Gas Springs - Seminar Report

INTRODUCTION

Gas springs provide controlled motion and speed for elements, such as lids and doors, that open and close. They typically rely on the fluid dampening of a gas such as nitrogen in the cylinder.  Important performance specifications for gas springs include absorber stroke, compressed length, extended length, maximum force (P1), and maximum cycles per minute.

            The absorption or damping action for gas springs can be compression or extension.  In a compression gas spring the shock absorption or dampening occurs in the compression direction.  In an extension gas spring the shock absorption or dampening occurs in the extension direction.  Important physical specifications for gas springs include the cylinder diameter or maximum width, the rod diameter, mounting, and body material.  The cylinder diameter or maximum width refers to the desired diameter of housing cylinder.  The rod diameter refers to the desired diameter of extending rod.  Choices for body materials include aluminum, steel, stainless steel, and thermoplastic.Common features for gas springs include adjustable configuration, reducible, locking, and valve.  An adjustable configuration allows the user to fine tune desired damping, either continuously or at discrete settings.  A reducible gas spring has an adjustment style for gas shocks in which gas is let out to permanently reduce force capacity.  In a locking gas spring the position can be locked at ends or in the middle of stroke.  Valves can be included for fluid absorbers, a valve or port, which can be used to increase or decrease fluid volume or pressure.Gas springs are a proven and reliable method of counterbalancing large covers and objects. They offer ideal capabilities for safely lifting, lowering and positioning heavy or cumbersome objects. More versatile than mechanical springs, gas springs offer your product the advantages of speed-controlled dampening, cushioned end motion, simple mounting, compact size, flat force curve, and a wide range of available forces.                                                                                                    

Product Information

A gas spring is typically comprised of the following parts:

·         Cylinder: Heavy gauge steel body; painted and cured to a glossy finish.

·         Piston Rod: Chromium-plated, hardened steel, precision-ground and highly polished.

·         Piston Assembly: Self-cleaning design automatically opens during each compression stroke to keep the piston area free of contaminants. Not offered by all manufacturers.

·         Sealing System: This is the area where most manufacturers differ in their approach. AVM uses a patented Triple-Lobe Rubber Seal, as well as a Rubber O-Ring Piston Seal.

·         Seal Backup System: Teflon ring, functions as a backup to the seal system, unique to AVM. Prevents seal wear.

·         Temperature Compensation: Optional feature, this module provides for an increase in the force when the temperature drops below approximately 40 F enabling the use of lower forces at room temperatures to provide easier closing efforts.

·         Nitrogen Gas Charge: Gas springs are charged with nitrogen most often to 1500 psi, but not more than 2500 psi. It does not react with any of the internal components. The amount of charge varies from 1/3 gram in the smallest springs to about 24 grams in the largest. Nitrogen is inert and is not flammable.

·         Glycol Fluid: Lubricant for internal components. Also provides dampening to slow down movement of liftgate just prior to full open. This is a high viscosity index synthetic oil with a pour point of -70 F.

Operating principle of a gas spring

The gas spring is a hydropneumatic adjusting element, consisting of a pressure tube, a piston rod with piston and appropriate connection fittings. It is filled with compressed nitrogen, which acts with equal pressure on differently dimensioned cross-sectional areas of the piston. This produces a force in the extension direction.This extension force can be exactly defined within physical limits through the appropriate selection of the filling pressure.

Properties

Ø  Gas springs always require some initial force to begin compression.

Ø  Gas springs in their “free length” require some initial force before any movement takes place.

Ø  This force can range from 20 to 250 pounds.

Ø  Gas springs have a controlled rate of extension.

Ø  Gas springs can have multiple extension rates within the same gas spring (Typically 2: one through the majority of the extension stroke, another at the end of the extension stroke to provide damping).

How the gas spring works

Ø  In its simplest form: the compression of the rod/piston into the tube/cylinder reduces the volume of the tube as it compresses.

Ø   When the cylinder is filled with gas, this constitutes the spring like force or action associated with gas springs.

Ø  The gas pressure on both sides of the piston are equal.

Ø   However, there is the small area of the shaft where the internal gas pressure does not exert any pressure. Therefore, the internal pressure times shaft cross-sectional area equals the output force exerted by the shaft.

CHARACTERISTICS OF GAS SPRINGS

1)LOCKABLE

For gas exchange between the two chambers,separated by the piston ,gas springs are equipped with a bore in the piston.However ,if the piston is equipped with a special valve,inorder to close this bore,the gas spring can be locked in any stroke position desired.In addition ,spring locking as well as rigid locking can be provided.                                                       

                                               

Spring locking

In the case of spring locking ,the gas spring is filled entirely with gas.Because of the gas compressibility,a spring effect (bounce) is obtained when the valve is closed.this ensures absorbing and damping of sudden impact or pulse-like peak loads(eg. In swivel chairs)

Rigid locking

In case of rigid locking the gas spring is filled with oil.The rigid locking effect is determined by the non-compressibility of oil.This allows rigid locking of the spring and thus the application;even when subjected to greater external forces.

2)DAMPING

 To provide the “comfortable” stopping of the application in the end position(eg. For tail gates in vehicles),in most application instances,end-position damping is provided.In addition,either the extension and compression stroke or only the movement in one direction can be damped.Damping can be achieved in either of two ways;either hydraulic or dynamic.

