6.0 Accessories:

6.1 Mechanical release system
6.2 Hydraulic release system
6.3 Metal fittings
6.4 Connecting parts

1.1 Principle of function
1.2 Measuring points / friction
1.3 Force progress
1.4 Nozzle function / speed control
1.5 Damping and presentation of different device positions
1.6 Temperature
1.7 Storage and durability
1.8 Tolerances and materials

1.1 Principle of function

Gas springs are used everywhere that weights have to be pushed, lifted, lowered, pulled or set into position. Due to the use of a modern nozzle technique, controlled motion speed is possible.

Advantages of Easylift gas springs:

• Controlled motion and speed
• Damping
• Maintenance-free parts
• Simple mounting options
• Flat gas spring curve
• Locking options

The gas spring essentially consists of a piston rod, seal, and a cylinder which is under pressure; it therefore acts as a force accumulator. We use nitrogen to fill the springs, and oil to grease them.

Due to the pressure which builds in the closed cylinder, there is overpressure contrary to the outside. As shown in the diagram below, both nozzle sides have the same gas pressure because of the nozzle boring. The piston rod area remains as an effective working area which is influenced by the pressure.

back to Index

The push-in movement of the piston rod begins from the starting point (SP) with a test speed of 750 mm / min. After having reached the end of the stroke, the piston rod runs out with the same speed. For a uniform and reproducible measurement of forces, we determined measuring points 5 mm before the beginning and end of the stroke. There is no movement at the measuring points F1 - F4; the measurement is therefore done statically.

During the dynamic measurement of gas springs, friction as well as flow resistances have to be overcome. In the diagram below, you can see the difference between the theoretical characteristic curve and the actual force progress. The dynamic friction results from the friction of the main seal in the guiding piece and a control-O-ring on the nozzle as well as from the flow resistance of the nozzle boring.

Besides the stated factors, the filling pressure (bar), the piston rod surface (Peak-to-valley height) and the axis parallelism of the parts also influence the height of the friction figures. The combination of these factors shows the smooth movement of the gas spring. (As a consequence, in stronger gas springs, there is a greater contact pressure of the sealing on the piston rod surface as in weaker gas springs).

When pushing in the piston rod for the first time, a break off (LK) of the seal can be noted. After a long period of inaction, the oil film under the main seal will be removed. After 1 or 2 movements the break-away force can be neglected.

Easylift gas springs are known for low friction figures reached through the use of modern sealing techniques and quality produced piston rod surfaces. These characteristics are a basis for a long durability of the gas spring and for the secure function in regards to low extension forces.

The height of the dynamic friction (FRdyn) can also be influenced by the motion speed. The faster the piston rod is pushed in and out, the higher the friction figures. The slower the movement progress, the smaller the friction. This figure can also be influenced by the diameter of the nozzle.

The stated points find their application in all Easylift products. In order to obtain usable test results, normally two tests are made before the actual measurement. After that, the reproduction of the measurement at approximately +20 °C is guaranteed.

F1: extension force with an extended piston rod
F2: extension force with a compressed piston rod
F3: insertion force with an extended piston rod
F4: insertion force with a compressed piston rod
FRdyn : Dynamical friction force
FRstat: static friction force
SP: Start point
LK: break power
OK: optimal known limit
P: progression

MP/FR

back to Index

The theoretical force progress has already been a topic of the previous mentioned points. By pushing in the piston rod, the gas pressure in the cylinder will be compressed by the volume of the piston rod. Contrary to gas, oil cannot be compressed. The progressivity or characteristic curve, meaning the difference between the force in compressed and extended position, can be influenced:

• The larger quantity of oil, the smaller the rest of the volume of gas, the higher the rise in pressure while pushing in the piston rod!
• The bigger and longer the cylinder, the bigger the gas volume in relation to the piston rod volume, the smaller the pressure increase while pushing in the piston rod!

For normal gas springs with standard lengths, please follow the progressivity figures below:

Generally, the following force progressivity shown in the force diagrams are possible:

• flat characteristic curve
• progressive characteristic curve through oil filling
• progressive characteristic curve through coil spring
• diminishing characterisitic curve through coil spring

back to Index

For the speed control of gas springs, we use a nozzle. This nozzle acts as a stroke limiter to the piston rod in extended position, or under certain circumstances, as a guide. Standard gas springs can be pushed in easily. Due to the diameter of the boring, the speed can be varied. The bigger the boring, the faster the push-out speed, and the smaller the boring, the slower the push out speed.

