versatility.
Multisectional Distributors: Combine several independent valves in one block to reduce the number and length of supply and return lines. Internal channels handle the distribution.
Port Identification: - P: Pressure connection (directly from the HPU). - T: Tank/Return connection (oil return to reservoir). - A and B: Connections to supply working actuators. - N: Port for serial connection of another distributor (can be plugged if unused).

Relief and Safety Valves

Overflow/Relief Valve: Maintains constant pressure in a specific part of the system. Excess liquid (when pump delivery exceeds system demand) is drained to the tank. The path is closed by a valve poppet held by spring tension; it opens when pressure overcomes the spring.
Safety Valve: Identical in construction and principle to the relief valve. Difference lies in function: a relief valve works continuously during normal operation, whereas a safety valve only acts during emergencies if pressure spikes. Safety valves are strictly regulated by laws, norms, directives, and regulations regarding their appearance, function, and parameter modification.

Hydraulic Lines: Rigid and Flexible

Rigid Lines: Used where connected elements are stationary and movement is absent. They are preferred when flexible hoses are at risk from sharp/rotating parts or for aesthetic purposes.
Flexible Lines (Hoses): Primarily used in drives and devices with moving parts, vibrations, or mechanical stresses.
Classification by Working Pressure: - Low Pressure: Up to $100\,\text{bar}$ . - Medium Pressure: $100-250\,\text{bar}$ . - High Pressure: $250-420\,\text{bar}$ . - Ultra-High Pressure: Above $420\,\text{bar}$ .
Special Properties: Includes high-temperature resistant, fire-resistant, increased electrical conductivity, high abrasion resistance of the outer layer, and chemical resistance.
Standard Flexible Hose: Typically made of rubber with a maximum pressure of $420\,\text{bar}$ and a temperature range of $-40\,^\circ\text{C}$ to $100\,^\circ\text{C}$ .
Standardization: Produced in inch sizes (internal diameter) and standardized by ISO (international) and EN (European) norms. Data like norm name, diameter, pressure, and production date are printed on the outer surface.
Reinforcement Types: No braid, textile braid (single or multi-layer), steel wire braid (single or multi-layer), and spiral steel wire braids.
Key Parameters: Outer and inner diameter (inches), maximum working pressure and burst pressure ( $\text{bar}$ ), temperature range, bend radius ( $\text{mm}$ ), and working environment description.

Handling and Assembly of Hydraulic Hoses

Operational Rules: - Never exceed fixed working pressures. - Avoid tensile (stretching) or twisting forces. - Do not kink/break the hose. - Avoid laying hoses over sharp edges. - Use drive-over bridges if the hose is on a roadway. - Maintain distance from heat sources or use thermal shields. - Do not load hoses with additional external weight.
Connection Types: Threaded (screw-in) connections, hydraulic quick-couplers, and Stecko connections.
Step-by-Step Hose Crimping Process: 1. Cut the hydraulic hose to the required length. 2. Place the ferrule (clamping sleeve) on the hose. 3. Insert the fitting (nipple/tail) inside the hydraulic line. 4. Set the crimping diameter and place the hose into the hydraulic crimping press. 5. Perform the crimp.