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Finding the right hydraulic accumulator

Finding the right hydraulic accumulator

(OP)
Hi

I have a application that requires a hydraulic bladder accumulator directly attached to a 1.5" x 6" dual stroke hydraulic cylinder for shock absorption. This cylinder would provide a constant pressure that equals the single shear bolt strength of a grade 5, 3/8" bolt, however any force greater than this would result in the cylinder retracting and the accumulator absorbing the volume and pressure. The accumulator would be directly mounted on the "cylinder port" end of the hydraulic cylinder. Could someone help me in calculating the type of accumulator this would require?

Thank you in advance.

RE: Finding the right hydraulic accumulator

You have mentioned a bladder accumulator and you have attached a picture of a diaphragm accumulator. You have then asked for help in calculating the type of accumulator.

There is no calculation to make, you need to pick one type.

"Cylinder End" is not a good enough description. It's better to say, rod end...as in the picture, or head end, which would be the other port on the cylinder.

I assume you mean you need help to calculate the size of the accumulator. This being the case, you need to confirm the following.

1) Which port will it go on?

2) What is the rod diameter?

3) What are the minimum and maximum pressures that cylinder will see in the application?

4) How quickly will the cylinder move?

All of the above are required to be able to calculate the volume and pressure of the nitrogen pre-charge.

RE: Finding the right hydraulic accumulator

Quote:

... any force greater than this would result in the cylinder retracting and the accumulator absorbing the volume and pressure.

Just connecting an accumulator to a cylinder doesn't do anything like that. You seem to be omitting, perhaps for brevity, a fairly fancy valve or two.


Quote:

.... provide a constant pressure ...

Accumulators don't really do that, either.

Mike Halloran
Pembroke Pines, FL, USA

RE: Finding the right hydraulic accumulator

I know what batdevice is trying to do, sort of, but I don't think he has it clear in his mind yet. It isn't clear what is moving.

First the accumulator must be connected to the cap or blind side of the cylinder. Not the rod side as shown. This way when the rod is pushed back the oil on the cap side can go into the accumulator. I know in the picture the accumulator is mounted on the rod side but when the accumulator is mounted on the cap side there must be some way for oil to flow to or from the rod side of the cylinder unless you only plan to expose it to air.

Second, the accumulator must be pre-charged above the system pressure. This means there will be NO oil in the accumulator most of the time. The accumulator must be able to keep the bladder from squeezing through the port. There are others on this forum that know more about which brands or types of accumulators that can do this.

The size of your accumulator looks like it would handle a pretty good bump. You need to know how much the oil will move into the accumulator when you get a severe bump and calculate the resulting volume of gas, N2. Then use P0*V0^1.4=P1*V1 to compute the pressure when the oil goes into the accumulator. P0 and V0 are the pre-charge pressure and accumulator volume. P1 is what you want to calculate. V1 is the V0 minus the volume of oil in the accumulator.

What is controlling the pressure?

Usually putting an accumulator between the valve and the cylinder is a big no-no. The accumulator will interfere with any motion control. Batdevice did not say how the piston is going to be moved back and forth when not in contact with the bolt or what is controlling the pressure when in contact. I wonder if the small accumulator is needed if he had a controller that can control the pressure.

We have many customers in the saw mill industry that use this technique to hold down boards with press rolls. This is a must know technique for planers. It is necessary because the boards are of slightly different thickness and may cause the press rolls to compress up to 1/4 inch as the press roll rolls over boards of different thickness.

Peter Nachtwey
Delta Computer Systems
http://www.deltamotion.com

RE: Finding the right hydraulic accumulator

Peter, I think you need to raise V1 to the 1.4 also.

Ted

RE: Finding the right hydraulic accumulator

Quote:

Peter, I think you need to raise V1 to the 1.4 also.
Yes

Peter Nachtwey
Delta Computer Systems
http://www.deltamotion.com

RE: Finding the right hydraulic accumulator

(OP)
Hi

Thank you for submitting information and comments regarding my conundrum.

