pressure control valves
- relief valve
- sequence valve
- pressure reducing valve
- direct operated counter balancing valve
- remotely operated counter balancing valve
- brake valve
- unloading valve
relief valve
- simple relief valve
- pilot operated relief valve
- used to limit pressure
unloading valve
- extension: clamp work piece and then form the work piece
- retraction: form retracts first and then unclamp
gravity hydraulic presses
- relief valve=counter balancing valve
- directly or remotely operated
- normally non passing
vertical press
- relief valve=counter balancing valve
- remotely operated, has a drain that comes from somewhere else
- direct always gives back pressure but remote sometimes doesnt
accumulators
storage devices of hydraulic energy
types of accumulators
- weight loaded
- spring loaded
- N2 gas type (inert gas)
N2 gas type accumulators
- diaphram
- pistontype
- bladder
precharge
-gas pressure of N2 inside of an accumulator when there is no fluid inside of it
use of accumulators
- develop system flow
- absorb shock
- maintain pressure
accumulator to develop system flow
- machine cycle (per min): 5 sec of work, 30 sec of charging accumulator, 25 sec of extra time
- working pressure: 500 to 1000 psi
- uses accumulator to get more speed
- accumulator will never charge with tandem center, must be block or flow center
- check valve makes sure fluid cant flow back
- if replace relief valve with unloading valve: saves horsepower/efficiency, will go back to tank at lower pressure
- always set accumulator to 100 psi below lowest working pressure
water hammer
high shock
inertia
reluctance of a body to start moving or stop moving once it is moving
accumulator to absorb shock
- set precharge of accumulators to 100 psi higher than relief valve
- if suddenly stop energizing, load wants to keep going and this can cause pressure spikes in lines
pilot operated valve
opens completely over a narrow pressure range, can operate over a wider pressure range without loss of fluid over the relief valve
resistive load
load that acts in opposite direction to actuator motion
overrunning load
load that acts in same direction to actuator motion
full flow pressure
what you set the relief valve to
direct acting relief valve
- only one poppet and one spring
- can withstand a higher pressure difference than pilot operated
pilot operated relief valve
- dart is held in place by the pilot spring, when the hydraulic force on dart becomes greater than the pilot spring force, the dart is unseated and fluid flows
- used for overpressure protection
- opens completely over narrow pressure range
cracking pressure
minimum upstream pressure at which the valve will operate
unloading valve
used to dump excess fluid into the tank at little to no pressure
unventing
piston moves enough to allow the dart to reseat
sequence valve
used to ensure that a certain pressure level is achieved in one branch of the circuit before a second branch is activated
