What does the pin carry?
The whole cylinder force passes through it.
The pin is loaded in three ways. It is sheared across its width where the forces of the eye and the clevis meet. It presses on the bore of the eye and on the holes in the clevis ears (bearing pressure). And it bends, because the eye pushes on the middle of the pin while the two ears hold its ends. A rod end with enough radial capacity can still be part of an inadequate assembly if the pin bends, the clevis spreads, the threads are not engaged far enough, or the shank is repeatedly side-loaded. Look at the pin as part of the load path.
How do you size the pin diameter?
Check shear, bending and bearing stress against the maximum cylinder force. Shear gives only the lower bound.
Take the worked example of a cylinder maker (Isohydraulic): a 100 mm bore at 210 bar produces about 165 kN. It assumes a safety factor of 3 and a pin shear strength of 400 MPa. These are the example's assumptions, not general values.
- Required shear area = force × factor ÷ shear strength = 165,000 × 3 ÷ 400 ≈ 1,238 mm².
- A pin in a clevis is in double shear: it is cut at two places, one in each ear. So each place carries half: about 619 mm².
- The diameter for that area is about 28.1 mm. That is the minimum from shear alone, not the pin to buy.
Bending usually asks for more. Take an illustrative geometry: an eye 35 mm wide between two ears each 20 mm thick, no side clearance. The pin acts as a beam loaded in the middle by the eye and held at the ears, and the peak moment is M = F ÷ 8 × (b + 2t), where b is the eye width and t the ear thickness, the same expression used in K05-Q1-05. For F = 165 kN, M = 165,000 ÷ 8 × (35 + 40) ≈ 1.55 kN·m. The bending stress is 32 × M ÷ (π × d³):
| Pin diameter | Bending stress | Bearing pressure on the eye bore |
|---|---|---|
| 30 mm | about 584 MPa | about 157 MPa |
| 35 mm | about 367 MPa | about 135 MPa |
| 40 mm | about 246 MPa | about 118 MPa |
| 45 mm | about 173 MPa | about 105 MPa |
A quenched and tempered alloy steel pin of roughly 700 MPa yield, with the same factor of 3, allows about 233 MPa in bending. That needs a pin of about 41 mm or more, not the 30 or 35 mm that shear alone would suggest. A gap between the eye and the ears raises the bending moment further (see K05-Q1-05), so spacers or a matching eye width matter. Bearing pressure on the eye bore and on the ear holes must be checked as well, using the ratings your parts' makers give. These numbers show the method; the geometry, material and factor of your joint decide the pin.
What material and finish should the pin have?
A hardened alloy steel, ground to size, with a shear strength suited to the load. A reported case shows what goes wrong otherwise: a loader cylinder with a generic rod end of no rated load and a mild steel pin suffered a loose thread, an enlarged pin bore and a cracked rod end. The fix was a forged clevis with a hardened alloy pin.
For aerospace spherical bearings, NHBB gives specific guidance: the pin should be heat treated to the required strength, then ground and polished to a surface finish of 8 µin Ra (about 0.2 µm) or better, with 17-4PH or PH13-8Mo stainless steel, or chrome-plated 4130 or 4340 steel, as suggested materials. That is aerospace practice and not a rule for a hydraulic cylinder pin. For a cylinder, use the hardened and ground alloy steel the cylinder maker specifies. The pin or bolt may also act as a journal, so provision for lubrication is needed.
What fit should the pin have in the bearing or bore?
Take it from the bearing maker. The same aerospace source recommends line-to-line to 0.001 inch loose (about 0.025 mm) between pin and bearing bore for its bearings. Other makers differ, and a bushed eye has its own bushing fit. Do not copy a number from one maker onto another's part.
Why does the clevis gap and the pin length matter?
Because a pin that does not fill the clevis, or an eye that floats in a wide gap, loads the pin and the ears unevenly. Match the clevis gap to the eye width, leaving a small side clearance, and make the pin long enough to pass through both ears and take the retainer. A wide gap increases the pin's bending moment (see above and K05-Q1-06).
How is the pin held in place?
By a retainer through or over its end, and the right one depends on pin size and risk.
A plain clevis pin is not threaded. It is retained at the open end by a split pin, an R-clip or a nut, which makes the joint quick to assemble and take apart.
- A split pin (cotter pin) goes through a cross-hole in the pin end. It is often used with a castle nut, a nut with slots the pin passes through, on a pin with a threaded end.
- An R-clip springs through the cross-hole.
- A bolted retainer with a locking plate is used on large pins.
Smaller pins are usually retained with a cotter pin and castle nut, mid-size pins with an R-clip or a bolted retainer, and the largest pins with a bolted retainer and locking plate, rather than a cotter pin alone. Treat that as the usual practice, and follow the cylinder maker's instructions for your joint.
What does a worn pin joint look like?
It rattles and hammers. This is pin bore elongation: looseness and hammering at the pin joint, caused by a pin that is too soft, too small, or overloaded. To check, measure the pin and the bore and look for ovality and galling. The fix is a hardened ground pin of the right diameter, with a replaceable bushing. See Q1-11 for measuring a worn bore and Q1-22 for inspecting the joint.