My apologies for the vague information. In answering Mr. PNachtwey questions, the hydraulic cylinder system would be mounted in this proposed design to be used to replace a 3/8 grade 5 (single sheer) sheer bolt that would "hold up" a heavy Concave device on a linkage mechanism used for threshing grain on a modern combine harvester. The current design has a sheer bolt that sheers when a foreign object enters the threshing mechanism and prevents severe damage to the combine components. The threshing mechanism drops an equivalent distance of the 6" stroke to a dead stop on the combine frame. Instead of using a sheer bolt, I am trying to design a system that uses either a bladder/diaphragm type of accumulator or controller could be used to allow for protection in this type of situation.

Yes, the accumulator should be connected to the cap or blind side of the cylinder, as it should be. There could be just an air bled valve on the rod side as this side of the piston would not require pressure. Unsure if a valve is required in this application between the accumulator and the cylinder port, however.

The accumulator should be pre-charged to the correct bar pressure that would allow the cylinder force to be the equivalent static force as the single sheer bolt strength of the bolt. However, the cylinder force should never exceed the sheer bolt strength and the accumulator system should return the threshing device back to it's original position after the foreign body has passed through. Therefore, it is possible this device could be staging many times a minute so I am unsure if 5% used in the Charles Law would be enough compensation.

I am unsure which type of controller could be used in this process. Would it have a separate circuit? Could a specific length of single braid 1/2 hydraulic hose be used instead of an accumulator to absorb this shock?

Thank you

RE: Finding the right hydraulic accumulator

It's not Charles' law, the pressure will not be constant. It's Boyles law, with a polytropic exponent added to compensate for any adiabatic processes involved.

From the sounds of your last post, it would seem that you only need to consider the minimum and maximum pressures. The min pressure being the pressure required to hold the rod against the underside of the plate. The max pressure being the pressure in the cylinder under full deflection.

The pre charge should be set 10% below the min pressure to ensure that there is always some oil in the accumulator when the thresher is running.

You can use a diaphragm, bladder or piston accumulator for this. You just use the equation to calculate the volume of gas required to give the desired performance.

A hose will not be able to absorb the same amount of energy as volume of gas. At least not in practical and reasonable terms anyway.

In terms of a system layout, don't you just need to have an isolator and drain valve to be able to service the system? When the system is on, the accumulator would just be on "standby"' ready to respond to the load on the plate.

RE: Finding the right hydraulic accumulator

Quote:

the hydraulic cylinder system would be mounted in this proposed design to be used to replace a 3/8 grade 5 (single sheer) sheer bolt that would "hold up" a heavy Concave device on a linkage mechanism used for threshing grain on a modern combine harvester.
OK, so no controller here.
In theory you should be able to fill the cap side of the cylinder with oil and pressurize the accumulator to whatever safety limit you want but eventually some oil will leak by the piston seals so the trapped volume of oil on the cap side would decrease over time. The accumulator would not come into play at all unless the pressure in the cap side rose above the pressure in the accumulator. Then the cylinder would start to retract but spring back as soon as the force of impact goes away. Until then the force the applied by the rod would be equal to the force applied to the rod. The piston would only retract a 0.5% per 1000 pounds of pressure increase. In other words the pressure would no be constant unless the load pushing on the rod is constant.

Another trick that might work is to put a hole in the piston. That would make the rod a little springier and provide a little damping until the pressure in the cylinder increased above the accumulator's pre-charge pressure. The orifice would keep the pressure on the rod side and the cap side equal most of the time so there would not be a need to worry about the oil seeping by the piston seals. Not there is only the rod seals to worry about.


It seems to me I am designing a shock absorber and there should be some out there that will fit Batdevice's needs.


Peter Nachtwey
Delta Computer Systems
http://www.deltamotion.com

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