Hydraulics Part 2 pumps and actuators Flashcards
Describe a dynamic pump
There is no positive seal between the inlet and discharge
Why must hydraulics use positive displacement pumps?
Because pressure must remain in the downstream lines
What are the three types of positive displacement pumps?
They are classified the name of the part that does the pumping
Piston
Vane
Gear
What three ways can the output be changed in a hydraulic pump?
Use a flow control and relief valve
Change the speed of the driver
Use a variable displacement pump
What are the different types of piston pumps?
In line axial piston, bent axis piston,rotary piston, radial piston
What are the two types of pumps?
Positive displacement, and non positive displacement, which is also known as dynamic
What is the name of the device that tilts and varies output in an inline axial piston pump?
Swash plate
What are the names of the two Pistons that regulate the pressure limiting swash plate
The top one (smaller one) is called the bias piston, and the other is called a compensator control piston
What Are the three ways for controlling speed?
Conventional, pressure compensated, load sensing pressure compensated
What must you do when installing a load sensing pump?
It must be set with 200-400 psi standby pressure
Why should you monitor fluid viscosity in a system?
A decrease in fluid viscosity can lead to an increase in internal leakage
How is the bent axis piston pump different from the inline axial pump?
The angle is what determines displacement. There is no swash plate, the angle of the cylinder block determines it
What are some benefits of the bent axis piston pump?
It is the most rugged design, it can withstand high vacuum at the inlet, so it can be mounted above the fluid level in the reservoir
What are some maintenance tips for piston pumps?
Always prime the pump, ensure the case is full before startup, always connect case drain, maintain fluid cleanliness
Describe how a rotary piston pump works
Fluid is ported in and out through the centre, while the cylinder block rotates around it
What is the centre area of a rotary piston pump called?
Pintle
What do the Pistons ride on in a Rotary piston pump?
A retaining ring, or reaction ring
How is displacement changed on a rotary piston pump?
By moving the reaction ring because the pintle is fixed in the middle
How does a radial piston pump work
The cylinder is stationary and the center cam is eccentric and rotates
How does a vane pump rotate?
It has a splined rotor fitted with sliding vanes
What are the two types of vane pumps
Balanced and unbalanced
What does the vanes ride on
The Cam ring
Describe the inlet and outlet ports on an unbalanced vane pump
There is one on each side 180° apart
What is an advantage and a disadvantage of the unbalanced vane pump
And advantages it has a movable cam ring which allows for variable displacement. Disadvantage is it requires large bearings and casing to support the unbalanced load
What two ways are the vanes held against the cam ring?
Centrifugal force, and hydraulic pressure at the root of the vanes
Why is the balanced vane pump much more compact?
It has two pumping chambers that are 180° apart so the forces at one outlet act against the forces on the opposing outlet
If the balanced vane pump rotor is centered how does it create pockets?
It uses an elliptical cam
What is the name of the cutout on the root of the vane?
Intra vane
What are some benefits of the Intra vane?
More stable, less noise, less maintenance
What are the two types of gear pumps?
Internal gear and external gear
How is the pumping action created in an external gear pump?
The meshing of the two gears creates the pumping action
How is gear pump displacement adjusted?
It is not adjustable without a fluid bypass. You must adust the rpm of the prime mover
Why are radial loads high on gear pumps?
Because of the highly unbalanced hydraulic pressures from the inlet to discharge side. Therefore they use heavy needle bearings or bushings to support those loads
What are the two types of internal gear pumps?
The crescent seal type, and the gerotor type
What is the purpose of the crescent seal on the internal gear pump?
It creates the positive seal between the inlet and discharge
Describe the gerotor internal gear pump
It’s used mainly as a charge pump, has a star shaped rotor with one less tooth than the lobes on the outer gear
Describe the difference between single acting and double-acting cylinders
Single acting only exert force in one direction, double acting is both ways
Describe the characteristics of a telescopic cylinder
It is a series of sleeves nested together, provides a long stroke but very compact when retracted. As it extends the speed increases and force decreases due to the diminishing diameter of each sleeve
Describe a differential cylinder
Has the cap end on one side, and rod end on the other. More force is created on the cap end due to the larger area.
What is a double rod cylinder?
