James T. Kull
Applications Engineering Manager Stabilus
Colmar, Pa.
Pressurized gas springs are used in a wide range of automotive, industrial, and furniture applications. While they remain the motion-control method of choice for many engineers, nagging doubts remain about their long-term performance. However, when designers properly size and mount gas springs, their life span meets expectations.
The key to properly using gas springs lies in understanding how they work. Basically, they are sealed pneumatic cylinders that impart forces to lift, lower, or hold components of mechanical systems, such as automobile hatchback doors and height-adjustable office chairs. They consist of pressure cylinders, pistons, rods, seals, nitrogen gas, oil, and end fittings.
Force characteristics
Pressurized nitrogen at 200 to 2,400 psi, and external atmospheric pressure at 14 psi, act upon rods connected to pistons that slide into and out of cylindrical bores. When the rods slide completely inside the cylinders, the rods occupy the greatest volume inside the cylinders. At this point internal gas pressure is at its maximum, and external forces hold the rods in place. When the forces are released, gas pressure pushes the rods from the cylinders and moves whatever mechanisms are connected to the gas springs. Cylinder output forces can be increased or decreased by charging the unit with higher or lower pressures of nitrogen.
The pistons and seals guide the rods in the cylinders, while oil damps motion. The pistons have small channels through which oil must pass as the pistons move inside cylinders. This forced passage of incompressible fluid produces a damping effect. Original gas springs used pistons with straight orifices to damp rod compression and extension. In these early gas springs, hole size controlled velocity. Unfortunately, orifices had to be extremely small and tended to clog easily. As a result, pistons with labyrinth orifices replaced straight-orifice pistons. Labyrinth pistons have larger openings but act much like straight-orifice pistons, using an oil-flow pressure head to damp motion. Longer labyrinth canals produce lower velocities, while shorter canals permit higher velocities.
There is also an emerging technology that has been developed for special applications. It is called dynamic damping and employs pistons without orifices. Instead, small grooves on the inner walls of the cylinders restrict motion. Damping can be adjusted by varying groove depth, typically 0.3 to 0.6 mm. Two-stage operation can be produced by double ramps that provide damping at both ends of the strokes.
Force losses
Gradual loss of the nitrogen gas causes a loss of internal pressure and, therefore, output force. Gas springs lose force from both static and dynamic nitrogen losses. Static losses primarily result from the permeability of rubber seals, which permit small amounts of gas to escape over time.
Dynamic losses result from cycling gas springs. Each extension and compression cycle pulls small amounts of gas past outer seals. This occurs because rod surfaces are deliberately made slightly irregular to retain oil films for seal lubrication. Although rods can be polished smooth to virtually eliminate dynamic nitrogen losses, seal lives would then be shortened considerably.
Given the inevitability of some static and dynamic losses, gas-spring manufacturers have searched for ways to extend useful lives of their products. One way is to select oversized gas springs, leaving a “force reserve.” This is the force a gas spring provides over the minimum needed for a particular application and normally constitutes a 30 to 50% safety factor beyond the minimum required force. This means a gas spring would have to lose up to half its output force before failure.
When calculating force reserves, operating temperatures are critical. As temperatures drop, gases contract, causing a loss of output force. Conversely, high temperatures cause gases to expand, increasing forces. Software programs are available to calculate gas-spring forces at minimum, maximum, and nominal temperatures, providing an indication of force reserves and the required size of a gas spring. Even though larger gas springs usually last longer than smaller ones, many applications have size limitations. In these instances, manufacturers must determine how best to retain smaller volumes of gas for longer periods of time. One way to do this is with different types of seal packages.
Seal systems
Gas springs have seals where the rods protrude from the cylinders. Typical seal packages contain guides, rubber lip seals, and oil cushions. Guides, at the outermost end of the cylinder, support the piston rods as they move in and out of the cylinders. Lip seals, adjacent to guides, keep contaminants out of the cylinders and prevent nitrogen gases from escaping. To minimize gas losses, lip seals have molded-in metal backup washers acting as barriers between the rubber seals and nitrogen gases. These washers, made of steel, are less permeable than rubber and reduce static force losses.
When rods extend and the gas springs are oriented with the rod end up, the seals do not get lubricated with oil. This can leave lip seals without lubrication and vulnerable to abrasion from rod surfaces. However, oil cushions within seal systems keep seals lubricated even when the rod is up, prolonging seal life.
