After a period of lurking, this is my first post ( As my username suggests, I am a tractorless suburbanite ). However, I have some small experience with hydraulics, and offer an analogy that may make the real issue here a little clearer. ( Or may simply confuse anyone foolish enough to listen to me... )
In an electrical installation, some sort of over current protection is ( almost ) always provided. In the most common case, circuit breakers are installed in a house. There is a main breaker that is sized to the total allowable current for the entire system ( typically 200A for a new home ) and individual breakers for each branch circuit. These branch circuits are installed such that the conductors size used can carry the entire current allowed by the breaker. Put another way, the breaker will trip before the current capacity of the wiring is exceeded. It does not matter how big a load you plug in- the wiring should NEVER overheat; the breaker should trip. Different circuits have different capacity breakers, but have wiring appropriate for the maximum load.
In a smaller system, there may only a single breaker ( fuse / thermal cutout, etc. ) protecting the entire system, based on the power supply capacity. In this case, every portion of the system, regardless of what portion of the total current load of the system it is designed to utilize, should be capable of carrying current up to the breaker. In other words, if any portion of the system fails in an overcurrent mode ( typically a short ) it will carry current up to the point required to trip the breaker.
The application of this in this case is as follows. The hydraulic pump has a certain design capacity. We tend to focus on the flow (GPM) rating, but the one we are interested here is the pressure (PSI). There should be a relief valve ( the hydraulic equivalent of a circuit breaker ) installed such to prevent the pressure from exceeding the capacity of the pump, typically by providing a path to the reservoir when the design limit is exceeded. If this is the only pressure relief in the system, then every component in the system must be rated to this pressure or higher. On the other hand, individual circuits may have independent reliefs provided, to limit the pressure on that circuit below the system limit.
For instance, if the pump is rated to 2000 PSI, there must be an appropriate relief provided to ensure that this pressure is never exceeded, in the form of a 2000 PSI relief valve. ( Ignoring all working rating, safety margin, etc. factors. I'm not designing a system here, just waving my arms a lot... ) Every valve, hard line, hose, etc. connected directly to the high side of the system must be rated at or above 2000 PSI. Now let's say that we have a cylinder that has a 1000 PSI rating on an implement to be attached downstream of a valve. When the valve is opened and fluid flows into the cylinder, as long as the cylinder can move at a rate equal to the incoming flow un impeded, no problem. But if cylinder is stopped, slowed, or the opposing force is sufficient, the 2000 PSI will cause the cylinder to fail. ( Bending a rod is actually a fairly good way to fail- blowouts of hydraulic systems can easily kill. ) The solution is to provide a separate relief for the circuit set at or below the 1000 PSI rating. This allows the system to be rated to provide high pressure to implements/attachments that are so rated, while allowing lower rated implements to be safely attached.
Sorry about the length, and no I am not a hydraulic engineer, so if someone better qualified wants to correct any errors in this post, I would welcome it.
Peter
"No tractor of any color, just green with envy"
