A trunnion is a cylindrical pin or mounting point that allows something heavy to pivot around a fixed axis. Think of it as a sturdy peg that sticks out from the side of an object, sits in a bracket or bearing, and lets that object tilt or rotate in a controlled way. Trunnions show up everywhere from bridges and industrial valves to hip replacements, but the concept has remained the same for over 600 years: support a heavy load while allowing it to move.
How a Trunnion Works
At its simplest, a trunnion is a short cylinder that protrudes from opposite sides of an object, creating a fixed axis of rotation. The trunnion sits inside a bearing or bracket, and the object can swing or tilt around that axis without sliding out of position. When the center of mass of the object sits right at or near the trunnion axis, the system stays balanced and requires very little force to move. The further the center of mass sits from the axis, the more force is needed, and the more stress the trunnion bearings absorb.
This is why trunnions are almost always placed near the center of gravity of whatever they support. Positioning them there minimizes the effort needed to adjust the angle and reduces the load on the mounting hardware.
The Origin: Cannon Design in the 1400s
Trunnions were first developed for artillery. By the early 15th century, European cannon founders began casting cylindrical lugs directly into bronze gun barrels. These trunnions, set just forward of the barrel’s center of gravity, served two purposes: they attached the barrel to its wheeled carriage, and they allowed gunners to tilt the barrel up or down by sliding a wedge called a quoin beneath the breech end. Before trunnions, adjusting a cannon’s elevation was crude and unreliable. With them, gunners could aim with real precision for the first time. By the 1420s and 1430s, this design was standard across European artillery.
Trunnions in Bridges
Bascule bridges, the kind that split open and tilt upward to let ships pass, rely on trunnions as their main pivot point. Each leaf of the bridge rotates around trunnions that are inserted through the webs of the main structural girders. London’s Tower Bridge is one famous example, using a roller-bearing trunnion bascule design. On a more everyday scale, the Mason Street Bridge in Green Bay, Wisconsin, has six bascule girders per leaf, all rotating around trunnions that bear enormous loads during each opening and closing cycle.
These trunnions must handle not just the dead weight of the bridge deck and its steel structure, but also any vehicles caught on the span during movement. The bearings around bridge trunnions are engineered for decades of service under loads that can reach millions of pounds.
Trunnion Ball Valves
In oil and gas, petrochemical, and power generation industries, trunnion-mounted ball valves are the standard for high-pressure pipelines. In a regular “floating” ball valve, the ball inside is held in place only by the seats on either side. Fluid pressure pushes the ball against the downstream seat, which works fine at low pressures but creates enormous friction and wear as pressure climbs.
A trunnion-mounted ball valve solves this by anchoring the ball at both the top and bottom with a fixed shaft, the trunnion. The ball can still rotate to open and close the valve, but it doesn’t get shoved sideways by fluid pressure. This design reduces wear on the seals, requires less force to turn, and maintains a tight seal even under extreme pressure and temperature. Multiple seal layers, including O-rings and seat rings, keep the valve leak-free in demanding conditions like steam, natural gas, and hydrocarbon service.
Hip Replacement Trunnions
In a total hip replacement, the artificial joint has a modular design: a metal stem that fits into the thighbone, and a ball (the femoral head) that snaps onto the top of that stem. The tapered connection point where the ball meets the stem is called the trunnion. It uses a Morse taper, a slightly angled cone that locks the two pieces together through friction when they’re pressed firmly into place.
This modular design gives surgeons flexibility to mix and match head sizes and stem lengths for each patient. But the trunnion also turns out to be a vulnerable spot. During normal walking and movement, tiny amounts of motion occur at that metal-on-metal junction. Over time, this micromotion, sometimes as small as 5 to 12 micrometers, can scrape away the protective surface layer on the metal and trigger a process called fretting corrosion.
Trunnionosis
When corrosion at the hip trunnion becomes a clinical problem, it’s called trunnionosis. The corroding metals release cobalt and chromium ions and tiny metal particles into the surrounding tissue. The body’s immune system reacts to this debris, and the resulting inflammation can cause real damage: destruction of the tissue around the joint, fluid buildup, formation of masses called pseudotumors, and even deterioration of the muscles that stabilize the hip.
Patients with trunnionosis most commonly experience pain and a limp. Some develop a skin rash from elevated metal ion levels, occasionally without any pain at all. In more severe cases, the tissue reaction can weaken the hip abductor muscles enough to cause the artificial hip to dislocate repeatedly. One documented case involved a pseudotumor 12 centimeters across with significant muscle damage. When trunnionosis is caught, revision surgery is typically needed. Surgeons can clean the trunnion surface and place a new femoral head, which has been shown to restore a stable connection comparable to a brand-new implant.
Aircraft and Military Applications
Trunnions also appear in aircraft structures, particularly in landing gear assemblies and structural ribs. On the KC-135 Stratotanker, a refueling aircraft the Air Force plans to keep flying until at least 2040, the trunnion support rib is a critical structural component that undergoes regular non-destructive inspection to catch fatigue cracks before they become dangerous. In military vehicles and truck trailers, trunnion assemblies serve as the pivot connection between a tractor and its semitrailer, with a trunnion plate and latching mechanism on the truck linking to a kingpin on the trailer.
Across all these applications, the underlying principle is identical to what 15th-century cannon founders figured out: a cylindrical pivot point, positioned near the center of gravity, that lets a heavy object rotate smoothly around a single axis while staying firmly attached to its support structure.