Gas Springs

What is Gas spring?

Have you ever tried lifting the trunk lid (sometimes called tailgate, hatch, or boot) of your car with just one finger? How come you can lift a heavy piece of metal and glass with so little force? The answer, if you didn't know already, lies in those clever piston-like hinges that support the lid either side. They're called gas springs (or gas dampers) and they make our lives a whole lot easier in all sorts of ways.

If you're sitting on an office chair right now, there's probably a gas spring underneath your body. Release the height lever and you'll feel (and probably hear) the gas in the spring being compressed as the seat gently falls down. Gas springs have loads of other uses too. Let's take a closer look at these handy gadgets and find out how they work!

Why do we need gas springs?

Suppose there were no springs on the trunk lid of your car. It would be really heavy to lift, for one thing. There'd be nothing to hold it up in the air when you wanted to load in your shopping, which would be a real nuisance. And, if you let the lid go, it would crash down onto your car's bodywork, probably doing a lot of damage in the process. Now we could put a normal metal spring on the lid, but that wouldn't help so much. It would need to be a very stiff and heavy spring, so it would take a huge amount of effort to lift the lid high in the air. The higher you lifted it, the harder it would get to lift any further. With the lid opened up fully, the spring would be stretched out so much that it would pull straight back down again!

How a gas spring works?

The basic idea gas spring

A gas springs is a bit like a super-sturdy version of a bicycle pump, only it's filled with pressurized nitrogen gas (the major constituent of the air around us) and oil and completely sealed up so they can't escape. The gas allows the spring to store energy, while the oil damps (slows and smooths) the movement of the piston and also provides lubrication. Just like in a bicycle pump, there's a tight-fitting piston mounted on a rod that can slide back and forth inside a cylinder (made from heavy gauge steel, not light plastic as in a bicycle pump).

Push on a gas spring and you force the piston rod and piston into the cylinder and this compresses the gas. Stop pressing and let go and the pressure of the gas pushes the piston back out again. So far, that sounds just like a bicycle pump—but it's working in a different way. Unlike with a bicycle pump, gas inside the cylinder can actually flow through or around the piston from one side to the other as it moves back and forward. Exactly how this happens varies from one design of gas spring to another; usually the piston has one or more holes or valves in it. Now if the piston can move through the gas, you might think it isn't compressing the gas at all. But don't forget that the whole cylinder is completely sealed. When the piston rod is inside the cylinder, it's taking up room that the gas previously occupied. In other words, when a gas spring is fully pushed in, you've compressed the gas inside by an amount equal to the volume of the piston rod. If the piston rod occupies virtually the whole cylinder, you can see that the gas is getting compressed quite substantially. The gas pressure can be very high, typically up to about 170 times normal atmospheric pressure!

The gas inside a gas spring can flow through or around the piston from one side to the other, but it can't escape from the cylinder. The whole system is sealed so, as the piston enters, the gas is compressed by a volume equal to the space occupied by the piston rod.

Gas springs as energy reservoirs

A gas spring's job is to make your life easy—and it does it by storing energy (when there's plenty available—usually when you're lowering something heavy) and releasing that energy (when you need extra help—usually when you're lifting something up). Think of a gas spring as a kind of mechanical battery that stores and releases energy by squeezing and releasing a gas and you can see why it's so useful.

What's happening with energy when you lift a heavy trunk lid that has no springs of any kind? There's a lot of mass in the steel and glass lid so it takes a lot of energy to raise it up against the force of gravity, which is constantly trying to pull it back down. Once the lid is high in the air, it has stored potential energy: you can release the lid and it'll crash straight back down again. If that happens, the potential energy is instantly converted into kinetic energy, as the lid accelerates, and then heat and sound energy when the lid smashes onto the car's body. What a waste!

Where does the force come from? In a gas spring, fluid at equal pressure pushes against both sides of the piston. But the inside of the piston (on the right) has a bigger area than the outside (on the left, where the blue piston rod takes up room). This means there's more force pushing on the inside than on the outside—giving a net outward or "output" force.

With a couple of gas springs on either side of the lid, it works a different way. Now, when you gently lower the lid, the weight of the metal and glass forces the pistons into the gas springs, compressing the nitrogen gas inside. As you lower the lid, the potential energy it had when it was up in the air is slowly converted into potential energy inside the gas springs and stored there. Next time you want to raise the lid, that potential energy is waiting inside the springs ready to help you. Release the lid catch, lift the lid gently, and the potential energy stored in the gas springs is slowly released. The pistons push out from the gas springs and help you lift the lid back up again.

