What is a hydraulic system?
A hydraulic system converts
mechanical energy into hydraulic energy and transmits it to hydraulic
actuators, such as cylinders, to perform mechanical work. It is often used for
applying large impact forces and lifting heavy loads. The hydraulic system
operates based on Pascal's law, which states that the pressure in an enclosed
fluid is uniform in all directions. According to Pascal's law, the smaller area
experiences lower intensity, while the larger area experiences a greater force.
This means that a hydraulic system can generate significant power with a
smaller input force. The diagram below illustrates Pascal's law.
In the diagram, a force F1
is applied to a piston with a cross-sectional area of A1. According to Pascal's
law, this force is transmitted in all directions through the fluid-filled
container. Therefore, the cross-section A2 also exerts a force, F2, equal to
F1. Since A2 is larger than A1, F2 will be higher than F1. This demonstrates
how a hydraulic system can achieve a large force (F2) by applying a small
amount of force (F1).
Hydraulic system with CNC
machines
A hydraulic system is commonly used with CNC machines for power transmission. It generates the required pressure to execute various machine tasks, including automatic tool and palette changes, spindle tool clamping/unclamping, etc. The specific configuration of a hydraulic system in a CNC machine depends on the type of work performed with hydraulic pressure. A hydraulic power pack unit and different hydraulic components are used to generate hydraulic pressure in a CNC machine. The selection of the power pack unit depends on the mechanical tasks that need to be performed with hydraulic pressure. The schematic diagram below illustrates a simple hydraulic system using a CNC machine.
In a hydraulic system,
hydraulic fluid is pressurized and delivered to different actuators through
control elements to ensure proper operation. Control elements and actuators
work together within a complete hydraulic system to carry out tasks
effectively. The following components are commonly used to build a standard
hydraulic system:
1. Hydraulic pump unit
2. Hydraulic fluid
3. Conveying elements
4. Control elements
5. Hydraulic actuators
6. Accumulator
1. Hydraulic pump unit
The hydraulic pump unit is a vital part of a hydraulic system. In CNC machines, hydraulic pump units are usually located separately from the machine body and deliver pressurized hydraulic oil to the machine through hydraulic hoses. Pressurized oil is then conveyed to hydraulic actuators for tasks such as automatic tool changes and workpiece loading/unloading, using different control valves. The image below shows a complete hydraulic pump or power pack unit.
A complete hydraulic pump
unit consists of the following elements:
• Hydraulic Pump
• Motor
• Breather cum filter
• Oil level gauge
• Section Steiner and return line filter
• Pressure regulator
• Pressure gauge
• Reservoir
• Heat exchanger
Hydraulic Pump - A hydraulic pump draws hydraulic oil from the reservoir and delivers pressurized oil to various hydraulic circuits through directional control valves. CNC machines commonly use piston, vane, and gear pumps to generate hydraulic pressure. Vane pumps, known for their smooth flow and compatibility with regular induction motors, are widely used in CNC machine hydraulic pump units. These pumps operate at a steady RPM, delivering a consistent amount of oil to the hydraulic circuits with each rotation. Pressure regulation is achieved through a pressure regulator, which controls the output pressure of the hydraulic pump unit. Some CNC machines may utilize other pump types, but their applications are limited. The image below shows a vane pump and its interior.
Motor - A 3-phase induction motor typically drives the hydraulic pump unit. The motor wattage is selected based on the capacity of the pump unit. The hydraulic pump unit runs continuously while the CNC machine is in operation, so high-efficiency induction motors are preferred to ensure continuous operation.
Breather cum filter - A breather cum filter is installed on the overhead plate of the oil reservoir. It maintains atmospheric pressure inside the reservoir and prevents the entry of oil sludge or unwanted materials during oil top-up. It is usually made of fine wire mesh and enclosed with a lid.
Oil level gauge - An oil-level gauge is mounted on the outside wall of the hydraulic oil reservoir, allowing the oil level to be visually inspected from the outside. It is usually made of transparent material and has markings indicating the minimum and maximum permissible oil levels inside the reservoir.
Suction Steiner & return line filter - A hydraulic pump unit is equipped with a suction strainer to prevent the entry of dirt or unwanted objects into the hydraulic pump through the hydraulic oil. The pump draws hydraulic oil from the reservoir through a pipeline, and the suction strainer is fitted to the pipeline opening, immersed inside the hydraulic oil. The return line filter is installed on the return pipeline opening, and placed inside the reservoir above the oil level. It prevents dirt or unwanted objects in the oil from passing into the oil reservoir. Both the suction strainer and return line filter are made of fine wire mesh.
