Inventor General Discussion

Inventor General Discussion

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JDMather
Posts: 27,488
Registered: ‎04-20-2006
Message 11 of 23 (565 Views)

Re: Dynamic Simulation: Use Spring Joint or Force?

01-28-2013 05:36 PM 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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JDMather
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Message 12 of 23 (560 Views)

Re: Dynamic Simulation: Use Spring Joint or Force?

01-28-2013 05:55 PM 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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JDMather
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Message 13 of 23 (556 Views)

Re: Dynamic Simulation: Use Spring Joint or Force?

01-28-2013 06:13 PM 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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LT.Rusty
Posts: 808
Registered: ‎10-23-2012
Message 14 of 23 (544 Views)

Re: Dynamic Simulation: Use Spring Joint or Force?

01-29-2013 07:34 AM 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?


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JDMather
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Message 15 of 23 (540 Views)

Re: Dynamic Simulation: Use Spring Joint or Force?

01-29-2013 08:54 AM 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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LT.Rusty
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Message 16 of 23 (536 Views)

Re: Dynamic Simulation: Use Spring Joint or Force?

01-29-2013 11:02 AM in reply to: JDMather

JDMather 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. 


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JDMather
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Message 17 of 23 (528 Views)

Re: Dynamic Simulation: Use Spring Joint or Force?

01-29-2013 12:40 PM 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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LT.Rusty
Posts: 808
Registered: ‎10-23-2012
Message 18 of 23 (521 Views)

Re: Dynamic Simulation: Use Spring Joint or Force?

01-29-2013 01:35 PM 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!


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JDMather
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Message 19 of 23 (484 Views)

Re: Dynamic Simulation: Use Spring Joint or Force?

01-30-2013 12:08 PM 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.

Please mark this response as "Accept as Solution" if it answers your question.
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LT.Rusty
Posts: 808
Registered: ‎10-23-2012
Message 20 of 23 (478 Views)

Re: Dynamic Simulation: Use Spring Joint or Force?

01-30-2013 12:13 PM in reply to: JDMather

JDMather 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.


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