Main Relief

[JVS Equip]

What is the function of the main relief on the valve body, how does it work, how is it adjusted and why would one adjust it in the first place?

 

[willie59]

On most excavator circuits, there are several relief valves, typically main relief valve and circuit relief valves. The primary purpose of the main relief valve is to protect the pump/pumps from excessive pressure on the supply circuit that would damage them.

 

[sultan]

Adding to what atco said, they are just spring loaded valves that open up by themselves when the pressure is too high. They can be adjusted by changing the tension on the spring by turning a head on the valve. You normally shouldn't mess with them unless you suspect they are set incorrectly. Some people like to turn up the relief pressure(s) to increase the force exerted by the machine, but turning it up too much can damage the machine.

 

[JVS Equip]

My reason for asking is I called out a mechanic from the local dealership when i had a recent problem "Stalling under load" and he says he adjusted the relief to the max, now the pressure reading from the computer is off by over 1000psi. Service centre in florida says i should have 5200 but i'm reading 4100. Now i think he was trying to increase the relief limit and instead lowered it. How hard is this going to be to reset without a guage or computer?

 

[John C.]

In most excavators built in the last ten to fifteen years the pumps destroke before they reach the main relief opening. In most machines now jacking up the main will not give you anymore power. You will have to adjust the pump cutouts in order to get more power.

So unless you really know what you are doing, you will be putting money in the pockets of people like ATCO. Even I wouldn't start jacking screws if I didn't know how the system is set up to be controlled. I did Komatsu excavators for years but would not touch a Cat, Kobelco, Link-Belt or any of the Korean units with out warning the machine owner that the risk was all his.

All the main reliefs I've seen in the last twenty years are pilot operated. Yes there is a spring inside that you adjust but the spring controls a pilot pressure differential to keep the cone seated. I've seen a few people pull cartridges apart and get surprised by how many small parts went flying.

 

[JVS Equip]

So increasing the power would be a long shot but could you decrease your power by lowering the relief setting? And is there a danger in low power?

 

[willie59]

My reason for asking is I called out a mechanic from the local dealership when i had a recent problem "Stalling under load" and he says he adjusted the relief to the max, now the pressure reading from the computer is off by over 1000psi. Service centre in florida says i should have 5200 but i'm reading 4100. Now i think he was trying to increase the relief limit and instead lowered it. How hard is this going to be to reset without a guage or computer?

Whoa! If you had a "stalling under load" problem, and had a wrench come out, and he dropped the main relief pressure 1000 psi +/-; then when your under load now you're more than likely dumping oil over the main relief valve instead of stalling. That's gonna heat up your oil really really fast.

 

[JVS Equip]

Actually i got the stalling problem solved which is why we are now checking and returning other adjustments to normal. The machine seems fine but since the "wrench" did that adjustment to the relief the computer reads a lower pressure and i like to go with recommended as much as possible so I'm about to spend some money to have a "qualified" mechanic reset the relief.


I'll post the find for the "Stalling" problem in the original thread.

 

[willie59]

Actually i got the stalling problem solved which is why we are now checking and returning other adjustments to normal. The machine seems fine but since the "wrench" did that adjustment to the relief the computer reads a lower pressure and i like to go with recommended as much as possible so I'm about to spend some money to have a "qualified" mechanic reset the relief.


I'll post the find for the "Stalling" problem in the original thread.

That's the proper thing to do bro. Like John C said, these pumps are designed to destroke before they reach critical pressures. On most of these circuits, the main relief is there to protect the pump from problems or failures that would put too much pressure on the circuit. In normal operation, main relief doesn't pop open often. Operating a main relief at too low a pressure can cause it to pop open too often, which will heat up your oil.

 

[Lee-online]

Reliefs may get week over time or get hung open from debris but they rarely cause a stalling issue. By adjusting it the tech put a band aid on the original problem but didn't find the real cause.

What machine do you have?

 

[MANUEL NAUT]

exactly, lee asked the right question. wich is your machine make and model?
the hydraulic systems designs vary according to the manufacturer. komatsu, hitachi use the load sense control systems meanwhile cat, jcb, volvo, hyundai, doosan use the negative flow control system in most of their excavators models.
most probably thing done by your mechanic is that he went down the horsepower curve of the hydraulic system by lowering the relief valve setting.
and like ATCO wrote this will heat the hydraulic oil temperarature, in many cases after 20-35 minutes of work.