Week4/Lesson 1 – - PowerPoint Presentation

Week4/Lesson 1 – Accumulators Fluid Power Engineering

Accumulators In this lesson we shallIntroduce hydraulic accumulators Discuss an example application of a hydraulic accumulatorExamine the types of accumulators often employed in hydraulic circuits

What is an accumulator anyway? It’s a little pressurized tank connected into the circuit It is charged with fluid during the non-working part of the cycle This fluid can then be combined with the pipe flow to increase the system’s total flow capacity during high-demand parts of cycle In this way we can get away with a much smaller pump than would be needed to deliver max flow in real time But this is only one reason to install an accumulator

What is an accumulator anyway? Different size accumulators

Example: commercial airliner A commercial airliner has a great need for hydraulic power… …but only during short phases of flight When the plane takes off… The deployed flaps and wing slats need to be retracted The landing gear needs to be retracted Large deflections of control surfaces on the plane are needed due to the low speed and thus the need for large deflections

Example: commercial airliner A commercial airliner has a great need for hydraulic power… …but only during short phases of flight Likewise, when the plane lands… The flaps and wing slats need to be deployed The landing gear needs to be deployed Large deflections of control surfaces on the plane are needed due to the low speed and thus the need for large deflections For stopping the plane need to apply brakes and also thrust reversers

Example: commercial airliner A commercial airliner has a great need for hydraulic power… …but only during short phases of flight During cruise flight at speed, little demand for hydraulic power Flaps/wind slats/landing gear stowed away High speed means small deflections of control surfaces have a big effect No braking

Example: commercial airliner And remember, on an airliner… …space and weight are critical… …so it would really be nice to use a smaller pump than is needed for maximum demand Enter the accumulator When charged, supplies extra flow in times of need And if pump is lost for one reason or another… …stored pressure energy in accumulator is on hand

Three types of accumulators There are three types of accumulators:Weight loaded Spring loaded Gas loaded (gas spring) 2 and 3 both used spring energy to work The compressed gas acts as a spring

Weight-loaded accumulator Weights stacked here… …push down on piston… …which compresses fluid here A weight-loaded accumulator maintains same pressure on fluid as fluid exhausts through the fluid port

Weight-loaded accumulator But a weight-loaded accumulator is only good for fixed installations Too big and heavy for mobile or aeronautical applications Weight-loaded accumulator is oldest type of accumulator

Spring-loaded accumulator Spring in top …pushes down on piston… …which compresses fluid here Accumulator open at top to let air enter and exit through a filter Disadvantage is that as spring expands, pressure drops in oil below

Gas-loaded piston accumulator Usually N 2 because no moisture Gas pushes down on piston… …which compresses fluid here Fluid enters and exits here N 2 charge put in here Can gang accumulators together for greater capacity Positive shut-off prevents N 2 from entering oil system When system is in an inactive phase, pump pushes oil back into accumulator Accumulator preloaded when empty, so that it exerts a pressure, even when most of the oil is gone

Gas-loaded bladder accumulator Usually N 2 because no moisture Gas pressure expands bladder… …which compresses fluid here Fluid enters and exits here N 2 charge put in here Positive shut-off prevents bladder from squeezing into attached oil line

Gas-loaded accumulators There are even accumulators that have no separation between the gas and the oilBut these have the disadvantage of pushing the gas into the oil, i.e. causing gas entrainment This makes the oil more compressible, which makes the response more spongy, oscillatory, and less precise

Accumulator symbols Weight-loaded Spring-loaded Gas-loaded

Gas-loaded accumulators There are even accumulators that have no separation between the gas and the oilBut these have the disadvantage of pushing the gas into the oil, i.e. causing gas entrainment This makes the oil more compressible, which makes the response more spongy, oscillatory, and less precise We ’ll discuss this more when we investigate the properties of the oil

Accumulator as extra power source This is a system in which the hydraulic pump supplies oil to a number of different actuators:Besides the extra power, the accumulators also help with reducing cross-talk, that is preventing demand by one motor from affecting the supply to other motors Also helpful is that if pump fails, still have some power to move motors a bit

Auxiliary power for rapid retraction Cylinder extends: At extension, cylinder stays for some time for an operation to be performed This allows accumulator to charge Valve deactivates Charged accumulator allows rapid retraction of cylinder Unloading circuit for pump

Can adjust at what pressure valve switches A modification At p max , pilot switches valve to bypass At pmin , pilot valve switches valve Could replace PRV with pilot-operated 2/2 valve

Accumulator to maintain pressure Or if you have a system that sits idle for long periods of time… The accumulator maintains pressure when system is idle At p max , pilot valve switches valve The switch turns the pump off when p reaches pmax …and it turns the pump on when p drops below pmin This is our normal unloading circuit

Accumulator as emergency power source If the pump fails or electricity to pump drive fails and need to retract cylinder… 3/2 valve switches to A-T Accumulator retracts cylinder Pump must work against this pressure when cylinder extends… But cylinder extension is also charging accumulator If power fails, solenoid will deactivate

Accumulator as pressure absorber This system has an emergency shut-off valve If the valve is shut suddenly, the pressure will rise due to the momentum of the fluid in the conduit The pressure spike will be absorbed in the accumulator As such, the accumulator acts like a capacitor in an electric circuit

How to size an accumulator? We’ll use a piston accumulator as an example Though a bladder accumulator works the same way Fluid N 2 N 2 Fluid State 1: Accumulator full of fluid State 2: Accumulator empty of fluid

How to size an accumulator? Use Boyle’s Law or   Here we’re assuming D T is small (slow, isothermal discharge) When the accumulator empties, so that …   … , the pressure goes down   Example: ⇒   So ⇒ Doubling the volume halves the pressure  

Sizing accumulator We want to use the accumulator below for extra flow to extend or retract the cylinder Qacc Q A = Q acc + Qpmp Q pmp QA At ends of stroke, pump charges accumulator When valve actuates, flow comes out of accumulator

Sizing accumulator   If we integrate this over time to extend cylinder   This tells us how much flow must come out of accumulator and pump during the extension

Sizing accumulator But there is another problem to consider. As the N2 in the accumulator expands, its pressure drops. We want the pressure in the accumulator to remain above the pressure in the cylinder at the end of its stroke Otherwise, at the end of the piston stroke, the pump will be feeding both the cylinder and charging the accumulator This will cause the cylinder extension to slow down The empty accumulator is charged to a certain pressure with N 2 This pressure must be greater than the pressure of the cylinder at the end of its stroke

Sizing accumulator There will be a pressure loss in the conduit between the accumulator and the cylinder Also, size the pipe between the accumulator and cylinder to accommodate the extra flow from the accumulator We’ll see later how to calculate pressure drops in pipes/tubes/hoses So place the accumulator near the cylinder to lower this loss

Sizing accumulator At end of piston stroke still need pacc > pA The load force has to be known   This gives us the pre-load needed on accumulator to prevent inflow into accumulator at end of piston stroke   This shows the reasoning needed to size an accumulator You need to look ahead and consider all states of the process You need to calculate preload on accumulator

Outside learning To better understand this subject matter, view the following videosWatch: AccumulatorsMechanical hydraulic basics course – accumulators Piston accumulators Don’ t forget to turn the closed-captioning on to be able to understand better the details of the lectures

32 End of Week 4/Lesson 1