Hydraulics
•The term hydraulics is used to specifically describe fluid power circuits that use liquids—especially formulated oils—in confined circuits to transmit force or motion.
•Hydraulic circuits:
–Hydraulic brakes
–Power steering systems
–Automatic transmissions
–Fuel systems
–Wet-line kits for dump trucks
–Torque converters
–Lift gates
Fundamentals
•Hydrostatics is the science of transmitting force by pushing on a confined liquid.
–In a hydrostatic system, transfer of energy takes place because a confined liquid is subject to pressure.
•Hydrodynamics is the science of moving liquids to transmit energy.
•We can define hydrostatics and hydrodynamics as follows:
–Hydrostatics: low fluid movement with high system pressures
–Hydrodynamics: high fluid velocity with lower system pressures
Atmospheric Pressure
•A column of air measuring 1 square inch extending 50 miles into the sky would weigh 14.7 pounds at sea level.
•If we stood on a high mountain, the column of air would measure less than 50 miles and the result would be a lower weight of air in the column.
•Similarly, if we were below sea level, in a mine for instance, the weight of air would be greater in the column.
•In North America, we sometimes use the term atm (short for atmosphere) to describe a unit of measurement of atmospheric pressure.
•Europeans use the unit bar (short for barometric pressure).
The basic principles of hydraulics
•Liquids have no shape on their own
•Liquids will not compress
•Liquid transmit applied pressure in all directions
•Liquids provide great increases in work force
Liquids have no shape on their own
Liquids will not compress
PASCAL’S LAW
Pressure in a fluid acts equally in all directions
How we apply Pascal’s law to make hydraulics work for us
Liquid transmit applied pressure in all directions
Liquids provide great increases in work force
Hydraulic Levers (1 of 2)
•Hydraulic levers can be used to demonstrate Pascal’s law:
– Pressure equals force divided by the sectional area on which it acts.
–(P=F\A)
–Force equals pressure multiplied by area.
–( F = P x A)
Hydraulic Levers (2 of 2)
•One of the cylinders has a sectional area of 1sq.” and the other 50 sq.”
•Applying a force of 2 lbs. on the piston in the smaller cylinder would lift a weight of 100 lbs.
•Applying a force of 2 lbs. on the piston in the smaller cylinder produces a circuit pressure of 2 psi.
•The circuit potential is 2 psi and because this acts on a sectional area of 50 sq.”, it can raise 100 lbs.
•If a force of 10 lbs. was to be applied to the smaller piston, the resulting circuit pressure would be 10 psi and the circuit would have the potential to raise a weight of 500 lbs.
Bernoulli’s Principle (1 of 2)
•Bernoulli’s Principle states that if flow in a circuit is constant, then the sum of the pressure and kinetic energy must also be constant.
–Pressure x Velocity IN = Pressure x Velocity OUT
•When fluid is forced through areas of different diameters, fluid velocity changes accordingly.
–For example, fluid flow through a large pipe will be slow until the large pipe reduces to a smaller pipe; then the fluid velocity will increase.
Bernoulli’s Principle (2 of 2)
Advantages of Hydraulic Systems
• Convenient method of power transmission over long distances
• Great flexibility
• Variable speed control
• Safety and reliability
Disadvantages of Hydraulic Systems
•Need for positive confinement
•Fire hazard if leaks occur
•Leaks in high pressure systems pose a safety hazard
•Adequate oil filtration must be maintained
a basic hydraulic system
move
•The pump which moves the oil
Do work
•The cylinder which uses the moving oil to do work
Hold the oil
•Check valves to hold the oil in the cylinders between strokes and to prevent oil from returning to the reservoir during the pressure stroke.
