If you have ever watched a giant grinding mill turn in a mine, a bascule bridge lift over a river, or a stadium floodlight get aimed with a quick twist, you have seen trunnion mounting at work. The word may sound antique, and in fact it dates back to cannon hardware, but the core idea is elegantly simple: support a heavy part on two short stub shafts so it can pivot smoothly around a fixed axis while carrying a serious load. This simple mechanism remains a popular choice, and its mount usage spans from industrial machinery to modern lighting systems with LED fixtures.
What is a trunnion mounting, mechanically?
A trunnion can be defined as a cylinder-shaped projection on an element. The axis of rotation is created by two trunnions arranged oppositely. The assembly that holds them is the trunnion mount. In many designs the trunnions sit in bearings or bushings inside yokes, pedestals, or supports. In other designs the trunnions are captured in pillow blocks bolted to a frame.
That arrangement isolates the pivot from the body of the part. It is possible for the rotating element to be large and difficult to handle, but it only requires two robust, small journals in order to achieve predictable movement. Loads travel through the trunnions and into the supports, not through the rotating shell, which makes design, installation, and maintenance easier. This mechanism is designed to ensure that the moving parts are guided along their intended path.
In some systems, the trunnion forms part of the structure that is stationary. In some systems, the trunnion forms part of the stationary structure. The geometry is the same either way: a clean, single-degree-of-freedom pivot that resists radial and axial loads for the expected motion range.
Where you find trunnion mounts
Industrial valves utilize trunnion mountings to stabilize the disk or ball under pressure. The ball of a trunnion mounted ball valve is held in place at both the top and the bottom by trunnions while the seats are moved against it. This design prevents the ball from moving downstream when differential pressure is applied, reduces actuation torque and increases seat life. This style is preferred by pipeline operators for large diameters with high pressures.
When a hydraulic cylinder is required to pivot while it extends or retracts, a trunnion mounting is often used. The trunnion is welded or bolted onto the cylinder and then pinched into a bracket or clevis on the frame. This keeps side loads out of the rod, minimizes misalignment binding, and allows a compact envelope in crowded machinery. All excavators, presses systems, and injection molders use cylinders mounted on trunnions. This particular mount usage has proven especially reliable in applications where alignment is critical.
Rotary kilns and dryers rest on trunnion rollers that support a massive shell. The rollers carry huge radial loads and define the axis for rotation. They also allow continuous turning. The alignment of these trunnions requires a special skill, as even small angular mistakes can cause hot spots, tire wear and structural stress.
At the feed and discharge end, mining grinding mills are operated by trunnion-bearings. These massive plain bearings carry the weight of the shell, charge, and process media. The trunnions must be cooled, lubricated, and sealed. The cost of mill downtime is high, so the simplicity and reliability of a trunnion-bearing becomes an advantage.
Foundry ladles and tilt frames ride on trunnions that let operators pour with precision. Here the mount must tolerate shock, thermal gradients, and abrasive environments. The geometry is intentional: the trunnions are placed so the center of gravity crosses the pivot axis at the right moment, giving good control during the pour.
Lighting fixtures are equipped with trunnion mountings for easy aiming. Trunnion brackets are the familiar U-shaped yoke found on many floodlights, including modern LED fixtures. It clamps to the luminaire and pivots using side bolts. Installers like it because the aiming angle can be set with one wrench, then locked with toothed washers or friction pads. Many technicians cite the ease of installation as one of their major advantages.
Bridge engineers use trunnions in bascule spans. The leaf pivots on large trunnions that are captured in bearings. The mount must be able to handle both static and dynamic loads, such as wind and traffic. It also needs to move with predictability.
In hot service, trunnions are used to support piping systems. The trunnion is a short section of pipe welded onto the main pipe and resting in a saddle, or shoe. It supports vertical loads while allowing axial expansion, which helps to maintain stress limits during thermal expansion.
Why use a trunnion mounting instead of alternative options
There are other ways to let heavy parts rotate. All of these have their place: spherical bearings and clevis pins. Trunnion mounts keep winning in harsh, heavy-duty service for a few core reasons.
Compact support with high stiffness.
Predictable rotation about a single axis.
Simple sealing and lubrication pathways.
Wear parts are easily replaced and can be inspected.
When you need a pivot that is rigid in every direction except the one you actually want to move, the trunnion mount stands out. It can resist thrust and carry radial loads. It is scalable from a 2-inch LED lighting yoke to a large 20-foot kiln wheel.
What the mount actually does
A trunnion mount supports, locates, and guides rotation. Those verbs matter. Support refers to taking the weight and any external forces. Locate refers to holding the rotating body in a precise position relative to its frame. Guide means to allow the desired motion without binding.
Controlling these three functions has real results: a lower actuation force in valves; less side load in cylinder rods; smoother rotation of drums and mills; and a clean aim and locking for lighting fixtures. Maintenance teams see the benefit as reduced wear and more predictable service intervals. The results have improved reliability in many cases.
Anatomy of a robust trunnion mount
Hardware varies according to industry, but building blocks are the same.
Journals: The trunnions themselves, typically forged or machined steel, sometimes stainless or alloy for corrosion or heat.
Bearings: plain bronze or polymer bushings or rolling element bearings for high speed and precision.
Housings: Yokes, caps, or pedestals that trap the bearings and transfer load into the frame.
Seals Labyrinths or lip seals to keep grit and grease out.
Fastening : Securing joints with retaining rings or tapered fits.
