FUNDAMENTALS OF HYDRAULIC SYSTEMS
Hydraulic systems mounted on trucks share common elements and operational principles, regardless of their specific use. These systems include a power source, reservoir, pump, directional control valve, and actuators, which work together to move and control fluid, thereby performing tasks. The process begins with mechanical power in the form of a rotating shaft, which is transformed into hydraulic power using the pump. This power is then directed through a valve to a cylinder or motor, converting it back to mechanical power. This conversion allows for the controlled application of force through fluid power.
The performance of Hydraulic Repair Near Me hydraulic applications hinges on flow and pressure requirements. The flow rate, measured in gallons per minute (GPM), influences the speed at which a hydraulic cylinder extends or a motor rotates, generated by the pump. Pressure, measured in pounds per square inch (PSI), dictates the force exerted, emerging when flow encounters resistance. While the pump doesn’t produce pressure, it can withstand it. The system’s horsepower (HP) requirements, essential for operation, depend on both flow and pressure, calculated as: HP = GPM × PSI ÷ 1,714.
Pascal’s Law, named after the 17th-century French mathematician and philosopher Blaise Pascal, is a cornerstone of hydraulics. It states that pressure applied to a confined fluid is transmitted instantly, uniformly, and undiminished to all parts of the container. As oil is nearly incompressible, any force applied to one end of an oil-filled tube or hose is instantly conveyed to the other end.
Understanding the distinction between flow and pressure is crucial for troubleshooting. Flow affects actuator speed, while pressure influences system force. A hydraulic system failing to lift a load likely faces a pressure problem, whereas slow performance suggests a flow issue.
Four key hydraulic laws also play a role:
- Pascal’s Law: Pressure applied to a confined fluid is evenly distributed throughout the fluid and container.
- Boyle’s Law: At constant temperature, the pressure of a fixed gas mass inversely varies with its volume.
- Charles’ Law: At constant pressure, the volume of a fixed gas mass directly varies with temperature.
- Henry-Dalton’s Law: The solubility of air in a fluid is proportional to the air pressure above the fluid.
Four types of pressure are integral to hydraulic systems:
- Atmospheric Pressure: Drives oil from the reservoir to the pump inlet.
- Neutral System Pressure: The flow resistance within the system, ideally kept under 300 PSI.
- Pump Operating Pressure: The pressure at the pump outlet needed to perform work.
- Relief Pressure: The threshold at which the relief valve opens to reduce system pressure.
These principles and laws govern hydraulic systems, influencing their design, operation, and troubleshooting.
EXPECTED HYDRAULIC PRESSURE IN TRUCK-MOUNTED SYSTEMS
The common misconception is that the operating pressure of a Hydraulic Repair Near Me hydraulic system is solely set by adjusting a relief valve. However, hydraulic pressure is influenced by multiple factors:
- The load that needs to be moved
- The displacement of the hydraulic motor or the piston area of the cylinder
- Mechanical and hydraulic efficiency of the design
In discussing Hydraulic Repair Near Me hydraulic systems, we frequently refer to the figure 231, which represents the number of cubic inches in one gallon of oil. This is crucial for understanding pump and motor ratings, which are given in cubic inch displacement (CID), similar to how car engines are measured. For instance, a 4 cubic inch pump displaces 4 cubic inches of oil with each rotation of its input shaft, affecting the flow rate (GPM). To achieve a flow rate of 20 GPM, a 4 cu.in. pump must rotate at 1,155 RPM.
Additionally, the number 231 is relevant when discussing hydraulic reservoirs, cylinders, and motors. It’s important to note that no hydraulic system is completely efficient due to friction and internal leakage in components, leading to energy loss and heat generation. New Hydraulic Repair Near Me hydraulic systems with cylinder actuators are about 85% efficient, and those with hydraulic motors are around 80% efficient.
There are two main types of Hydraulic Repair Near Me hydraulic systems: open center and closed center. Open center systems have continuous flow and intermittent pressure, while closed center systems have continuous pressure and intermittent flow. Each system type is characterized by its specific directional control valve construction and hydraulic circuit design.
Volumetric efficiency and mechanical efficiency are crucial in evaluating a Hydraulic Repair Near Me pump’s performance. Volumetric efficiency measures the actual output flow against the theoretical output, while mechanical efficiency assesses internal power losses as a percentage of input power. Overall efficiency is determined by combining these two efficiencies.
The prime mover in Hydraulic Repair Near Me mobile hydraulics, typically the truck engine, provides the mechanical power for the hydraulic system. This can be achieved through a power take-off (PTO), belts from the crankshaft pulley, or a tubular driveshaft assembly. Various PTO designs exist, each suited to specific applications and meeting distinct torque, horsepower, and speed requirements of the system.
Drivelines for these applications must be balanced, tubular, and capable of handling high torque loads. A common choice is the Spicer 1310 series or an equivalent. Using solid bar stock in crankshaft drive applications is not advisable.
Driveshaft angularity is a critical factor. It’s essential to maintain a shallow angle, typically under 7°, and ensure the pump input shaft is parallel to the engine crankshaft within a margin of 1½°. Additionally, the yokes at each end of the shaft should be in phase or aligned. Neglecting these aspects can cause driveshaft vibrations and damage to the pump.
In cases where full engine horsepower and torque are needed, such as with large vacuum or concrete pumps, auxiliary engines, either gasoline or diesel, are used. These engines often have one or more hydraulic pumps attached to their output shaft.
Belt-driven or “clutch” pumps are commonly used in applications like wreckers and bucket trucks. They serve as an alternative to PTO systems, especially in vehicles lacking PTO apertures or where access to the transmission PTO aperture is blocked. The clutch pump is driven by belts from the crankshaft pulley, using an electric clutch similar to those in car air conditioner compressors.