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Dynamic Simulation: Use Spring Joint or Force?

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Message 1 of 24
LT.Rusty
3755 Views, 23 Replies

Dynamic Simulation: Use Spring Joint or Force?

I'm designing a hinged cover for something right now that will use a pair of gas extension springs  for assisted opening.    I'm currently working to figure out how much force it will take to open the cover using different values of spring - it's available in 50, 100, 150, 200, etc., lb versions. 

 

I've tried running simulations using a force applied to the moving end of the gas spring, and I've trued running simulations using a spring joint applied between the fixed and and the moving end of the gas spring, and I'm getting dramatically different results between the two.  For instance, using a 200 lb force shows me that I need about 25 extra lbs of force applied to the handle of the cover to open it, where when I use that same 200 lbs on a spring joint, I need 50-75 lbs to keep the cover closed, because it will open on its own.

 

Which is the correct direction to go here?

Rusty

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Message 2 of 24
JDMather
in reply to: LT.Rusty

Attach your assembly here.

I think a better approach might be to find the Unknown Force using Jack (between two points).

 


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Message 3 of 24
LT.Rusty
in reply to: JDMather

I have been using unknown force, actually.  I've been setting up all the joints and / or forces, then using unknown force to find the the force needed to move the cover at any given point on a 90 degree rotation of the cover.  What I'm trying to do is figure out what force to use on the gas springs to bring the force needed to start the cover opening to right around 15-20 lbs.

 

Here's the parts attached.  There's no simulation stuff built in this one, as it's an EXTREMELY simplified thing that I just threw together now.  All the parts are rigged for motion though, and you'll have no trouble figuring out my design intent here I don't think.

Rusty

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Message 4 of 24
JDMather
in reply to: LT.Rusty

First thing I notice is that your mechanism is overconstrained.

You only need one of the cylinders to do the analysis.

 

This one will be a good exercise for my class (our next meeting will be Wed) so it will be a couple of days before I present their solution. Maybe Hugh will present his solution before then.  (BTW - I didn't see how you set up the Unkown Force in the assembly you attached.  Gravity was not even defined?)


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Message 5 of 24
LT.Rusty
in reply to: JDMather

I didn't include the unknown force stuff - I didn't actually set up any simulations on the assembly that I attached earlier.  Gravity would, of course, be defined as "down" relative to the model.  I used one of the frame corner edges to define it.

 

As to the overconstrained part ... both cylindrical constraints on the gas springs show up as overconstraints.  Even if I suppress one of them, the other one is overconstrained still.  I'm honestly not sure why they're overconstrained at all, though?

Rusty

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Message 6 of 24
LT.Rusty
in reply to: LT.Rusty

This is how I set up the unknown force operation.  I'm looking to find out how much force it will take on the pick hole (in this case, in the real part it's actually a handle) to open the cover, when using gas springs that have a certain pre-set force. 

 

 

 

unknownforce.JPG

Rusty

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Message 7 of 24
LT.Rusty
in reply to: JDMather

Okay, so I think I understand what the overconstrained issue is, but I don't really understand how to solve it, or why Inventor thinks that it's overconstrained in the first place.

 

The ball joints at the end of the gas spring parts were originally created by McMaster-Carr as separate components, but when I imported the STEP files I used COMBINE to consolidate them into the main pieces.  I used insert & mate constraints to assemble the threaded sections of the ball joints into the cover.  These constraints prevent the gas springs from rotating.

 

The overconstrained condition comes up because the cylinder constraint used to mate the two parts of the gas spring together thinks that the two halves should be free to rotate around the axis of the cylinder, but the insert & mate constraints used are preventing this from happening.

 

Is there a different type of joint that I should be using, rather than cylindrical? 

Rusty

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Message 8 of 24
JDMather
in reply to: LT.Rusty

I recommend you get the Wasim Younis book.

Overconstrained means something different than in assembly environment.

 

Think of it like this - a door only needs one hinge (Revolution Joint) to work.


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Message 9 of 24
LT.Rusty
in reply to: JDMather

I understand that overconstrained means different things in DS than in sketches or assembly, and yes, I understand that a door only needs one hinge to work.  My problem is that each half of that gas spring needs one hinge to work, as well as a cylinder constraint between the two halves.  Inventor is getting angry because the two halves are not free to rotate around the long axis of the gas spring.  If the two halves are free to rotate around the long axis, then the assembly itself will not work, and things start falling apart. 

 

The only way that I've come up with to ensure that no overconstrained conditions attain in this simulation is by using a point-to-point instead of an insert on the 5/16" threaded end of one half of the gas spring.  This causes the whole thing to start coming apart as the spring halves rotate around each other ... but hey, at least it's not overconstrained.

Rusty

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Message 10 of 24
JDMather
in reply to: LT.Rusty

As I start to take a look at this -

I don't understand the purpose of Work Axis 1, 2 & 3 in the assembly - so those I will delete.


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Message 11 of 24
JDMather
in reply to: JDMather

I only need one ROD and BODY for the DS so the second instance will be Suppressed.

Suppress any constraints between these redundant parts and the Frame and Cover.


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Message 12 of 24
JDMather
in reply to: JDMather

Delete the axis-axis constraint between the ROD and the frame.

Replace with axis of ROD connector to point of frame hole circle.  (this would translate to a point-line joint)

Add Mate Fush between XZ planes of the BODY and ROD.  ( translates to prismatic joint)


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Message 13 of 24
JDMather
in reply to: LT.Rusty


@LT.Rusty wrote:

I'm designing a hinged cover for something right now that will use a pair of gas extension springs  for assisted opening.   


