Fifth Wheel Couplings — 2″ & 3.5″ Semi-Trailer Saddles
A fifth wheel coupling is the load-bearing joint between tractor and semi trailer, carrying the entire kingpin load through a forged top plate and spring-loaded jaw lock. We build fixed and slider fifth wheel assemblies in 2 inch and 3.5 inch kingpin bore, sized for line-haul, heavy-haul, and off-road duty, and export them as complete assemblies ready to bolt onto your mounting brackets.
Specifications
| Kingpin bore | 2″ (50 mm) standard duty / 3.5″ (90 mm) heavy-haul |
|---|---|
| Static capacity | 24,000 kg – 55,000 kg depending on model |
| Mounting type | Fixed (bolt-on bracket) or slider (air-operated, 500 mm+ travel) |
| Top plate material | Forged carbon steel, induction-hardened wear strips |
| Lock mechanism | Single or double jaw with secondary safety latch |
| Tilt / articulation | ±12° tilt, ±90° horizontal oscillation |
| Mounting height | 1,150 – 1,450 mm (adjustable with brackets) |
| Standard compliance | SAE J700, 10-hole bolt pattern |
| Surface finish | Primer + gloss enamel or hot-dip galvanized |
What's in a Fifth Wheel Coupling
The fifth wheel plate is a single forged or fabricated casting shaped to cradle the trailer's kingpin and take the vertical, longitudinal, and lateral loads generated in braking, cornering, and hill starts. Under the plate sits the jaw assembly — a spring-loaded throat that closes around the kingpin shank and locks it in place, backed by a secondary lock so the jaw can't release under vibration alone. A release handle on the side, sometimes cable-routed to the cab, pulls the lock open for uncoupling. The top surface carries grease points and, on higher-mileage units, a low-friction wear plate so the trailer can pivot through a turn without scoring the steel. We pair every fifth wheel with a matching king pin ground and hardened to the same bore tolerance, since a mismatched pin-to-jaw fit is the single biggest cause of premature wear on a saddle coupling. Many buyers search for a 5th wheel coupler or fifth wheel coupler rather than "coupling" — same assembly, different regional term, and we ship the identical forged plate and jaw either way.
2 Inch vs 3.5 Inch Fifth Wheel — Picking the Bore
Most on-highway trailers in Africa, the Middle East, and Southeast Asia run a 2 inch fifth wheel (50 mm kingpin), rated for gross combination weights up to roughly 36 tonnes on a standard tandem tractor. Once you move into logging, mining haul roads, tanker convoys, or oilfield rig-moving trailers where combination weight climbs past 60 tonnes, the 2 inch pin runs out of shear and bearing capacity and the industry moves to a 3.5 inch fifth wheel (90 mm kingpin). The 3.5″ assembly uses a heavier forged top plate, a wider jaw throat, and a reinforced mounting bracket to match — you can't simply bolt a 3.5″ top plate onto a 2″ mounting frame and expect the load path to hold. If your fleet mixes both trailer types, tell us the axle configuration and typical payload and we'll spec the coupling to the combination, not just the pin size on paper.
Fixed Mount vs Slider Fifth Wheel Assembly
A fixed fifth wheel bolts straight to the tractor frame at one position and never moves — simplest, cheapest, and the right call for a dedicated tractor-trailer pairing that never changes axle loading. A slider fifth wheel assembly rides on rails and can be locked at different fore-aft positions, typically over 500 mm of travel in 100 mm increments, released by an air cylinder pulling the locking pins. Sliders let a driver shift weight between the drive axles and the trailer axles to hit bridge-law axle weight limits without reloading cargo, and they let one tractor tow trailers with different kingpin setback. The tradeoff is more moving parts to inspect — slide rails, locking pins, and the air line all need to be checked on schedule, whereas a fixed mount only needs the jaw and grease points watched.
Sourcing from a Fifth Wheel Manufacturer
We forge and machine fifth wheel top plates and jaw assemblies in-house at our Liangshan facility, so bore tolerance, jaw spring rate, and heat treatment are controlled from raw billet through final assembly rather than bought in from three different subcontractors. Every coupling is proof-loaded and gauge-checked before it ships, and we keep tooling for both 2″ and 3.5″ bores plus the common SAE J700 bolt patterns used by Fontaine- and JOST-style mounting brackets, so a replacement plate from us drops onto an existing bracket without adapter work. Container-load orders get palletized with grease already applied to the plate and jaw — no re-machining or fitting needed at the yard.
Installation, Greasing, and Wear Checks
A new fifth wheel coupling should be greased at the plate surface and jaw pivot before its first coupling and then on a fixed interval — weekly for line-haul, more often in sandy or wet operating conditions where grit works into the jaw throat. Check jaw-to-kingpin clearance with a feeler gauge or the manufacturer's go/no-go gauge every service interval; clearance past spec means the jaw has worn enough that the kingpin can shift under braking, which shows up as a knock or clunk on deceleration. The jaw lock and secondary safety latch are the coupling locks for trailer and tractor that actually hold the connection together under load, and they're the parts worth inspecting most closely at every service — a worn or sticking lock is a far more urgent issue than surface wear on the plate itself. Also check the mounting bolts to the bracket for torque and the bracket-to-frame welds or bolts for cracking, since a coupling that's perfect on its own can still fail at the mounting interface. See our fifth wheel coupling guide for a full inspection checklist and torque values by model.
Frequently Asked Questions
How do I know if I need a 2 inch or 3.5 inch fifth wheel?
Are your fifth wheel couplings compatible with Fontaine and JOST mounting brackets?
What's the difference between a fixed and a slider fifth wheel assembly?
How often should the fifth wheel plate be greased and inspected?
What load capacity do your fifth wheel couplings carry?
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