Ch61 Principles of Hydraulics

What is the minimum speed the pump must rotate to supply 5.8 hp if it has 3 in3 of displacement per rotation?

1340 in3/min /3 in3 = 447 rpm P1760

Centrifugal pumps typically pressurize fluids to ____ psi and are not used for high-pressure hydraulic systems.

61-80 psi; p1763

3. Which of the following is responsible for producing pressure in a hydraulic circuit? A. circuit resistance B. Hydraulic pumps C. Prime movers D. Linear actuators

A. Circuit resistance;

4. Which of the following types of positive-displacement pumps operates at the highest pressure? A. Gear pumps B. Vane pump C. Piston pump D. Variable displacement pump

A. Gear pump; positive or fixed displacement pump.

10. Technician A says petroleum-based hydraulic fluids need additives to work well. Technician B says because synthetic hydraulic fluids have a high VI index, they thin out at high temperature and require additives. Who is correct? A. Technician A B. Technician B C. Both Technician A and Technician B D. Neither Technician A nor Technician B

A. Tech A, p1771; Petroleum-based hydraulic fluids are refined mineral oils with additives to improve fluid properties and characteristics. Petroleum oils offer advantages, such as low cost, wide availability , good lubricity, and low toxicity. Synthetic fluids are chemically stabile and have a low VI. This means the fluid viscosity does not change very much over a wide range of temperatures. Synthetic fluids are used in extreme operating temperature or pressure conditions where petroleum-based stock oils quickly break down or have performance problems.

What is a consequence of insufficient hydraulic fluid in the reservoir? A. The system runs erratically B. High system pressure C. High system flow D. System contamination

A. The system runs erratically, p1794

aeration

Air in the fluid.

____ is the formation of air or gas bubbles at the inlet of the pump because the pump does not completely fill with fluid. A. Displacement B. Cavitation C. Aeration D. Pressurization

B. Cavitation, p1793

In an electrical solenoid, what is used to push the valve spool into position? A. Sliding plate B. Hand-operated lever C. Plunger D. Spring

C. Plunger

Three-position valves will have a spring on each end of the spool to return it to its center position when the valve is not actuated. In what center position are all four ports connected? A. Float B. Closed C. Tandem D. Open

D. Open

9. Which fluid has the most viscosity when cold? A. ISO 32 B. SAE 15w C. ISO 150 D. SAE 50

D. SAE 50, 220 centistokes at 40 degrees Celsius. p1770-71, Table 61-1; Common hot SAE viscosity ratings are SAE20, SAE30, and SAE50. Cold ratings are designated by a W suffix such as 5W, 10W, 15W, and 20W. A lower ISO viscosity number is less viscous than a higher number since ISO viscosity rating numbers increase with higher oil viscosity. Temperature changes an oil's viscosity. Multi-grade oils have a lower viscosity index than single-weight oils. That means multi-grade oil viscosity does not change as dramatically with temperature.

PTOs are generally activated using a cable or air cylinder controlled ___ or ___ by the driver from inside the cab.

Electrically or mechanically; p1761

An accumulator is an

Energy-storage device. Since hydraulic fluid cannot be compressed, the hydraulic energy must be Changed into mechanical or pneumatic energy.

The latest PTOs used on buses, trucks, and emergency vehicles use a ___.

Wet disc clutch; p1761

Pascal's Law

states that "pressure applied to a fluid in one part of a closed system will be transmitted without loss to all other areas of the system."

Hydraulic accumulator

A device that stores hydraulic energy and acts as an emergency power source in the event of system pump failure.

Which of the following mechanisms consists of a solenoid-operated valve that allow s or disallows flow depending on the valve's activation or non-activation by an electric current? A. Rotating-plate B. Cartridge C. Spool-type D. Sliding-plate

B. Cartridge

There are several factors that can damage a hydraulic system. Excessive speed, heat, and pressure can result from ____. A. Pump cavitation B. Operator error C. System contamination D. Fluid leakage

B. Operator error, p1793

Tubing

Metal pipe that is intended to be bent and shaped to fit an application.

Chafing

Wear or abrasion due to prolonged constant friction.

Vane pumps and piston pumps

(But not gear pumps) can have a variable displacement capability. Variable displacement pumps have a Mechanism that allows the displacement to be changed.

A weight-loaded accumulator has

A cylindrical body with a piston separating the fluid from the mechanical energy force. A weight on top of the piston (or a very heavy piston) provides the force on the piston to expel the fluid from the accumulator.

