Where does the shank fail, and why?
Most failures of threaded fasteners are fractures through the male thread. The reason is geometry. Along the thread, the cross-section is smallest at the bottom of the thread groove, the minor diameter. Everything the joint carries has to pass through that narrowed section.
The rod end's shank is the same case. The shank stress area and the stress in the transition to the eye are two of the five things that limit a rod end (Q1-02; the five-factor breakdown is NHBB's). A rod end is rated for a pull or a push along the shank, so the shank is loaded in tension or compression, and the bending that comes from anything else is a separate problem.
How strong is the shank in pull?
Start from the stress area. For a male shank, a simple, cautious form is:
stress area = π ÷ 4 × (minor thread diameter)²
Take a shank with a minor diameter of 10 mm. The area is π ÷ 4 × 10² = 78.5 mm². If the material's tensile strength were 600 MPa (an example value; take the real figure from the material certificate), the section would break at about 78.5 × 600 = 47 100 N, or 47 kN. With a design factor of 5, the working load is about 9.4 kN (K06-Q1-02).
Two cautions. First, a female shank has a different area, a ring between the outer shank and the thread's major diameter, so do not reuse the male figure. Second, the static figure is a strength number. It does not tell you how many cycles the shank will survive, which is the next problem.
Why does a small side load matter so much?
Because bending grows with the offset, and the section resists bending much less than it resists pull. For a round shank, the section modulus is W = π × d³ ÷ 32. A load F acting at a distance e from the shank axis gives a bending moment M = F × e.
Take the same 10 mm shank with a 5 kN load and an offset of only 5 mm:
- axial stress F ÷ A = 5 000 ÷ 78.5 = 63.7 MPa
- bending stress M ÷ W = (5 000 × 5) ÷ 98.2 = 254.6 MPa
The offset makes the bending stress about four times the axial stress, so the shank fails in bending long before the pull alone would matter. These figures are illustrative, but the pattern is real. An angled or off-axis force adds bending stress in the shank and its thread, and a shank loaded off-axis again and again is a common cause of failure. The practical lesson is to keep the load in line with the shank, and to fix any misalignment at its source (K06-Q1-10).
Why does it break at the thread and not along the plain shank?
Because the thread root concentrates stress. A fatigue crack starts at the place of highest stress, which is usually the root of a thread. For bolts, Bolt Science puts the stress there at about 4 to 6 times the stress in the middle of the thread. A fully rounded root lowers the concentration and extends fatigue life.
The same logic applies to the thread on a rod end shank, and to the transition where the shank meets the eye, which is a limit in its own right.
What makes it worse?
Bending. Anything that moves the load line away from the shank axis adds bending, as the example above shows. Misalignment, a bent pin or a clevis that spreads all do it.
Repeated loading. Fatigue needs a change in stress. With no change there is no fatigue. A rod end that sees pull and push thousands of times can crack at the thread root long after the static check looked safe.
Too little thread engaged. The load is then carried by fewer threads, so the stress per thread goes up (K04-Q1-23).
A connection that loosens. A loose lock lets the joint move and load the shank unevenly (K06-Q1-05).
When does the static figure fall short?
Whenever the load reverses or repeats. The static stress area tells you the strength under one application of load. It does not show fatigue life. If the joint sees many cycles, ask the maker for a fatigue rating of the exact part, and treat the static figure as the upper limit, not the design point.
What should you do about it?
Choose a size whose stress area leaves margin for the real load and the design factor (Q1-02). Keep the load along the shank. Screw in enough thread and keep the connection locked. Where the load reverses, ask for fatigue data. At each inspection, look at the shank and the eye for cracks at the thread root and at the transition (Q1-09).
What loads the shank sideways on a suspension or steering link?
A male shank on a suspension or steering link is where the bending in this article tends to show up, for two reasons.
The housing can load the shank sideways. At the end of suspension travel, a rod end that runs out of tilt stops being a free joint. The housing presses on the bracket or spacer, and the bracket then pushes sideways on the housing. That side force acts on the shank through the lever arm between the contact point and the thread root. An illustrative case with the 10 mm minor-diameter shank used above: a side force of only 1 kN, acting 30 mm from the thread root, gives M = 1 000 × 30 = 30 000 N·mm. With W = π × 10³ ÷ 32 = 98.2 mm³, the bending stress is about 306 MPa, roughly half of the 600 MPa example tensile strength, from one contact and before any pulling load is added. Repeated at every full droop or bump stop, that is fatigue loading at the thread root. The first cure is angle margin (Q1-06); a bigger shank only treats the symptom.
Links in compression are slender. A long tube link with a rod end in each end can buckle before the shank or the bearing yields; check the tube as well as the shank (K06-Q1-02).
A bent shank, or a crack at the first engaged thread found after an impact, is a replace-and-investigate sign, not a straighten-and-reuse one. For the buckling check, see the tube buckling calculator; the vehicle application is in automotive and racing rod ends.