Inventor General Discussion

Inventor General Discussion

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*Expert Elite*
LT.Rusty
Posts: 852
Registered: ‎10-23-2012
Message 1 of 23 (950 Views)
Accepted Solution

Dynamic Simulation: Use Spring Joint or Force?

950 Views, 22 Replies
01-28-2013 07:29 AM

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?


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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)


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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JDMather
Posts: 28,200
Registered: ‎04-20-2006
Message 2 of 23 (948 Views)

Re: Dynamic Simulation: Use Spring Joint or Force?

01-28-2013 07:34 AM 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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*Expert Elite*
LT.Rusty
Posts: 852
Registered: ‎10-23-2012
Message 3 of 23 (937 Views)

Re: Dynamic Simulation: Use Spring Joint or Force?

01-28-2013 08:08 AM 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.


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JDMather
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Message 4 of 23 (928 Views)

Re: Dynamic Simulation: Use Spring Joint or Force?

01-28-2013 08:36 AM 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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LT.Rusty
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Registered: ‎10-23-2012
Message 5 of 23 (920 Views)

Re: Dynamic Simulation: Use Spring Joint or Force?

01-28-2013 09:14 AM 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?


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LT.Rusty
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Registered: ‎10-23-2012
Message 6 of 23 (918 Views)

Re: Dynamic Simulation: Use Spring Joint or Force?

01-28-2013 09:17 AM 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


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LT.Rusty
Posts: 852
Registered: ‎10-23-2012
Message 7 of 23 (903 Views)

Re: Dynamic Simulation: Use Spring Joint or Force?

01-28-2013 12:09 PM 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? 


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JDMather
Posts: 28,200
Registered: ‎04-20-2006
Message 8 of 23 (892 Views)

Re: Dynamic Simulation: Use Spring Joint or Force?

01-28-2013 03:09 PM 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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*Expert Elite*
LT.Rusty
Posts: 852
Registered: ‎10-23-2012
Message 9 of 23 (888 Views)

Re: Dynamic Simulation: Use Spring Joint or Force?

01-28-2013 04:39 PM 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.


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JDMather
Posts: 28,200
Registered: ‎04-20-2006
Message 10 of 23 (883 Views)

Re: Dynamic Simulation: Use Spring Joint or Force?

01-28-2013 05:35 PM 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.

Please mark this response as "Accept as Solution" if it answers your question.
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