This article will give an in-depth discussion about Gas Springs.
The article will bring more understanding about:
A gas spring functions as a hydro-pneumatic component that accumulates potential energy by compressing gas within a sealed cylinder, which is controlled by a moving piston.
Unlike traditional springs, a gas spring does not depend on elastic deformation. It operates as a self-contained system, requiring no additional gas after it has been initially charged with inert nitrogen and assembled.
Regardless of the gas spring’s orientation, the pressure around the piston remains uniform because the rod's cross-sectional area is relatively small. The force (F) generated by a gas spring is determined by the pressure difference (P) between the internal and external pressures, applied to the rod’s cross-sectional area (A). When used in high-pressure environments, the pressure difference ΔP should be carefully considered.
Gas springs can be analyzed in various contexts as follows:
It represents the greatest extent of movement of the rod, measured from its fully retracted position to its fully extended position.
This refers to the overall length of the gas spring, measured from the center of one end fitting to the center of the opposite end fitting.
This measurement indicates the complete closed length, extending from the center of one end fitting to the center of the adjacent end fitting. If end fittings are not specified, it refers to the length from the rod end to the tube end.
This portion of the tube is designed with grooves to help secure the guide and seal assembly. Its purpose is to protect the seal package from being damaged by the piston during extension.
In various industries and applications, gas springs might be known as gas props, gas struts, or gas lifts. These components are commonly used to support or counterbalance moving loads. The term "nitrogen gas springs" is also used, as nitrogen is the standard gas used within these springs. Nitrogen is favored due to its inert and non-flammable nature, ensuring it does not react with the internal components.
Gas struts are typically used in vehicle suspension. They have a coil spring to support the vehicle’s weight and a shock absorber to absorb as well as dampen vibrations and shocks. A push or pull force is provided through the pressure that is applied to the spring. The gas strut supports the vehicle, absorbs the impact generated from surface inconsistencies, and aids in the wheel turning.
Gas springs equipped with damping features are often called gas dampers or gas shocks, depending on the industry and application. These gas shocks not only support dynamic loads but also regulate the motion of the system.
Dampers, or "shocks," utilize viscous friction to control movement. They typically work alongside an external spring or with moving components like panels and doors. The damper generates a force opposing the motion, which is directly proportional to the velocity of the moving mass.
The main distinction between gas springs and coil springs lies in their compression methods. Coil springs store energy through physical deformation, while gas springs rely on pressurized gas. Coil springs are often preferred for their lower cost, tunability, quicker rebound time, and lack of concerns about leaks or broken seals. In contrast, gas springs offer smooth and silent operation due to their internal pressure system.
With gas springs, force increases exponentially as they are compressed with less force being necessary to compress them, initially. As a gas spring approaches the end of its stroke, the amount of compression force that is required increases. In operations where several springs are necessary, gas springs are used over coil springs because it takes fewer gas springs to do the same job as multiple coil springs. Since coil springs are physical mechanical devices, they do not operate smoothly and produce a certain amount of noise.
Gas springs deliver greater force and require a shorter stroke compared to coil springs, making them more efficient for certain tasks. They are known for their high reliability, excellent performance, and safety. Essentially, gas springs provide lifting force at a slower speed, needing fewer units to achieve the same results.
The operation of a gas spring relies on nitrogen gas acting as an elastic medium, combined with a 50% mixture of oils such as transformer oil and turbine oil for sealing, lubrication, and pressure transmission. As the piston rod moves into the cylinder during compression, the internal volume of the gas decreases, leading to a proportional rise in pressure, in accordance with Boyle’s law. As a result, the force exerted by the gas spring reaches its peak when the rod is fully compressed.
According to the diagram above, as the piston moves from the fully extended position P1 to the fully compressed position P2, the pressure (indicated by the solid line) increases while the volume (shown by the dashed line) decreases.
The K-factor, or gas spring progression, is a crucial parameter representing the difference in force between the extreme rod positions P1 and P2. Gas springs can achieve a very low K-factor, typically ranging from 1.05 to 1.8, compared to mechanical springs. Since gas springs are pressurized to the necessary force at position P1, this force is immediately available. Therefore, it is important to consider the force at P1 when calculating the gas spring force at any position. In this context, F denotes the gas spring force, k represents the spring constant in N/mm (force change per unit of compression), and X indicates the deflection distance in mm.
In addition to lubricating the piston, seals, and piston rod, the oil within a gas spring also plays a crucial role in controlling the spring's velocity as it reaches the end of its extension stroke. The oil helps to decelerate the spring and mitigate shock loading upon full extension, preventing potential damage, failure, or injury. Damping is accomplished by regulating the flow of gas and oil through the piston. Optimal damping occurs when the piston approaches the internal oil column near the full extension point, especially when the spring is mounted in the rod-down position, which is typically preferred.
Factors affecting damping:
Operating temperature influences damping in two main ways. As the temperature rises, the force within the spring increases and the oil's viscosity decreases. This leads to a faster extension of the spring and reduced damping. Conversely, at lower temperatures, the force in the spring decreases while the oil's viscosity increases. This results in slower spring extension and enhanced damping.
