Double shear is a loading condition where a fastener, such as a bolt or pin, is cut by force across two planes at the same time. Picture a pin passing through three stacked plates: the middle plate pulls one direction while the two outer plates pull the opposite direction. The pin resists being sliced at two separate cross-sections, which means it can handle twice the load of the same pin loaded in single shear.
How Double Shear Works
A shear force is one that tries to slide one part of a material past another, like scissors cutting paper. In a double shear setup, the fastener sits through three connected parts instead of two. The load splits across two shear planes, so each plane only carries half the total applied force. This is the core advantage: a bolt or pin in double shear has double the shear capacity of the same fastener in single shear, all else being equal.
The classic example is a clevis joint. A U-shaped bracket (the clevis) wraps around a flat tongue, and a pin passes through all three layers. When a pulling force is applied, the pin gets sheared at two locations, one on each side of the tongue. The force distributes symmetrically, which also reduces the bending that can twist or cock a fastener sideways.
Double Shear vs. Single Shear
In single shear, a fastener connects just two plates. The entire load acts on one cross-section, and the offset between the two plates creates an eccentric load that tries to bend the fastener and pry the joint apart. This bending moment is a real problem in high-load applications because it concentrates stress at the shear plane and can cause premature failure.
Double shear largely eliminates that problem. Because the outer plates are symmetric around the middle plate, the forces balance on either side of the fastener. There is little to no net bending moment, which means the fastener works in nearly pure shear, the condition it’s designed for. The connection can handle a greater load because the stress is shared across two planes and the fastener isn’t fighting a secondary bending force at the same time.
Where Double Shear Joints Are Used
Aerospace engineering relies heavily on double shear. NASA specifications require a minimum double shear strength for aerospace fasteners, tested under pure shear loading, because flight structures demand maximum strength from every gram of hardware. Wing fittings, control surface hinges, and landing gear linkages commonly use clevis-style double shear pins.
The same principle shows up in steel construction for bridges and buildings, automotive suspension links, heavy equipment pivot points, and any mechanical joint where a pin or bolt must carry significant shear loads. Whenever engineers need to get the most capacity out of a fastener without increasing its diameter, double shear is the standard approach.
Calculating Double Shear Strength
The basic formula is straightforward. Shear stress equals the applied force divided by the total area resisting it. For double shear, that area is twice the cross-sectional area of the fastener:
Shear stress = Force / (2 × cross-sectional area of the fastener)
So if a pin has a cross-sectional area of 0.2 square inches and is loaded in double shear, the effective resisting area is 0.4 square inches. A 10,000-pound load would produce a shear stress of 25,000 psi on each plane rather than the 50,000 psi you’d see in single shear.
One critical detail: the strength depends on whether the bolt’s threads fall within the shear planes. The threaded portion of a bolt has a smaller effective cross-section than the smooth shank because the thread grooves remove material. Engineering practice often assumes conservatively that threads are in the shear plane, which gives a lower calculated strength. In real design, you want the smooth, unthreaded shank to pass through the shear planes whenever possible. Proper grip length selection ensures this.
How Double Shear Joints Fail
The fastener itself can shear through if the load exceeds its material strength, but that’s only one failure mode. The plates matter too. Bearing failure occurs when the pin crushes into the hole, elongating it over time. This is actually the most desirable failure mode in structural design because it happens gradually. You can spot the deformation before the joint falls apart completely.
Net-tension failure is more dangerous. This happens when the plate tears across its narrowest section, through the bolt hole, perpendicular to the load. Shear-out (or tear-out) is another sudden failure where the material between the bolt hole and the plate edge rips away. Both of these are catastrophic, meaning they happen fast and without much warning. Good joint design spaces fasteners far enough from plate edges and from each other to prevent these modes, pushing the joint toward the slower, more predictable bearing failure instead.
Material Requirements for Pins and Bolts
The material a fastener is made from determines its shear strength. Clevis pins used in military and aerospace applications follow strict standards. Low-carbon steel clevis pins (SAE 1010 or 1111) must meet a minimum ultimate shear strength of 41,000 psi. Stainless steel versions made from AISI 416 or 430F are held to a higher minimum of 61,000 psi. Some pins are surface-hardened to resist wear at the shear planes, while others are left untreated depending on the application.
For high-load applications, the test fixtures themselves use hardened steel inserts with a surface hardness of Rockwell C65, ensuring the test block doesn’t deform before the pin does. This gives you a sense of how seriously the shear plane conditions are controlled: the surrounding hardware has to be significantly harder than the pin being tested.
Design Safety Factors
Engineers never design a joint to work right at its theoretical limit. European structural codes apply a safety factor of 1.25 for bolted connections in shear, meaning the calculated capacity of the joint must exceed the expected load by at least 25%. This accounts for variations in material quality, manufacturing tolerances, and unexpected load spikes.
In practice, you also need to account for whether the joint will experience static loads (constant pull) or cyclic loads (repeated loading and unloading). Fatigue from cyclic loading can cause cracks to initiate at stress concentrations around bolt holes well below the static shear strength. Double shear joints are more fatigue-resistant than single shear because the symmetric loading reduces the bending cycles that accelerate crack growth, but fatigue life still needs to be evaluated separately from static strength.