General things to remember
- Pascal's Law - Pressure exerted upon one point or area of a fluid body immediately translates to all portions of the body and acts with equal intensity.
- Pressure always seeks to move the fluid from a high-pressure to a low-pressure point.
- Pressure acting upon an area becomes a force. Equal forces acting in direct opposition to one another cancel or neutralize each other.
- Pressure drop across an orifice is needed to cause flow. No pressure drop = no flow. Pumps do not create pressure, only flow. Flow restrictions create pressure. Oil flows through the path of least restriction.
- Hydraulic oil is practically non-compressible – 0.4% at 1000 psi, 1.1% at 3000 psi by volume.
- Oil required to move a cylinder – Piston Area x Stroke.
- 231 in3 = 1 gallon
- 2.5 feet of oil = 1 psi
Causes of cylinder rod leakage:
- cleanliness during installation
- nicks and cuts on rod
- improper lubrication
- over-tightening of seal
- seal mounted upside down
- contamination, especially during rod retraction (need bellows in dirty environments)
- chemical and heat degradation
Effects of high air content in hydraulic oil:
- spongy response
- increased heat load (air temperature increases when compressed)
- oxidation and thermal breakdown of oil
- reduced oil viscosity
- cavitation corrosion
- high noise
- decreased efficiency
- Need to filter oil right out of a new barrel for use with proportional valves as it is not clean enough.
- Use 6-12 micron filters on the oil supply side to proportional valves.
- Use 25 micron filters on the return run to the tank.
- Filters with alarms on them should alarm when filters are at 90% of normal pressure drop. If the pressure differential is too great, oil will bypass the filter and contaminate the entire system.
- When bringing new filters online, always bleed air out before reinstalling the cap.
- Using multiple filters in sequential decreasing filter size significantly increases overall filter life.
Accumulators in hydraulic circuits are used for several purposes – to dampen hydraulic pulsation, shocks and noise and/or to provide a reservoir to draw from when actuator movements exceed the capacity of the pump or supply system. Types of accumulators include bladder, diaphragm, and piston construction.
Accumulators are often overlooked during normal maintenance. They should be checked at least annually. To check the charge pressure of an accumulator, the supply pump has to be shut off and the system pressure drained out at the accumulator.
A special connection is located at the top of the accumulator (accumulators should always be mounted vertically to reduce bladder wear).
The pressure in the accumulator depends on its function when running. For vibration/shock reduction purposes, the accumulator pressure should be approximately 60% of the minimum working pressure. For reserve flow purposes, the pressure is closer to 90% of the minimum working pressure. The lower the accumulator charge, the more free oil will be in it.
Accumulators are charged with nitrogen. Never use air or oxygen to charge any type of accumulator as it can create an explosive atmosphere under pressure.
Finally, a quick method to check accumulator charge is to shut off the supply pump. If the accumulator stays charged, slowly open the drain valve and watch the rate of pressure reduction. When the pressure suddenly drops to zero, this is the pre-charge of the accumulator.
For more information about improving the operation of your hydraulic systems, contact your Valmet representative.