How To Identify If A Gas Spring Is Failing

There are 4 typical traits that identify a gas spring is failing

If an internal seal has developed a fault then you will notice less force in the spring and maybe see excess oil on the rod or body. Some slight oil is normal

When you are opening a door / hatch you will have to apply more effort to open the device as the gas spring will not be performing fully

The door / hatch to be held open will start slowly drop, the gas spring will not be strong enough to hold open your application

The rod will no longer operate within a smooth manner and will create a juddery motion when opening / closing

Common reasons for a gas spring fo fail

There are many reasons why a gas spring may fail, listed below are some of the most common

Ideally a gas spring rod should be

Clean and free from grit or debris that may ingress into a seal. Damage to the rod, including pits, dents and scratches can all lead to premature seal wear

Away from extremes of temperature which may impact the internal components or vary the pressure of the gas inside

Shielded from salt water or corrosive elements which could cause rust or discolouration, or use high grade 316L Stainless Steel if unavoidable

Used within the limits for which it is designed i.e. not over stressed

Installed correctly so that the gas spring operates within its maximum efficiency. Incorrect specification and fitting can lead to premature wear and ultimately earlier than expected failure

What to do if a gas spring is failing

Depending on the type and quality of gas spring there are a few options available if your gas spring fails

For lower cost / lower quality units unfortunately the most effective approach is to safely dispose of and replace the faulty units. Depending on the application and the frequency at which you are changing the gas springs it might be worthwhile evaluating if a higher specification / better quality gas spring would provide a suitable better value replacement. The engineers at Alrose will happily evaluate your existing gas spring application

Specifying and correctly fitting a gas spring does require a professional engineering approach. Over the years we have come across numerous applications where

Gas springs have failed because they were initially installed incorrectly

The weight of the hatch / door has been massively underestimated resulting in an incorrect gas spring being specified. When you then add in that incorrect fitting locations and brackets are used it is a recipe for component failure

The Alrose Technical engineers and design team will happily review both the specification and fitting position. Why not call one of our team and send them through your design criteria

Using a high quality gas spring (such as an Alrose Product) it maybe possible to increase or decrease the pressure of your existing spring. More about this in another article…

How to choose your gas spring?

Decision support

1 - Gas spring pressure/force

The pressure required for the gas spring is calculated by substituting the values ??used in the following formula:

F1 = ((m x RH)/(2 x N x x2) + 5) x 9.81 = gas spring pressure expressed in Newtons

F1 = gas spring pressure/force

N = number of gas springs desired

m = weight (kg) of the tailgate/object to be lifted, manoeuvred

RH = length (in metres) of the tailgate object to be lifted, manoeuvred

x2 = distance (in metres) between the point of application of the gas spring on the tailgate and the rotation axis of the tailgate.

The number 5 represents a constant force of 5 kg/p, equivalent to rubbing actions.

Example:

For a hatchback with the following characteristics and values:

N = 2

m = 50 kg

RH = 1,50 m

x2 = 0.40 m

F1 = ((50 x 1.5) / (2 × 2 × 0.40) + 5) = 9.81 x 508N

In this case, since the tolerance is +/- 50 N, we would recommend two 550N gas springs.

Why use a gas spring?

A gas spring can do a similar job as an ordinary metal spring, though it has a number of advantages. Because of the high pressure of the gas inside it, a gas spring can be much more compact than a metal spring that would provide the same amount of force. Gas springs expand and contract more smoothly than metal springs and can be designed to open and close at an exact and constant speed (unlike metal springs, which contract faster when they are extended further and can be very unpredictable). Mechanically, gas springs are simple and have few moving parts, so they are relatively cheap, extremely reliable, and often last many years without any maintenance at all. Metal springs are more likely to break through repeated stretching and releasing (loading and unloading) because of fatigue.

Gas springs sound great, don't they? But that doesn't mean you'll find them everywhere. Some things that look very much like gas springs turn out to be a little bit different inside. Doors often have elbow-like closing devices fitted onto their top edge that pull them shut automatically. At a glance, they look very much like gas springs but, inside, they use a combination of one or two hydraulic (oil-filled) pistons, a rack-and-pinion gear, an ordinary coiled metal spring, and (sometimes) a cam. Some "gas springs" use a mixture of gas and liquid in their pistons; the one illustrated below is an example.

Gas springs, dampers and adjustment systems for the automotive industry In line with the future trend towards safety, gas spring products are used in many functions in the car to aid with opening and lowering the tailgate, to adjust the seat position and especially to provide protection in the event of accidents: DLC products thus offer more active safety for the passengers.

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