It is a non-differential cylinder so it produces equal force in speed in both directions. It can push and pull simultaneously
What is The difference between the rod seal and the rod wiper?
The rod seal keeps the fluid inside the piston, and the rod wiper keeps contaminants out
What is the purpose of the tandem cylinder
It is used for very high force and short stroke requirements
Describe cylinder cushion
When the plunger comes to the end it plugs off the flow of fluid which is then forced to go through a small controlled opening that is adjustable similar to a meter out flow control
What is the purpose of the cylinder cushion
It is used to prevent mechanical shock
What is the purpose of a stop tube
It is a metal collar that fits over the piston rod next to the piston it is used to reduce sideloading on longer rods
What is the two types of Rod supports and what is their function
Center support, and tie rod support. They are used to provide rigidity for long stroke cylinders
What is the most recommended port, and what kind of threads does it have
The O-ring boss or orb for short is the most recommended. It has a straight thread and then O-ring seal and seat
Where should you mount the bleed ports on a cylinder
You should try to mount the ports on top which allows any air trapped in the system to rise and escape through the port
Why should you check for hot spots on the cylinder?
Because it is an indicator of internal leaking of the piston rings
What is a hydraulic motors purpose
It converts fluid power into rotary motion
There are three types of motors what are they?
Low-speed, high torque (LSHT) for smooth operation under heavy loads
High-speed, low torque(HSLT) for operating continuously at constant high speeds
Limited rotation motors-used in some robotics
Describe the limited rotation motor symbol
It’s a half circle with two arrows pointing up
How is displacement measured
Measured in cubic inches per revolution.
Describe torque
It is the rotational force of the motor. It is expressed in foot-pounds
What three things does torque depend on?
Inlet pressure
Surface area
Radius of rotation
When you gain torque, what do you lose
Lose speed, and vice versa
What does motor speed depend on?
Flow rate, volume
What are the three types of internal gear motors
Direct drive gear motors
Orbiting gear Motors
Roller vane gerotors
How is a sliding vane motor different from a vane pump?
The vane motor has springs under it, to make contact with the cam, as it starts too slow to use centrifugal force
The axial piston motor adjusts torque with the swash plate how?
At maximum angle it has low speed, high torque, and minimum angle provides maximum speed, low torque
Describe the radial piston motor
It is used in severe service, it’s a low-speed high torque motor. It is known as a Staffa
What 5 things does hydraulic fluid do?
Lubricates, seals, energy transfer, heat transfer, Flushing contaminants
What other considerations must be taken with hydraulic fluid
It must be compatible, resist foaming, be fire resistant, and have a high viscosity index
Describe viscosity index
The ability for oil to maintain its properties over a range of temperature
What is the relativity between control and compressibility of fluids?
If the fluid is less compressible, it gives more accurate control. Some systems will use a more compressible oil, to act as a shock absorber
What are some characteristics of petroleum based fluids
They have a high viscosity index, are low cost and have low toxicity
Give some characteristics of water-based fluids
There more expensive than petroleum based, they’re heavier than petroleum based, they oxidize faster, and the absorb water readily
Give some characteristics of synthetic fluids
They are much more expensive, they have a high resistance to oxidation, a very high viscosity index, and increased life. Most synthetic fluid’s require special seal and hose material
What is the most prominent type of vegetable oil used
Soy oil
What are the main functions of the reservoir tanks
The reservoir tank is to hold a supply of fluid. It also warms or cools the fluid as needed, de-aerates the fluid, allows contaminants to settle out, reduces turbulence, and acts as a platform for the pumps and valves
What does the air filter on a reservoir filter down to?
3 µm
How is the tank made?
It is pickled and oiled (soaked in oil)
How big is the reservoir for an open circuit tank?