Actually you don't need to have either of these in the assembly to calculate how much force would be required to open the cover unless the weigh enough that their weight needs to be considered in the calculations.


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Message 14 of 24
LT.Rusty
in reply to: JDMather

The work axes are irrelevant for the simulation.  They were used initially for locating the pivot points for the gas spring.  2 & 3 were used with a mate constraint to each other with a max and min distance equal to the max and min length of the gas spring.  #1 was used (with 2 & 3) to project a point in a sketch in the assembly (now deleted) that was the center-point for an arc describing the travel of the cover-side pivot (Work Axis 2), while a pair of arcs centered on #3 described the max and min lengths of the spring.  Repositioning #2 and #3 slightly showed where to put them to ensure that the arc of #2 stayed within the max and min extensions.  With that sketch deleted, they don't really serve any further function.

 

 

Thanks for the constraint setup.  That was messing with my brain for 2 days, trying to come up with a way to do that that was not over constrained.  I still don't understand why this isn't overconstrained but the other one was - they're both accomplishing the same thing.

 

 

Now, I've tried to set up the unknown force a couple different ways.  I've used pre-set external load forces on the hydraulic parts along the long axis of the gas spring, with the unknown force on the pick hole, acting on the revolution joint between the cover and frame.  I've set force joints, both jack and spring, between the pivot points of the (deleted) gas spring, with the same unknown force on the pick hole.  I've set a constant force on the pick hole, with the unknown force being a jack between the spring pivot points. 

 

In all setups I'm coming up with dramatically different numbers. Which setup should I believe, or is there a different way that I should be looking at this?

Rusty

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Message 15 of 24
JDMather
in reply to: LT.Rusty

Q1 How much does the lid weigh?
Q2 Are you lifing the entire weight or is part of the weight supported by the mechanism? (trick question)

 

You only need the frame and the lid to get the unknown force.

 

As far as the different answers you are getting - I would need to see the different assembly files.


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Message 16 of 24
LT.Rusty
in reply to: JDMather


@Anonymous wrote:

Q1 How much does the lid weigh?
Q2 Are you lifing the entire weight or is part of the weight supported by the mechanism? (trick question)

 

You only need the frame and the lid to get the unknown force.

 

As far as the different answers you are getting - I would need to see the different assembly files.


 

 

Q1 - in this example, something like 90 lbs.  The actual cover is closer to 150 lbs.

Q2 - of course you're not lifting the entire weight.  Much of it remains supported by the hinges.  You only need to overcome the moment of inertia, by a combination of the gas springs and lifting on the pick hole. 

 

 

As far as the different results, I just ran through all the possibe rigs again, and I'm getting roughly the same results using a force between the two halves of the gas spring as I do using a force joint of, of equal power, set up as a jack between the two pivot points.  It appears to be when I use a spring (rather than a jack) that the issues happen.  Since I'm getting almost identical results between the other two methods, I'm guessing those are the right answers, and the spring joint is causing issues because spring power is expressed in lbs/inch, rather than lbforce. 

Rusty

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Message 17 of 24
JDMather
in reply to: LT.Rusty


@LT.Rusty wrote:
 

Q1 - in this example, something like 90 lbs.  The actual cover is closer to 150 lbs.

.....and the spring joint is causing issues because spring power is expressed in lbs/inch, rather than lbforce. 


You changed the problem on me!
I thought from your originial post that you were trying to put way more helping force than you needed.

 

With the lid closed the spring is at minimum length.

Put the length of the spring at max open position as the Free Length (or a bit more).

This puts the preload on the spring.  The amount of force will be the (free length - minimum length)*the stiffness lbs/inch rating for the spring force.    So: if Stiffness is 10lb/inch and the change of spring length (compression) is 6in the force is 60lbf for the spring.


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Message 18 of 24
LT.Rusty
in reply to: JDMather

Ah, see now there you go, inserting common sense into this thing again.  I didn't even think about the compression being a conversion issue.  In retrospect it's perfectly obvious ...  :facepalm:

 

Thanks!

Rusty

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Message 19 of 24
JDMather
in reply to: LT.Rusty

Did you get a solution that makes sense?

 

We experimented in class and it turns out this is a much more interesting problem than I originally thought.

 

Going back over the thread it is not really clear what direction is "Up"?

 

Is up in the Positive Y direction?

Negative Y.PNG

 

Or

Is "Up" in the Negative Z direction?

Positive Z.PNG

 

 

It turns out the initial angle of the gas spring is critically important.
If it is too close to parallel to the lid even an enormous force by the spring would not aide the operator.

So in addition to defining "Up" we also needed to move the connection on the frame down as much as possible to increase the initial angle.


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Message 20 of 24
LT.Rusty
in reply to: JDMather


@Anonymous wrote:

Did you get a solution that makes sense?

 

We experimented in class and it turns out this is a much more interesting problem than I originally thought.

 

Going back over the thread it is not really clear what direction is "Up"?

 

Is up in the Positive Y direction?

 

 

Or

Is "Up" in the Negative Z direction?

 

 


 

 


Yeah, I got answers that make enough sense for me to work with, at least.  I think.  Your assertion that it was more interesting than you initially thought makes me wonder, though ... what about it was particularly odd or interesting?

 

Up is Positive Y.

Rusty

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