Store the oil
•A reservoir to store the oil
direction
The control valve directs the oil
Protects the system
The relief valve protected the system from high pressures
summary
•To summarize:
–The pump = generating the force
–The cylinder = working force
–The valve = oil control
–The reservoir = oil storage
Comparing hydraulic system
• two major types of hydraulic systems are used today:
1.Open-center system
2.Closed-center system
Open-center system
This system requires that the control valve spool be open in the center to allow pump flow to pass through the valve and return to the reservoir. The pump we have used supplies a constant flow of oil and the oil must have a path for return when it is not required to operate a function
Closed-center system
Valve spool closed in center to dead end pump oil in neutral
Hydraulic Components
•Pumps
–Fixed Displacement
–Variable Displacement
•Valves
–Flow control
–Pressure relief
•Actuators
–Cylinders
–Motors
•Hoses
•Other
–Reservoirs
–Filters
–Heat Exchangers
–Accumulators
•Stores hydraulic pressure
•Absorb shock, maintain pressure, develop system flow
fluid
•All hydraulic systems use a fluid to perform work.
•All domestic manufactures require a petroleum-based fluid.
•Some import manufactures require the use of a synthetic fluid.
•ATF is dyed clear red, and darken when burnt, or become milky when contaminated by water.
FLUID ADDITIVES
•Zinc, phosphorus, and sulfur are commonly added to reduce friction.
•Detergent additives, keep the parts clean.
•Dispersants keep contaminants suspended so they can be trapped by the filter.
•ATF is subjected to many tests, oxidation resistance, corrosion and rust inhibition, flash and flame points, and resistance to foaming.
FLUID REACTIONS
•Chemicals in fluids may react with seals.
•Fluids are tested for compatibility with a specific transmission.
•Incompatibility can result in swelling or shrinking seals.
•ATF is also tested for the ability to mix with other brands.
Advantages of Using Oil As a Hydraulic Fluid.
•Non-corrosive
• Superb lubricant
Pumps
•Fixed Displacement
–Vane
–Lobed
–Advantages: cheap and simple!
–No control over amount of flow output for given rotational speed
•Variable Displacement
–Piston-type
–Advantages include ability to adjust flow output by varying swash plate angle
–Expensive and mechanically complex
Fixed-Displacement Pumps
•A fixed-displacement pump will move the same amount of oil per revolution with the result that the volume picked up by the pump at its inlet equals the volume discharged to its outlet per revolution.
•This means that pump speed determines how much hydraulic oil is moved.
•Fixed-displacement pumps are commonly used for applications such as:
–Lift pumps
–Power steering pumps
–Transmission pumps
–Lube pumps
Variable-displacement Pumps
•Variable-displacement pumps are positive displacement pumps designed to vary the volume of oil they move each cycle even when they are run at the same speed.
•They use an internal control mechanism to vary the output of oil— usually with the objective of maintaining a constant pressure value and reducing flow when demand for oil is minimal.
Gear Pumps
•Gear pumps are widely used in mobile hydraulics because of their simplicity.
•They are also widely used to move fuel through diesel fuel subsystems and as engine lube oil pumps.
•Three types of gear pumps are used:
–External gear
–Internal gear
–Rotor gear
External-gear Pumps
•Two intermeshing gears are close-fitted within a housing.
•One of the gears is a drive shaft and this drives the second gear because they are in mesh.
•As the gears rotate, oil from the inlet is trapped between the teeth and the housing, and is carried around the housing and forced from the outlet.
Internal-gear Pumps
•A spur gear rotates within an annular internal gear, meshing on one side of it.
•Both gears are divided on the other side by a crescent-shaped separator.
•When an external gear is in mesh with an internal gear, they both turn in the same direction of rotation.
•As the gear teeth come out of mesh, oil from the inlet is trapped between the teeth and the separator and is carried to the outlet and expelled.
Rotor-gear Pumps
•A rotor-gear pump is a variation of the internal-gear pump.
•An internal rotor with external lobes rotates within an outer rotor ring with internal lobes.
•No separator is used.
•The internal rotor is driven within the outer rotor ring. The internal rotor has one less lobe than the outer rotor ring, with the result that only one lobe is fully engaged to the rotor ring at any given moment of operation.
•As the lobes on the internal rotor ride on the lobes on the outer ring, oil becomes entrapped: as the assembly rotates, oil is forced out of the discharge port.
Vane Pumps
•Vane pumps are also used extensively in hydraulic circuits.
–Truck power-assisted steering systems use vane pumps.
•A slotted rotor fitted with sliding vanes rotates within a stationary liner known as a cam ring. There are two types:
–Balanced
-Unbalanced
Balanced Vane Pumps
•As the rotor rotates, centrifugal force moves the vanes outward.
