Materials strength question

[Dawson]

This question relates to clamping a dovetail to a telescope and the bolts used for that…

I’ve got M4 bolts, 25mm long, made of A4-70 stainless steel.

How much mass could one bolt support before it snapped or deformed? I guess the two force options would be a weight hanging off the bolt pulling it in its long axis. The other would be the weight hanging off the bolt, and stressing it across is short axis / diameter.

I’ve read values of 600 MPa but what does that mean in terms of kg hanging off, or across, an M4 bolt?

Thanks for any answers and I appreciate I’m asking for a very complicated topic to be condensed into a single kg answer, per bolt.

James Dawson

[Roy Hughes]

James,
Typing “how much weight can an m4 bolt hold” into Google gave…
“An M4 bolt’s weight-bearing capacity depends on several factors, including its grade, the material it’s made of, and the type of load it’s subjected to (tensile or shear). However, a general estimate for a common grade 8.8 M4 bolt is around 300 kg (661 lbs) in tension and potentially 150 kg (330 lbs) in shear. This assumes the bolt is properly tightened and not subjected to excessive vibration or impact.”
from the AI (always suspect, may have the figures the wrong way round!).
But if the material you are bolting into is aluminium that would fail in tension/pull out well before that.
Not a lot of use I’m afraid – everything else is above my brain grade.
Roy.

[Nick James]

James,

The maths is simple but probably not very useful for your application.

Tensile strength is measured in Pascals and a Pascal is a Newton per metre squared. If a rod has tensile strength T [N/m^2] and diameter d [m] then it will support a weight which is a product of its tensile strength and area, i.e. W = T * pi * d^2 / 4 [N]. The material will permanently deform long before it breaks.

Taking your example of T = 600MPa and d = 0.004m then W is around 7500 N which corresponds to a mass of 750 kg at the Earth’s surface.

[Dr Paul Leyland]

I am not a mechanical engineer, but wouldn’t the thread give way before the body of the bolt? I’ve seen far too many stripped threads in my life.

[Dawson]

Thanks all. I had done extensive reading but with my limited knowledge and comprehension of such matters it was impossible to just get an answer in kg. I’m grateful to Nick for coming up with 750kg, which puts my mind at ease, especially as there are four bolts, and as pointed out the aluminium of the dovetail would likely fail first. Paul, I was worried more about shearing across the bolt than longitudinal force and stripping the threads. It was interesting reading about it all though, and I had no idea it was so involved and different thread counts present different shearing capabilities.

Thanks all!

James

[Author]

Posts