GAS SPRINGS - DOC by swenthomasovelil

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									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.




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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.




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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


  Gas springs always require some initial force to begin compression.
  Gas springs in their “free length” require some initial force before any movement
     takes place.
  This force can range from 20 to 250 pounds.
  Gas springs have a controlled rate of extension.
  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).




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How the gas spring works
    In its simplest form: the compression of the rod/piston into the tube/cylinder
      reduces the volume of the tube as it compresses.
    When the cylinder is filled with gas, this constitutes the spring like force or
      action associated with gas springs.
    The gas pressure on both sides of the piston are equal.
    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.




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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                              Rigid locking




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.




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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                                   Dynamic damping




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



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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


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       Progressive spring characteristic           Degrssive spring characteristic




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




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TERMS ASSOCIATED WITH GAS SPRINGS
Crossover - Self-Rise - Self-Close


   Crossover is the point in the opening cycle where the gas prop takes over all the
   lifting action (self-rise).At this point no further assistance is required by the
   operator for the door to reach the fully open position.There is a corresponding
   crossover position for the closing cycle where the door will fall to the closed
   position with no operator assistance (self close).The actual angle at which these two
   events occur are usually separated by a few degrees.The separation is due to
   friction in the gas spring internal components and connectors and with the hinge.


Self rise angle
   The self-rise angle is the angle at which the gas spring will lift the door without any
   assistance from the operator. For most systems this will take place between 10°
   and 30° from the full closed position.This angle will become greater as the
   temperature falls from ambient and will be smaller as the temperature rises.




Self-close angle
   Self-close is the angle at which the door will close without any assistance from the
   operator.Self-close is related to self-rise.The only reason these two angles are not
   exactly the same is due to friction.One of the sources of friction is friction internal
   to the gas spring.Another source is the friction in the hinge or hinge system.



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Hump
   Hump is defined as the difference between the force required to begin closing the
   liftgate and the maximum force required to close the liftgate at any point in the
   closing cycle.
Closing and opening efforts
   Cold closing and opening efforts
   Room temperature closing and opening efforts
   Hot closing and opening efforts
Opening and Closing Efforts
   For the most part, opening and closing efforts are dictated by one thing: liftgate
   weight.The lighter the liftgate the easier the opening and closing efforts will be at
   all temperatures.Currently, opening and closing efforts for an automotive liftgate
   above 50 pounds total weight should be 12 to 15 pounds to open and close.
   Acceptable efforts would be in the 15 to 18 pound range.Typically cold hold open
   effort is set to 3.0 pounds at -30C for an automotive liftgate or hatch.
Relation between self rise angles and efforts
   For most systems self rise angles take place between 10° and 30° from the full
   closed position. This angle will become greater as the temperature falls from
   ambient and will be smaller as the temperature rises.If the designer tries to make
   the self-rise angle small it will tend to make the closing efforts high. This is because
   in order for the system to have a small self-rise angle the output force in the
   compressed position will have to raise. Raising the compressed force tends to raise
   the extended output force at some proportional rate.
Life of a Lift Support
   All Lift Supports lose output force over time.When estimating the life of a Lift
   Support, one must first determine how much force the support can lose before the
   application becomes unacceptable. The time it takes to lose this amount of force is
   considered to be the life of the support.




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  Factors that affect the rate of force loss are:
    Size of the support
    Orientation
    Amount of cycles
    Ambient temperature
Safety
Buckling
   Buckling of a gas spring will not occur if the stroke meets the recommended length
   requirements of the chart shown.It is based on the EULER equation for long
   slender rods, and the design limitations of overall spring length, for smaller shaft
   diameters.




   The pressure in a gas spring is determined by: pressure = output force/shaft area.
   The shaft areas are as follows:
   6 mm shaft is .0491 sq.in
   8 mm shaft is .0779 sq.in
   10 mm shaft is .1217 sq.in




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Burst pressures
   Burst pressures of gas springs are recommended to be a minimum of 5 times the
   charge pressure to meet design requirements.


Side Loading
   Side loading is tested as shown in Figure. The gas spring should withstand the
   loads as shown in the table below for a given shaft diameter.




Shaft    0 - 10 10 - 20 20 - 30 30 - 40
Diameter inches inches inches inches
6 mm     40 lbs 20 lbs n/a      n/a
8 mm     79 lbs 40 lbs 27 lbs 20 lbs
         154
10 mm             77 lbs 52 lbs 39 lbs
         lbs




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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:

  a flat spring rate flat spring rate (lower change in forces), even for high forces
     and long strokes
  a compact design,
  straightforward assembly mounting to other equipment
  definable linear, degressive or progressive spring characteristic
  damping of the adjustment motion without additional damping components,
  infinitely-variable locking
  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.




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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.


       Three types of locking gas springs

        Flexible             Good resistance to rod being pushed of
                             pulled.
        Rigid in             Rigid when rod is being pulled, high resistance
        tension              to rod being pushed.

        Rigid in             High resistance to rod being pulled, rigid when
        compression          rod is being pushed.




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Tension gas springs
   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.




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Operating Conditions
   TEMPERATURE         -40°C (-40°F) TO +80°C (176°F)


   ALTITUDE            VACUUM TO PRESSURIZED CHAMBER


   HUMIDITY            0 TO 100%


   CORROSION           AS PER TESTS
  (SALT SPRAY)         BODY PASSES 240-480 HOURS
                       PASSIVATED 96 HOURS


   DUST & DIRT         PASSES FORD ES TEST WITH FINE COAT
                       OF ARIZONA ROAD DUST APPLIED TO
                       CYCLING SHAFT EVERY 1500 CYCLES


   ULTRAVIOLET         A 10% LOSS OF GLOSS OCCURRED WHEN
                       TESTED TO GM SPECIFICATION


   VIBRATION           0-100 HERTZ RANGE ONLY IF NO
                       BUILDUP OCCURS
   HEAT                NOSE TEMPERATURE NOT TO
                       EXCEED 25°F


   CYCLE FREQUENCY     ABOVE AMBIENT DURING RAPID
                       CYCLING




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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



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Applications




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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.




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REFERENCE
www.stabilus.com/produts.htm
www.mechanicalcomponents.com/gassprings/learnmore.htm
www.arvinmeritor.com/gassprings.htm




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