# Five Terms

Nobody was born with a brain containing the full compendium of hydraulic knowledge already imprinted, and even the top engineers at Parker, Bosch Rexroth or Eaton couldn’t tell a servo valve from a hole in the ground when they were twelve. Even if you have been around the business for a while, but not yet at the level of said engineers, there might be some terms you’ve heard and not understood, but have been too embarrassed to ask for clarification.

I’ve got five hydraulic words/terms and their explanations for you. If you know all five of these terms, then you probably work at Parker, Bosch Rexroth, Eaton or something….

1. Bulk Modulus – Bulk modulus describes the compressibility of matter, or more specifically, the incompressibility of matter.  Everything is compressible, even steel, and bulk modulus is the figure that represents just how hard it is to compress something. Bulk modulus is essentially the pressure it takes to reduce the volume of matter a given amount.

Bulk modulus is important to factor into highly accurate or utterly massive hydraulic systems. Because of oil’s compressibility, we must factor how much the oil compresses when calculating motion. Compressibility will cause a delay in motion when the control valve is opened, and will cause a delay in stopping when the control valve is closed. Also, systems with large volumes of pressurized oil can see problems related to decompression shock, resulting in damaged components as the oil returns to near-ambient pressure.

2. Beta Ratio – Beta Ratio, as it relates to fluid power, describes the efficiency of a hydraulic filter. It is ratio of dirt particles measured before a filter to the number of particles measured after the filter. The micron rating must be expressed to be valid, because every filter has an epic beta ratio when you’re filtering marbles.

If a filter element is β10 1000, it means if a thousand 10 micron particles enter the filter, only one particle will be measured after the filter. This filter would be 99.9% efficient in one pass.

3. Hysteresis – Hysteresis in fluid power describes the difference in performance from when a  (usually electrohydraulic ) component “remembers” its actuated state compared to its relaxed state. For example, when you fire a solenoid valve’s coil, and then remove current to that coil, the magnetic field still exists and tries to keep the valve shifted slightly. The difference between where the valve stays shifted and its intended position is its hysteresis. Less hysteresis is better.

4. Through Centre – If you’re a mobile hydraulic specialist, you can skip over this one, but if you’re an industrial specialist, you may want to read on. A mobile valve, such as a monoblock valve, has three internal sections; the power/pressure core, the tank core and the through centre core. The through centre core simply allows for power beyond, or series, capability.

A mobile valve has a through centre core regardless of standard spool configurations. For example, you can have all ports blocked (P, T, A and B blocked in neutral), yet still have a fixed pump flowing fluid through the valve where it joins with the tank core and then plumbed back to tank. If a power beyond plug is installed – which separates the tank and through centre cores - this very same valve can provide downstream flow similar to a tandem centre industrial valve, where fluid flows to from P to T in neutral, only the valve will still have P, T, A and B blocked. To be turned into a true closed centre valve, the power beyond adapter must be installed and the power beyond port plugged.

5. CentistokeIf you try to search for the definition of a centistoke, you will get web pages of engineering technical jargon or ridiculously obscure definitions such as this actual result from an online dictionary: “a unit of kinematic viscosity that equals one hundredth of a stoke.” Seriously?

Centistoke describes the kinematic viscosity of a fluid, or in simpler terms, how thick the oil is as it flows. The test rig for measuring kinematic viscosity is simply a graduated cup (with accurate measuring lines) with a calibrated hole in the bottom. The sample fluid is added to the cup, and the time it takes for the given fluid to drain out the hole will tell us how many centistokes (cSt) the liquid is.

#### Discuss this Blog Entry 0

##### Post new comment
What's Fluid Power Tips?

Tips, advice, and opinion on fluid power related topics for education and entertainment, with a splash of humor for good measure.

### Josh Cosford

Josh Cosford is a certified fluid power hydraulic specialist with Cambridge, Ont. office of The Fluid Power House Inc. Contact him at  joshc@fluidpowerhouse.com or call (519)-624-7109.