Hydraulic cylinder

A device that uses hydraulic fluid pressure and converts it to linear mechanical movement. Also referred to as a ram.

Telescoping cylinder

A linear single acting cylinder extending from a series of nested cylinder sections.

Swashplate

A plate in a variable displacement pump the tilts and changes the stroke length of axial pistons.

Pipe

A rigid tube of metal, plastic, or other substance, used to conduct a gas or fluid flow

Variable displacement pump

A type of pump with the ability to change output volume as in a pump with a mechanism that allows the output to vary.

Fluid___ is caused by outside air entering the fluid. A. Aeration B. Cavitation C. Displacement D. Motion

A. Aeration, p1793

Which is the most common directional control valve mechanism for hydraulic applications? A. Spool-type B. Sliding plate C. Rotating plate D. Cartridge

A. Spool-type

6. Technician A and B were examining some milky-colored hydraulic fluid. Technician A says the fluid is likely water-contaminated. Technician B says a vacuum pump should be used to evacuate the pressure in the fluid reservoir to correct the problem. Who is correct? A. Technician A B. Technician B C. Both Technician A and Technician B D. Neither Technician A nor Technician B

A. Tech A

Tech A says that as with every heavy-duty vehicle system, preventative maintenance is critical for the hydraulics on a vehicle. Tech B says that preventative maintenance is unnecessary for hydraulic systems. Who is correct? A. Tech A only B. Tech B only C. Both Tech A and Tech B D. Neither Tech A nor Tech B

A. Tech A only

1. Technician A says a single-acting cylinder relies on the weight of the load to return it to the original position. Technician B says a double-acting cylinder is also load dependent. Who is correct? A. Technician A B. Technician B C. Both Technician A and Technician B D. Neither Technician A nor Technician B

A. Tech A, p1756-59, 1783. Increasing the surface area of the output cylinders (double-acting cylinder) multiplies force but shortens distance traveled. In a single-acting cylinder, the oil flows into only one side of the cylinder, normally to extend the cylinder, and can be retracted by oil flow. Cylinders are returned to their original positions by either the load or a spring. In a double-acting cylinder which has two oil ports, oil flows into one port to extend the cylinder, and oil flows into the other port to retract the cylinder.

4. Technician A says to detect hydraulic pinhole leaks, a piece of cardboard should be used. Technician B says pinhole leaks can be identified by wearing gloves and checking for fluid by "wetting" the gloves while moving one's hand around potential leak points. Who is correct? A. Technician A B. Technician B C. Both Technician A and Technician B D. Neither Technician A nor Technician B

A. Tech A, p1775; Never check for hidden leaks with a hand. Always use a piece of cardboard or paper to avoid hydraulic injection of fluid through the skin.

10. The flow characteristics of lines and hoses used in a hydraulic circuit is best predicted by: A. The Bernoulli principle. B. Pascal's Law C. Line length and outside diameter. D. Law of Energy Conservation.

A. The Bernoulli principle, p1774; Fluid pressure and velocity vary as the line narrows or widens. When fluid velocity increases, fluid pressure must drop if the volume is to remain constant. As the line narrows, fluid velocity increases and the pressure decreases. As the line widens, the fluid velocity decreases and the pressure increases. The Bernoulli principle applies to all three types of hydraulic lines, pipes, tubes, and hoses. Pascal's law also applies to the working and burst pressure of hoses and pipe. Working pressure is the maximum pressure a hydraulic conductor can safely hold. Burst pressure is working pressure plus a safety factor for unintended pressure changes. Increasing the inside diameter (ID) of a hose reduces its working and burst pressure because the conductor has a larger ID surface area acted on by fluid pressure.

Cavitation is the formation of

Air or gas bubbles at the inlet of the pump under low pressure when the pump does not completely fill with fluid. The term is also used to describe the collapse of air and gas bubbles in the pump, which erodes metal and creates a distinctive noise.

Most hydraulic coolers are ___ type and very similar to engine coolant radiators.

Airflow; p1773. When hydraulic fluid in the reservoir is cold, a thermostatic or pressure-based bypass valve directs warm returning fluid directly to the reservoir. As fluid temperature rises and viscosity decreases, a greater volume of fluid is passed through the cooler.

Where are positive displacement pumps used?