Pressure Regulator - The hydraulic pump unit converts mechanical energy into hydraulic energy and transmits it to various hydraulic actuators, such as cylinders, to perform different machine tasks through the movement of the cylinder piston. Once the cylinder piston has completed its movement or achieved its intended purpose, the work is considered finished, and further piston movement is not feasible. However, since a positive displacement hydraulic pump continues to generate constant pressure, it increases the pressure within all hydraulic actuators. This can potentially damage or crack hydraulic actuators or hoses due to the high pressure. To address this issue, a pressure regulator is always utilized in conjunction with a hydraulic pump. The pressure regulator redirects excess pumped oil directly to the oil reservoir via a separate pipeline, thus mitigating the problem.
Pressure gauge - A pressure gauge is commonly used in a hydraulic system to measure the pressure inside a hydraulic line. The hydraulic pump unit usually employs burden tube-type panel-mounted or bottom-entry-type pressure gauges. In some cases, a gauge isolator is connected to a hydraulic pressure gauge to display the hydraulic pressures within different hydraulic circuits using a single pressure gauge unit. By rotating the gauge isolator dial to a selected position and pressing it, the corresponding hydraulic line pressure can be determined. The image below shows a typical hydraulic pressure gauge used in hydraulic power pack units.
Reservoir - The reservoir or tank of a hydraulic pump unit stores hydraulic oil, which is directly drawn by the pump from the reservoir through a pipeline and returned through a return line. All components related to the hydraulic power pack unit are typically mounted on top of the reservoir. A drainage system is usually located at the bottom of the reservoir to facilitate complete oil drainage, if necessary.
Heat exchanger - As the hydraulic pump unit continuously operates and generates hydraulic pressure, the temperature of the oil within the hydraulic circuit gradually increases. Since the pressurized fluid circulates within a closed path, there is no opportunity for heat dissipation. Increased heat inside the oil alters its properties and viscosity, which can affect overall hydraulic pressure. Moreover, heated oil can damage seals and O-rings associated with hydraulic actuators. To dissipate heat, a hydraulic heat exchanger is employed within the hydraulic power pack unit. A typical heat exchanger unit used in CNC machines consists of a pump, radiator, and blower-type fan. The pump draws hydraulic fluid from the tank and passes it through the radiator, while the fan blows air over the radiator to dissipate the heat.
2. Hydraulic fluid
Hydraulic fluid, also known as hydraulic oil, plays a critical role in a hydraulic system, and the system's overall performance depends on it. Commonly, hydraulic fluids are based on mineral oil, and using clear and high-quality mineral oil enhances the efficiency of a hydraulic system. Viscosity is another crucial parameter for hydraulic oil. Low-viscosity fluid increases leakage within the hydraulic system, while high-viscosity fluid leads to pressure drops within hydraulic lines and valves. Selecting the appropriate viscosity for the fluid is important, considering the nature of the hydraulic pumps, valves, and actuators used in the machine. The VG number (Viscosity Grading) indicates the density of hydraulic oil, with larger VG numbers representing thicker oil. In the United Kingdom, ISO VG is used for grading hydraulic oils, and ISO VG 32 or 46 is commonly used with different CNC machines. Some regular brand hydraulic oils that perform well with CNC machine hydraulic systems include Servo System 32, Servo System HLP 32, and Servo CIROL 46. Hydraulic oils should also possess good lubricant properties to reduce wear and tear and prevent rust in valves and other hydraulic components. Typically, petroleum-based fluids are suitable for most hydraulic systems.
Pressurized fluid from the hydraulic pump is conveyed to the actuators through two different types of pipelines: steel-type and reinforcement-type hydraulic pipelines. CNC machines use both types to transport pressurized fluid. Reinforcement-type pipelines are suitable for movable elements, while steel pipelines are suitable for fixed operating parts of the machine. These hydraulic pipelines provide flexibility in movement and can be routed through the machine body easily. The pipelines or hoses consist of three layers. The inner layer, made of synthetic rubber or thermoplastic, carries the pressurized fluid. It is supported and reinforced by braided wire sheaths tightly embedded over the inner piping layer. The ability of the hose to withstand pressure depends on the strength of the braided wire binding. Finally, the outer part of the hose is made of water and oil-resistant synthetic rubber. The selection of hydraulic pipes depends on the system's pressure and the specific part of the machine where they will be used. The image below shows the inner part of a reinforcement-type hydraulic hose.
