I'm not going to say it's a thing specific to just excavators, but the majority of them I've worked on, of all brands and sizes, have port reiefs on them. That said there is also a main system relief on them also that protects things like the pumps, etc from over pressurization. Even then some of the main reliefs are designed in such a way that pilot pressure is directed to the top of them to increase their setting when a function that requires a higher than normal pressure is activated. I've seen this happen on the travel function on the ones that have it. The main relief on a system is located within the valve body and is prior to te valve spools for the individual functions. Like I said it's main purpose is to protect the pump. With the port reliefs they are located on the downstream/function side of the valve spool. This is done in order to protect the cylinders and structure of the function. For instance the port relefs on an excavator would allow a pressure spike from something like a rock falling into the bucket or onto the end of the dipper to dissipate back to the tank instead of bending the rod on a cylinder or breaking a weld on the boom.
Like I said, in the most basic terms, the main relief is designed mainly to protect the pump and supply side of the system. The port reliefs are designed to protect the individual functions.
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