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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Today's Featured Article - What Oil Should I Use? - by Francis Robinson. I keep seein this question pop up over and over again in discussion groups all over the web. As with many things there are often several right answers and a few wrong ones. Some purist I'm sure will disagree to no end with what I will tell you but most of us out here in the real world don't really care do we ? Some of them only bring their noses down out of the air long enough to look down them anyway. If you are like me you are only doing this old tractor stuff because you enjoy it. You
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