Chapter 13

1. Chapter 13 Hydraulics

2. Objectives (1 of 2) • Explain fundamental hydraulic principles. • Apply the laws of hydraulics. • Calculate force, pressure, and area. • Describe the function of pumps, valves, actuators, and motors. • Describe the construction of hydraulic conductors and couplers.

3. Objectives (2 of 2) • Outline the properties of hydraulic fluids. • Identify graphic symbols. • Interpret a hydraulic schematic. • Perform maintenance procedures on truck hydraulic systems.

4. 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

5. Pascal’s Law • Pressure applied to a confined liquid is transmitted undiminished in all directions and acts with equal force on all equal areas, at right angles to those areas.

6. 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

7. 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).

8. Force • Force is push or pull effort. • The weight of one object placed upon another exerts force on it proportional to its weight. • If the objects were glued to each other and we lifted the upper one, a pull force would be exerted by the lower object proportional to its weight. • Force does not always result in any work done. • If you were to push on the rear of a parked transport truck, you could apply a lot of force, but that effort would be unlikely to result in any movement of the truck. • The formula for force (F) is calculated by multiplying pressure (P) by the area (A) it acts on. • F = P x A

9. Pressure Scales • There are a number of different pressure scales used today but all are based on atmospheric pressure. One unit of atmosphere is the equivalent of atmospheric pressure and it can be expressed in all these ways: • 1 atm = 1 bar (European) = 14.7 psia = 29.920 Hg (inches of mercury) = 101.3 kPa (metric) • However, each of the above values is not precisely equivalent to the others: 1atm = 1.0192 bar 1 bar = 29.530 Hg = 14.503 psia 10 Hg = 13.60 H2O @ 60° F

10. Torricelli’s Tube • Evangelista Torricelli (1608–1647) discovered the concept of atmospheric pressure. • He inverted a tube filled with mercury into a bowl of the liquid and then observed that the column of mercury in the tube fell until atmospheric pressure acting on the surface balanced against the vacuum created in the tube. • At sea level, vacuum in the column in Torricelli’s tube would support 29.92 inches of mercury.

11. Manometer • A manometer is a single tube arranged in a U-shape used to measure very small pressure values. • It may be filled to the zero on the calibration scale with either water H2O) or mercury (Hg), depending on the pressure range desired. • A manometer can measure either push or pull on the fluid column. Examples: • Crankcase pressure • Exhaust backpressure • Air inlet restriction

12. Absolute Pressure • Absolute pressure uses a scale in which the zero point is a complete absence of pressure. • Gauge pressure has as its zero point atmospheric pressure. • A gauge therefore reads zero when exposed to the atmosphere. • To avoid confusing absolute pressure with gauge pressure • Absolute pressure is expressed as: psia. • Gauge pressure is usually expressed as: psi or psig.

13. 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)

14. 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.

15. Flow • Flow is the term we use to describe the movement of a hydraulic fluid through a circuit. • Flow occurs when there is a difference in pressure between two points. • In a hydraulic circuit, flow is created by a device such as a pump. • A pump exerts push effort on a fluid. • Flow rate is the volume or mass of fluid passing through a conductor over a given unit of time. • An example would be gallons per minute (gpm).

16. Flow Rate and Cylinder Speed • Given an equal flow rate, a small cylinder will move faster than a larger cylinder. If the objective is to increase the speed at which a load moves, then: • Decrease the size (sectional area) of the cylinder. • Increase the flow to the cylinder (gpm). • The opposite would also be true, so if the objective were to slow the speed at which a load moves, then: • Increase the size (sectional area) of the cylinder. • Decrease the flow to the cylinder (gpm). • Therefore, the speed of a cylinder is proportional to the flow to which it is subject and inversely proportional to the piston area.

17. Pressure Drop • In a confined hydraulic circuit, whenever there is flow, a pressure drop results. • Again, the opposite applies. Whenever there is a difference in pressure, there must be flow. • Should the pressure difference be too great to establish equilibrium, there would be continuous flow. • In a flowing hydraulic circuit, pressure is always highest upstream and lowest downstream. This is why we use the term pressure drop. • A pressure drop always occurs downstream from a restriction in a circuit.

18. Flow Restrictions • Pressure drop will occur whenever there is a restriction to flow. • A restriction in a circuit may be unintended (such as a collapsed line) or intended (such as a restrictive orifice). • The smaller the line or passage through which the hydraulic fluid is forced, the greater the pressure drop. • The energy lost due to a pressure drop is converted to heat energy.