2 to 3 times the pump output
What is the purpose of the baffle
The barrier to direct flow from the return line away from the pump inlet. It prevents turbulence, promote cooling, promote settling, allows air to escape
How is the return line directed in the tank
Return flow was directed outward towards the tank wall away from the pump inlet
What is the inlet filter purpose
It filters down to about 100 µm. It’s purpose is to remove large contaminants that could cause Pump failure
When you see a hydraulic symbol with arrows pointing in or out what does it mean
Arrows pointing in means heat is moving in (heater)and arrows pointing out the heat is leaving(cooling)
What are the three types of reservoirs
Open, pressurized, closed circuit
Describe a pressurized reservoir
The supply of low pressure regulated air is pumped into the tank approximately 1 to 3 PSI above atmosphere. You use these when airborne contamination is too great for the breather, or atmospheric pressure is not adequate to push fluid into the pump. It also can solve cavitation problems
Describe a closed circuit reservoir
Most of the return flow goes directly back to the pump from the actuator. The reservoir holds approximately three quarters of the charge pump output
What are the four main reservoir mounts
Flat top, L-shaped, overhead stack, vertical mount
Describe the vertical reservoir mount
It is the most compact design, it increases cooling because the pump is immersed in the fluid, the pump system stays cleaner, less noise and sideloading and a positive suction head
What are the four common sources of contamination of oil
New Oil, built-in, ingressed, generated
Ingressed=taken into system through leaky seals, dirty piston rods
Generated=wearing parts
How do you prevent fine filters from getting clogged too quickly?
Install a coarse filter before to trap the bulk of the contaminants
Why should you never use galvanized fittings in the hydraulic system
The zinc flakes off and gets in the system and reacts with the fluid
What is the best way to get away from thread debris
Use O ring boss fittings
What is iso range code?
A way to describe three different particle size levels in a sample
Eg 15/9/8
What is the particle sizes we measure for?
2, 5, 15 µm
What size microns do the most damage in the system
14,6, 4 micron particle sizes
What is the difference between a strainer and a filter
A strainer is usually a wire mash with pore sizes larger than 50 µm. It is used at the pump inlet another feature is that you can wash it and reuse it
What components does a filter consist of
Metal case containing filter element, a bypass valve, a bypass indicator
What are the two types of filter elements
Pleaded fiberglass cloth supported by metallic mesh, or depth type media.
How does depth type media work?
It does not have a constant pore size, it captures dirt by forcing the flow through a torturous where the good becomes trapped these filters are not washable and must be discarded after use
Why is fiberglass superior to paper as a filter?
The glass fibers can be produced much more finely, they can be laid uniformly, they resist chemical action of the fluid
At what colour indicator should you change the hydraulic filter?
As soon as the indicator turns yellow. This minimizes the amount of unfiltered oil through the system
What must the strainer equipped with
A bypass to ensure full flow to the pump when the strainer clogs
Where should pressure line filters be located?
Before any contaminant intolerant device such as servo, proportional, instrumentation
How much slow should be through the return line filter to control contamination in the entire system
20% of the total system volume should pass through the filter per minute
What are some of the requirements of the return line filter
Must be able to handle the maximum return line flow with minimum pressure drop. Must be a full flow filter however I return line filter is equipped with the bypass valve that is preset to divert flow
What is the advantage of an off-line filter
Free of pressure pulsations, can be located where it is convenient to service
When must you use an off-line filter system
When you use a pressure compensated variable displacement pump that is running at standby for extended periods
When you see a brown sticky substance on a valve what is it
Gum and varnish
Explain the beta ratio and filter efficiency
It is how many contaminants does it take until one passes through. For example a beta ratio of 10 means for every 10 particles one will go through for a 90% efficiency
A beta reading of 20 means for every 20 particles that get filtered one gets through for a 95% efficiency
What methods can be used to remove water from a system
Drain it out of the hydraulic reservoir, coalescent filters, centrifuge, absorbent filters, desiccant air breathers
What is the main cause of oxidation
Heat. For every 10°C you increase the temperature of the petroleum based fluid above 60°C the oxidation rate doubles
When O-rings are used as stationary seals how much of their diameter must project above the groove?
10%
What is the purpose of an accumulator
It gives the supplementary flow when demand needs it
What kind of air is put into an accumulator
Nitrogen
Accumulator precharge varies with applications define them
For energy Storage applications pre-charged to 90%
For pulsation dampening applications precharge to 70%
For shock absorption applications precharge to 75%
What is another name for the swashplate
Yolk
What instruments are required to check the system
Flow meter, pressure gauge, thermometer, tachometer
What is the mechanism that protects the pressure gauge called
Snubber
What is another name for a pipe or tube?
The fluid conductor
How was all tubing and hoses measured?
OD
What happens when the ID of tubing is decreased
The velocity decreases
