How much load is going through the bolts

[Stickyman]

Wow, awesome replies guys, sorry I have missed them until now.

I ended up making that brace but added a section above to connect them and hang the slings.

These are welded straight to the frame as I did not have any flat bar suitable and the steel stores are closed for xmas.

I got caught up in the moment, and forgot to take all the measurements of this test, but I think it performed much better.

Here is a video of 220kg.

 

[Stickyman]

On second view, the barbell falls just over 1 plate width.

The free fall would have been just over 450mm.

 

[nvn]

Stickyman: Which plate were you referring to, in post 32, when you said "1 plate width"?

The mass is falling and hitting almost a hard stop. The stiffer you make the system, the more the impact force skyrockets. I agree with the post by munster; a damper or shock absorber would be good. How about if you attach, say, ten pieces of velcro to the sling, then make ten small folds in the sling, attaching the velcro, like an accordion? Then when the mass falls, the velcro must disconnect along the way, thereby reducing the barbell descent velocity.

I agree with the post by 256bits; one of the highest stresses currently occurs in the horizontal square tube "foot," just before it connects to the lower end of the upright square tube. The static normal stress on this cross section is, sigma1 = 0.5(825 mm)(425 kg)(9.81 m/s^2)(50 mm)/(1 827 092 mm^4) = 47.06 MPa. But sigma1 must still be multiplied by a dynamic amplification factor, which we do not know yet, but which could be, say, 5 to 20. Any of these values overstress that cross section. (And that cross section is also subjected to some torsional shear stress, but we do not have all the structure dimensions yet.) Therefore, the diagonal brace I mentioned in post 29 would be helpful. But also, the damper, mentioned above, is critical. Otherwise, I currently doubt you could go up to a 425 kg mass without exceeding the structure allowable stress.

 

[Stickyman]

Thanks for the reply NVN.

The plate I am referring to is a "weight plate", sorry, gym talk.
A weight plate/disc is 450mm in diameter.

I realize that the stiffer you make the frame, the higher the stresses become, that's why I was happy with the level of "bouncing" it did.
Not so much that it was going to flip, but enough to "help" dissipate the energy from the falling barbell.

The velcro idea sounds plausable, but I really like the idea of adding a car/motorcyle shock absorber to the frame, but that will take some thinking.

The uprights are joined by a (loose) clevis joint, 50x50x3mm RHS (just below the clevis), and a 100x100x3mm post that the slings now hang from.

As far as the bracing the footprint goes, there are not a lot of options on this style of monolift, at least on this particular one.

In future, I could modify it to allow bracing at the bottom.

What you need to work out a SFL on the current design? Also, out of curiosity, what do you do for work NVN, you seem very intelligent, at least to a carpenter like me haha.

 

[Stickyman]

What about something like this?

http://safetysling.thomasnet.com/item/gripper-mesh-slings/g-35-heavy-duty/4005

 

[Stickyman]

http://www.yatesgear.com/climbing/screamer/

 

[nvn]

Stickyman: What did you mean by "SFL" in post 34?

 

[Stickyman]

Sorry mate, safe work load.

Safe Work Load.

What dimensions would be needed for us to find out what the structure could if "X" amount was dropped from 750mm in its current set up?

Also, what did think about the shock absorber in post 36?

 

[nvn]

Stickyman: For one thing, if you could measure the static deflections described in post 18 (paragraphs 2 and 3) and post 30, for your current setup, that would be helpful.

Regarding the post 36 sling, my first impression was, it could be expensive, because it is not reusable. And, it seems it might be too short when contracted, and too long when extended, to be usable for the barbell sling (?). And the post 35 sling might be too elastic (?), but I am not sure.