What is a tube adapter, and what is it for?
A tube adapter (a weld bung, in the weld-in form) is a threaded or bored fitting that joins a tube to a rod end or clevis. It turns a plain tube into a push-pull link with an adjustable, standard end. It does not belong to the same family as compression or flare fittings for fluids.
What are the ways to join a rod end to a tube?
Three routes cover most machine linkages: threaded on, or welded or pinned where a threaded attachment is not feasible. A threaded part can be replaced in the field and lets you fine-tune the length with a jam nut; the other two give that up.
Weld-in adapter. A threaded plug or boss is welded into the tube end. The load goes from the rod end into the adapter's thread, then through the weld into the tube wall.
Pinned or bolted insert. A plug with a thread or a tab sits inside the tube and is held by a cross pin or a through-bolt. The load goes from the rod end into the plug, then through the pin into the tube wall as bearing stress and shear.
Threaded-in insert. The tube has an internal thread, or is thick enough to take one, and an insert screws in. The load goes through the thread into the tube wall.
What does each one give up?
| Weld-in adapter | Pinned or bolted insert | Threaded-in insert | |
|---|---|---|---|
| Load path | Thread → weld → tube wall | Thread → plug → pin → tube wall | Thread → tube wall |
| Strength limited by | Weld and the heat-affected zone | Bearing and tear-out at the pin holes | Thread strip and tube wall thickness |
| Heat on the tube | Yes | No | No |
| Needs a qualified welder | Yes | No | No |
| Can it be taken apart? | No, cut out | Yes | Yes, if the thread is not seized |
| Effort | One part, then welding | Several parts and drilling | A thick-walled tube or a tapped tube end |
| Main risk | Heat distortion of the thread; fatigue at the weld toe | Holes weaken a thin wall | A thin wall cannot take the thread; loosening |
For the welding side, K09-Q1-04 covers heat, distortion and alignment as principles, and K09-Q1-07 covers weld cracks. This article is about when to choose welding at all.
How strong is a pinned joint in a thin wall?
The weak point is the hole. The pin presses on the wall edge of the hole, and the area that carries it is the pin diameter times the wall thickness, on each wall.
Take a 6 mm pin through a tube with a 1.5 mm wall, with both walls loaded, carrying 5 kN. The bearing area is 2 × 6 × 1.5 = 18 mm², so the bearing stress is 5 000 ÷ 18 = 278 MPa. That is a large stress on a thin wall, and it has not yet counted the stress concentration around the hole or the tear-out to the tube end. Doubling the wall thickness halves the stress. A larger pin diameter helps too. This is why pinned inserts suit thicker-walled tubes, and why a thin-walled tube may need an internal plug or a reinforced sleeve.
The numbers are an illustration only, not an allowable stress. Check the real wall, pin and material.
What about a square or rectangular tube?
A round weld-in adapter does not fit a flat tube face. Square and rectangular tubes are usually closed with an end plate, a machined plug that fits the inside, or an insert with a pad. The load path in these cases depends on how the plate or plug is attached, so treat each as one of the three routes above and check the same limits.
Which route should you choose?
Start with whether the link will ever need to be taken apart in service. If it will, avoid welding and choose a pinned or threaded route. If you do weld, you need a qualified welder and a weld procedure, and without them a welded adapter is a risk, not a saving. Then compare the wall with the thread you need, because a thin wall limits both a threaded insert and a pinned one (Q1-02). Ask whether the load is cyclic: weld toes and pin holes both start fatigue cracks, so look at the failure order in Q1-04. Last, think about what a failure costs. If a cracked adapter is expensive to replace, choose the route that you can inspect and replace (Q1-08).
How does this apply to a vehicle link or chassis tube?
A weld-in adapter is a common way to give a tube link a threaded end on a vehicle, because the rod ends and jam nuts then set and adjust the link length. The load path is the same as in a machine: thread, weld, tube wall (Q1-04). The vehicle adds two things. The link sees impact as well as steady load, and a failed link or mount is a safety failure. Chassis tubes, mount tabs and link adapters in sanctioned racing or in a regulated vehicle may also be covered by rules on tube material, wall and welding, so the joint is chosen to the vehicle maker's data or the organiser's current rules, and the welding to a qualified procedure (K09-Q1-04). The weld-in tube adapters page is the product page and the tube adapter buyer's guide covers selection.