Almost all truck and bus hydraulic systems. Fixed displacement pumps not only have a fixed output that varies only with speed, but each rotation of the pump produces a specific volume of fluid, that changes linearly with speed. For example, if a positive displacement pump produces 5 in3 of fluid each time it rotates, the flow rate from the pump is easily calculated by multiplying rpm times the pump's rated displacement. This pump displaces 500 in3 of fluid at 100 rpm and 5000 in3/min at 1000 rpm. When graphed, output volume is linear compared to speed.

Cylinder

An actuator that converts hydraulic power into linear mechanical force. Also known as ram.

A directional control valve is

An important component within a hydraulic system. Its function is to Direct hydraulic oil flow within the circuit.

Trunnion-mounted

Articulating connection points on the ends of a hydraulic cylinder.

5. Which of the following calculations is used to correctly predict the output force of a single acting cylinder? A. Pressure x volume B. Pressure x piston surface area C. Work/time D. Rpm x pump displacement / rotation

B. Pressure x piston surface area; p1756; Force = Pressure x Area

Tech says that the lower parts of the spool (the parts with the large diameters) are called lands). Tech B disagrees saying the raised parts of the spool are called lands. Who is corrects? A. Tech A only B. Tech B only C. Both Tech A and Tech B D. Neither Tech A nor Tech B

B. Tech B only

8. Technician A says the same pressure applied over different-sized surface area produces the same level of force. Technician B says the same pressure applied over different-sized surface areas produces different levels of force. Who is correct? A. Technician A B. Technician B C. Both Technician A and Technician B D. Neither Technician A nor Technician B

B. Tech B, p1758; The ratio of input force to output force depends on the relative differences in surface areas of a hydraulic circuit. The input force of 50 lbs generates 39.28 psi with a one-inch master cylinder bore. Acting on various surface areas, wheel cylinders having half, the same or, twice the surface area of the master cylinder bore each produces a different output force. Force is either multiplied or reduced 1:0.5, 1:1, or 1:2 demonstrates the mechanical advantage ratio which corresponds to each cylinder area ratio.

7. Technician A says a defective relief valve could cause the hydraulic cylinders to move slowly and lack normal force. Technician B says a variable displacement pump with a high swash-plate angle could produce those symptoms. Who is correct? A. Technician A B. Technician B C. Both Technician A and Technician B D. Neither Technician A nor Technician B

B. Tech B, p1766 and 1790; A fixed displacement piston pump has a non-adjustable component called a swashplate that is tilted at an angle and connects to the backside of each piston with a connecting rod and articulating joint. The pump driveshaft turns the cylinder block and the swashplate, but the swashplate angle remains stationary or fixed. The arrangement causes the pistons to move back and forth in the block's cylinder bores to move fluid in and out of the pump. Pump output flow is directly proportional to input shaft speed. The variable-displacement version of this pump uses a mechanism to tilt or change the angle of the swashplate. When verticals, no reciprocating piston action takes place in the cylinders. Without a swashplate angle, both the block and swashplate turn using little energy. By tilting or changing the angle of the swashplate, reciprocating action takes place. The displacement of the pump increases as the swashplate angle moves from vertical to an angle of a maximum of 30 to 40 degrees.

2. Technician A says check valves allow fluid flow in both directions. Technician B says check valves prevent flow in one direction and allow free flow in the opposition direction. Who is correct? A. Technician A B. Technician B C. Both Technician A and Technician B D. Neither Technician A nor Technician B

B. Tech B, p1790; Check valves are used to block fluid flow in one direction but allow flow in the opposite direction.

3. Technician A says oxygen should be used to charge an accumulator. Technician B says the accumulator should be fully charged before it is removed from the system. Who is correct? A. Technician A B. Technician B C. Both Technician A and Technician B D. Neither Technician A nor Technician B

B. Tech B, p1791-92; Oxygen is not suitable for use in gas-charged accumulators, since these gases will ignite and explode when contacting pressurized petroleum based hydraulic fluids. Nitrogen, an inert gas, is used.

5. Technician A says before servicing a hydraulic system, the directional control valves should be cycled through all positions and centered. Technician B says booms and buckets should be placed in a resting position and any pressurized hydraulic components relieved of pressure. Who is correct? A. Technician A B. Technician B C. Both Technician A and Technician B D. Neither Technician A nor Technician B

C. Both Tech A and Tech B, p1775-76;

6. Which type of pump is best adapted for a hydraulic system using a closed-center directional-control valve? A. Gear pump B. Variable displacement pump C. Vane pump D. Belt-driven pump

C. Vane pump, p1786; In a closed center system, flow is intermittent, but pressure is continuous. These systems commonly use a variable displacement-type pump that senses the change in pressure or flow or both pressure and flow to increase pump output.