Viscosity, by definition, is the measure of a fluid's resistance to flow and shear. Oil, which typically has high viscosity, becomes less viscous as the temperature rises. This reduction in viscosity means the fluid flows more easily and offers less resistance to objects moving through it, such as the piston in a gas spring. Therefore, a higher viscosity of the fluid results in increased damping of the gas spring.
When the oil volume in a gas spring is high, the spring will quickly engage the oil damping zone, resulting in a slower extension speed.
The pour point is the temperature at which the oil becomes semi-solid and loses its ability to flow. Once this pour point is reached, the oil in the gas spring solidifies, preventing the spring from completing its full stroke and inhibiting damping.
Metering controls the rate of extension or compression by adjusting the sizes of piston orifices to create restrictive flow paths. Regardless of the method used, the goal is to induce a pressure drop across the piston to regulate the extension speed. A larger piston orifice or shorter flow path reduces the pressure drop, lessens flow restriction, and increases the speed of spring extension.
Breakaway friction is another factor impacting spring performance. It occurs when a gas spring has been stationary for an extended period, such as several hours. During this time, lubrication moves away from the seals, and the rubber settles into tiny cracks and crevices within the metal due to the pressure in the cylinder. The initial use of the spring requires extra force to overcome this static friction, as the rubber is displaced from the cracks and crevices.
A gas spring should never be used under the conditions listed below, as doing so could lead to an explosion or other malfunction that may result in serious accidents or product issues.
A gas spring is made up of several key components, each essential for its proper and safe functioning. The diagram below shows these parts.
Rods are available in three types: polished carbon, precision-ground, or stainless steel. Their surfaces are treated to enhance wear and corrosion resistance. The rod’s length is always greater than the spring stroke but shorter than the tube length.
Carbon steel can be treated in various ways, such as salt bath, chrome plating, or nitriding. Chrome-plated rods may face chemical compliance issues due to chromium. In contrast, black nitrided rods offer a smoother surface and equivalent corrosion resistance compared to chromium-plated rods.
Nitrotec surface treatment is a specialized method for rods and offers several advantages over other treatments, including:
The gas spring tube is constructed from carbon steel, powder-coated steel, or stainless steel, all designed to withstand high pressures. Key factors affecting the gas spring’s durability and burst pressure include the internal surface finish and the tensile strength of the tube.
Constructed from plastic composites, the guide and seal package provides a bearing surface for the rod and prevents gas leakage and contamination ingress. Guides can also be made from materials such as zinc or brass, incorporating a suitable bearing sleeve. Rubber is the standard material used for seals.
The piston assembly, made from zinc, aluminum, or plastic, is crucial for maintaining the integrity of the piston-to-rod connection, which is vital for safety and preventing the rod from being ejected from the spring. It also regulates the gas spring's rate of extension and compression.
The end plug seals the tube end of the gas spring and also secures the end fitting to the tube.
Nitrogen is used in gas springs due to its inert and non-flammable properties, ensuring it does not react with any of the internal components.
The following outlines the different types of gas springs:
This gas spring type includes a rod connected to a piston inside a sealed cylinder. The cylinder is charged with high-pressure nitrogen, which creates significant force. These gas springs are known for their reliability and compact design, making them ideal for various lifting and counterbalancing tasks.
These gas springs allow the piston rod to be locked at any position along its stroke. The locking mechanism is activated by a plunger connected to the piston rod. When the plunger is pressed, the rod functions as a compression gas spring. Releasing the plunger locks the rod in place at the chosen position. Key components of locking gas springs include the piston rod, cylinder, piston valve, guide, and seals, among others.
These gas springs are tailored for specific applications and are suited for a range of industrial uses. They come in various materials, including EPDM, polyurethane, Viton, and others, to meet different requirements.
These gas springs feature an extra shroud mechanism encircling the rod. This mechanism locks the gas spring in place when it is fully extended.
Tension and traction gas springs apply some force or maintain the tension that helps to provide tension on the belt drive and mechanical assemblies. These springs operate in a way that is directly opposite to the way that compression gas springs operate. The springs come in a variety of sizes and stroke ranges, which aid in determining its force range and k factor. These types of springs are manufactured from stainless steel. They control the pulling and adjustment as per requirement.
These gas springs are ideal for use in corrosive environments and are available in various types, including compression and locking springs. Unlike micro gas springs, all stainless-steel gas springs are equipped with a release valve. This valve assists in the operation by releasing gas once the desired force is achieved.
These gas springs are designed to control the speed and motion of the spring using hydraulic oil. They are commonly used in applications where movement is directed, such as with doors, lids, and covers. They offer a load capacity ranging from 10 to 150 lbs and come with various speed characteristics to suit different application needs.
This chapter will explore the advantages and uses of gas springs.
