Hydraulics

Four different types of hydraulic cylinder mountings:

- Foot and Centerline Lug Mounts - Rectangular Flange Mount - Square Flange Mount - Trunnion Mount - Clevis Mount - Flush Side Mount - Intermediate Fixed Trunnion - Extended Tie Rod - Double Rod End

Key Advantages of Hydraulic Fluid Power

- mechanical advantage (force or motion) -accurate, repeatable, precise motion -constant force, torque produced -reversible motion -variable speed -good power to weight ratio

Three-way Valve

-3rd port provides return flow to reservoir

Four-way Valve

-Allows both working and return flow through same valve position

Unloading Valve

-NC -allows full flow at low pressures = open, low pressure flow path for full pump flow to reservoir -pilot line senses high pressure branch -isolated from valve by check valve

Sequence Valves

-NC -controls order of operation of two branches of a system in a pressure sequence -pressure in one branch controls flow in a secondary branch

Deceleration Valve

-NO -provides automatic speed control of cylinder -must drain spring cavity of any leakage past spool = avoid hydraulic lock -prevents damage to valve -cam has free play at each end of range of motion

Pressure Reducing Valve

-NO -used to operate one or more branches of a circuit at pressures lower than system pressures -senses downstream pressure and responds to it

Four ways to save Pump Energy Costs and Reduce Heat Generation:

-Pressure Compensated Pump -Tandem Center DCV -Unloading Valve -Accumulators

Two-way Valve

-Serves as shut-off valve -NO or NC

DCV Attributes

-Valve porting = # of ways -Number of positions (envelopes) -Methods of actuation -Neutral position = unactivated position

Purpose of a stop tube and when is it appropriate to use:

-When requirements call for unusually long strokes with rod in compression (push load) -Stop tube keeps piston and rod separated, reduces loading and moves fulcrum point

Meter-in Flow Control Circuit

-controls rate of flow into actuator -rod side pressure is very low due to return flow back to reservoir

Meter-out Flow Control Circuit

-controls rate of flow leaving cylinder -creates back pressure on actuator

Gerotor Pump

-eccentrically mounted inner gear drives outer gear -inner gear has one less tooth, rotates faster and creates increasing and decreasing volume

Proportional Control Valves

-electro hydraulic proportional valves -solenoid produces force proportional to current in coils; controls position of valve -provides directional control and flow control or pressure control

Oxidation Rate depends on:

-fluid chemistry -amount of oxygen -fluid temperature -amount and type of particulate contamination -component materials

Heat Exchanger

-heat generation due to fluid friction, pump, control valves and pressure relief valve -must dissipate heat at rate heat is generated -excessive temperature causes increased oxidation rate

Cartridge Valves

-installed in a cavity in a ported manifold block -integrate various cartridge valves and other components into single, machines, ported manifold block Advantages: -flexibility -compact -easily maintained -low cost -simplifies physical circuit

Working Pressure

-maximum safe operating pressure with factor of safety applied

Mixed Film Lubrication

-not sufficient film thickness to completely separate surfaces -fluid carries part of load, asperities carry remainder = has lowest coefficient of friction

Pressure Control Valves

-only pressure reducing valve actually controls pressure -the rest sense pressure and automatically adjust to respond to the senses pressure - secondary effects: flow rate (decreased), direction of flow

Check Valves (Application)

-positive lock for holding suspended loads (safety) -safety bypass; provides alternative flow paths around equipment -facilitates maintenance

Burst Pressure

-pressure at which hoop stress within pipe equals yield strength of pipe material -plastic deformation begins to occur

Pressure Compensated FCV

-purpose is to create a pressure drop that keeps dP constant across needle valve -maintain output flow within +/-%5 of setting over wide pressure range

Full Film Lubrication

-thick enough to prevent any asperity contact -virtually no surface wear

Four Main Functions of Hydraulic Fluids

-to transmit power -to lubricate internal moving parts of components -to seal small clearances within components -to act as a medium for heat transfer

Counterbalance Valve

-used with vertically operating cylinders supporting heavy loads -prevents uncontrolled load drop -safety device

Accumulator Types:

1) Weight-loaded accumulator 2) Spring-loaded accumulator 3) Gas-loaded accumulators