Needle valves are the most

Common type of flow control valve. They are used to Control and maintain the hydraulic fluid at a set and required pressure inside the hydraulic system.

Excessive speed, excessive heat, excessive pressure, and

Contamination can all damage a hydraulic system.

A spring-loaded accumulator has a

Cylindrical body with a piston to separate the fluid and the mechanical energy force. A strong Spring provides the force on the piston to expel the fluid from the accumulator.

7. What is the inside diameter of a hose 100R4-18? A. 1" B. 18 mm C. 18 Gauge D. 1 - 1/8"

D. 1 - 1/8", p1778; Hose dimensions are based on a "dash" number size system where each dash equals 1/16". This means a 100R1-16 hose number is dash 16 (-16) is equal to one inch.

8. Which hose is most likely to have the largest internal diameter? A. A return circuit hose B. A medium-pressure hose C. A high-pressure hose D. A suction side hose

D. A suction side hose, p1779; The supply or suction line form the tank or reservoir to the pump has very low or even negative pressure that requires a large ID to provide adequate flow of fluid to the pump inlet.

2. Which of the following components in a hydraulic system stores hydraulic energy? A. Actuator B. Reservoir C. Pump D. Accumulator

D. Accumulator, p1791. A hydraulic accumulator stores hydraulic energy.

1. Which of the following devices converts fluid pressure into mechanical force? A. Hydraulic pump B. Prime mover C. Directional control valve D. Linear actuator

D. Linear actuator, p1756, Fluid pressure is converted into mechanical force by hydraulic actuators, a term given to system output devices, such as cylinders and motors. P1763, Hydraulic pumps extract the mechanical energy from the prime mover, converting it to hydraulic energy present in fluid flow. Pumps only produce liquid flow or volume but do NOT produce pressure.

9. Technician A says a corroded steel line conducting high-pressure hydraulic fluid can be replaced by a thicker walled, non-corroding copper line. Technician B says a galvanized line is a better choice for replacing the steel line. Who is correct? A. Technician A B. Technician B C. Both Technician A and Technician B D. Neither Technician A nor Technician B

D. Neither Tech A nor Tech B, p1776-1777; Never use galvanized pipe or fittings for hydraulic system applications. The zinc coating can react with the hydraulic fluid and flake off the pipe walls and fittings, resulting in fluid contamination. Never use copper tubing for hydraulic system applications. Copper does not have adequate tensile or burst strength to tolerate high hydraulic pressure.

Flow from a positive displacement pump, also called a ___ , varies according to rotational speed.

Fixed displacement pump; p1763

Hp is

Flow x pressure / 1714. The constant 1714 is industry used. It equals the energy required to pump 1714 gallons per minute (gpm) at 1 psi (1 hp = 1714 gpm x 1 psi). A hydraulic hp is also equal to 231 in3/min at 1 psi. Hydraulic pump displacement is commonly measured in cubic inches of displacement per rotation. For example, a hydraulic system requires a flow rate of 10 gpm at an operating pressure of 1000 psi. The hydraulic hp requirement is: 10 x 1000 / 1714 = 5.8 hp 5.8 hp x 231 in3/min = 1340 in3/min P1760

The Bernoulli principle describe the behavior of a

Fluid, gas, or liquid as it flows through a pipe, tube, or hose. Bernoulli stated that If the flow of a liquid remains constant, the pressure or speed of fluid must change, for flow volume to remain constant. Changing one of the two flow measures (up or down) affects the other oppositely.

Never use galvanized pipe or fittings

For hydraulic system applications. The coating can React with the hydraulic fluid and come off the pipe walls and fittings, causing contamination and damaging components.

Seamless pipe

Formed in one piece; lacking seams, thus smooth and regular.

Hydraulic systems fall into two basic types: open loop and closed loop. In an open-loop system, fluid flows

From a reservoir to the pump, through the system, and back to the reservoir, where it is directed to the pump again. In a closed-loop hydraulic system, no reservoir is used. Fluid circulates from the pump, through the system, and directly back to the pump.

