# Using 2 bolts M12 or 4 bolts M6?

1. Aug 19, 2010

### cherish

Hi there,

I'm not sure that using the same type of 2 bolts M12x1.5 (L=30mm) or 4 bolts M6x1 (L=25mm) & place equidistant in a circle as picture below, which can bear force better & why?

Last edited by a moderator: May 4, 2017
2. Aug 20, 2010

### Studiot

Is this coursework?

If so, here's a hint.
Think about what is acting on the joint besides force. Think in 3D.

3. Aug 20, 2010

### cherish

hi Studiot, certainly this's not coursework, just a small problem that i design it for a gearbox. First time i try to use 4 M6 as mentioned above & my boss agreed with solution but later i found a guide from catalogue they using 2 M12 in stead as picture attached, just a bit wondering that the total diameter of these two solutions is almost the same but not sure which is stronger :grumpy:

While looking for an answer i made a test on Inventor software & the result is that: M6 appeared in failure with axial force 750N but M12 is compliance with 1500 N axial force (even if 2500N is still fine). by the way not understand essence of situations.
P/S: one more test
one M12 can bear an axial force P = 2500N & 7 bolts M6 can bear an approximately axial force (P = 2500N)

Thanks many in advanced & have nice weekend!

Last edited: Aug 20, 2010
4. Aug 20, 2010

### Studiot

OK what I am talking about is moment.

A single bolt cannot cope with moments due to forces applied across the joint or slight angular misalignments.

Two bolts can cope with moments in one plane ie along one axis

3 or more bolts can cope with moments applied in two planes at right angles and therefore all such moments.

Does this help?

5. Aug 20, 2010

### Mech_Engineer

Think about it this way- a bolt's strength is based on it's shank's cross-sectional area (not diameter)... so what has more cross-sectional area, 2 12mm bolts, or 4 6mm bolts?