Accumulator Applications

1. Auxiliary Power Source =stored oil delivered by pump can do portion of work cycle -intermittent operations 2. Leakage Compensator -for internal or external leakage during extended periods in which system pressurized but not in operation 3. Emergency Power Source =if safety dictates that cylinder be retracted in event normal power is lost 4. Hydraulic Shock Absorber -elimination or reduction of high pressure pulsations or hydraulic shock loading sudden stoppage of fluid flow

Explain the operation of Gear Pumps

1. Partial vacuum created at inlet port as gear teeth un-mesh (increasing volume). 2. Atmospheric pressure pushes fluid into the inlet port from the tank. 3. Fluid is trapped between gear teeth and housing. 4. As gears rotate, fluid is carried in fixed equal quantities around to the discharge port. 5. Fluid at discharge port is "squashed" out as teeth re-mesh together and the volume between teeth decreases.

How does a Positive Displacement Hydraulic Pump work?

1. Pump generates an increasing volume in the suction side, causing a partial vacuum. 2. Atmospheric pressure, or pressure in the tank, pushes fluid from the tank and into the intake port of the pump. 3. Pump creates a decreasing volume at the pressure side, forcing fluid out via the discharge port.

Needle Valves

3 factors affecting flow across an orifice: 1. Orifice size 2. Pressure drop across orifice 3. Fluid viscosity

Pressure Compensated Axial Piston Pump

= automatic method for variable volume flow -uses pressure feedback to adjust swash plate angle and volume displacement from maximum flow to zero -used to maintain steady pressure on system, adjust flow on demand

Accumulators

=Stored potential energy -quick secondary source of fluid power capable of doing useful work

What is a cylinder cushion and what is its purpose?

A cushion placed at the piston end of a cylinder used to slow down the piston near the end of its stroke.

What is a double-rod cylinder and what is it used for?

A cylinder in which the rod extends out of the cylinder at both ends. Typically used when equal displacement is needed on both sides of piston, load coupled to each end of cylinder, mounting cams to operate limit switches.

Oxidation Stability

Ability of fluid to resist chemical degradation by reaction with atmospheric oxygen.

Filters (Operation)

Air line filters -installed in pneumatic system pipeline: i) downstream of dryer/separator ii) near connection of tool or actuator Deflector vanes - causes air to swirl Shroud - ensures even air flow over filter element Baffle plate - creates 'quiet zone' at bottom bowl Bowl with drain (filter serparator) - handles water and dirt - centrifugal force from deflector throws water droplets and solid particles to wall

Shuttle Valves

Allows system operation from two alternate power sources = ensures backup power

Explain the operation and features of Vane Pumps

As rotor turns, vanes are thrown out by centrifugal force, and track along outer ring. As vanes make contact with ring, positive seal formed between vane tip and ring. As rotor turns, an increasing and decreasing volume is formed within the ring. Port plate separates low pressure and high pressure sides. Fluid trapped in voids between vanes, ring and motor.

Name the two basic types of piston pumps:

Axial Piston Pump Radial Piston Pump

Piston Motors

Bent axis piston motor Radial piston motor - medium & heavy duty applications - can handle overrunning loads and provide braking at very low speeds

Flow Directing Valves

Bypass flow diverts part of flow back to reservoir.

Sequencing Circuits

Causes operations in a circuit to behave in a pressure sequence.

Check Valves

Controls direction of fluid flow. -Standard: one directional -Pilot operated: allows reverse flow; pilot line senses high pressure

DCV's

Direct flow or prevent flow through selected lines

Regulators

Ensures constant pressure is available for a pneumatic system.

Lubricators

Ensures proper lubrication of internal moving actuators, valves, and components. - function is to inject small amount of oil into air stream

Differentiate between first-, second-, and third-class lever systems used with hydraulic cylinders to drive loads.

First-class: Lever fixed-hinge pin located between the cylinder and load rod pins. Second-class: Load rod pin located between the fixed-hinge pin and cylinder rod pin of the lever. Third-class: Cylinder rod pin located between the load rod pin and fixed-hinge pin of the lever.