•Fluid is trapped between the crescent-shaped “chambers” formed between vanes.
•The size of these chambers are continually expanding and contracting as the rotor turns.
•Oil from the inlet is trapped in the space between two vanes.
•As the rotor continues to turn, the chamber contracts until it is aligned with the outlet and the oil is expelled.
•This action repeats itself twice per revolution because there are a pair of inlet ports and a pair of discharge ports.
Unbalanced Vane Pumps
•This has the same principle as the balanced version, with the exception that the operating cycle only occurs once per revolution because it has only one inlet and one outlet port.
•The disadvantage of the unbalanced vane pump is the radial load caused by high pressure that is acting on the discharge side of the rotor and none on the inlet side because the inlet oil is under little or no pressure.
Piston Pumps
•There are a wide variety of piston pumps, beginning with the most simple and including some of the more complex pumps used in hydraulic circuits.
•There are three general types of piston pump:
–Plunger pumps
–Axial piston
–Radial piston
•Plunger-type pumps are seldom found on hydraulic circuits, but the latter two are used on systems that demand high flow and high-pressure performance.
Plunger Pumps
•A bicycle pump is an example of a plunger pump as are the fuel hand-priming pumps used on many diesel fuel systems.
•A plunger reciprocates within a stationary barrel. Fluid to be pumped is drawn into the pump chamber formed in the barrel on the outward stroke of the plunger.
•This fluid is then discharged on the inboard stroke of the plunger.
Axial Piston Pumps
•A rotating cylinder with piston bores machined into it rides against an inclined plate.
•The pistons are arranged parallel with the pump drive.
•The base of each piston rides against a tilted plate known as a swashplate or wobble plate which does not rotate.
•They provide a method for controlling the tilt angle of the swashplate.
•Fluid is charged to each pump element as the piston is drawn to the bottom of its travel.
•As the cylinder head rotates, the piston follows the tilt of the swashplate and is driven upward forcing fluid out of the discharge port.
Radial Piston Pumps
•Radial piston pumps are capable of high pressures, high speeds, high volumes, and variable displacement. However, they cannot reverse flow.
•Radial piston pumps operate in two ways:
–Rotating cam
-Rotating piston
Piston Pumps
•Work well at PSI’s of 2000 or more
•Single piston pump used in bottle jacks
•Require several pistons working together to generate enough volume for tractor applications Necessarily involve many moving parts
Valves
•Valves are used to manage flow and pressure in hydraulic circuits.
•There are three basic types of valves used in hydraulic circuits.
–Pressure control
–Directional control
–Volume (flow) control
Directional Control Valves (2 of 3)
•Directional control valves direct the flow of oil through a hydraulic circuit. They include:
–Check valves
–Rotary valves
–Spool valves
-Pilot valves
Directional Control Valves (3 of 3)
•Check valves
–A check valve uses a spring-loaded poppet. It permits flow in one direction and prevents flow in the other.
•Rotary valves
–A rotary spool turns to open and close oil passages. Rotary valves are commonly used as pilots for other valves in systems with multiple sub-circuits.
•Spool valves
–A sliding spool within a valve body to open and close hydraulic circuits. Spool valves are used extensively in hydraulic systems and automatic transmissions.
•Pilot valves
–Pilot valves may be controlled mechanically, hydraulically, or electrically.
Hydraulic Valves
•Responsible for controlling and directing flow
–Directional flow control valves
•Spool and internal passages dictate flow paths
–Proportional flow control valves
•Variable orifice
•Gate, globe, and needle valves
–Check valves (similar to a diode in an electrical circuit)
Pressure Relief Valves
•Found in all hydraulic circuits
•Normally closed
•Can be pilot operated (i.e. Spool is biased by both spring and fluid pressure…less override)
•Provides protection against exceeding system pressure rating which can damage components
Cylinder
Piece of equipment that transfers hydraulic power into mechanical movement in one or two directions only.
Single-acting
Refers to a hydraulic cylinder that works in one direction only.
Double-acting
Refers to a hydraulic cylinder that pushes and pulls.
