A pin’s a pin, right? Only two measurements to consider – length and diameter.
But that’s not quite right; there are two types of pins used in excavators, and they’re not interchangeable. Using the wrong pin type can, in fact, cost money and time (which is also money).
Factory, or OEM, pins are the ones that shipped with the excavator. These are made of induction hardened steel. They join the dipper and linkage to the coupler (or in the case of no fitted coupler, directly to the bucket).
Bucket, or attachment pins are fitted to the bucket (or attachment). These are usually mild steel, with no additional hardening.
The first answer is rotation. The OEM pins pass through the dipper and the linkage and are bolted to the coupler. That means that when the coupler is curled or extended, the pin (fixed to the coupler) rotates inside the hardened bushes of the excavator. These joints must be kept lubricated, and the steel used has to be tough. If mild steel pins were used in this application, they would quickly wear down and the joints loosen, requiring regular pin replacement (and associated downtime).
Mild steel attachment pins are bolted to the bucket and picked up by the coupler. But rather than rotating in their coupler housings, they’re held fast. Induction hardened steel is not necessary for these pins.
One answer is price; why pay extra for hardened pins when non-hardened are sufficient?
But the main reason, again, has to do with wear. Although the bucket pins are held fast by the coupler, there are still tiny movements when the attachment is jolted or moved. When two types of steel rub together, the weakest of the two will take the wear load. The jaw of the coupler and the face of the safety wedge—which are in direct contact with the pin—while made of tough steel, are not induction hardened. A hardened steel pin will ultimately cause excessive wear in these vital and expensive coupler parts.
A mild steel pin, on the other hand, will take on any wear – and is much cheaper to replace.