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How much load is going through the bolts

by Stickyman
Tags: bolts, load
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Stickyman
#1
Dec20-13, 11:35 PM
P: 20
Hey guys.

Im building a special type of squat rack for my gym, and need some input on the bolts to use.

I have used high tensile M16 bolts on all joints, but am not sure if it is enough, hence me joining this forum and asking you guys.

Attached is a picture, and Ill explain the lines ive used it a bit more detail.

The white circles are M16 bolts.

The red arrow is a pivot point.

The yellow line is 880mm long, the pivot point to load.

The blue line is 1000mm long.
A Barbell sits inside the green slings. You squat up to 500kg, the bar could fall a maximum of 1000mm.




So, the question is;
If a weight of 500kg falls inside the slings, which is 880mm away from teh pivot point, how many kilograms of force will be on the 2 bolts at the pivot point? Is 2x M16 bolts big enough?

Many thanks for your time.
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Stickyman
#2
Dec20-13, 11:35 PM
P: 20
This may help https://www.facebook.com/photo.php?v=10151835569993015
Baluncore
#3
Dec21-13, 01:41 AM
Sci Advisor
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The answer to your question requires knowing how long it takes the falling mass to stop. That is determined by the flexibility of the structure.

If the structure has no flexibility and the falling load stops instantaneously, then the force on the bolts will be infinite.

Stickyman
#4
Dec21-13, 03:23 AM
P: 20
How much load is going through the bolts

The link in my 2nd post has the video of my dropping the bar with 400lb from about 1m height.
Baluncore
#5
Dec21-13, 04:08 AM
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Then you can measure how long the mass takes to fall and how long it takes to stop.
I cannot access facebook and I avoid watching videos.
AlephZero
#6
Dec21-13, 10:24 AM
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I think you need some professional advice for designing this, since obviously a falling 500kg weight could cause a serious injury to the user.

As Baluncore implied, the force on the bolts will depend very much on how flexible the green (fabric?) slings are. Beyond that comment, IMO you need to do some proper calculations based on the full details of the structure, not just a photo and a couple of dimensions. Personally, I wouldn't like to guess if the bolts or something else would fail, and it would be a guess from the (lack of) information we have.

You could easily be talking about seriously high loads here. The multiplication factor from stopping the falling mass could easily be 10 or 20 times. Them you have a lever arm with a ratio that look like about 6:1

So your 500kg weight could be putting a load of 50 tonnes or more into the top of your frame. But that number is just a (plausible) guess - don't use it to design anything!
Stickyman
#7
Dec21-13, 06:35 PM
P: 20
Quote Quote by AlephZero View Post
I think you need some professional advice for designing this, since obviously a falling 500kg weight could cause a serious injury to the user.

As Baluncore implied, the force on the bolts will depend very much on how flexible the green (fabric?) slings are. Beyond that comment, IMO you need to do some proper calculations based on the full details of the structure, not just a photo and a couple of dimensions. Personally, I wouldn't like to guess if the bolts or something else would fail, and it would be a guess from the (lack of) information we have.

You could easily be talking about seriously high loads here. The multiplication factor from stopping the falling mass could easily be 10 or 20 times. Them you have a lever arm with a ratio that look like about 6:1

So your 500kg weight could be putting a load of 50 tonnes or more into the top of your frame. But that number is just a (plausible) guess - don't use it to design anything!

Thanks for the reply mate.

Sorry about the lack of information, I have no idea on engineering, which is why I'm posting here.

The green straps have 0 stretch in them.

I dropped 180kg from 1m into the straps, and guessed around 10cm of flex from the structure.


This is a reply I got from Facebook;

Okay, sorry man I've been at Ikea..

Basically, without knowing the angle of the member indicated by the yellow line, I've just had to do the calculations under the assumption that it acts perpendicular to the uprights.

The impact force of a 500kg object falling 1m = 49000 N

To find the moment force about the bolt you multiply that by 0.88m, which = 43120 Nm

Which is about 4397 Kg/m. So across 240mm that's giving me like.... 18,320 Kg of force distributed between the two bolts.

This is by no means the full story though, to give you an accurate measurement I would need to determine the compressive force on the front bolt and the tensile force on the back bolt, as well as whether or not they're in single or double shear. Which would still only give you a static result, from there you could then start looking at what's happening to the bolts and welds and **** dynamically.

Patrick, that's not plastic deformation, that's the "recoil" of the falling object. And 0.1m movement in the bar after impact seemed pretty reasonable based on Scott's test video. Plus, the lower that value is (distance traveled after impact) the higher the impact force becomes. So if you take it down to say.. 5-10mm the impact force value becomes something like 490^12kN

^^Completely agreed with Patrick on that. The rating of 8.8 grade M16 bolts isn't worth jack **** if they're subjected to multi-planar force (heavy **** stressing it in several directions at once), which a half-ton impact will certainly do. So if someone fails while going heavy, it's worth replacing them.
nvn
#8
Dec21-13, 08:47 PM
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Stickyman: In your video, I saw your system deflecting about 60 mm, not 100 mm like you estimated in post 7. I currently estimated 60 mm, unless you have more accurate data. Therefore, using a 60 mm deflection, I currently obtained a system stiffness of k = 704 250 N/m.

