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Hydraulic Valves: Directional, Proportional, and Servo

For  decades, hydraulic valves have operated in more or less the same simple way: they flipped back and forth, opening first one line, and then another. Normally, one pipe was the ‘do nothing’ line, and one was the ‘do something’ line. But as our understanding of hydraulic systems increased, our desire to manipulate them in more precise manners piqued — and in response, different kinds of valves developed. Here’s a brief summary of the three major types of hydraulic valve.


Directional Valves

Also known as ‘switching valves,’ or more colloquially, ‘bang-bang valves’ because of the noise they made when switching, these are the valves described above. Directional valves have evolved over the past decades, however, from a simple left-right decision to a single valve that can contain several different outputs. Many also moderate the speed of the hydraulic fluid by altering the aperture through which it can flow.


Directional valves are useful, but simple: every change in direction, flow, or pressure requires its own directional valve, making even modestly complex hydraulic circuits enormous in size and expensive to produce.


Proportional Valves

Proportional hydraulic valves use solenoids to allow the valve to take any desired position between and including ‘closed,’ ‘left output,’ and ‘right output.’  This means they can adjust the flow to any proportion between the two outputs. This gives the ability to put speed, flow, and directional controls all on a single valve, dramatically reducing the space taken up by a complex circuit.


In addition, the ability of a proportional valve to adjust the speed of a circuit anywhere between ‘stop’ and ‘full power’ means that a single source and a single hydraulic pump can be used to power a wide variety of hydraulic devices, even if they require entirely different flow speeds and/or pressures to operate. That meant not only were circuits smaller, but there were fewer circuits necessary for any given set of jobs.

Servo Valves

Servo valves aren’t new — they’ve been around since the 1940s — but they’re rare, because they’re expensive. They operate using a combination of input pressure from the hydraulic line and electronic controls to create a valve that doesn’t ‘bang-bang’ — it moves smoothly and accurately. The net effect is that a servo valve lasts a very long time, responds quickly to controls, and has low hysteresis compared to the other two valve types.


Hydraulic Cylinders: When Hysteresis Isn’t Funny

There’s a common phenomenon in literally 100% of hydraulic systems that simply must be taken into account, and it has the misfortune of bearing a name that makes it sound like something we should all be grinning at: hysteresis.


What Is Hysteresis?

Simply put, every single kind of hydraulic cylinder, hydraulic valve, or hydraulic end-device (winch, servo arm, whatever it may be) that has at least two directions of movement suffers from the same problem. That being, the pressure required to move them from one position and the pressure required to move them back aren’t the same pressure.


Let’s take the simplest example: a valve designed to open when pressure gets too high — a ‘release valve.’  As the pressure behind the release valve builds, it will remain steady until that pressure reaches a certain point — say, 500 psi — at which point the valve opens. As the pressure decreases, however, the valve does not then close at 500 psi; it will only close when the pressure hits a lower point — say, 480 psi. The difference between those two pressures is called the ‘hysteresis’ of the valve, and it’s most often given as a percentage — 480 being 96% of 500, we would say the hysteresis of the valve is 4%.


Hysteresis is also rate-dependent, so for example, bringing a given hydraulic cylinder slowly from one pressure to another will result in less hysteresis than if you just crank the control all the way over to the other side in one swift action.


Why is Hysteresis a Problem?

Hysteresis is a problem primarily because of the way in which the human mind thinks — we expect, for example, that if we put all of the settings on a given hydraulic circuit to the same positions they were in last time, we’ll get the same result. But because of hysteresis, the result you get as you increase the pressure to get to Point X can differ significantly from the result you get if you reach Point X by decreasing the pressure — and the results can vary even more if you increase and/or decrease the pressure quickly rather than gently.


The end result is that confident operators can do everything they are ‘supposed to’ in order to achieve a specific end result, and end up missing that end result by enough to cause a disaster on the job site. Minimizing hysteresis — and constantly monitoring it — are critical goals for every job site that relies on hydraulic circuits, especially when lives could be endangered by an error.


