Where Does a Tube-Adapter Joint Fail First: Rod-End Thread, Adapter Thread, Weld or Tube, and How Do I Check Each Under Tension, Compression and Cyclic Load?

What are the four links?

A tension or compression load goes from the rod end into the tube by this path:

1. The rod end's male thread (the shank).
2. The adapter's internal thread, which holds the shank.
3. The weld, or a cross pin, joining the adapter to the tube (Q1-01).
4. The tube wall, which carries the load along the link.

Each link has its own capacity, and the joint is no stronger than the smallest. The usual design rule for a threaded joint is that the screw should break before the threads strip, which means checking the tensile stress area of the male thread and the stress area of the female thread. The same logic applies here. A thread that strips, or a weld that tears out, fails suddenly. A shank that stretches and breaks, or yields first, gives some warning.

Link 1: How strong is the rod end's thread?

By the area at the thread root, the minor-diameter area. For a male shank that area is the square of the minor thread diameter times π ÷ 4, and the shank and its transition are the limiting features (K01-Q1-03 covers this in full).

For an M12 × 1.25 thread the minor diameter is 12 − 1.22687 × 1.25 = 10.47 mm, so the area is π ÷ 4 × 10.47² = 86.0 mm². If the shank's tensile strength is 700 MPa, its ultimate tension capacity is 86.0 × 700 = 60.2 kN. (The usual tensile stress area, π ÷ 4 × (d − 0.9382P)² = 92.1 mm² here, is a little larger; this article uses the minor-diameter area, as K01-Q1-03 does.) This is an illustration of the method; the real figure depends on the grade of the rod end.

Link 2: Will the adapter's thread strip?

It depends on how much thread is engaged. The shear area of an internal thread grows with the engagement length. A simple approximation, cautious for an internal thread and good for a first check only, is

A = 0.5 × π × d × Le

where d is the thread diameter and Le the engaged length. Simplified formulas like this give only an estimate (K05-Q1-04). Take the shear strength of the adapter's steel as 0.6 times its tensile strength.

Take a steel adapter with a tensile strength of 400 MPa, so the shear strength is 240 MPa.

At 12 mm, the thread strips at 54 kN, below the 60.2 kN that breaks the shank. The adapter's thread is the weak link. The engagement at which the two are equal is 60 200 ÷ (0.5 × π × 12 × 240) = 13.3 mm, or about 1.1 × d. Below that, the soft internal thread fails before the shank. The general rule of 1 × d for steel is a starting point, and a softer adapter material needs more (K04-Q1-23).

Link 3: What about the weld?

The weld carries the whole load from the adapter into the tube wall. It shears around the adapter's circumference, so its capacity is the weld's throat times its length times its shear strength. For a 25 mm adapter, the length is π × 25 = 78.5 mm, so every millimetre of throat gives 78.5 mm² of shear area. The throat, and the weld metal, are set by the weld drawing and a qualified procedure, so weld size and procedure come from the weld drawing and a qualified procedure (K09-Q1-04).

Heat is the other effect. A weld alters the tube and the adapter around it, and the thread inside the adapter can distort (K09-Q1-04). After welding, check that the rod end still threads in freely by hand.

A pinned adapter replaces the weld with a pin and the tube wall's bearing strength (Q1-01).

Link 4: What does the tube carry?

In tension, the tube's cross-section. A tube of 25.4 mm outside diameter and 3.0 mm wall has an area of π × (25.4 − 3.0) × 3.0 = 211 mm². At a tensile strength of 400 MPa that is 84.4 kN. This is more than the 54 kN thread above, so in this example the tube is not the limit.

In compression, the tube can buckle long before it yields, and the adapter ends change the length that counts. That is a separate calculation, set out in Q1-10.

Why does the static check not decide a cyclic load?

Because a joint that carries a repeated load fails by fatigue at a much lower stress. Fatigue cracks start at the highest-stress point, usually the thread root, where stress is concentrated about four to six times (fastener-engineering site Bolt Science gives that range), and fatigue needs a change in stress. The other starting places are weld toes, holes, undercut and changes of thickness. An off-axis force also puts bending into the shank and its thread.

So for a cyclic load, check three things beyond the static numbers: the range of load, not only the peak; any bending from misalignment (K06-Q1-10); and the thread root and weld toe as the places to inspect (K09-Q1-07). Take the design factor from your own standard.

Can the adapter turn in the tube?

A welded adapter, or one held by a cross pin, cannot turn in the tube unless the weld or the pin fails. An insert held only by its thread can turn, and tightening a jam nut puts a torque through whichever kind it is. If the adapter has wrench flats, hold it while tightening (K11-Q1-05). Loosening of the rod end itself is covered in K06-Q1-05.

Does a vehicle link bend as well as pull?

Two things: bending and impact. A vehicle link is often not loaded on its own axis. A link with two rod ends is a two-force member, so bracket offset and swing alone do not bend it; the bending comes from a link bent to clear the axle or a tyre, a shock or anti-roll-bar mount part-way along the tube, or a rod end driven against its tilt limit so the housing pushes a moment into the tube. Illustrative numbers: a 2 kN load that acts 40 mm off the axis of a 25.4 × 2.4 mm tube gives a moment of 2,000 × 40 = 80,000 N·mm. The tube's section modulus is 913 mm³, so the bending stress is 87.6 MPa, against an axial stress of only 11.5 MPa for the same load. Where that moment reaches the end of the tube, the bending stress is highest at the surface right next to the weld toe, which is where fatigue cracks start in welded joints (TWI compares welded and unwelded fatigue behaviour). Impact loads add to this and cannot be calculated from the static check. So inspect the weld toe and the thread root first (K09-Q1-07), and take any design factor from measured loads or the vehicle maker's design data. The weld-in tube adapters page is the product page.

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