Are you sure you want to design the pivot point bolts for a 500 kg mass, dropped from a height of 1000 mm? When I checked this, the dynamic amplification factor was 18.0, and the M16 property class 8.8 bolt, located at your pivot point (red arrow) shown in post 1, was stressed to 380 % of the allowable stress, which exceeds 100 %, and therefore would be grossly overstressed. (And this assumes one end of your bolt has threads in the shear plane.)

The above is quite different from your test video, which used only a 182 kg mass dropped from a height of 650 mm. Therefore, what is the maximum mass and drop height you want the bolt to withstand?
Stickyman
#9
Dec21-13, 09:01 PM
P: 20
Quote Quote by nvn View Post
Stickyman: In your video, I saw your system deflecting about 60 mm, not 100 mm like you estimated in post 7. I currently estimated 60 mm, unless you have more accurate data. Therefore, using a 60 mm deflection, I currently obtained a system stiffness of k = 704 250 N/m.

Are you sure you want to design the pivot point bolts for a 500 kg mass, dropped from a height of 1000 mm? When I checked this, the dynamic amplification factor was 18.0, and the M16 property class 8.8 bolt, located at your pivot point (red arrow) shown in post 1, was stressed to 380 % of the allowable stress, which exceeds 100 %, and therefore would be grossly overstressed. (And this assumes one end of your bolt has threads in the shear plane.)

The above is quite different from your test video, which used only a 182 kg mass dropped from a height of 650 mm. Therefore, what is the maximum mass and drop height you want the bolt to withstand?
Awesome, that's such a great reply, thank you.

In real life, the most weight that will be attempted will be 380-425kg.
The fall into the straps in normally much shorter than in the vid I posted, I'm just trying to make it as safe as possible.

I could move up bolt sizes if needed.
nvn
#10
Dec21-13, 10:09 PM
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Stickyman: So would the maximum drop height be about 800 mm? Do you want to use ISO property class 8.8 bolts? Or property class 10.9 bolts?

Can you post the dimensions (mm) of your lug and clevis plates, at the pivot point bolt (red arrow in post 1)? From a front view, it will look something like my attached file. Dimension t1 is the thickness of your lug, t2 is the thickness of your clevis plates (one clevis plate on each side of the lug), and gp is the gap between the lug and each clevis plate (when the lug is centered between the clevis plates).
Attached Thumbnails
lug-clevis-03.png  
Stickyman
#11
Dec21-13, 10:21 PM
P: 20
I can use what ever bolts I need to, but currently 8.8.

The plates are all 10mm thick, and there is 8mm clearance gaps either side..... A little to loose I know :(
nvn
#12
Dec21-13, 10:33 PM
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Stickyman: Your current gaps are terrible. I would not want those gaps to be more than about 0.5 mm, each. Is that something you could redesign?
Stickyman
#13
Dec21-13, 10:58 PM
P: 20
I can close them up easy enough.

Will it make the structure that much weaker as is?

This whole thing has been made with a 5" grinder, drill press and a mig.

I don't have top gear.
nvn
#14
Dec22-13, 12:27 AM
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Stickyman: For a 425 kg mass, dropped from a height of 800 mm, for the pivot point bolt, it currently appears you would need t1 = 24 mm, t2 = 12 mm, gp = 0.5 mm, and an M30 x 3.5, property class 8.8, bolt.
Stickyman
#15
Dec22-13, 12:34 AM
P: 20
Awesome.

Thank you very much.

If it's not a huge trouble, can you give me a brief run down on the calcs?
nvn
#16
Dec22-13, 01:05 AM
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P: 2,121
Stickyman: That would be too lengthy and too detailed.

I want to point out though, the mass and drop height (and force) corresponding to post 14 is far more force than what you tested in your video. Even though the force in your video is much less, you can see, in the video, the structure looks like it is already being pushed to limits. I am doubtful your structure and weldments could withstand the far greater force corresponding to post 14. See what I mean?
Stickyman
#17
Dec22-13, 01:23 AM
P: 20
I do mate, I do indeed.

That's why I posted.

I'll redo the bolts as per your recommendation, and thicken t1 and t2, whilst moving the clearance to .5mm.

I'm also going to shorten lever arm ( yellow line) from 880mm to 600mm.

Thanks for your help mate, much appreciated.
nvn
#18
Dec22-13, 11:37 AM
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Stickyman: First let me mention, I think energy losses in this particular system are no more than 2 or 3 percent, and are therefore negligible.

Secondly, if you could measure the deflection for us, as follows, it would be appreciated. Place the empty steel barbell rod (with no disk-shaped masses installed yet) in the green slings, and measure the exact distance, in mm, from the floor to the bottom of each green sling.

Next, install any amount of disk-shaped masses onto the barbell rod, preferably a high mass, or fairly high mass (such as your 180 kg, or 350 kg). (And let us know exactly how much mass, in kg, you installed.) Now measure the exact distance from the floor to the bottom of each green sling.

If you could do this, and post this data, it would enable us to compute the system stiffness k1, and would be really interesting, and important.

Third, I think I now notice that your structure has a rear brace only on one side, where a hydraulic actuator is installed. This is quite different from what I thought yesterday, and it would cause the structure to be loaded very unevenly, causing a vast majority of the load to go to the side containing the rear brace. This makes it much more difficult (and inaccurate) to compute. If you are going to keep the structure designed asymmetrically like this, then we would need to rethink the answers, and it would become more complicated.


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