Firestone Airmount Isolators: Good Vibrations

Is there really such a thing as ‘good vibrations’ when you’re in an industrial setting? Perhaps somewhere, but by and large, what you want is some way to keep the vibrations at bay, away from the fragile bits and pieces. The traditional solution for this problem is ‘solid state’ — in that it’s a metal spring, or some other form of flexible-but-solid material such as hard rubber that has give, but isn’t fluid.

The Firestone Airmount isolator is a beast of a different stripe, because the load-bearing element is quite literally air. The unique four-layer-but-two-ply construction of the vulcanized rubber ‘bubble’ keeps the air in place, but doesn’t itself hold any weight whatsoever. This gives the Airmount some incredible advantages no other isolator has, like:

  1. Incredibly Low System Frequencies: Most isolators have system frequencies around 8 hertz; the Airmount can go as low as 60 cycles per minute — that’s 1 hertz — and it can drop even lower with the addition of an optional auxiliary reservoir.
  2. Constant System Frequencies: Whereas most ‘solid’ solutions have a system frequency that changes as the load gets greater (and thus requires the load to be evenly balanced across all isolators), the Airmount’s natural system frequency is independent of weight — so your load can be imbalanced without affecting the Airmount’s ability to reduce vibration.
  3. Wide Weight Range: Airmount isolators can handle as little as 100 up to as many as 100,000 pounds per mounting point.
  4. Compact, Controllable Height: The Airmount isolator stands a mere 2.5 inches high (at minimum), giving much better isolation than a metal spring at 1/2 to 1/4 the height — but because the height is controlled by its internal air pressure, if you need your Airmount to run a little taller, that’s not a problem, either.
  5. Noise Reduction: Unlike solid solutions, the Airmount transfers almost no noise from one side to the other, and generate even less noise on their own.

Because the Airmount is so compact and so effective at isolating vibration, it’s often used twice: once between a piece of heavily-vibrating machinery and the structural elements around it, and then again between the structural elements and any particularly sensitive equipment on the same system. Used thusly, they promote both structural stability and can dramatically extend the life of sensitive equipment compared to conventional solutions.

Firestone Airstroke Actuators vs. Air Cylinders

Firestone Airstroke Actuators vs. Air Cylinders

When you have a pneumatic system being used to lift, push or pull– whether it’s a scissor lift or conveyor belt take up– the default tool for the job is an air cylinder. An air cylinder is essentially a closed, lubricated tube with a piston and rod assembly inside of it that moves outward as air is forced into the cylinder.

However many pneumatic system applications can be better designed using an Airstroke Actuator. The Airstroke Actuator has several distinct advantages over the classic air cylinder, including:

  1. Cost: The Airstroke Actuator can cost as little as half of an air cylinder that delivers similar force — and the cost savings goes up as the cylinders get bigger!
  2. Lifespan: Airstroke Actuators are designed and manufactured using Firestone’s patented Airide spring technology, provide a lifespan meaningfully longer than that of an air cylinder.
  3. Maintenance-Free: Try saying that about an air cylinder! Airstroke actuators require no lubrication, no maintenance, and in fact have no internal piston and no seals to break down.
  4. Friction-Free: Because there are no seals sliding inside, there is no ‘breakaway’ friction to overcome with an Airstroke Actuator — the response is smooth and instantaneous.
  5. Curved Motion: Without the need for any special machinery, an Airstroke actuator can easily extend along a 30-degree arc.
  6. Compact Size: Airstrokes have a much lower profile when fully retracted than a traditional air cylinder — between 2.2 and 5.5 inches, and can achieve long stroke lengths of up to 14 inches!
  7. Works In Multiple Mediums: While we’re comparing them to an air cylinder here, the Airstroke cylinder can just as easily become a Fluidstroke cylinder (don’t tell Firestone we called them that!) — because they function just as well using a fluid medium.

So What Is an Airstroke?

