Learning Forum

[mvcad]

Hi All,

I have designed and built a set of 4 mobile lifting jacks for locomotives. Each Jack will be placed on each corner of the locomotive to lift it. The Client has ask me to do a dynamic analysis to find out if a mass hanging from on overhead crane impacting any jack would be strong enough to tip it to the ground. As the Jacks are mobile, they are not fixed to the ground. I wonder if this is possible using Explicit Dynamics. or perhaps Rigid body analysis. All examples I have found are focused on the object impacting a fixed geometry. but in my case I am concerned about the object being impacted. i.e. if I compare this with a car crashing a wall I would want to get the effects on the wall.

I have read that if I am not interested in deformations, a rigid body analysis would suffice. Is this correct?

Anyone cant point me to an example of something similar? or perhaps a guideline on how can I achieve this?

I also want to be able to add point mass on my model. but I cannot do it in explicit dynamics. or at least in V18.1
See attached image: Thanks in advance.

[peteroznewman]

Inserting your attached photo for the benefit of ANSYS staff who are not permitted to open attachments

There are two ways to think about a crane with a hanging mass that makes contact with this structure, to answer the question of whether the structure will tip over.

The first way is statics. If the crane moves the mass very slowly, and the hanging mass makes contact at the point P on the structure, and the crane keeps moving, the cable suspending the mass will develop an angle. Say the cable is 10 m long, and the weight suspended is 100 kN, once the angle of the cable goes past 3 degrees, the side force will exceed the 5.3 kN tipping load. If the weight is less, then the angle will have to be more.

The second way is work-energy analysis. Now you have to specify the mass and velocity of the hanging mass to calculate the kinetic energy. A 10,000 kg mass moving at a velocity of 1 m/s has a kinetic energy of 5,000 J.  Now calculate the potential energy or work done to rotate the jack until the center of mass is above one side of the base plate.  The figure above doesn't show where the center of mass is, but say it was 1 m above the baseplate. If you rotate the jack 25 degrees, the center of mass will be at the tipping point above one side of the baseplate. The center of mass will have been lifted 0.107 m.  The work done lifting 32 kN to this height is 3,400 J.  Therefore the impact of a mass larger than 3,400 kg traveling at 1 m/s has enough energy to tip over the jack.

You could build a Rigid Dynamics model where a wrecking ball of some mass impacts the structure at some velocity, but you would have to make sure your structure had the correct mass, which you may not have bothered to do for the Static Structural analysis you have done so far.

[mvcad]

Thank you Sir, this is a very detail response. I really appreciate you time. this is pretty much everything I need to do. I have to do other scenarios like a 4T forklift hitting the jacks at 5km/hr but with your guidelines this is easy to do. Thanks again.

 

Mario.

[mvcad]

"Therefore the impact of a mass larger than 3,400 kg traveling at 1 m/s has enough energy to tip over the jack."

Quick one. does the height where the mass impacts the jacks have any relevance? I cannot see it from your explanation, but from a static point of view the higher the force the easier would be to tip the jacks. Is this correct? in my figure I put it at 2865mm.

[peteroznewman]

The work-energy analysis assumes 100% of the kinetic energy goes into lifting the mass against gravity to the tipping point. Obviously, the impact has to be at a height that allows tipping to occur. It is also true that this is a limit analysis. The transfer of energy from kinetic to potential energy will not be 100% efficient, so a lot more energy may be required to actually tip over the structure.