Differentiate between Fixed Displacement and Variable Displacement Pumps

Fixed Displacement: Pump ejects a fixed amount of fluid into the hydraulic system per revolution of pump shaft rotation. Variable Displacement: The flow rate and outlet pressure can be changed as the pump operates. (Low-pressure, high-volume flow applications)

Differences between extension and retraction strokes of a double acting cylinder in terms of force capability and movement speed:

Force Capability - higher during extension stroke since pressure acts over full face of piston. Speed - higher during retraction stroke since less area and volume displaced during travel. -acting on annulus area of piston/rod end

Reservoirs

Functions: a) Reservoir capacity based on floor are of pump. -when all actuators are fully extended -enough fluid for pump to continue pumping -allow fluid to drain back to reservoir when system shut down b) Fluid conditioning = significant function -retention time to allow, i) allow air to leave fluid ii) allow heat dissipation (large surface area) iii) baffles to help retention time - allow dirt, particles to settle - allows water to settle - reduce turbulence c) Return line below min. fluid level in reservoir - avoid foaming of oil d) suction line near bottom to avoid vortex formation (whirlpool)

Types of full rotation motors

Gear Motor (external) LSHT Gerotor Motor Vane Motor -spring doves vanes out following cam surface

Name three popular construction types of positive displacement pumps:

Gear pumps Screw pumps Vane pumps Piston pumps

Explain the operation and feature of Piston Pumps

Generate pumping action by causing pistons to reciprocate within a piston bore. (Generates increasing and decreasing volume by means of pistons reciprocating in and out of a cylinder barrel as the shaft of cylinder barrel turned by prime mover) Complex in design with closer tolerances, and higher pump costs.

Aftercoolers (Operation)

Heat exchanger is used to extract some of the heat of compression. -pass cool air or water through internal tubes -'counter flow' of compressed air through shell (casing) -internal baffles control velocity and turbulence for high heat transfer rates

What are two ways of expressing pump size?

Horse Power? Pump delivery

How to eliminate Pump Cavitation

Keep suction line velocities below 4ft/s (1.2m/s). Keep pump inlet lines as short as possible. Minimize the number of fittings in the inlet line. Bury inlet and return lines. Use a large reservoir.

Aftercoolers

Located between compressor and receiver. -cools hot air from compressor before it enters receiver tank. -conditions air (1st step) - removes moisture, and solid contaminants drop out

What is a telescoping-rod cylinder and what is it used for?

Made up of short concentric tubes. Used where long work strokes are required, short and compact when retracted.

Flow Control Valves

Meters fluid flow at required rate. Controls speed of actuators by controlling rate of flow out of device.

Center Positions

NC: -holds actuator position -used in parallel circuits Tandem Center: -unloads pump -avoids energy waste, heat generation -disadvantage: time delay before pressure available to actuator Open Center: -unloads pump and actuators are not held in position -cylinder may drift Float Center -motor can freewheel, and coast to stop

Operation of Pressure Compensation

Once maximum allowable pressure is reached, pump flow is reduced or stopped in order to maintain pressure.

Direct Acting Relief Valve

Pressure acts directly on spring mechanism. Cracking pressure: valve begins to open and divert flow. Full flow pressure: max opening of valve Reseat pressure: pressure at which valve seals off flow

Why must positive displacement pumps be protected by relief valves?

Pressure provides work energy to overcome flow - if no flow allowed due to blockage or closed valve, pressure will increase in a positive displacement pump until something happens. E.g. Internal leakage increases, something breaks, or relief valve opens.

Features of Piston Pumps

Pros: - highest overall performance - high pressures - high efficiency - variable volume displacement, independent of shaft speed - power to weight ratio > 4 - can be driven at high speeds - no lateral loads on pump axis Cons: - expensive - loud - not competitive in small size - complex construction (lower reliability)

Features of Vane Pumps

Pros: - inexpensive - reliable, few moving parts - self adjusting for vane wear - variable volume displacement control Cons: - medium pressure - low efficiency - very loud - power to weight ratio ~ 2 - leakage losses across rotor face between bronze wear plates and pressure ring

Reason for Pressure Compensation

Protects against excessive pressure buildup and prevents pump from being overloaded.

What is a positive displacement pump? How does it differ from a centrifugal pump?

Pump ejects a fixed volume of fluid per revolution of the pump. Centrifugal pumps have a flow rate and output pressure that can be changed during operation.

What parameters affect the noise level of a positive displacement pump?

Pump speed

Pump Cavitation

Pump suction lift is excessive and the pump inlet pressure falls below the vapour pressure of the fluid. As a result, air or vapour bubbles, which form in the low-pressure inlet region of the pump, are collapsed when they reach the high-pressure discharge region. This produces high fluid velocity and impact forces, which can erode the metallic components and shorten pump life.

Cavitation

Rapid vaporization and subsequent collapse of cavities (bubbles) in the liquid.