As it can be seen in the diagram below, the nozzle has two borings, each of them with a different size. Through these, as well as through the ring slot between piston and cylinder can flow gas. Due to the swimming mounted control-O-ring, the flow can be controlled through the ring slot. When pushing in the piston rod, the O-ring seals up the ring slot on the side of the guiding piece. The gas can flow from the cylinder end side through the small boring and the open ring slot over the big boring to the piston rod side. While the piston rod is pushed out, the O-ring seals up the ring slot on the opposite side. From the guiding piece side, the gas can flow first through the big boring and the ring slot. On the cylinder end side - and therefore relevant for the speed - the gas can only flow through the small boring. For this reason, the push-out speed of the standard gas spring is controlled.

Drawing 1.4.1

Besides the nozzle installation which is damped in the push-out direction, there are also the following variations possible:

• damped in push-in direction
• damped in both directions
• no damping in either direction

The last specification is normally used for example for a height adjustment of tables. The gas spring acts only as a weight balance and should not have it’s own braking action.

back to Index

Due to reasons of lubrication, it is often recommended to install and store gas springs with the piston rod downwards. In order to guarantee installation, storage at any position as well as a long durability, Easylift uses a special guiding piece with a grease chamber. This quality feature assures an optimum amount of lubrication for the seal, even if the gas spring is installed with the piston rod upwards (see diagram below).

Drawing 1.5.1

In some applications, the slowdown effect of the push-out movement on the last millimeters or centimeters of the stroke is very comfortable. This hydraulic end damping is often used to open heavy machine covers. Here, the gas spring has to be installed with the piston rod showing downwards so that oil can build up at the guiding piece. When the piston rod moves into the push-out direction, the nozzle will make contact with the oil. The thicker oil has to be pushed through the nozzle borings, and the end damping effect arises. The length of the end damping effect can be determined individually to customer requirements by filling in different oil quantities (see diagram [a] below).

If the gas spring is installed horizontally, a detailed end damping, as described in the last point is not possible. The oil inside spreads out horizontally on the cylinder ground, causing more or less controlled end damping. The damping effect will be minimized in addition, when the nozzle boring works in the upper area of the horizontally installed gas spring (see diagram [b] below).

The following construction is suitable for controlled damping in the push-out direction in the horizontal position. Here, the cylinder is divided into a oil, and a gas chamber by a floating piston the same as the rigid blocking gas springs (see diagram [c] below). Contrary to the gas spring with the floating piston shown in diagram [c], there are special applications when gas spring dampers must ensure a controlled damping effect, despite rotating motion, when it is loaded shortly with high force in the push-in direction.

For applications which have to be damped in the last part of the stroke movement in the push-in direction, the gas spring has to be installed with the piston rod showing upwards (simple construction; without floating piston). The oil can accumulate at the end of the cylinder side. In order to get an intensive damping effect, the position of the nozzle is recommended to be damped in push-in direction or damped on both sides (see diagram [d] below).

In all variations, the damping effect can be intensified by using nozzles with small borings. The oil can also be influenced by its viscosity whereas side effects (for example temperature) have to be noted.

back to Index

Standard gas springs are made for an environmental temperature of -30 °C to +80 °C. Moreover, special seals for temperatures of -45 °C to +220 °C are available. To exceed the temperature of the figures by some degree in the static area is normally not dangerous. In case of a dynamic load, special seals are recommended before reaching the critical value.

back to Index

Easylift gas springs can be stored in every position. Loss of pressure due to long storage or storage in a horizontal position is not to be expected. We have not had any negative experiences in more than 30-years history of production. But there can be a stick-slip effect which can require a higher effort of force (break-away force) for the first cycle.

Easylift gas springs are generally tested on a performance of approx. 70000 - 100000 complete strokes. However they must not lose more than 5 % pressure. (In the automobile field, the requirement lies at approx. 50000 strokes). Depending on the application, the stated durability can be considerably lower or higher. In praxis, 500000 cycles have indeed already been reached.

Generally, you have to note that the durability is mainly determined by the final product and its handling. Side forces, overload, damage, considerable dirt, paint mist, etc. shorten the durability extremely. Moreover, the gas spring should in be chosen in the middle pressure range of the respective size (partial-load range). A maximum force as well as the use in extreme temperatures influence the life of the spring in a negative way.

back to Index

Length tolerance: ± 2.5mm, within series production ± 1 mm
Force tolerance: + 40N / -20N or 5 - 7%.

Usually, Easylift gas springs are made with + 20N. The filling tolerances are normally small. However, they are overlaid by measuring tolerances. Due to the measuring instruments and influences on the measuring points such as friction, speed, temperature, there can be differences.

Materials:

• Piston rod: Steel, hard chromed
• Cylinder: Steel powder-coated / zinc
• Filling: Nitrogen
• Connectors: Steel, zinc-plated
• Sealing: special mixtures on NBR or PU-basis
• Oil: Hydraulic oil
• Release levers: Stainless steel

back to Index