19. Work • Work occurs when effort or force produces an observable result. • In a hydraulic circuit, this means moving a load. • To produce work in a hydraulic circuit, there must be flow. • Work is measured in units of force multiplied by distance, for example, in pound-feet. • Work = Force x Distance

20. 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.

21. Bernoulli’s Principle (2 of 2)

22. Laminar Flow • Flow of a hydraulic medium through a circuit should be as streamlined as possible. • Streamlined flow is known as laminar flow. • Laminar flow is required to minimize friction. • Changes in section, sharp turns, and high flow speeds can cause turbulence and cross-currents in a hydraulic circuit, resulting in friction losses and pressure drops.

23. Types of Hydraulic Systems • Hydraulic systems can be grouped into two main categories: • Open-center systems • Closed-center systems • The primary difference between open-center and closed-center systems has to do with what happens to the hydraulic oil in the circuit after it leaves the pump.

24. Open-center Systems • In an open-center system, the pump runs constantly and oil circulates within the system continuously. • An open-center valve manages flow through the circuit. When this valve is in its neutral position, fluid returns to the reservoir. • An example of an open-center hydraulic system on a truck is power-assisted steering.

25. Closed-center Systems • In a closed-center system, the pump can be “rested” during operation whenever flow is not required to operate an actuator. • The control valve blocks flow from the pump when it is in its “closed” or neutral position. • A closed-center system requires the use of either a variable displacement pump or proportioning control valves. • Closed-center systems have many uses on agricultural and industrial equipment, but on trucks, they would be used on garbage packers and front bucket forks.

26. Calculating Force • In hydraulics, force is the product of pressure multiplied by area. • Force = Pressure x Area • For instance, if a fluid pressure of 100 psi acts on a piston sectional area of 50 square inches it means that 100 pounds of pressure acts on each square inch of the total sectional area of the piston. The linear force in this example can be calculated as follows: • Force = 100 psi x 50 sq. in. = 5000 lbs.

27. Hydraulic Components • Reservoirs • Accumulators • Pumps • Valves • Actuators • Hydraulic motors • Conductors and connectors • Hydraulic fluids

28. 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

29. Gas-loaded Accumulators • The gas and hydraulic oil occupy the same chamber but are separated by a piston, diaphragm, or bladder. • When circuit pressure rises, incoming oil to the chamber compresses the gas. • When circuit pressure drops off, the gas in the chamber expands, forcing oil out into the circuit. • Most gas-loaded accumulators are pre-charged with the compressed gas that enables their operation.

30. 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

31. 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.

32. 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

33. 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.

34. 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.

35. 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.

36. 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

37. 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.

38. 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.

39. 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.

40. 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.

41. 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.

42. 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

43. 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

44. Directional Control Valves (1 of 3)

45. 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

46. 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.

47. 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.

48. Single-acting Cylinders • Hydraulic pressure is applied to only one side of the piston. • Single-acting cylinders may be either: • Outward-actuated: When an outward-actuated cylinder has hydraulic pressure applied to it, the piston and rod are forced outward to lift the load. When the oil pressure is relieved, the weight of the load forces the piston and rod back into the cylinder. • Inward-actuated: When an inward-actuated cylinder has hydraulic pressure applied to it, the rod is pulled inward into the cylinder. • One side of a single-acting cylinder is dry. The dry side must be vented so that when oil pressure on the pressure side is relieved, air is allowed to enter, preventing a vacuum. • A ram is a single-acting cylinder in which the rod serves as the piston.

49. Double-acting Cylinders • Double-acting cylinders provide force in both directions. • Pressure is applied to one side of the piston to either extend or retract the cylinder; the oil on the opposite side returns to the reservoir. • Double-acting cylinders may be balanced or unbalanced. • Balanced double-acting cylinder • The piston rod extends through the piston head on both sides, giving an equal surface area on which hydraulic pressure can act. • Unbalanced double-acting cylinder • A piston rod is located on one side of the piston. There is more surface area on the side without the rod because the rod occupies part of the space on the other side.

50. Vane-type Cylinders • Vane-type cylinders may be found in some much older hydraulic systems. • A vane-type cylinder provides rotary motion. • Double-acting vane-type cylinders can be used in applications such as backhoes because they enable a boom and bucket to swing rapidly from trench to pile. • An alternative to one double-acting vane cylinder for this application would be a pair of opposing cylinders.