# Mechanical joint that break at specific force

I am designing a joint which I want to break at a specific force. I want to design a 2 parts connected by a joint that breaks when a certain amount of force is applied and thus prevent any damage to those individual components themselves. I also want the joint to be easily replaceable without replacing the individual parts themselves. I'm hoping to 3D Print the joint.

I have seen some petrol stations that have pipes attached such that they will break and cut-off fuel supply if they are pulled harshly(i.e. By someone who forgot to take the nozzle off their car and tried to drive away). I don't know the name of the mechanical linkage they use. But I'm hoping to achieve a similar affect.

I would appreciate if you could point me to relevant information sources and design of such type of linkages.

• What type of force- compression, tension, shear, etc? If you know the material properties, you just size the cross section appropriately to handle that specific load. Aerospace collars (nuts) are designed with a thin middle cross section so that the nut side is installed to a certain torque and the driven end is sheared off once that torque is achieved.
– jko
Commented Apr 28, 2021 at 12:56
• Shear pins are sometimes used for this, in a joint that would rotate freely when the moment on it exceeds limit and the pin(s) are broken in shear as the name implies. They can also be used without the rotating joint, if the direction of the force is predictable. Commented Apr 28, 2021 at 13:14
• @jko it is Tension force Commented Apr 28, 2021 at 16:51
• Just to help you in your search, in the US at least this feature of a design is usually called "breakaway." Commented Apr 29, 2021 at 23:26

Another option is the shear pin. It was first thing that came to mind (Pete W suggested it also already).

A joint involving a shear pin (in tension and compression) would look something like the following (here its called a clevis pin, and sometimes you find it as hinge pin).

## force calculation

The force required for the shear pin to fail is basically determined by the smallest cross-section subjected to shear. Although theoretically you can have any cross-section, usually , (to allow for rotation) the shear pin is cylindrical. So the Force for a cylindrical shear pin will be calculated by:

$$F_{max} = \sigma_s \cdot A$$

where:

• $$\sigma_s$$ is the shear stress at failure
• $$A$$: is the smallest cross-section under stress.
• For a filled cylinder $$A=\pi r^2$$
• for a hollow cylinder like the following

## 3d Printing relevance

The really good thing about this and 3D printing is that you can fine tune the exact force that you want. With 3D printing you can design a hollow shear pin and adjust :

• thickness of walls
• printing orientation
• generate different cross-section geometries like

It is called a "fuse" connection, sometimes called the "fuse block", "fuse plug" depending on the trade.

Essentially it is simply a weakened link/connection between two components or a necking area in a location within an element, which is often seen in the tension test, so the failure can be triggered and limited to the weakened region without damaging the components by its sides.

For a fuse connection to remain replaceable, you can design a bolted connection with the flanges much stronger than the bolts, which will fail at a specified stress level, then you only replace the bolts rather than the entire service line.

• Always heard them called "shear pins" or "shear bolts" and a very common use is in agricultural equipment. So if a baler picks up a stone then the pin or bolt shears limiting the damage. Commented Apr 29, 2021 at 20:12
• @Solar Mike The "fuse plug" was taking a page from the electrical fuse box for the same safety concerns. It can be called anything that maintains the same function - trigger the protection when the system is overloaded.
– r13
Commented Apr 29, 2021 at 20:52

You could use a link like this figure. A narrow and thin strap welded to two large tabs with holes for bolt or shaft connection to the parts to be attached.