Hydraulic Cylinders
•Provides linear actuation
•Single vs. Double acting
•Single vs. Double ended
•Complexity varies, but basic operation and principles the same
•Basic design terminology
–Stroke length
–Piston diameter
–Piston rod diameter
Hydraulic Cylinders
•Can generate very large forces
•Contamination of hydraulic oil and proper maintenance directly affect the life of a hydraulic cylinder
Hydraulic Motor
Receives power from moving fluid to transfer hydraulic power to mechanical rotating force.
Hydraulic Motors
•Provides rotary actuation/torque
•Can be thought of as a pump working in reverse
•Motor ratings
–Displacement
–Torque capacity
–Speed
–Max pressure
Hydraulic Motors (1 of 2)
•The function of hydraulic motors is the opposite of hydraulic pumps:
–Pump
•It draws in oil and displaces it, converting mechanical force into fluid force.
–Motor
•Oil under pressure is forced in and spilled out, converting fluid force into mechanical force.
Hydraulic Motors (2 of 2)
•There are three categories of hydraulic motors:
–Gear motors
–Vane motors
–Piston motors
•All hydraulic motors rotate, driven by incoming hydraulic oil under pressure.
Gear Motors
•External gear
–An external-gear motor is driven by pressurized hydraulic oil forced into the pump inlet, which acts on a pair of intermeshing gears, turning them away from the inlet, with the oil passing between the external gear teeth and the pump housing.
•Internal gear
–An internal-gear motor is similar to an internal-gear pump. The motor drive shaft is connected to the inner rotor.
Hydraulic Motors
•Constant displacement
–Vane (can be variable)
–Gear
–Piston
•Variable displacement
–Axial piston
•High Speed, Low Torque (HSLT)
•Low Speed, High Torque (LSHT)
Hydraulic Motors
•Advantages over electric motors
–Instant reversal of rotation of motor’s shaft
–Can be stalled without damage
–Torque control through operating speed
–Dynamic braking is easily accomplished
–Weight to horsepower ratio of .5 lb/hp compared to 10lb/hp for electric motors
Hoses
•Used to connect the various components
•Analogous to wires in an electrical circuit
•Compliance of the hose can affect the effective bulk modulus
•Modeled as a fixed volume
Hydraulic Hoses (1 of 2)
•The size of any hydraulic hose is determined by its inside diameter.
•This is sometimes indicated as dash size in 1⁄16-inch increments.
•Each dash number indicates 1⁄16 inch,
–a #4 dash hose would be equivalent to 4⁄16 inch or 1⁄4 inch.
•Dash size Nominal diameter
–# 4 = 1⁄4 inch
–# 6 = 3⁄8 inch
–# 8 = 1⁄2 inch
–# 10 = 5⁄8 inch
–# 12 = 3⁄4 inch
•A large-diameter internal hose has to be stronger to sustain the working pressures of a hydraulic circuit.
Hydraulic Hoses (2 of 2)
•Another consideration for hose selection is that the hose must be compatible with the hydraulic fluid used in the system.
•There are four general types of hoses used in hydraulic circuits:
–Fabric braid
–Single-wire braid
–Multiple-wire braid (up to 6 wire braid)
–Multiple-spiral wire (up to 6 wire spirals)
Hydraulic Oil
•Responsible for transmitting hydraulic power from source to actuator
•Dissipates heat!
•Lubricates parts
•Seals clearances
•Bulk modulus, viscosity, and density are the most important properties
–Petroleum based most common
Piston
Internal component of a hydraulic cylinder that is moved in a linear motion by the action of fluid introduced into the cylinder.
Reservoirs
•A reservoir in a hydraulic system has the following roles:
–Stores hydraulic oil
–Helps keep oil clean and free of air
–Acts as a heat exchanger to help cool the oil
•A reservoir is typically equipped with:
•Filler cap
•Oil-level gauge or dipstick
•Outlet and return lines
•Baffle(s)
•Intake filter
•Oil filter
•Drain plug
Actuators
•Hydraulic actuators convert the fluid power from the pump into mechanical work.
•A hydraulic cylinder is a linear actuator.
•A hydraulic motor is a rotary actuator.
