Hydraulic Relief Valves

[TomG]

I was looking in the hydraulics section of a Northern Tool catalogue. I noticed that the input sections for spooling valves have a specification for relief valve pressure. I wonder if this refers to a relief valve in the spooling valve or refers to something that is adjusted to be compatible with the pump's relief valve.

Every now and then I see something that reminds me how little I know about something. However, reminders from time to time probably keep me safer. I'd appreciate a response that helps fill in some of the basics to my knowledge of hydraulics.

Thanks.

 

[MChalkley]

To be considered a relief valve, it has to open and dump fluid to tank to "relieve the pressure" once it exceeds its preset pressure. Often it's nothing more than a ball valve held in the closed position by a spring. To adjust the pressure you add or remove shims behind the spring. Sometimes they're made to be easily adjustable by backing the spring with an adjustment screw.

If the valve has a relief valve, it will usually state its relief pressure; if not, it will often list the valve's maximum operating pressure. In the case you refer to, it sounds like the valve has its own relief valve.

Mark

 

[TomG]

Thanks Mark. Several of the valves in the Northern catalogue had specs for relief valves that are adjustable over a range. I can't exactly figure out what a relief valve in the input section of a spooling valve would do that the pump relief valve wouldn't. Maybe it's a fail-safe idea, or maybe some pumps used in industrial applications don't have their relief valves. At least I now have a read that maybe I'm not completely oblivious to the basics.

 

[MChalkley]

TomG - Sometimes it's helpful to be able to limit pressure to whatever you're controlling with the valve to a value less than the overall system pressure determined by the tractor's relief valve. That's when you would need the additional relief valve. Otherwise, it's just redundant protection.

Mark