4. Control Elements
Control elements function in
conjunction with a hydraulic system in a CNC machine. The primary purpose of
these elements is to effectively serve hydraulic actuators and regulate
hydraulic pressure lines according to machine requirements. Hydraulic pressure
lines are managed using different types of valves, and there is a range of
hydraulic valves that are used in CNC machine operations. Let's discuss some
fundamental control valves specifically related to CNC machine operation.
What is a Hydraulic Valve?
A hydraulic valve is
responsible for converting hydraulic energy obtained from a pump unit into
motion and forces, which are applied to an actuator. It plays a crucial role in
regulating the hydraulic actuator and different types of control valves within
a hydraulic system. There are three main types of valves used in hydraulic
systems to control the pressure, flow rate, and direction of the pressurized fluid:
a) Direction control valve
b) Pressure control valve
c) Flow control valve
a) Direction Control Valve
A direction control valve is
used in hydraulic systems to force pressurized fluid in a specific direction.
These valves are typically used to control the start, stop, and switching of
fluid flow direction. Various types of direction control valves are available,
including manual or solenoid-operated valves. In CNC machines, check valves and
solenoid-operated directional control valves are commonly used.
Check valve - A check valve is a simple directional control valve used in hydraulic systems. It blocks or resists the fluid flow in one direction and allows free flow in the opposite direction. These valves are also known as "non-return" valves. The following picture shows a non-return valve and its interior.
Direct-operated direction control valve - These valves are commonly used in CNC machines to control the fluid force in a specific direction. A solenoid coil actuates the valve with the movement of an iron plunger inside it. When current flows through the solenoid coil, the iron plunger becomes magnetized and tries to eject from it. Spring tension is applied to resist the ejection force, allowing the plunger to move only against the spring tension. When the current flow ceases, the iron plunger retracts to its initial position due to the spring tension. The plunger inside the solenoid coil acts as a direction control spool of the valve, controlling the direction of pressurized fluid and regulating the movements of the spool. Sometimes, one or two solenoid valves are used to control the spool movements. The internal layout of a direction control valve varies based on its type, such as 2/2, 3/2, 4/3, and 5/3, depending on the configuration and motion of the pressurized fluid being controlled. The following picture shows a 4/3 directional control valve commonly applied in CNC machines.
In line with the previous picture, a 4/3 directional control valve has four ports and can be placed in three distinct positions or have three different conditions. Output ports 'A' and 'B' are always connected to an actuator (in this case, a cylinder), port 'P' is associated with the pump pressure line, and port 'T' is a return line that carries the returning fluid to the tank. Here, 'S1' and 'S2' are two solenoid coils used for the movement of the inner spool in both directions. The valve can have three different situations, as illustrated in the following pictures.
In Picture-1, when both Solenoid coils S1 and S2 are deactivated, ports P, T, A, and B are blocked,
resulting in no fluid flow through the output lines A and B and no movement of
the actuator (which remains in its last state). In Picture-2, when solenoid S1
is activated, the movement of the inner spool will be on the left side,
connecting the pump pressure line P to port A. Hydraulic pressure will reach
the forward side of the cylinder, pushing the cylinder piston inward. The fluid
accumulated inside the bottom part of the cylinder will return to the tank
through port B, which is already connected to port T. Similarly, if solenoid S2
is activated (Picture-3), the movement of the inner spool will be on the
right-hand side, connecting pressure line P to port B, which will push the
cylinder piston outward. The accumulated fluid inside the upper part of the
cylinder will drain to the tank through port A, as the T line is connected to
port A. The alternating operation of both solenoid valves reverses the
direction of the hydraulic pressure line, thereby successfully controlling the
functioning of the actuator (in this case, the cylinder).
Indirect or pilot or servo-operated direction control valve - In this type of valve, a small valve called the indirect or pilot valve is used to control the larger main valve. The structural configuration of these valves is designed to ensure that the fluid force inside the pilot valve is lower than the mainline pressure and that the pressure reaches the pilot valve through a restricted channel. The low-pressure line inside the pilot valve is controlled by a solenoid coil from the outside, and this controlled low-pressure line is used to control the higher-pressure line passing through the main valve. These types of valves require minimal electrical power to operate the solenoid coils since low-pressure fluid control is necessary inside the pilot valve. Pilot or servo-operated solenoid valves are used when there is a need to control a large amount of fluid pressure.