In short, an Airstroke actuator is a vulcanized rubber tube, carefully shaped so that as it collapses, it folds in on itself in as compact a manner as possible. As air (or another medium) flows in, the tube’s shape causes it to extend firmly and evenly in a specific direction. So next time you are designing a pneumatic system that requires up to a 14 inch stroke consider the Firestone Airstroke Actuator for the job.

Choosing the Right Pneumatic Fitting

There are a few different kinds of pneumatic fittings on the market, and choosing the right one isn’t always easy. You need to know the tubing being used, the operational air pressure, the thread type on the receiving fitting (if any), the amount of vibration the fitting is expected to endure, and more.


The Most Common Fittings

  • A Compression Fitting relies on a nut or other similar device being screwed down over a ferrule, compressing it. The compression of the ferrule causes it to bow inward, compressing the tube, the fitting, and the receiving fitting together to form a tight seal. Compression fittings are appropriate for copper, aluminum, and plastic tubes, and are themselves usually made of brass.
  • Specialty Compression Fittings such as Compress-Align, Metru-Lok, Poly-Tite, and so on use a similar mechanism to a generic compression fitting, but generally have preassembled parts that are designed to ‘play nice’ with a wider variety of tubing, allowing the use of tubes made of TFE, PFA, PEA, thermoplastic, steel, and so on to be used.
  • Hi-Duty Flareless Fittings are a kind of compression fitting made specifically for high-pressure applications (exceeding one thousand PSI.)
  • Push-Connect Fittings use a similar mechanic to Compression Fittings, but instead of a metal ferrule, they use a (usually vulcanized rubber) O-ring that will regain its original shape when released. This allows the fitting to be undone as easily as it was created, but creates fittings that are by necessity used for lower-pressure applications. They are generally made from nickel-plated brass, though some composite fittings made of glass-reinforced nylon are available as well. Push-connect fittings are easy to disconnect and reconnect, making them excellent for any environment where frequent changes are necessary. They are usable with copper, brass, steel, thermoplastic, nylon, polyurethane, and some less common tube types.
  • Barbed Fittings don’t rely on anything outside of simple friction to keep the tube attached; as such, they tend to be used for very low-pressure jobs only — anything higher would pop the tube off of the fitting!  Barbed fittings are generally made of either brass or thermoplastic, and are only used with tubes that have a little bit of ‘give’ to them: polyethelene, rubber, or GPH.

There are several other types of fitting available, naturally — the more exotic the job conditions, the more unusual the fitting. These are only the most commonly available, but they are appropriate for the vast majority of jobs.


The Power of a Hydraulic Power Unit

Lumberjacks. Miners. Search and rescue crews. What do they have in common? Well, they’re obviously some of the most hardcore people in America, but other than that, they also all work some with extreme machinery in their daily jobs. But where does a man find enough power to lift a massive tree, move huge volumes of earth, or drag a car out of a canyon? The answer is that the machinery they use are almost universally driven with a hydraulic power unit — a motor that converts the motion of liquid into mechanical force.

A hydraulic motor, at its essence, is fairly simple: a reservoir of hydraulic fluid, a power unit, and a machine that can be moved by any form of rotation. The power unit — usually a small electric device — pressurizes the fluid. Because fluid cannot be compressed, any amount of pressure from the pump causes the fluid to move, usually through a series of valves designed to make sure that the fluid can’t move backwards and harm the pump.

At the far end of the line, the fluid moves into a piston, which extends as it fills up, and voila! — a log is lifted easily off of the earth, swung about, and dropped onto a waiting truck. Or, perhaps, the fluid moves through a propeller, which generates a rotational force that is then fed into a series of gears that slows the rotations-per-minute but adds a huge amount of torque, and the winch on the far side of those gears pulls a truck out of a lake. The number of potential applications is huge.  After the fluid has done its job, it returns back to the reservoir along a different line, ready to be used again the next time the pump comes online.

Electric motors can only generate more power by being built bigger — so to get an electric motor to haul a ton of earth directly, you need one that is absolutely massive. But by using a hydraulic power unit — which is, ultimately, an electric motor, just used in conjunction with a hydrostatic system — the same job can be accomplished with a much smaller device.

