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23-Emerald IV

## Spring with lift off?

How does one create a spring with lift off?  If I place a block on top of a spring, compress it, and then let it go, the spring should launch the block into the air to some height and then gravity should pull the block back down on top of the spring.  In Mechanism, the spring seems to act like both a compression spring and an extension spring.  As soon at the block exceeds the free length of the spring, the spring begins actively pulling it back down.  This isn't realistic.  How do I model a compression spring with lift off so the spring stops pulling down on the block after it's thrown into the air?

1 ACCEPTED SOLUTION

Accepted Solutions
14-Alexandrite
(To:TomU)

The springs in MDO are idealizations of the real world, so they are limited to always be "connected" to two bodies or act directly on a joint axis. If you wish to model the behavior you describe, you can do it. One way is to model a third body that has a "liftoff" cam pair to the body that the spring is compressed by. This way, after the constraint is removed from the body connected to the spring, the 3rd body will accelerate and loose contact with the body attached to the spring.

The 2nd way is to model the spring as multiple bodies and have a liftoff cam pair created between the top coil and the body you would like to "launch". This method would allow you to capture the non-linear spring dynamics when they exist for your system. I did this countless times many years ago for engine valve springs with very accurate results for high-speed dynamics.

Good Luck!

5 REPLIES 5
14-Alexandrite
(To:TomU)

The springs in MDO are idealizations of the real world, so they are limited to always be "connected" to two bodies or act directly on a joint axis. If you wish to model the behavior you describe, you can do it. One way is to model a third body that has a "liftoff" cam pair to the body that the spring is compressed by. This way, after the constraint is removed from the body connected to the spring, the 3rd body will accelerate and loose contact with the body attached to the spring.

The 2nd way is to model the spring as multiple bodies and have a liftoff cam pair created between the top coil and the body you would like to "launch". This method would allow you to capture the non-linear spring dynamics when they exist for your system. I did this countless times many years ago for engine valve springs with very accurate results for high-speed dynamics.

Good Luck!

23-Emerald IV
(To:ChrisKaswer)

Thanks Chris!  I've got it working.

On a slightly different note, any idea why this would be grayed out?

23-Emerald IV
(To:TomU)

Since the community seems to be unable to play videos created from Creo Parametric, here is the same video uploaded to YouTube:

14-Alexandrite
(To:TomU)

Yeah - the original video posted won't play on this site...YouTube does though. Looks like you have it working very well!

17-Peridot
(To:ChrisKaswer)
i think there are some tricks to getting video to work. limited format. moderation. i think attaching a zip file is most universally successful.