Using 2 bolts M12 or 4 bolts M6?

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Discussion Overview

The discussion revolves around the comparison of using two M12 bolts versus four M6 bolts in a mechanical design, specifically for a gearbox application. Participants explore the implications of bolt size and quantity on load-bearing capacity and structural integrity, considering both axial forces and moments.

Discussion Character

  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant questions which configuration (2 M12 bolts or 4 M6 bolts) can bear force better and seeks clarification on the reasons behind it.
  • Another participant suggests considering additional factors acting on the joint beyond just force, emphasizing a three-dimensional perspective.
  • A participant shares results from a test conducted using Inventor software, indicating that M6 bolts failed at 750N while M12 bolts withstood 1500N, suggesting a significant difference in strength.
  • Further testing by the same participant indicates that one M12 bolt can bear an axial force of 2500N, while seven M6 bolts can also bear approximately the same axial force, raising questions about the comparative strength of the configurations.
  • One participant introduces the concept of moments, explaining that a single bolt struggles with moments due to forces across the joint, while multiple bolts can better manage these moments in different planes.
  • Another participant points out that a bolt's strength is determined by its shank's cross-sectional area, prompting a comparison of the cross-sectional areas of the two configurations.

Areas of Agreement / Disagreement

Participants express differing views on the strength and suitability of the two configurations, with no consensus reached on which is definitively better. The discussion remains unresolved regarding the optimal choice for the application.

Contextual Notes

Participants mention various factors such as axial force, moments, and cross-sectional area, but the discussion does not clarify the assumptions or definitions underlying these considerations.

cherish
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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?

Thanks many in advanced!

[PLAIN]http://img824.imageshack.us/img824/7971/123ok.jpg
 
Last edited by a moderator:
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Is this coursework?

If so, here's a hint.
Think about what is acting on the joint besides force. Think in 3D.
 
Studiot said:
Is this coursework?

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

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

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:
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?
 
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?
 

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