A most important factor is machine design, and structural design is the rigid fastening together of different components..This should include the following considerations..

There are many methods of fastening items together including

These notes relate primarily to the bolted joint.   The bolted joint is a very popular method of fastening components together.  The prime reason for selecting bolts as opposed to welding, or rivets is that the connection can be easily released allowing disassembly, maintenance and/or inspection..

The bolts /screws are generally used in groups to fasten plates together.   A bolt is a screwed fastener with a head, designed to be used with a nut.   A screw is a fastener designed to be used with a formed female thread in one of the components being attached.

These notes generally relate to bolts and nuts and hex headed screws..

A bolt can be loaded in one of three ways

Note: Conditions where bending loads are imposed on the bolt e.g. non-parallel bolting surfaces, should be avoided.

A bolt is primarily designed to withstand tensile loading while clamping components together.   Ideally the bolt should only be loaded in tension.  Any forces tending to slide the clamped components laterally should be withstood by separate means..

Holes for bolts are generally clearance holes and the best design of bolt is one with a reduced shank diameter (waisted shanks).   Joints in shear depending on the bolts to withstand the shear load are not really rigid.   Significant relative sideways movement must take place before the bolt shank can take any shear load (hole clearance).  It is also likely that in the case of components attached by a number of bolts that one bolt would be loaded first and this bolt would have to yield before the other bolts take their share of the shear load....

Bolts taking significant tensile and shear load need to be engineered to withstand the combined stress..

In structural engineering the codes identify the use of High Strength Friction Grip Bolts (Ref BS 4604 Pts 1-2:1970).   The bolts are tightened to a specified minimum shank tension so that transverse loads are transferred across the joint by friction between the plates rather than by shear across the bolt shank.

In mechanical engineering / machine engineering, items are often accurately located using dowels /locating pins.   When installed these dowels /locating pins should be engineered to withstand any traverse loads.   A recent innovation is to provide dowel bushings.  These are used in conjunction with bolts which pass through the inside of the bushing after it has been installed.   Separate holes for locating pins are eliminated.    The hardened bushings absorb shear loads, isolating the bolts from these forces.

If the choice is made that bolts/screws are to take shear load the joint should be arranged that the threaded portion of the bolt/screw shank is not taking the shear.

Important Note:
The calculations below are based on the unrealistic assumption that there is no friction forces between the plates which are clamped by the bolts.  The calculations are therefore conservative (safe)..

For bolts joints loaded in shear - three stress areas result-

Single Shear..

Shear Stress = 4 . F / π. d ²
Compressive Stress = F / (d . t)
Plate Shear Stress = F / (2.c.t)

Double Shear ..

Shear Stress = 2 . F / π. d ²
Compressive Stress = F / (d . t)
Plate Shear Stress = F / (2.c.t)

The stresses are adjusted based on the number of bolts / screws used for the joint..

Consider a bracket taking an offset load F (N) at a radius R (m).  The bracket is secure using a number of bolts each with a Area A(m² ).   The bolts are located around a centroid position each with a radius from the centroid of r_n(m) and a horizontal/vertical position relative to the centroid of h_n /v_n (m) . ( bolt is designated by the subscript "n". )

The offset load is equivalent to a vertical force (F) + moment (F. R) at the centroid of the bolts...

Each bolt is withstands a vertical shear force      F_nv = F / No of Bolts.
Each bolt also withstands a shear load      F_nm = F.R. r_n / (r₁² + r₂²...r_n²)
The total horizontal force on each bolt      F_th= F_nm . v_n / Sqrt(h_n² + v_n² )
The total vertical force on each bolt     F_tv= F_nv + F_nm . h_n / Sqrt(h_n² + v_n² )
The total shear load on each bolt     F_t= Sqrt (F_th² + F_tv²)
The resulting bolt shear stress     τ _t = F_t /A


The shear stress in each bolt is calculated to ensure the design is safe..

Each Bolt withstands a shear Force     F_s = F_v / (Number of bolts)
The resulting shear bolt stress     τ _n = F_s /A

Note: Each bolt is assumed to withstand the same shear force.

If there are x bolts( numbered n = 1 to x). Then the tensile force withstood be each bolt is designated F_nt i.e F_1t,F_2t, F_3t....F_xt
A selected bolt (n) withstands a tensile force of     F_nt = ( F_v. R_v + F_h. R_h) . V_n / (V₁² + V₂²....V_x² )
The resulting tensile bolt stress     s_n = F_nt /A

The notes on this page Assuming all stresses developed only as a result of bracket loading i.e zero preload and zero residual bolt torque...

Maximum principal tensile stress in the bolt

Maximum principal compressive stress in the bolt

Maximum shear stress in the bolt

The notes on this page In order to estimate the design factors of safety it is necessary to consider the failure modes.    The preferred failure criteria for ductile metals is the "Shear Strain Energy Theory" (Von Mises-Hencky theory).   For a stress regime associated with a bolt i.e pure tensile stress s_x combined with shear stress τ _xy.   The Factor of safety relative to the material tensile strength S_y..is calculated as follows

Factor of Safety = S_y / ( s_x² + 3 .τ _xy² ) ^1/2

These stresses do not include for the stresses developed in preloading the bolts.  The residual shear stress from bolt tightening should also considered (added).   The actual tensile preload force should be considered following the principles identified on the pages addressing this topic