Hydraulic damping

Inorder to enable gas exchange between both chambers of the pressure tube separated by the piston,the piston is provided with a bore.however if the pressure tube is partially filled with oil and the gas spring is mounted with the piston rod pointing downwards(in this event the oil collect on the seal and guide element of the gas spring),thus at the end of the stroke the oil must flow through the bore in the piston.Due to the viscosity of the oil,the flow resistance is greater than that of gas,and therefore motion is damped.

Dynamic damping

Dynamic damping allows the gas spring to be mounted in almost any orientation.Control of the extension speed of the gas spring is achieved by providing a longitudinal groove inside the pressure tube.In this case the piston does not have a flow conduit so that the gas flows through the groove cross-section.The groove geometry determines the extension speed;the smaller the groove cross-section becomes,the slower the extension or compression speed is.In this way the extension speed is controlled up to the end of the stroke and ensures a gentle stop of the application.By varying the groove geometry,it is possible to pre-define the motion speed of the piston rod over the effective stroke.

3)SPRING CHARACTERISTICS

The spring characteristic is the means of measuring the change in spring force of the gas spring over the entire stroke.Arealistic spring characteristic is illustrated(Force-Stroke diagram).The difference between the force during extension and the force during compression is the product of dynamic friction force.In difference to mechanical springs,the flat and linear spring characteristic is typical for gas springs.

 When an external force exceeds the force (F3) of the extended gas spring,the piston rod is retracted (compressed)back in to the cylinder.If the extension (F2) is greater than the external force,the piston rod of the gas spring is extended.The increase in the characteristic is determined by the force ratio F2/F1 and is also known as spring characteristic.Standard gas spring have a spring characteristic of between 1.2 and 1.4 (depending on application,various values can also be predefined).

SPECIAL SPRING CHARACTERISTIC

Several application demand specially defined force requirements.For eg. In certain applications the end stroke position may require greater spring force than that of  the main stroke run.The standard linear spring characteristic of a gas spring can be adapted to various requirements by adding mechanical coil springs                   

Progressive spring characteristic

Inorder to achieve a progressive spring characteristic,a mechanical coil spring is placed between the piston and bottom of the pressure tube.Since the gas spring is supported by the coil during a part of its extension stroke,the gas spring force is increased in its compressed state.

Degressive spring characteristic

By installing a coil spring on the piston rod, the gas spring force is reduced during extension at the end of the stroke by the force of mechanical coil spring.This results in what is known as a degressive spring characteristic.Thus the spring force of the extended gas spring is less than that of a standard gas spring

ADVANTAGES OF A GAS SPRING

Gas springs are used to provide counterbalance and force assistance in applications requiring a convenient and reliable adjustment function.Compared to mechanical springs,for many applications gas springs offers remarkable features which include:

  v  a flat spring rate flat spring rate (lower change in forces), even for high forces and long    strokes

v  a compact design,

v  straightforward assembly mounting to other equipment

v  definable linear, degressive or progressive spring characteristic

v  damping of the adjustment motion without additional damping components,

v  infinitely-variable locking

v  elastic or rigid behavior in locked position.

Gas springs have a number of advantages over coil springs.

1.   They can offer a much higher force in a smaller package than coil springs.

2.   On compression they do not bounce back, and the extension rate can be controlled, giving a smooth return.

3.   Typically gas springs have a low compression rate, but if required this can be increased.

4.   With a wider range of end fittings available, gas springs can be easier to fit.

5.   A wide range of additional features can also be offered.

Different types of gas springs

Micro Gas Springs

Micro compression gas springs offer users many advantages due their small size and low force.

 The table below shows standard sizes.

Micro springs are also available in 316 stainless steel and in custom strokes and lengths.

Locking Gas Springs

A locking gas spring incorporates a mechanism to enable the rod to be locked at any point in its travel. This locking mechanism operates when the plunger rod is depressed by opening a valve in the piston.When the plunger rod is released the valve closes and the passage of oil or gas is prevented, locking the piston in that position.

Tension gas springs sometimes referred to as traction springs, these units operate the opposite of compression gas springs.They retract rather than extend.Examples include doors and access panels hinged horizontally at the bottom and any type of cover or lid that must be pulled open or pulled shut.Tension gas springs also find many uses as tensioners on mechanical assemblies and belt drives.

Applications

Automotive

·         Trunk lids

·         Hatch lids 

·         Engine hoods

Consumer

·         Desks

·         Tool boxes

·         Sewing machines

·         Folding tables

·         Seating

·         Cabinet doors

Electronic

·         Printer covers

·         Printers

·         Money sorting equipment

·         Copy machines

Health & Fitness

·         Angle adjustment

·         Resistance equipment

·         Height adjustment

·         Treadmill

Marine

·         Engine covers

·         Folding beds and tables

CONCLUSION

The goal is the same with either type of spring; to move or resist the movement of some object.Gas springs in fact can be used in many applications where mechanical springs are applied because of their compact size and accurate adjustment.Gas springs are now achieving greater importance with greater variations being incorporated in it for specialized applications. More versatile than mechanical springs, gas springs offer your product the advantages of speed-controlled dampening, cushioned end motion, simple mounting, compact size, flat force curve, and a wide range of available forces.