Material choice, heat treatment, and surface finish on the trunnions strongly influence life. Hardened journals with proper lubrication tolerate higher contact stress and resist brinelling. In hot service clearances and lubricants must take into account expansion and viscosity. Engineers can extend the life of their equipment by carefully managing installation and bolt torques.
Applications and what the mount is doing
The table below outlines the types of movement and loads that the mount can handle.
Application area
Motion type
Dominant load
Key design focus
Ball valves
Quarter-turn indexing
Pressure thrust and actuator torque
Seat load control, sealing, torque reduction
Hydraulic cylinders
Small-angle pivoting
Side load avoidance
Rod alignment and bracket stiffness
Rotary kilns/dryers
Continuous rotation
Radial load of shell and product
Alignment, tire-trunnion contact, thermal growth
Grinding mills
Continuous rotation
Radial and some axial
Seal integrity, lubrication and cooling
Foundry ladles
Intermittent tilting
Shock and thermal
Heat shielding and center-of-gravity placement
Floodlights
Set-and-lock aiming
Light friction and vibration
Anti-slip features, corrosion resistance (especially in LED systems)
Bascule bridges
Controlled lifting
Static plus dynamic
Balance, bearing life, redundancy
Pipe Supports
Limited motion due to expansion
Vertical Support
Wear pads for sliding surfaces
This variety shows why "trunnion mount" is not a single catalog part. Engineers tailor the design to the specific job, keeping in mind the mounting guidelines.
Advantages that matter in practice
After a project or two with these mounts, the benefits become obvious.
Load path clarity: Short, direct load paths limit bending and reduce stress concentrations.
Alignment control: The axis of rotation can be machined and checked with high accuracy, which improves repeatability.
Serviceability: Bearings and seals are accessible, and trunnions can be re-bushed or replaced without tearing apart the whole assembly.
Scalability : The same concept can be used at both plant and lab scales, only the proportions and materials are different.
There is also a hidden advantage: trunnions make it easier to separate structure from function. The rotating shell can be optimized for the process and the mounting for the mechanics. They are then connected with interfaces that are well defined. This careful installation process has often led to fewer operational issues over time.
Design notes that prevent trouble
The axis is the key to a smart layout. Place trunnions to keep the center of gravity where you want it through the motion range. In a tilting application, small changes in pivot location can turn a controllable pour into a runaway.
Choose bearings based on the environment, speed and load. Plain bearings are best for low-speed and heavy loads, particularly in areas where shock is possible. Rolling element bearings save space and reduce friction at higher speeds, but they are more sensitive to contamination and misalignment.
Think through axial restraint. If thermal growth is expected, one end should float. If thrust loads are alternated, both ends require thrust bearings or shoulders that can handle the thrust.
Early planning is important for lubrication. Grease channels, oil baths, or automatic lubrication systems should be built into the mount from the start. Seal selection must match temperature, chemicals, and grit level.
Corrosion protection matters. Coatings, stainless alloys, or sacrificial sleeves can keep journals and housings healthy in coastal plants or chemical service.
Installation and maintenance
Good mounts fail from bad installs more often than from weak design. Surface preparation, cleanliness, and alignment checks are your friends. Bore-to shaft fits must be in accordance with the drawing. Shims should be tight and stainless, and torque specifications should be accurate, not guesswork. It is important to ensure that the mechanism works perfectly by ensuring a well-organized installation.
After running, monitor temperature and vibration. A bearing's temperature rising can indicate misalignment, or that the bearing is lubricated. The wear patterns of bushings can tell you a lot about the load distribution. Monitoring conditions can be done with a thermometer on a hand and a check list.
Retorquing bolts after the first load cycle is a cheap form of insurance. So is a spare bushing kit on the shelf. Quick-change bearing cap and split housings are useful in process-critical equipment.
Safety and Standards
In many fields, trunnion designs are anchored to codes. Pipeline valve makers follow widely adopted valve standards that define torque, pressure testing, and materials. Hydraulic cylinder mounts follow common dimensional series so trunnion brackets line up with frames. Bridges use national specifications for moving parts and bearings. https://general-valve.com/what-is-a-trunnion-ball-valve/ for pipes are designed according to MSS and piping codes.
Document the load cases, safety factors and standards if your application involves the public or hazardous fluids. Good practice in one industry may be risky in another.
Selecting the right trunnion mount
After you have determined the load and motion, selecting a mount is easy. Keep the checklist short and decisive, and aim for clarity over complexity.
Load Rating with Margin
Speed-specific bearing type
Seal strategy for the Environment
Measure alignment features
Parts and service access are available
For the procurement of materials, finishes and fittings, be specific. Define allowable misalignment, target torque for set-and-lock brackets, and lubrication intervals. Vendors do their best work when the expectations are crisp and the installation guidelines clear.
Quick examples that answer the original question
What is the purpose of a trunnion mounting? This mount is used wherever a heavy component must pivot or rotate around a fixed axis reliably and with control. Here are a few images to make it real:
Quarter-turn valves: The trunnion steadies the ball so upstream pressure does not shove it off center.
Machine cylinders: The trunnion lets the cylinder body pivot instead of bending the rod.
Rotating drums: Trunnions carry the weight and define the spin axis for continuous processing.
Aiming brackets : The yoke of a floodlight, which often incorporates LED technology, is a trunnion. It holds an angle to prevent vibration.
Moveable bridges: The whole span rotates around large trunnions supported in bearings.
You will find trunnion mountings all over the place once you start to look for them. These are silent pieces of hardware that perform a repeatable mechanism. This is what good mechanical design should look like.
What is the purpose of a Trunnion Mount? Essential Uses Uncovered
·
8 min read