____ pumps are the most common design in use for truck-mounted hydraulic systems. These pumps typically build pressure to ___ psi.

Gear, 2500; p1764

Piston pumps are

High-pressure pumps that maintain a constant, regular oil flow. They can operate more efficiently with High and low shaft speeds, can tolerate higher pressures over longer periods, and are generally less noisy than gear pumps.

Gear pumps consist of a

Housing, a drive gear (driven by the shaft), a Driven gear, and other mechanisms, such as pressure plates.

Hydraulic systems on trucks and buses take mechanical rotary power from a prime mover and convert it to a ___ pressure inside a ___ pump.

Hydraulic, hydraulic; p1761

Regular preventative maintenance prolongs the

Life fo the system, reduces system down-time, operational and repair costs, and Potentially eliminates preventable failures. ideally, preventative maintenance should be performed when a machine is not scheduled to be unit use.

Pumps only produce ____ or volume, but do not produce ____. Pressure is created only when ___ is restricted or stopped.

Liquid flow, pressure, flow; p1763

A ___ is a ___ device that is frequently used on commercial vehicles to engage and disengage a pump drive mechanism front eh engine or transmission supplying mechanical power.

Power-take off (PTO) is a gear-driven device; p1761

The main types of pressure-control valves are

Pressure relief valves, unloading valves, sequencing valves, pressure-reducing valves, brake valves, and counterbalance valves.

The purpose of a hydraulic pump is to

Provide the system with a constant supply of hydraulic fluid. It does this by producing flow in the system. The fluid flow is used by actuators in the system to perform work.

Engine-mounted PTOs today are often driven from the ___ of the engine because ____ from the ___ of an engine are lower compared to a front-mounted take-off.

Rear, torsional vibrations, rear; p1761.

A vane pump consists of a

Rotor containing sliding vanes in slots. The rotor is Offset from the centerline of the housing.

Rarely are hydraulic pumps belt-driven because ___

The belt drive system has limitations to transmit torque. Maximum output capabilities are no more than 15 gallons per hour (gph) displacing 2.5 in3/rotation. ; p1763

Hydraulic actuators convert

The fluid energy back into mechanical energy to move the load. They can come in two forms: Linear actuators, such as hydraulic cylinders or rams, and rotary actuators or hydraulic motors.

cavitation

The formation of air bubbles in fluid caused by lower boiling point produced by reduced pressure.

burst pressure

The maximum pressure a manufacturer desig- nates a hose to withstand before a particular hose explodes or ruptures.

Viscosity

The measurement of a resistance to flow of a liquid.

Working pressure

The normal range of pressure within a hydraulic system while the system is being operated.

Undercuts

The smallest diameter in a spool; Used to create a flow path when the valve is open.

The main differences between pipe and tubing are

Their construction materials and applications. Piping is intended for industrial applications requiring Long, straight runs, and is not normally bent or shaped, other than in smooth turns. tubing is designed to be bent and shaped to accommodate the installation on the machine.

Pascal's law state that pressure applied to a fluid in one part of a closed system will be

Transmitted without loss to all other areas of the system." This means that pressure applied to a fluid in one part of a closed system is transmitted equally to all other areas of the system.

Vital functions of hydraulic fluid include

Transmitting energy lubricating components, preventing rust and corrosion, sealing clearances, carrying solid contaminants to filters, and providing electrical insulation for certain applications. These capabilities make hydraulic fluid one of the most important components of any hydraulic system.

Oxygen is not suitable for

Use in gas-charged accumulators, because It may cause an explosion if it comes into contact with the hydraulic fluid. Nitrogen, which is almost inert in its natural state, is commonly used.

The fundamental operating concept of all hydraulic applications is the

Use of fluid to transmit power. One of the main advantages of hydraulic systems is that they can be designed to lift very heavy objects through the use of mechanical advantage and by using the fluid as the transfer medium.

Check valves are

Used to prevent flow in one direction, but Allow free flow in the opposite direction.

Accumulators are

Used to store energy, and, therefore, can pose significant safety hazards. Understand the safety hazards and always use appropriate safety precautions.

More efficient, but more complex and expensive, ____ displacement pumps have a mechanism to change the pump ___ independently of rotation speed.

Variable, flow; p1763.

The two measures used for fluid flow are

Velocity and pressure. When velocity increases, the pressure must Drop if the flow is to remain constant. Fluid pressure and speed vary as a hydraulic line narrows or widens.