Gas springs offer numerous advantages, including: a high force range up to 12000N, diverse designs and sizes, unlimited cycle lifespan, no need for external power sources, the capability for opening angles greater than 90 degrees, silent operation, and relatively low cost. Here are additional details on the benefits of gas springs:
Gas springs require minimal maintenance compared to traditional coil springs. Consisting of components like pistons, seals, and fittings enclosed within a cylinder, they need no cleaning, oiling, or lubrication. Their self-contained design simplifies maintenance, as there are no external parts to manage.
Gas springs are designed for durability, often outlasting other types of springs. Unlike coiled metal springs that rely on mechanical operation, gas springs are protected by a gas-filled cylinder, which reduces wear and tear. They offer approximately 100,000 strokes and have a longer lifespan, making them a cost-effective choice as they do not need frequent replacement.
Many gas springs come with a locking feature. While some gas springs are non-locking, you can select models that offer locking capabilities. This feature provides an advantage over traditional springs by allowing precise positioning and stability.
Gas springs offer ergonomic benefits by providing smooth and controlled action under compressive force. The piston retracts smoothly into the cylinder, making them suitable for applications like office chairs and other furniture where ergonomic design is crucial.
Gas springs come with a variety of mounting options, including ball and socket, rod end, clevis, eyelet, threaded, tapered end, and bumper or rod end unattached.
Gas springs are used in various applications, including:
This chapter will cover factors to consider when selecting gas springs and the key points for their installation.
When selecting a gas spring, the following factors should be considered:
Several key performance specifications must be evaluated when selecting a gas spring, including compressed length, absorber stroke, extended length, maximum force (P1), and maximum cycles per minute.
When selecting a gas spring, it is important to consider the following physical specifications:
This specifies the desired diameter of the housing cylinder.
This specifies the desired diameter of the extending rod.
The available mounting options include:
This mounting uses a spherical bearing that allows for multi-axis rotation. It features a ball end with an integral threaded stud.
These involve mating fork mounts.
The cylinder side has a tapered end that fits snugly into a corresponding tapered mounting hole.
This option includes standard threads at the ends for mounting or attaching accessories.
Here, the rod and load are not directly attached, but the rod has a bumper at the end to engage with the moving load.
This refers to the material used to manufacture the gas spring. Common materials include aluminum, steel, stainless steel, and thermoplastic. For applications near saltwater or in food and medical environments, materials with rust-inhibiting properties or UV coatings may be chosen.
It is crucial to know the weight and dimensions of the object being moved. A spring scale can be used to measure the weight for optimal results.
The gas spring's range of motion should not be restricted, so the body must fit properly within the assembly.
This indicates the temperature range within which the gas spring can function effectively.
Understanding the load's geometry and weight is essential for determining the appropriate mounting position, required counter-balancing force, and potential compromises. Mounting the gas spring with its center of gravity close to the pivot point helps in predicting its operation more accurately.
The longevity of a gas spring depends on proper lubrication of its seals. For optimal performance, the spring should be installed with the rod facing downward (refer to figure 2.1 below) or with the rod guide positioned lower relative to the cylinder attachment.
In applications such as car boots, the spring may pivot between the fully open and closed positions during the opening process. It is crucial to install the spring with its rod facing downward when fully compressed inside the cylinder (see figure 2.1 below).
This orientation is recommended because it aids in proper lubrication of the guide and seals while ensuring effective braking. The rod surface maintains gas pressure and should be protected from damage caused by blunt or abrasive objects and corrosive chemicals.
When installing the spring, avoid placing strain on the seal by ensuring proper alignment of the lower and upper fittings. Maintaining this alignment throughout the stroke is essential. Jointed fittings that allow for alignment adjustments can be used. Rigid attachments may transmit vibrations to the seals, so it is important to leave a small gap between the fixing screws and the attachments and to use at least one jointed attachment for fixing the spring. Smooth pins are preferred over threaded bolts to prevent thread crest damage.
Ensure that the thrust force of the gas spring exceeds the pulling forces to prevent exceeding the normal rod sliding speed. The recommended operating temperature range is -30°C to +80°C. In damp or cold environments, frost on the seals may affect the gas spring’s performance and longevity.
Gas springs are designed to assist with lifting or counterbalancing heavy loads for either the operator or the structure they are installed in. Any other usage requires a careful assessment by the designer or manufacturer to ensure safety and durability.
Side loads on the spring or rod are a primary cause of gas spring failure. It is crucial to avoid any side contact with the gas spring, as even minor contact can lead to premature failure. Sometimes, users might intentionally apply force to the side of the gas spring to support another mechanism, which can result in early spring failure.
Twisting forces should also be avoided on the end connector of a gas spring. To prevent twisting failure, a ball and socket connector is used to eliminate any torque on the connector.
Gas springs have a diversity of benefits over other types of springs due to their design and manufacture. They can be used in many applications including office furniture and industrial equipment. To choose the right gas spring for a particular application, one must know the specifications of the gas spring. Gas springs are long lasting as long as they are used under the correct specified conditions.
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