Midterm Flashcards
Common uses of hydraulics
Marine - cargo handling, steering, elevators, capstans, emergency shutdown, etc
construction - cranes, dump trucks, etc
cars and bus - steering, transmission, brakes
Hydraulics meaning
Greek: “hydro” - water, “aulis” - pipe
a power system that uses liquid to provide the force to do work
pneumatic system
system that uses air to provide force to do work
force definition
any influence capable of causing change in the motion of a body
A hydraulic pump provides force via fluid to a hydraulic actuator
changes in motion
- cause a body to move
- retard or stop a body that is moving
- change in direction
Weight definition
weight is the result of gravity pulling on an object. it is always a downward force. In a hydraulic system, the fluid in the reservoir, in the lines or in a component
has weight. This is true whether the fluid
is moving or not.
Mass definition
- All objects have mass (substance).
- Mass is a measure of the amount of matter in an object and its resistance to change.
- The mass of an object determines how much force is required to change its motion.
- Mass is measured in pounds (lbs) or kilograms (kg).
Resistance definition
Any force which can stop or retard the movement of a body is a resistance. Examples of resistance are friction and inertia during acceleration.
friction as resistance
Frictional resistance is always present between the contacting surfaces of two objects when they are moving across one another.
inertia as resistance
Inertia is the reluctance of a body to change in its motion.
Inertia is directly related to the quantity of matter in a body. The more mass or matter an object has, the heavier it is, and consequently the harder it is to move.
energy definition
A force which can cause a body to move is energy.
inertia as energy
Inertia, being the reluctance of a body to change in its motion, can also be energy.
A moving body exhibits a reluctance to be stopped, and can, therefore, strike another body and cause its motion.
law of conservation of energy
energy cannot be created or destroyed - it can only be
transformed.
For instance, electrical energy may be changed to several other forms. Depending on what device or appliance is plugged into the outlet, electrical energy
changes to light energy, heat energy, mechanical energy, or sound energy.
A certain amount of energy is expended, when doing work, to over come friction or resistance. This energy is not lost, but changed into heat.
kinetic energy
energy in motion
potential energy
stored energy
types of energy in a hydraulic system
electrical, hydraulic, heat, potential, kinetic
work definition
- Work takes place when a body or object is moved through a distance by force.
- The amount of work accomplished is expressed in foot-pounds (ft-lb) or Newton-meters (N-m)
power definition
Power is defined as the amount of work, done in a given amount of time. usually minutes, sometime seconds.
pressure definition
Pressure is a measure of a force’s intensity. To determine pressure the total force is divided by the area (square inches or square centimeters) on which it is acting.
- pounds per square inch (PSI)
- bar (bar)
- kilopascals (kPa)
Bernoulli’s Principle
the total energy of the fluid always remains constant. Therefore, when fluid flow in a hydraulic system increases the pressure must decrease.
efficiency definition
a system’s ability to change energy from one type or source to another with the minimal of loss.
hydraulic system definition
a closed piping circuit in which a liquid under controlled pressure is used to do work.
basic hydraulic system
- Reservoir - to store the hydraulic fluid.
- Pump - to provide fluid pressure to the system.
- Control valve - to direct the flow of fluid.
- Actuating unit - such as cylinder to do the work.
- Suitable hydraulic fluid.
- Piping or tubing to move the fluid.
additional hydraulic system components
- Filters
- Pressure-regulating valves
- Accumulator
- Check valves
- Hand pump
- Pressure gages
- Relief valves
- Piping & Tubing
- Control valves
hydraulic fluid functions
to transmit power from one point in the
system to another and to lubricate the contacting surfaces of the mechanical working parts of the system.
viscosity
Saybolt Universal Seconds (SUS) or Centistokes (eSt).
most important physical property of hydraulic fluid. thickness or resistance to flow and its measure of the fluid’s internal friction. affects a fluid’s ability to lubricate two contacting surfaces.
International Standard Organization (ISO) has established a table of ISO grades for industrial oils based on viscosity ranges at different temperatures.
too high viscosity problems
- High resistance to flow
- Increased power consumption
- High temperature caused by friction
- Increased pressure drop due to resistances
- Possibility of sluggish or slow operation
- Difficulty in separating air from oil in reservoir
too low viscosity problems
- Internal leakage
- Excessive wear and even seizure of internal parts
- Pump efficiency may decrease
- Leakage losses may result in increased temperatures
fire hazard
Most hydraulic fluids are petroleum based, with a fairly high vapor and flash point.
