How much weight does a liftarm hold?

[Stefaneris]

Hello Technic people

I'm currently building a Zeppelin which should be hanging form the ceiling. The weight is less than 3kg (about 6 pounds), and there is a frame with techinc bricks on the inside. The whole construction is held together with pins and I use two times two liftarms (so there are four liftarms) to have a two ropes connecting the Zeppelin with the ceiling (at least, thats the plan).

Now my question: Since I think that the critical part are the four liftarms, which might break. Can these bricks hold the weight for a certain time (several days)?

Here's a picture of my work in progress:

Zeppelin WIP 3 by StefanEris, auf Flickr

[knotian]

I believe so, but you may have to think about bending moments. I would suggest a strong beam be built at the bottom of the model and the hanging fixtures be secured to it rather than the skin.

Ed

[zux]

Just 4 liftarms? Your model seems to be quite big, so I would assume it has quite sturdy frame and 4 liftarms doesn't sound like enough for this. I would be more concerned about the lack of proper frame than the strength of liftarms.

Do you have picture of your internals?

 

[Davidz90]

I think it should work ok as long as the zeppelin is more or less hanging on them. In pure tension or compression, even a single liftarm can withstand 3+ kg. I had a gravity powered clock where the 2.5 kg weight was hanging on two liftarms. There was no damage after a week. 

[Stefaneris]

3 hours ago, knotian said:

I believe so, but you may have to think about bending moments. I would suggest a strong beam be built at the bottom of the model and the hanging fixtures be secured to it rather than the skin.

Ed

Thanks for your suggestion, there is a strong beam on the inside, where the liftarms are fixed. The skin itself doesn't need to support the whole weight.

3 hours ago, zux said:

Just 4 liftarms? Your model seems to be quite big, so I would assume it has quite sturdy frame and 4 liftarms doesn't sound like enough for this. I would be more concerned about the lack of proper frame than the strength of liftarms.

Do you have picture of your internals?

 

Thanks, see the following pictures.

1 hour ago, Davidz90 said:

I think it should work ok as long as the zeppelin is more or less hanging on them. In pure tension or compression, even a single liftarm can withstand 3+ kg. I had a gravity powered clock where the 2.5 kg weight was hanging on two liftarms. There was no damage after a week. 

Thank you for sharing your experience.

 

Here are two pictures showing the internal structure. I build the zeppelin in two halves, so I can transport it better. There are four liftarms (the black ones) which connect these two halves.

http://Zeppelin WIP 4 by StefanEris, auf Flickr

http://Zeppelin WIP 5 by StefanEris, auf Flickr

[zux]

Ok, there was a little misunderstanding - your structure has more than 4 liftarms. It looks sturdy enough to be kept for far more than few days.

[Stefaneris]

Thanks, that's what I was hoping to read. I already had the feeling that it was stable (otherwise I wouldn't take such a picture) but it's sometimes better to ask about a second opinion.

[BusterHaus]

@Stefaneris From what I can see in the pictures, the 15L LBG beams look to be unsupported close to the string attach point.  You could make your structure much stronger by using a few beams to create triangles.  You can use any combination of the ones below:

 

Edited July 22, 2018 by BusterHaus

[Stefaneris]

@BusterHaus Thanks for your input. This looks like a good idea, I will add some more beams to my construction.

[schraubedrin]

If you want to stiffen up your construction you could try to put the beams further on the outside of the construction.

The distance of the beams to the centreline  influences the stiffness to the power of three.

[nerdsforprez]

As for the beams themselves (not the internal structure/chassis) I think you are fine.  WHen I was first beck from my dark ages i had a custom Millennium Falcon that I had made. Nearly the exact same size as your creation.  Nearly 3000 grams exactly, and the mounting points were nearly the same as yours.  Because mine was mounted from a wall, and therefore gravity acting differently than just pulling straight down, I actually think the forces on my MF were even greater.   It held for many months before crashing down.  But this was my own fault, and not because of the Lego.  I actually had not accurately hit a stud in the wall, and the MOC had pulled straight out of the sheet rock. But the Lego mount, similar to your mounting points, was entirely intact.