Impact Idlers vs. Impact Saddles vs. Elastic Bands

If you’re in an industry that involves using conveyor belts to move lots of potentially-hazardous stuff from one place to another, you’ve probably encountered that horrible moment where some piece of debris being dumped onto the belt hits just right and punctures a hole. If you’re unlucky, you’ve seen that debris, still stuck it its own hole, carried all the way to a pulley or crossbar, where it got stuck and proceeded to slice a long gash right down the middle of your conveyor belt.


There has to be something to keep that from happening, right? Of course there is. In fact, there are three common variations:

  • Impact Idlers: several sets of three large pulleys that sit below the site of impact. Each set has one pulley angled up on either side, with a third pulley flat and lower in the middle. The earliest attempt to reduce conveyor belt damage.
  • Impact Saddle: several sets of U-curved iron bars lined with dense polyurethane squares.
  • Elastic Bands: several sets of thick elastic bands with a few polyurethane pads riveted on.


The Problems with Impact Idlers

Impact idlers don’t sit snug with your conveyor belt; their three flat planes mean that significant gaps occur in the corners, where the belt pulls away from the idlers. A sharp piece of something that lands in that gap can still easily puncture the belt.


The Problems with Elastic Bands

Elastic bands do fit snug with the conveyor belt, so puncture points aren’t a problem. However, elastic bands don’t hold the shape of the belt; they adjust to it. That, in turn, means that in order to use elastic bands, you have to position them between two sets of idlers that force the belt into the desired shape, meaning you can’t use them in situations where space is limited. Also, when the bands do require repair or replacement, you have to shut down the entire operation. Both idler pulleys and saddle pads can be replaced on the fly.


The Problems with Impact Saddles

By and large, we believe impact saddles are the best solution for most situations, but they aren’t entirely without their problems. Unlike idlers and elastic bands, an impact saddle has one shape; it’s not adjustable. They’re available in any given shape, but once they’re installed, any on-the-fly changes you make are going to have to take their existing shape into account.




How The ISO Guide to Hydraulic Filter Performance Works

ISO — the International Organization for Standardization — releases guides for almost every conceivable area of industry on Earth. Some of these are easy to understand, others are packed into language so dense and jargon-filled that it takes an industry expert just to read the introduction. We thought we’d shed a little light on hydraulic filters for everyone by unpacking the ISO codes for cleanliness, which are used for virtually every kind of air and/or liquid filter, including hydraulic ones.


How Small is a Micron?

The core unit by which the ISO measures ‘uncleanliness’ is the micron, which is a measure of distance. Filters are measured by how many particles of what diameter (in microns) they allow through. But before we get into that, let’s talk about what a micron is. Unfortunately, they’re so small, we have to describe them, because simply telling you “one millionth of a meter” doesn’t have any meaning. So here are three quick examples to help you understand:

  • The average human hair is about 70 microns wide.
  • A single grain of standardized table salt is about 120 microns wide.
  • The eye of a standard American sewing needle is a whopping 1,230 microns across.


Particulate in Microns

The ISO standards for particulate break them down into three categories:

  • 4 microns and smaller (about the size of a larger bacterium)
  • 5-6 microns (about the size of a deoxygenated red blood cell)
  • 7-14 microns (about the size of a mold spore)


Presumably, particulate larger than that only gets through a filter if that filter is critically compromised.


The Confusing World of ISO Codes

The ISO tests a filter, and they assign one code for each of those categories, in a string like this: 20/17/14. The ISO code is, unfortunately, not intuitive. In short, an ISO code of ’10’ means ’10 or less of this size of particle per milliliter of volume (after filtration).’  Every number you go up from 10 doubles the “X or less” number; every number you go down from 10 halves it.



So when you see a hydraulic filter with a three-digit rating like 32/12/4, you can look at it and say to yourself “OK, so after the fluid has been filtered, it’s got about 32 bacteria, 12 blood cells, and 4 mold spores in it.” Of course, what those particles actually are is often more important than the size (if they’re aluminum, you’ve got significantly greater problems than if they’re water — but that’s a different post altogether.




