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Discussion in 'Calculations' started by KevinC, May 24, 2014.

1. KevinCWell-Known Member

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Hi all,

I'm a mechanical engineer. I'm here stuck for more than a week now. Hope someone can shed a light onto my problem.

I'm working on a machine that is driven by a Pneumatic Piston that creates a tangential force (assume it is always tangential) at a distance from a pivot. Then after it's at a certain location, it hits a damper and slows down to stop.

I'm trying to find the Piston Force. Assume I know the angular decceleration rate (from sensor).

Since it's a Pneumatic Piston at motion, I do know know the actual force provided by the piston (I know it's pressure times area, but we don't know the instantaneous pressure at the point), so I need to solve all the forces in order to get the piston force.

So now I come to the damper,
The datasheet only state the max drag, and the absorption characteristic/stroke. It doesn't give me any equations to work with.
Max absorption energy, Recovering power of the piston rod, max drag are just some big names I don't know how to make use of. Obviously I won't be using the biggest number from each field. How do I find the force in this damper at a given stroke length?

..........|<-........ (Piston Force)
..........|...........
......-> |........... (Damper)
..........|...........
..........o........... (pivot)

If someone can shed some light on how they calculate dampers, it will be great. (Not the damping ratio/factor type of stuff from school, I seriously can't make correlation from those to these damper here)

Thank you very much for at least reading this.

Kevin C

2.
3. LochnagarWell-Known Member

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Hi Kevin,

I think the "problem" you have is that the resistance force - shown on the graphs (on your hyperlink) - is not a constant value - and if I am understanding your question - you are wanting to know what the force is - but according to the graphs it is a variable value - and varies with time - so it is not a constant value.

I am going to take a guess that the pneumatic cylinder and the damper are acting on a lever - which is connected to a "large mass" which rotates - and the pneumatic cylinder rotates this "large mass" - and the damper reduces its rotational speed. If my guess is correct - there may be a possibility of recording the variation in the rotational speed of this large mass - and therefore work backwards to compute the resistance force that the damper is imparting to reduce angular acceleration of this large mass. However, the accuracy of this method will depend on whether you can ignore the "dynamics/inertia" of the pneumatic cylinder in your angular acceleration calculations. If the cylinder is small by comparison to the large mass - then yes, but if not then no.

Another possibility - might be to remove the damper from the machine - and simulate the resistance force - by applying a displacement to the end of the damper - at the same speed - as the machine applies to the damper. You can then measure what this force is.

The the only other alternative would be to use a load cell and data recorder to measure the variation in force with time - but whether it is feasible to get a load cell in position - is hard to say (the dimensions appear quite small) - and is likely to be the most expensive option.

Hope this helps.

4. KevinCWell-Known Member

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Need to understand damper graphs, or Piston Behavior

hi Lochnagar:

Thank you so much for replying. Variable value in the graph is not a problem for me, but the problem is that I don't know How to use the graph. I can assume the max drag value given is the peak of the absorption graph, and approximate the rest of the value from there, but since I know damping force is related to impact velocity to the damper, I can't assume a slow impact, small mass will have the same damper reaction as fast impact, large mass. Just trying to see how can I find info on this piece of puzzle.

Thanks for the testing suggestions. You've guess the part right, the actual problem is a part in a machine (this piece) that someone else design broke off. Now I'm responsible to find theoretical bases in whether a certain fix will work. The actual part is more like an L shape, but this doesn't matter as I can deal with all the other shape/size/dynamic/stress and other stuff, just the piston and damper is killing me, so I'm simplifying the geometry so that I can isolate the problem.

I have assumed a certain angular acceleration, so now I'm just down to two unknowns. I can't put any measurement device on the part as I don't have access to it. So I can only work with the damper datasheet or pneumatic piston's characteristic. But I definitely need some help in those areas.

Kevin C

5. LochnagarWell-Known Member

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Hi Kevin,

I am going to take a guess that the angular acceleration of your object is possibly a function of time - as opposed to a constant value. So what you need to do is measure the angular velocity over time - and from that you will be able to determine if the angular acceleration is a constant or not - by looking at the change in velocity over time. It is not a good idea to assume things - unless you have got hard evidence to support your assumption.

I would be inclined to "ignore" the data sheet - because from what you are saying you have no idea on the design of this system nor have you have any idea on the set up of the "damper". So to work out what the set up of the damper is - you need to measure things - and work backwards from there - to work out what values have been used from the data sheet.

Can you measure the linear velocity over time of this damper? If you can do this - then you can simulate this on the work bench by imposing a force at the same linear velocity as what you measured - and then measure the force at the same time.

However, if you don't have a lot of data on the object that the cylinder and damper are acting on - you are probably going to struggle solve this whole problem - since you will need to have data on it's inertia - and the position of its inertia - if you are to make a mathematical model of this problem - and see how the effect of changing the damping force effects the angular acceleration.

If you don't have this data - then you might be as well to tweak the orifice on the damper (which appears to be variable according to the data sheet) - within its safe operating range - to see how this effects the deceleration time - which appears to be what you are trying to do. However, be careful if you chose this option - and always remain within the allowable settings of the damper. Be very careful of allowing larger accelerations - because of the knock on effect this might have on other parts of the system. Think carefully of possible consequences of your actions before pursuing this option.

