TechArchive

Original article date: May 1999

Geoff Coleman of Industrial Gas Springs looks at improvements in the design of the industrial gas spring over two decades.

How things change! Under 20 years ago a designer seeking to benefit from the advantages of a gas spring in a design would find that options were limited to a small range of springs designed for the automobile trade. Such springs were designed to suit a narrow market and were not appropriate for most industrial users who needed longer life greater temperature range and springs which could be provided in a wider variety of performance characteristics.

With the supply problem solved users then began to use gas springs to replace conventional springs in numerous applications. However in many cases the user did not know the required spring force and could not specify the spring required.

A solution to this problem is the adjustable gas spring which is achieved by fitting a Schrader valve into the end plug of the gas spring and changing it to deliver a force in excess of the calculated or estimated requirement. It enables the user to reduce the pressure in the spring progressively until the optimum spring force for the specific application is reached.

The spring can either be used in this condition or returned to the factory for the force to be measured so that the ideal spring can then be manufactured.

The method of fitting the valve ensures that in use the valve is hidden and tendered tamper-proof. However since the valve can be difficult to adjust on the larger springs a side-fitting adjustment valve has been introduced. This permits the valve to be adjusted in situ.

Whilst this method has worked well for existing applications for new applications many users have experienced difficulty in finding the optimum position for the mounting points of the springs. In response to this Industrial Gas Springs (IGS) has introduced a design service to specify these points and the optimum spring. Initially this took a couple of hours on a drawing board but it has become impossible to keep up with demand so a computer program has now been written. Initially only straightforward applications were covered but it reduced the time to handle the application to 10 minutes making many thousands of calculations iteratively until the optimum solution was found.

The program has now been further developed to cover many different applications. It makes full use of research data and mounting design data. Graphics have also been incorporated so that diagrams can be generated to show the final mounting positions and the hand forces required throughout the stroke of the spring. Once the spring design has been completed the user can be supplied with a sample to confirm its suitability before production quantities are delivered.

One of the early problems encountered with automotive and standard gas springs was that they must be mounted rod down to prevent gas leakage with corresponding reduction in performance and premature wear. This was overcome by a design modification which can now be incorporated in most products permitting the spring to be mounted in any attitude. A typical example of this application is the overhead passenger lockers in aircraft where the failure rate of automotive-type springs was unacceptably high due to their horizontal mounting. By using the new design this type of failure could be eliminated.

More demanding applications in the food medical chemical and marine sectors have led to the introduction of springs manufactured in 316 stainless steel. These springs provide maximum resistance to corrosion and may also be supplied with a range of seal material to suit demanding applications.

Originally only springs with forces between 40 and 1200 could be catered for but this range has now been extended to cover applications from 10 to 10 0 Similarly with the introduction of new materials the temperature range over which the springs can operate has gradually been extended. It is now possible to supply springs to operate in temperatures from -50 to +360degC.

Typical gas springs are low rate springs the force increases by 20% to 40% as the spring is compressed due to the reduction in free space for the gas to occupy as the rod moves in. By using smaller rod diameters larger tube diameters and short rod-to-tube extensions the spring rate can be made almost constant. Alternatively the opposite approach. coupled with large fill volumes of oil can achieve high spring rates. Hence both higher and lower spring rates can be offered and if required an almost constant force spring can be provided to balance a straight vertical lift.

Synergy between the designs of dampers and gas springs has enabled springs with variable damping characteristics to be produced for those who seek controlled force. The damping characteristics of springs can be altered by modifying the internal piston orifices selecting a damping medium from a range of oils and silicones or even by the inclusion of internal coil springs. By using different volumes of these damping mediums the length of stroke over which damping occurs can be tailored to suit the application.

A compression spring can also be offered as a locking spring. The spring through a lever operation can be locked in any position throughout its stroke. There are three variants of locking springs. The flexible type permits a small amount of “bounce” in the locked position. This type of arrangement is ideal for a seat back recliner where the small element of bounce adds to comfort. The semi-rigid type often called rigid by many suppliers has less bounce whereas the rigid locking spring has no bounce at all. The rigid type is ideal for applications such as height adjustment on an operating table where no movement is permissible.

A recent patented innovation is the provision of an overload feature on the compression locking spring. This feature can be supplied to work in either tension or compression at a force to suit the application. It is designed to prevent damage to equipment where the users tries to force the movement of the spring against the lock. When the applied force exceeds a pre-determined amount the overload device overrides the locking mechanism permitting the spring to act as if it were a non-locking type. When the force is removed the spring reverts to its normal locking action.

Tension springs have been added to the range for applications where the use of a compression spring is impractical. These springs resist a force trying to extend the spring and contract once the force has been removed. Tension springs are more complicated than the standard compression spring and have a shorter life.

The latest applications for gas springs are from the space industry. Springs have been designed to withstand the trauma of vibrations and stress during launch and the radiation and temperature extremes associated with space. This is an application where zero failure has to be assured. For one application the springs were used to replace exploding bolts and to provide a similar service. This required the springs to extend almost explosively in the shortest possible time to jettison the fairing protecting the satellite during the launch.

Another application where gas springs were specified was for the deployment of the antennae on one of the largest satellites ever made. To avoid damaging the antennae during operation the springs had to expand at a linear rate for over 35s.

• Industrial Gas Springs
• 0181 646 6595
• Geoff Coleman

May 1999