How strong are 1/4 and 3/8 thread screws like on tripod heads, magic arms,mounts,etc?

A

analogs

Well-known member
I am always worried about my gear on top of a small tripod head or mount being held by only 1/4 thread screw.

How much force or weight can it actually hold?

I know that even though gear can be 12 kgs in total on top of a tripod head the balance of the gear on the head and once you start doing things like tilt where you actually apply force or momentum that the actual weight or strain on the 1/4 screw changes.

Can someone enlighten me on this and how much stronger is 3/8 compared to 1/4?

Are there ways of minimizing the strain on the screw or what is the best way of piling your gear on top of a 1/4 or 3/8 screw?

Thanks
 
analogs said:
I am always worried about my gear on top of a small tripod head or mount being held by only 1/4 thread screw.
Click to expand...

You shouldn't be. A 1/4" screw can take way more stress than you think. If the screw and the nut you are screwing into are both in good shape, I wouldn't be surprised a run of the mill cheap steel 1/4" screw could hold half a ton or more. The screw is the size it is to make it easy to handle, and because that size screw is extremely common, and therefore cheap. It has absolutely nothing to do with the amount of stress it can handle, because just about anything is overkill.

The trick with a screw joint is to make it tight. The screw is then loaded only in tension, which is its strength. The tension in the screw supplies the normal force between the tripod and the camera bottom surface. The force of friction is proportional to the normal force (friction force = (friction coefficient)(normal force)).

For more, see the Wikipedia entry on static friction.

If the screw is not tight, you can possibly load the screw in shear. Shear strength is not great in screws, but it's still likely going to take hundreds of pounds of force to shear a 1/4" screw in half.

As long as you have several threads engaged (three or more to be really safe) and the screw is in decent shape I wouldn't worry.
 
The torsional in-line strength of even the ¼" thread is surprisingly high. The issue is that the material it's made from. Steel vs aluminium v some of the alloys with bronze in them, and who knows what a tripod thread is actually cut into! The critical thing is to not allow it to bend. so it's the base that surrounds the thread that matters. If it's small then bending forces start to play with a heavily loaded top - even just the small layer of rubber can allow back and forth movement, which fatigues the bolt so it fails.⅜" threads have a thicker body, and a greater circumference so the surface area means they're stronger and stiffer.

¼" capacity in terms of tensile strength depending on material goes from 1750 to 3800lbs compared to 4200 to 9300lbs for ⅜" size - which show that the actual capacity is pretty huge. The trouble comes when you apply your high school physics (levers, wedges, force etc) and think about the distance from the thread out to a pan bar tip from the thread, and the small area of plate where the bolt goes through. You've got a pretty good crow bar (pry bar? Wrecking bar US???) two feet of pan bar and an armful of force is a really large force at the thread.

If you look at old broadcast wedge plates for the old huge, heavy cameras, you can see how ⅜ bolts were quite strong enough because the size and design of the wedge plate made sure that multiplication of force was well under control.
 
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