Hope this helps.

6. miltonbrWell-Known Member

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Hi Kevin:
Not sure if I understand the question. The pressure value acting in the cylinder and the cylinder size are not known?

7. KevinCWell-Known Member

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I know the whole design, and I have the whole Solidwork model. So the size is known. I know the setting of pressure valve (80psi), so the input pressure is known as well (although this piece of information is no use). Worst part is that it goes through long lines and valves and such. Because the fact is that when the pressure applied, the whole thing expands, accelerate. The whole pressure in the cylinder drops to an unknown pressure. This is why I can't rely on the input pressure, especially it's so far up in the upstream.

Last edited: May 28, 2014
8. KevinCWell-Known Member

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Hi, Lochnagar:

i can justify my instanteneous acceleration assumption within reasonable uncertainty +-25% ish, so this part is good (or at least good enough. I've been analyzing the whole thing everyday for the past 2 weeks. Actually am looking at the Solidwork model, and running FEA and hand calc on it for analysis. Since I have the Solidworks model, inertia, CG, and all the shape data are good on hand, and I can verify all those.

But the sad part is I'm actually unable to test the part, I don't have it on hand as it's at a site (although is close to my work place, but I don't have access to go there very easily). Nor I have the time to wait for a setup.

Since I have two variable (Piston force, and damper force) and assumed an acceleration. Piston force is not easliy figured out. I can only see whether I can find info on the damper on the datasheet.

9. LochnagarWell-Known Member

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Hi Kevin,

If you have the Solidworks model - with real densities applied - therefore giving real inertia's - then that is a good starting point. Within Solidworks - you can "Add In" Solidworks Motion - and perform a kinematic analysis.

What we don't know much about at the moment is the pneumatic cylinder or the pneumatic circuit. This cylinder will probably be controlled by a solenoid valve - activated by a switch or a timer - or possibly a bit of software. It might be worth finding out - what the "control system" is for this system is - and the location and pressure setting of any relief valves . I am going to take a guess that the solenoid valve remains activated - to control the "device" - clockwise - and then the solenoid is changed direction - to control the device - in the anti-clockwise direction. You need to look at the pneumatic circuit schematic to bottom this out. If the cylinder is still rotating the "device" - (and there are no local relief valves coming into action) - then the "damper" is working against the cylinder and the inertia of the "object".

So if you know the change in angular velocity over time of the "object" (from the sensor you mentioned earlier) - you should be able to work backwards to quantify what retardation force the "damper" is providing - with Solidworks Motion. You may have to make one or two iterations - until you get the kinematic model to match the real world problem.

(Your guess of the acceleration of +/-25% - is a total error of 50%. To me that is not very accurate).

Hope this helps.

10. KevinCWell-Known Member

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Yes, the pneumatic actuator is control by a bi-directional solenoid valve that controls the pneumatic actuator in both direction.

The problem here is still the fact that I'm having two unknowns here. Even knowing the acceleration. All that is saying the fact that the force differences provides the acceleration. It doesn't gives me the actual number for any of the two unknowns here.
The acceleration could be (in m/s^2):

acceleration : 5, 3, 1, 2, 4

But as long as the value differences of the two forces provide this number for acceleration, it can have an infinite number of solution pairs.

damper force/mass: 10, 11, 12, 14, 16
Piston force/mass: 15, 14, 13, 16, 20

damper force/mass: 5, 1, 6, 2, 20
Piston force/mass: 10, 4, 7, 4, 24

It will still provide the same acceleration/decceleration profile, but they gives me a drastic difference in terms of the internal stress of the part. So it still comes back to the question of what is the force of piston or damper.

I know the acceleration is not the most accurate thing, but it's accurate enough for all intend and purposes. =)

11. LochnagarWell-Known Member

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Hi Kevin,

Have you bottomed out the pneumatic circuit - and if there are any local relief valves or local pressure regulators that come into play? Does the solenoid valve - go into a mid position - where both sides of the cylinder are open to exhaust - once it goes into the damper (I suspect not - but that is why you need to fully understand the pneumatic circuit - and its controls logic).

The force the pneumatic cylinder exerts will equal the pressure times the piston cross section area (if it is the full bore side) - or the piston cross section area minus the rod area times the pressure - (if it is the annular area). The system will have a regulator - but there may also be other regulator or pressure reducing valves - for this particular cylinder. The other side of the cylinder should be to the "exhaust" port. There is sometimes a little drag between the piston and the cylinder (due to the seals) that you need to factor in.

So if you know the inertia of the object (you say you have a full Solidworks model), leaverages to the cylinders and the damper - and you have the output of your sensor - which you say is giving rotational speed with time - you have only now got one unknown - the resistance provided by the "damper".

If you have Solidworks Motion - which you may well have - if you have Solidworks Simulation - you can make a model of this system to determine the one unknown. Yes, you will have to assume a value for the resistance force provided by the damper to begin with. Then you will see if the object reduces in speed at the same rate as the real world object - then if it doesn't you will need to increase the resistance force - until you get convergence. Solidworks Motion is very powerful - since you can model forces as a function of time - as opposed to just a constant value - but maybe leave that for later - until you have got a simple model up and running.

Hope this helps.