Rotary Actuators

Rated according to: -output torque -volumetric displacement and RPM -maximum operating pressure

Dryers (Operation)

Refrigeration: i) wet compressed air entering dryer passes through a heat exchanger and is chilled by liquid refrigerant. ii) remaining moisture in compressed air is held as water vapour iii) 'dry air' passes immediately into a reheater Absorptive dryer (chemical): Water vapour absorbed into chemical agents (pellets) -solids initially -liquefy as water absorbed -resultant liquid drips through screen Adsorptive dryer (mechanical): -removes water by using a desiccant material e.g. silica gel or activated alumina -moisture captured and held in pores of bed material = mechanical process, no chemical change in desiccant

Filters

Remove particulate contaminants. -function is to avoid damage to sensitive pneumatic components (valves, actuators) -often made of some type of paper/cellulose/fibre

Dryers

Removes enough moisture to ensure air will not become saturated as it moves through pneumatic distribution system. -relative humidity must be low so that pressure few point will not be reached Used as required to keep liquid water levels at a manageable level -installed downstream of air coolers

What is the difference between a single- and double-acting cylinder?

Single-acting: Can only exert a power stroke in one direction. Double-acting: Can exert a power stroke in both directions.

Demulsibility

The ability of a hydraulic fluid to separate from water.

Why does the rod of a double-acting cylinder retract at a greater velocity than it extends for the same input flow rate?

The difference in flow-path cross-sectional area accounts for the difference in velocities. Greater area equates to slower velocity for the same flow rate.

Why are centrifugal pumps so little used in fluid power systems?

There is a great deal of clearance between the rotating impeller or propeller and the stationary housing. As the resistance of the external system starts to increase, some of the fluid slips back into the clearance spaces, causing a reduction in the discharge flow rate.

Fail Safe Circuits

Two Purposes: 1) prevent injury to operator 2) prevent damage to equipment a) Protection from inadvertent cylinder extension = prevents uncontrolled vertical cylinder falling b) Overload Protection = protects system components against excessive pressure due to excessive cylinder loading c) Two-handed Safety System = protects operator from injury

External Gear Pump

Two gears rotating, un mesh and mesh to create increasing and decreasing volumes.

What is meant by a balanced design hydraulic pump?

Two pressure quadrants oppose each other and balance forces acting on the shaft.

Pressure Control Valves

Two uses: 1. Safety device for over pressure 2. Pressure limiter for max force or torque on actuator - high heat generation across relief valve

Regenerative Circuit

Used to speed up the extending speed of a double acting cylinder.

Needle Valves

Used when precise control is not required and when system pressure is constant.

Pilot Operated Relief Valve

Uses pressure imbalance to operate valve. -controls pressure in pilot chamber above disk on main spool -operates more smoothly -cracking pressures are 90-95% full flow pressures Pressure override ~ 100psi

How are single-acting cylinders retracted?

Via an internal spring.

Flow Divider Valves

a) Priority Flow Divider - ensures essential system functions are performed at expense of non-critical functions b) Proportional Flow Divider - allows a constant flow to each circuit

Gas-loaded Accumulators

i) non-separator type - gas pushes directly on oil - absorption of gas in oil due to lack of separation ii) separator type =most common type -physical barrier between gas and oil a) Piston accumulator b) Diaphragm accumulator -diaphragm secured in shell -small weight to volume ratio =exclusive use in airborne applications c) Bladder accumulator -bladder fitted with gas valve element -can be removed through shell opening

Features of Gear Pumps

• Fixed displacement pump - requires pressure relief valve in the system to release flow and avoid damage of components. • Cheapest type of hydraulic pump • Unbalanced design with one inlet and one discharge port - results in high side loads on the pump axis and greater mechanical friction. • Gear noise can be reduced on external gear pumps by using helical gears or twinning gears (offset 2nd set by half a tooth). Pros: - inexpensive - reliable, only 2 moving parts - simple in design, compact Cons: - fixed displacement units - volumetric efficiency rapidly reduced by wear - lowest level of performance - medium pressure - low efficiency - very loud - power to weight ratio ~ 2

Laminar Flow

• The no-slip condition is responsible for the development of the velocity profile. • Shear stress in a flowing fluid and the resulting pressure loss in flow is due to the velocity gradient at the pipe wall. • Maximum velocity at pipe centerline • Velocity profile is parabolic in shape - low velocity gradient at wall - lower shear stress in fluid near wall • Lower pressure drop along piping • Reduced power loss in whole system - less viscous resistance to fluid flow