Quick-Disconnect Pneumatic Fittings – Modern and Legacy

It’s a pretty basic concept: you have something that moves air, and you need to attach it to something that has to have air pressure in order to do its job. You need to be able to detach that tool quickly, so you can’t use screws or other complex machinery. Enter the standard thumb-latch connector, and your problems are solved! Seems like there’s not much there that needs changing — but pneumatic fittings have in fact evolved in recent years.


The Old Guard: Metal Latch Couplings

More than 55 years ago, when pneumatics were first becoming industry standards across the world, the metal latch coupling was born. Cast of strong metals with as few moving pieces as possible, they held up against the extreme changes in internal air pressure easily. After a few years of trial-and-error to establish what the standardized sizes would be, metal latch couplings became so ubiquitous that they seemed to be a foregone conclusion. You need to connect pneumatics, you use a metal coupling, done deal.

But then, along came plastic and changed everything.


Plastic Latch Pneumatic Fittings

Plastic is lighter — significantly lighter — than metal, and its ability to remain strong even when more thinly sculpted allows for a more ergonomic fitting. Furthermore, plastic is resistant to corrosion in a way that metal isn’t, which made the plastic coupling much more durable under a variety of different industrial conditions.

And plastic wasn’t done yet.


Plastic Twist-Lock Connectors

Twist-lock connectors were the logical extension of the Luer-taper fittings of the previous century: simple pneumatic fittings that could be slipped together and locked tight with a mere quarter-turn. Because they are nearly as durable as a plastic latch fitting but don’t require a latch or button to activate, twist-lock connectors can be used in smaller spaces, or in implements like sphygmomanometers (blood pressure cuffs) where the much heavier plastic latch would cause usability problems.


With quick-disconnect pneumatic fittings evolving from the near-indestructible and extraordinarily reliable metal latches that are still industry standard after more than half a century of use to the the very small, very light, and easy-to-operate plastic twist-lock, there are very few places where pneumatic connections are an engineering challenge these days — and we’re all better off for it.


Introducing The Industrial Conveyor Belt Cleaner

Industrial conveyor belts are some of the hardest-working devices in America, shipping tons of debris ranging from freshly-dug earth to literal garbage of unknown composition thousands of feet every minute. They suffer from their efforts, too, in a variety of ways. Perhaps most dangerously, sharp items can puncture them upon landing — and if those things are still stuck in the belt when it hits a pulley or a brace, they can lodge and end up ripping a long slice out of the center of the belt, making it nigh unto useless.


That’s one of the chief reasons that every wise industrial engineer builds conveyor belt cleaners into every system in his demesnes. Conveyor belt cleaners come in three basic types:

  • Primary Belt Cleaners (also called ‘pre-cleaners’) sit directly opposite the discharge pulley, below the angle of discharge, and ensure that nothing remains on the belt as it ‘rolls over’ and begins its trip back to the impact saddle to receive more load. Primary conveyor belt cleaners can be as simple as a static rubber blade scraping the belt as it travels past or as complex as a whirling array of brushes arranged in nested helixes that constantly turn against the direction of the belt’s travel.
  • Secondary Belt Cleaners sit just back from the discharge pulley, with the belt pulled more tightly over the blade, and again, remove any carryback from the surface of the belt as it makes its return trip. The term ‘secondary’ doesn’t imply that they only work in tandem with a primary cleaner (though they certainly work best in that circumstance); it simply implies that the cleaner doesn’t touch the belt simultaneously with the discharge pulley.
  • Plows rest on the inside of the conveyor belt, preventing the inside of the belt from carrying any spillage back. These are particularly important because debris on the inside of the belt will get jammed in as the belt passes over the pulleys on its way back to the impact saddle, potentially causing much more damage than mere carryback. Generally a single diagonal blade but often seen in chevrons or similar shapes, a plow is virtually mandatory any time you have spillage landing on the inside of the belt.


Modern conveyor belt cleaners add years of life to industrial conveyance systems, and many can themselves go for months or even years without any appreciable maintenance needs of their own.