b) Pressure control valve
Pressure control valves are
used in nearly every hydraulic system and serve various functions such as
maintaining restricted system pressure, maintaining predefined pressure in a
specific part of the hydraulic circuit, etc. These valves regulate the pressure
by controlling the opening of a throttling orifice inside the pressure control
valve. Commonly, three types of pressure control valves are found in hydraulic
systems:
- Pressure relief valve
- Pressure-reducing valve
- Sequence valve
Pressure relief valve - A pressure relief valve is used to protect different hydraulic components from excessive pressure. Its primary function is to limit the system pressure within a predefined range. It is a closed-type valve that opens when the pressure exceeds a certain value and redirects the excess hydraulic oil back to the tank. As shown in the picture, a pressure relief valve has two working ports: one connected to the hydraulic pump and the other directly linked to the hydraulic tank. The valve has an adjustable spring, and its tension can be adjusted from the outside using an adjustable knob. The spring positions a "poppet" on the opening of the pressure line, and the poppet remains at the same hydraulic system pressure by adjusting the knob or spring tension. When the pressure exceeds the set limit, the poppet moves from its position, allowing excess hydraulic oil to flow back to the tank, thus reducing the system pressure and returning it to its original state. The following picture shows a pressure relief valve and its interior.
Pressure-reducing valve - A pressure-reducing valve is used when the hydraulic system requires a lower pressure in a specific part of the hydraulic circuit. Typically, this type of valve is used in hydraulic branch circuits where the pressure needs to be different from the primary hydraulic pressure. It is also an open-type valve with an adjustable spring and a movable spool. The following picture shows a pressure-reducing valve.
Sequence valve - After the primary process has been successfully completed, a sequence valve diverts the pressurized fluid to another hydraulic circuit. In its normal state, the sequence valve allows fluid to flow freely to the primary circuit to perform its function. Once the primary function is executed, the pressure inside the circuit increases and is sensed through a pressure-sensing channel, pushing a spool against the force exerted by a spring. As a result, the spring is compressed and shifts the valve spool, allowing pressurized oil to flow into a secondary circuit. The following picture shows a sequence valve and its interior.
c) Flow control valve
The operating speed of a hydraulic actuator is crucial for achieving the desired output and controlling the momentum of the actuator. A flow control valve is used to regulate the flow of hydraulic fluid by controlling the diverging area of the valve opening through which the hydraulic fluid passes. The operating speed of a hydraulic actuator can be increased or decreased by adjusting the valve opening area. Flow control valves are used in three ways within a hydraulic line to control the operating speed of an actuator: Meter-in-circuit, Meter-out-circuit, and Bleed-off-circuit. In a Meter-in-circuit, a flow control valve is positioned between the output pressure line of the pump and the incoming pressure line of an actuator. In a Meter-out-circuit, a flow control valve is installed between the actuator's outgoing pressure line and the tank line. In a Bleed-off-circuit, a flow control valve is used to connect the actuator's pressure line and return line. The following picture shows a flow control valve and its interior.
5. Hydraulic actuator
Hydraulic actuators perform
multiple functions in a CNC machine, such as automatic tool changing, automatic
palette changing, etc. They convert hydraulic energy into mechanical power, and
the power generated by an actuator depends on factors such as hydraulic oil
flow rate, pressure drop across the actuator, and overall efficiency. Hydraulic
actuators are classified as linear, rotary, and semi-rotary actuators,
depending on the type of actuation. Hydraulic cylinders and motors are commonly
used as actuators in CNC machines. The following pictures show hydraulic
actuator functions in different CNC machines.
6. Accumulator
An accumulator stores a significant quantity of pressurized hydraulic fluid, and the additional fluid pressure is instantly released to perform a specific operation in a hydraulic circuit when needed. In other words, the hydraulic circuit requires some additional fluid pressure for a moment, which is provided by an accumulator. There are three main types of hydraulic accumulators: bladder type, diaphragm type, and piston type. Among them, the bladder-type accumulator is commonly used in the hydraulic circuit of a CNC machine. A bladder-type accumulator consists of two chambers inside a rigid steel container. A rubber bladder filled with pressurized nitrogen gas is positioned inside one chamber, while the other chamber is connected to the hydraulic circuit. For example, if there is a pressure drop within the hydraulic circuit, the pressurized nitrogen gas-filled bladder provides additional pressure to the hydraulic circuit for a brief period. A spring-loaded valve is used with the gas-filled bladder to prevent it from escaping the steel container when there is no pressure inside the hydraulic circuit. The following picture shows a bladder-type accumulator and its internal components.
