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I am working on a project designing a pendent speaker. The speaker will be housed in a steel ball and suspended from the ceiling using an M6 eyebolt component. The overall weight to be suspended will be under 10 lbs at maximum. When installed correctly this load will be applied straight down with minimal sheer force.

The eyebolt component I have sourced is rated for lifting up to 150 lbs. My steel ball component has a wall thickness of .118", into which I am tapping an M6 thread. There is obviously some tolerance variation in thickness but visual inspection shows at least two complete threads being tapped at this thickness at all points. It seems like the weakest link here is probably the thread tapped into the steel.

Part of the design is to have speaker directionality easily adjustable, so being able to quickly screw and unscrew the eyebolt to multiple of these tapped holes from the outside is a requirement. Hoping this doesn't mean that I have to weld nuts on the interior or anything crazy like that, as that will be prohibitive fabrication wise.

Is this thread sturdy enough to safely hold the speaker? How do I calculate the maximum safe load based on the depth of the material being threaded?

Here are the parts I am using for reference:

Steel Ball: https://www.kingmetals.com/Catalog/ItemContent.aspx?ItemNumber=2338&CatalogId=c39&CatalogDetailId=197&NSM=Y

M6 Eye Bolt: https://www.mcmaster.com/3107T41/

Emmett Palaima
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  • Bolts and screws are common enough that there are tables... – Abel Oct 13 '23 at 22:41
  • @Abel wym? I think your comment got cut off – Emmett Palaima Oct 13 '23 at 23:03
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    M6-1 (1 tells you 1 mm pitch) .188 in = 4.7mm > 3 mm (3 threads) so you should be able to use full proof load of bolt as long as the steel isn't something terrible. Look up proof load table of your bolt – Abel Oct 13 '23 at 23:12
  • @Abel thanks! how did you determine that 3 threads number? – Emmett Palaima Oct 14 '23 at 03:32
  • @Abel also forgive me for the beginner question here but all the proof load tables I can find are in strange units I don't recognize (kN). How do I determine a maximum suspended weight in lbs based on these values?

    Looking at these table: https://structx.com/Design_Table_004c.html https://international.optimas.com/technical-resources/iso-metric-thread-proof-loads/

    For the lowest rating that would give 4.52 kN which online translates to 1016.1364228 lbs. Far beyond the 150 from the McMaster rating.

    That's all good if that's the reality, but can you confirm my understanding of this?

     – Emmett Palaima Oct 14 '23 at 03:38
  • @Abel how did you determine that 3 threads would be sufficient for the whole rating I mean? – Emmett Palaima Oct 14 '23 at 04:28
  • I haven't calculated the 3 threads rule since I was in school. If I recall correctly (not really trustworthy these days), it could be derived from shear stress or something like that. Perhaps someone younger has it fresher in their mind. It's held pretty widely as a rule of thumb though https://www.physics.wisc.edu/ishop/faq.html#:~:text=How%20many%20threads%20do%20I,the%20diameter%20of%20the%20screw. – Abel Oct 14 '23 at 13:08

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You found that the proof load is 1000 lb for your bolt if it were of a particular grade. That should be about right for the threads. You do have to pay attention to some additional factors.

I did the math in the comment using .188 instead of .118 so you're actually just barely under 3 threads. Take some off for that. Fat finger to 800 lb.

Torquing the bolt down can eat up a lot of this! Do not tighten more than needed. For every turn (2pi radians) you screw down 1mm (.04 in). That means for every 1 lbf-in of torque, you can consume 1*2pi/.04~=157 lb of this. That's a pretty steep price for friction to keep a bolt in when you don't have much to begin with.

This load applies to that grade of material. While the bolt shouldn't have too much issue meeting it, and your wall should be close initially, your newly tapped thread may be prone to rust. If it is important for the long term, do something to protect it (loctite can seal to keep air and moisture out depending on how it's applied too!)

Suspended loads can swing. If you expect any swinging treat it like a pendulum and add centripedal force calculations to your 10 lb.

Bottom line: lubricate your tapped hole preferably with something that will compensate for lack of friction later (loctite blue? It's practically glue) and install your bolts finger tight.

If you need it to be removable, consider rivet nuts. No welding is needed, and you only need to access one face to install. Each reinstall is a chance someone uses that bolt to tear out the threads.

Abel
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