In a previous post we looked at Design Considerations of Bolts in Tensile Loading.
Now let’s look at shear loading. We will consider a similar scenario where two pieces of ductile steel are bolted together with a single bolt. The two pieces of steel experience a constant force (F) which attempts to shear the members in opposing directions. For this example the following properties and assumptions hold true:
- The effective cross-sectional diameter of the bolt is 3/4″
- The bolt proof strength is 80 ksi
- The bolt ultimate tensile strength is 100 ksi
- The endurance strength of the bolt is 20% of proof strength (16 ksi)
- The static coefficient of friction of steel-on-steel is 0.7
- The dynamic coefficient of friction of steel-on-steel is 0.6
- The Shearing Force (F) in this example is 18,000 lbf
Let’s look at two scenarios. Scenario – A involves the bolt being loosely hand-tightened (this means that before the shearing force ( F) is applied to the plates, the bolt experiences zero stress, and after the force (F) is applied, the bolt absorbs 100% of the applied force). Scenario – B involves pre-loading the bolt to 80% of the bolt’s proof strength before the shearing force (F) is applied to the plates. If the bolt tension (T) can create enough Friction Force (Ff), the shearing force ( F) will not affect the bolt but will transmit solely between the two steel members.
Below are the resulting stresses and factors of safety (FS) seen by the bolts in each scenario:
|Scenario A (No Bolt Preload)||Scenario B (With Bolt Preload)|
This type of bolted joint is commonly called a Slip-Critical Joint because it is damn critical that the joint does not slip. In fact, the bolt is able to transmit more load that it is capable of carrying by enlisting the help of Friction (not that I would recommend designing it this way necessarily). By placing the members in tension (bolt preload), there is enough friction force for the loading to transfer safely from one member to the next without adding stress to the bolt. This type of joint largely eliminates fatigue loading on the bolts themselves. Friction is good, no?