What is a weld-on rod eye, and where is the weld?
It is an eye that becomes part of the rod end or a mounting bracket by welding, instead of being screwed on. It is used where the eye has to be built into a rod, a base bracket or a mounting structure (Q1-02), and the welding is done at manufacture, on a prepared, unhardened weld zone, to a qualified procedure. That is different from welding an eye onto a finished cylinder rod in the field, which the sections below advise against.
The weld is therefore part of the load path. Every push and pull of the cylinder, and any bending from misalignment, goes through it.
Where is welding acceptable on a cylinder?
On external attachments that do not hold pressure, and where the heat can be kept away from seals and machined surfaces.
Welding is routine at the factory on external, non-pressure features such as clevises, rear lugs and mounting brackets, and a replacement rod eye is normally threaded on, not welded.
Three places should not be welded, because heat there causes harm:
- A finished rod. Many rods are induction-hardened medium-carbon steel. Welding softens the hardened surface and creates a brittle heat-affected zone, a typical starting point for rod fracture under bending load. The heat-affected zone is the metal next to the weld that was heated and changed.
- The chrome surface. Arc strikes and spatter destroy the hard chrome and the protection beneath it, and the damaged spot then tears the rod seal on every stroke.
- The barrel, piston and precision bores. Heat distorts them.
If a rod eye breaks off, the correct repair is a new rod with a threaded or machined eye, or a complete new rod assembly. This is why welding on an existing cylinder is a decision for the cylinder maker or a qualified shop.
What should a welding procedure cover?
The principles matter more than any number you could copy. A qualified welding procedure specification, written for the actual steel grade and wall thickness, should set the preheat and the other conditions. ISO 15614 or ASME IX are the usual bases, and numbers should not be guessed from a forum, because the right value depends on carbon content, thickness and restraint.
The principles are these:
- The procedure, not the welder, sets the heat. The consumable type, preheat, bead sequence and cooling are taken from the welding procedure document for that part.
- Strip and clean first. The cylinder is stripped of seals and cleaned of paint and grease, because contamination is the main source of hydrogen, which causes delayed cracking.
- Check alignment before the final weld. Tack the part, measure its pin axis against the bore centre line, correct it, then finish.
- Inspect after welding. Use visual inspection, plus dye-penetrant or magnetic-particle testing of the weld and heat-affected zone.
What really differs between a factory weld and a field weld?
The metal, the sequence and the control. The table sets the two side by side.
| Weld-on eye made at the factory | Eye welded onto a finished cylinder rod | |
|---|---|---|
| State of the steel | Weld zone prepared to bare, unhardened metal (plating and hardened case removed, or the weld placed outside the hardened length) | Induction-hardened, chrome-plated, ground |
| What follows the weld | Preheat and any post-weld heat treatment set by the procedure, then final machining and inspection | Nothing. The heat-affected zone stays as it cooled |
| Seals and piston | Not yet fitted | Present, and exposed to the heat |
| Procedure | Qualified for the steel and thickness | Usually not qualified for that steel |
| Alignment | Fixtured and machined to the pin axis | Held and tacked by hand |
| Inspection | Planned, with surface testing | Often visual only |
The reason the field version fails is metallurgical. A carbon steel that is heated well above its transformation temperature and then cooled quickly forms a hard, brittle structure. On a hardened rod the weld creates exactly that zone, right next to a surface that was made hard for wear, and with no later heat treatment to temper it. Hydrogen cracking needs three things at once: hydrogen (from moisture, grease or paint), that brittle structure, and tensile stress. A field weld on a rod supplies all three, and the cracks may appear hours or days later.
Geometry also matters. Take an illustrative 40 mm rod carrying 100 kN. A full-penetration joint puts the load through the full 1,257 mm² section, which is 80 MPa, the same as the rod. A fillet weld around the rod with a 5 mm leg has a throat of about 3.5 mm, so it carries the load through only about 445 mm² of weld, an average stress of about 225 MPa, nearly three times the rod's stress. These dimensions are not a procedure, but they show why the joint design is part of the qualified specification and not left to the welder.
How is a weld inspected?
In three stages. Visual inspection can be applied before, during and after welding. Before welding, it checks joint preparation, alignment and cleanliness. Alignment problems make a sound weld harder to produce and add stress concentration that shortens fatigue life. During welding, it checks that the procedure is followed. After welding, it looks for surface defects.
Visual inspection finds only what is on the surface. Cracks are the most serious. The American Welding Society (AWS) describes weld cracks as fracture-type discontinuities with sharp tips, the most severe form of discontinuity and generally the primary cause of catastrophic failure in components. Some cracks cannot be found by visual inspection, which is why dye-penetrant and magnetic-particle tests are used. Dye-penetrant testing uses a dye that seeps into surface cracks. Magnetic-particle testing uses iron powder that gathers at cracks in steel.
How do weld failures show up?
| Sign | What it can mean |
|---|---|
| A crack along or beside the weld | A weld crack. Hydrogen cracks form hours to days after welding, often under the bead where they cannot be seen. Toe cracks start at the edge of the weld, where residual stress is highest |
| The eye sits slightly out of line with the bore | Misalignment. One example is a clevis welded 1° off-axis, which side-loads the rod on every cycle and wears the rod bearing and seal in months |
| Rod bent or scored near the weld | Distortion or side load from the weld position |
| Seals leaking soon after a repair | Heat damage to seals, or side load from poor alignment |
| Eye breaks off at the weld | Softening and brittleness in the heat-affected zone, or fatigue at the weld toe |
See Q1-10 for how side load damages the rod and seals.