However as with most oils, when sprayed as a mist under high pressure can be very combustible
petroleum based oil advantages
- Excellent lubricity and anti-wear properties
- More oxidation resistant at high temperatures
- Higher viscosity index
- Naturally protects against rust and corrosion.
- Dissipates heat well.
petroleum based oil disadvantages
burns
water based oil advantages
- Highly fire resistant
- Excellent cooling characteristics
- Inexpensive
water based oil disadvantages
- Low maximum operating temperature (120 F)
- Poor rust and corrosion protection
- Hard to incorporate additives
- Must be careful to use only proper seal materials.
synthetic advantages
- Highly fire resistant
- Excellent cooling characteristics
- Operate well at high temperatures
- Suitable for higher pressure systems
synthetic disadvantages
- Do not operate well in low temp systems
- May require special seals
- Very expensive
Reservoirs
one of the most important components
- stores fluid for use in system
- Acts as a heat exchanger, dissipating heat through its walls.
- Allows entrained air to rise and escape from the fluid.
- Allows solids and impurities to settle out of the fluid.
- May be used as a platform to mount the pump and components.
Reservoir components
- Oil Level Gage – To monitor fluid levels
- Breather Assembly – A vented and filtered breather to accommodate the air exchange from the constant change in temperature and pressure.
- Filler Opening – To add fluid to the system, is usually fitted with a removable fine mess screen.
- Clean-out Plates – to accommodate the servicing and cleaning of the sediment from the bottom of the reservoir.
- Baffle Plate – These helps slow the migration of hot returning oil to the pump suction.
- Line connections and fittings – usually piped below the surface of the fluid to reduce foaming.
- oil heater
- water oil cooler
- air oil cooler
function of hydraulic fluid
- Transmit Power
- Cool or dissipate heat
- Lubricate moving parts
- Seal clearances between parts
90 % Hydraulic failures are due to poor fluid condition
silting
Very fine particles, smaller than a component’s mechanical clearances that collect in the clearances and block lubrication.
contamination - types and sources
solid particles, heat, air and gas, chemicals, water.
comes from inside and outside the system
contamination - sizes
large - block openings
medium - block or cause wear
small - erode surfaces
heat as contaminant
causes fluid to wear out faster
mechanisms of wear
- Abrasion – Particles grinding between moving parts.
- Erosion – High velocity particles striking Surfaces
- Adhesion – Metal to metal contact
- Fatigue – Repeated stress of a surface
- Cavitation – Restricted flow to pump inlet
- Corrosion – Foreign substance in fluids (chemicals or water)
- Aeration – Gas bubbles in fluid
mechanical clearance size
high pressure - up to 5 microns
low pressure - up to 20 microns
filter cart
portable filtration. supplements the filters in a system. should be used when adding fluid to a system.
in system filters
– pump suction pipe (inlet) this is usually a strainer.
– pump discharge pipe (pressure line).
– return pipe line.
– kidney loop.
pressure line filter
• Filter must be designed for pressure line application.
• Can trap much smaller particles than an inlet or suction line filter.
• Might be used where system components are less dirt-tolerant.
• Changing a pressure line filter requires, securing the system.
(Remember to Lock-out / Tag-out)
return line filter
– Also good at trapping very small partials.
– Is nearly a must in a system with high-performance components.
– Need to be full-flow, capable of handling full system flow with minimum of pressure drop.
– Must be fitted with internal bypass, be sure to check the pressure setting.
Establishing & Maintaining Desired Cleanliness considerations
ALL systems must have an establish target cleanliness level.
Consider: – Environment – Pressure – Duty Cycle – Location of filter in the system – How clean the system needs to be
What is the ISO requirement for the most critical component, which is typically the pump
filter considerations
– Minimum size of particles to be controlled. – The quantity or weight of the particles to be held. – Flow rate capacity. – Pressure rating. – Pressure drop. – Type of condition indicator. – Specify ISO Beta Code – Specify Micron size – Specify GPM – Filter Mounting type
filter definition
a device whose primary function is the retention, by some porous medium, of insoluble contaminants from fluid.
strainer definition
a coarse filter, usually metal, with pores larger than 50 microns.
beta ratio definition
the Beta ratio of a filter during steady state flow is the count upstream divided by the count downstream of fine test dust.
filter/strainer mounting types
cartridge, bowl, screw-on
