What does SFC on a bolt mean

Strength calculation of a bolt and pin connection

If you want to determine the strength of a bolt or pin connection through a stress calculation, this process is complex. This has the following causes:

  1. the preload as a result of the impact and the superimposed deformation of the pin and component under the load
  2. the appearance plastic deformations without affecting the transmission behavior
  3. the Deformation-dependent adjusting contact and notch stresses for bolt-tab connections with play
  4. uneven distribution of the load on the individual pins in the case of several rows of pins, according to the deformation behavior of the components
Load profile of a bolt connection

 

In order to still achieve a reliable result, assumptions and simplifications are made.

Assumptions:

  • Neglect of deformation
  • linear stress distribution present

Simplifications regarding the cause of failure shear

In the next figure you can see a shaft-hub connection that is ensured by a bolt.

Shaft-hub connection with bolts

 

The mean shear stress is defined by:

Click here to expandmean shear stress: $ \ tau = \ frac {F} {A} = \ frac {4 \, \ cdot \, F} {\ pi \, \ cdot \, d ^ 2} $

For transverse pins in shaft-hub connections, the associated peripheral force $ F_u $ is calculated at the interface. The circumferential force is defined by:

Click here to expandCircumferential force: $ F_u = \ frac {2 \, \ cdot \, T} {D} $ with $ T $ = torque

The circumferential force $ F_u $ is divided accordingly into $ 2 \ cdot \ frac {F_u} {2} $.

For this reason, the shear stress equation is obtained as follows:

Click here to expandShear stress: $ \ tau = \ frac {F_u} {2 \, \ cdot \, A} = \ frac {T} {A \, \ cdot \, D} = \ frac {4 \, T} {\ pi \, \ cdot \, d ^ 2 \, \ cdot \, D} $

The following applies to the permissible shear stress $ \ tau_ {perm} $:

Click here to expandpermissible shear stress: $ \ tau_ {zul} = \ frac {\ tau_F} {\ nu} \, \, \, $ with $ \, \, \, \ nu = 2 $ to $ 4 $

$ \ nu $ is the required security.

Click here to expand Double fit before, both parts must be able to withstand the same load, otherwise there will be an asymmetry.

Simplifications regarding the cause of failure shear and bending

In the next illustration you can see a hammered bolt that is loaded by a force $ F $.

Bolts with additional bending stresses

Both shear stress and bending stress occur.

The Shear stress results as above from $ \ tau_a = \ frac {F} {A} $.

The additionally occurring bending stresses due to the force $ F $ are new. The bending stress is formally described by:

Click here to expandBending stress: $ \ sigma_b = \ frac {M_b} {W_b} = \ frac {F \, \ cdot \, l_F \, \ cdot \, 32} {\ pi \, \ cdot \, d ^ 3} $
  • $ l_F $ = length of the lever arm
  • $ M_b $ = bending moment
  • $ W_b $ = bending resistance moment

According to the shape energy change hypothesis (GEH), the following applies to the equivalent stress:

Click here to expandEquivalent stress: $ \ sigma_v = \ sqrt {\ sigma_b ^ 2 + 3 \ cdot \ tau ^ 2} \, \, \, $ with $ \, \, \, \ nu = 2 $ to $ 4 $

If there is only a very short restraint, as in the next figure, the bending is neglected in pin connections.

Clevis under load

The effective support is estimated for the simplified calculation of the bending moment. In our illustration, the maximum bending stress is $ \ sigma_ {max} $ at point A of the clevis shown:

Click here to expandmaximum bending stress: $ \ sigma_ {max} = \ frac {M_b} {W_b} = \ frac {1} {2} \ cdot F \ cdot (\ frac {a} {2} + \ frac {b} {4} \ cdot \ frac {1} {W_b}) $

At the interface B, however, the bending stress $ \ sigma_ {bB} $ is defined by:

Click here to expandBending stress: $ \ sigma_ {bB} = \ frac {M_b} {W_b} = \ frac {1} {2} \ cdot F \ cdot \ frac {a} {2} \ cdot \ frac {1} {W_b} $

Simplifications regarding the cause of failure surface pressure

In this load variant, surface pressure results from the superposition of shear force and bending.

Surface pressure on the pin

In the illustration you will again see a pin with a spring attached to its head, at the end of which a force $ F $ acts.

The shear force is calculated from:

Click here to expandShear force: $ P_d = \ frac {F} {s \, \ cdot \, d} $

The bend $ P_b $ is obtained by rearranging the equation of moments. The equation of moments around the point (yellow) has the form:

Click here to expandMoment equation: $ P_b \ cdot \ frac {s} {2} \ cdot \ frac {2} {3} \ cdot d \ cdot \ frac {s} {2} = F \ cdot (l_F + \ frac {s} {2} ) $

If one solves the moment equation for $ P_b $, one obtains:

Click here to expandBend: $ P_b = 6 \ cdot F \ cdot \ frac {l_F + \ frac {s} {2}} {s ^ 2 \, \ cdot \, d} $

With the equations for the shear force and the bending, we can finally set up the equation for the maximum surface pressure, which results from both:

Click here to expandmaximum surface pressure: $ P_ {max} = P_d + P_b = \ frac {F} {s \, \ cdot \, d} (1 + 6 \ frac {l_F + \ frac {s} {2}} {s}) $