One of the key advantages of direct-acting relief valves, in comparison to their pilot-operated cousins, is fast response. And their fast response makes direct-acting reliefs the valve of choice in situations where pressure spikes are possible and must be minimized. Secondary relief valves in an actuator circuit and the ‘spike relief’ for a variable-displacement pump are common examples.
But as a general rule, direct-acting relief valves suffer more from pressure override than their pilot-operated counterparts. Pressure override is the difference between the valve’s full flow pressure and its cracking pressure.
For example: if a relief valve is set so that it cracks open at a pressure of 1200 PSI and pressure rises to 1800 PSI when full flow from the system or sub-system is passing across it, the pressure override is 600 PSI (1800 – 1200 = 600).… continue reading »
If you know something about load sensing hydraulic systems, the above headline appears to be a contradiction. After all, load sensing circuits match pump flow to actuator demand. And this is supposed to increase the hydraulic system’s efficiency. Not reduce it.
HOWEVER, under certain conditions a load sensing hydraulic system can be VERY inefficient. Let me give you a real-life example. The manufacturer of an orchard mister decided to modernize the machine’s hydraulic circuit. The original circuit featured two fixed displacement hydraulic pumps. One pump driving the mister fan. And the other pump driving the chemical pump.
In the upgraded circuit, these two fixed displacement pumps were replaced with a more efficient, variable-displacement pump with load-sensing control.… continue reading »
What is an acceptable level of oil loss for a hydraulic machine:
c). 5% or more?
In a perfect world, the right answer would of course be zero. But in reality, the true answer is 5% or more–of tank oil volume per year. And the reason I can say this with confidence is because total hydraulic oil consumption is composed of losses from:
1. Slow leaks; weeps and drips.
2. Sudden and catastrophic conductor failure–usually hoses.
3. Opening of the system–usually for change-out of components.
When these three types of losses are considered together, it’s all too easy to get up to and beyond an oil consumption of 5% of tank oil volume per year.… continue reading »