Some manufacturers use multilobe seals consisting of guides, separate plastic back-up washers and rubber seals. These seals work relatively well under vibration and in rod-down applications. But, they fail quickly under high cycling and in rod-up positions. In multilobe seals, back-up washers are not molded in. This leaves full face areas of rubber seals exposed to pressurized gases, resulting in higher static-force losses. Also, because multilobe seals have no oil cushions, they are vulnerable to abrasion when the rod is up.
The shortcomings of multilobe seals were evident in recent tests. After extended cycle testing of a premium lip-sealed, and a multilobe-sealed, gas spring through 40,000 full strokes, those with multilobe seals suffered force losses of 25%, while lip-sealed gas springs showed losses of less than 5%.
Another type of seal package, and one that offers superior performance, is referred to as a twin-seal package. These contain two lip seals in fixed positions with small amounts of oil between them. The first seal serves as a filter, collecting dust and other contaminants to protect the integrity of the second seal. The synergistic effect of two seals working in tandem more than doubles gas retention of single-seal assemblies. These designs increase gas spring life under adverse operating conditions involving extreme temperatures, dirt, dust, vibration, and high cycling. Twin seals also reduce static force losses.
Rod surfaces
Just as important as seals are rod surfaces, where imperfections create potential leak paths. Some manufacturers try to prevent them by chrome plating rod surfaces. Unfortunately, chrome is susceptible to corrosion that damages seals and allows nitrogen to escape. Moreover, effective chrome plating requires absolutely clean surfaces. Any contaminants, such as machining oils, cause poor adhesion, creating “pipes” between rods and chrome through which nitrogen can leak.
A better solution for preventing nitrogen loss is to immerse the rods in liquid-nitriding salt baths, creating tough, ductile epsilon nitride compounds on rod surfaces. Under these compounds are diffusion zones of suspended nitrogen, which improve the fatigue properties of rod surfaces. After initial salt baths, rods are polished and reimmersed, providing proper surface finishes and superior wear and corrosion resistance.
During these processes, the high temperatures of the salt baths anneal the core of the rods while surface hardening their outer shells. This process nitrides the rods, allowing them to flex. Chrome-plated rods, on the other hand, tend to be more brittle. Rods also expand slightly during nitriding, filling small surface inclusions for smooth seal interfaces. In recent ASTM B117 salt-spray tests, nitrided rods were compared with chrome-plated rods. Results showed the corrosion resistance of nitrided rods exceeded that of chrome-plated rods by a factor of eight-to-one.
Force analysis
Proper mounting and selection of appropriate gas-spring types are the keys to reliable long-term performance. To achieve this, designers must analyze forces, including weight of the object to be lifted, force of the gas spring, and force applied by the operator. These forces can be calculated from moment equations for static equilibrium.
∑Mo = 0 = –W X D1 + F1 X D2 + F2 X D3
where ∑Mo = sum of moments about origin; W = weight of object; D1 = horizontal distance from CG to pivot point; F1 = gas spring force; D2 = length of spring arm; F2 = force applied by operator; and D3 = distance from handle to hinge point.
The center of gravity (CG) in this context is the point at which an object is perfectly balanced in all directions. Accurate determination of the CG is essential for force analysis. For gas-spring applications, designers only need to find 2D CGs. Complex shapes are reduced to simple geometric shapes to easily locate their CGs. Homogeneous items of consistent density have CGs at their physical centers when they are circles, squares, or rectangles. CGs must be calculated for arcs, semicircles, triangles, trapezoids, parallelograms, fillets, and parabolas. Once CGs are located for all components, their individual weights are added and combined to obtain the overall CG for the system.
When calculating gravity moments, bear in mind that they always pull vertically downward from the CGs. Also, CGs move closer to pivot points as objects rise. Spring moments are calculated as perpendicular distances from pivot points to center lines of spring forces passing through mounting points.
Mechanical advantage is the moment arm that is acted on by the machine (in this case a gas spring) to apply force to the machine. When gas springs raise and lower covers such as automobile hatchbacks, mechanical advantage is designed to match the torque requirements of the system with the output force of the gas springs. As with levers, larger mechanical advantages require lower gas spring forces, and vice versa. Mechanical advantages remain constant when associated with components such as handle arms because operators apply tangential forces at fixed points.
Designers must also decide the magnitude and location of gas springs. Short, high-force gas springs mounted near pivot points are less sensitive to force losses than are long, low-force gas springs mounted closer to handles. But, high-force gas springs aren’t always an option. For instance, when raising a plastic cover, 250 lb of pressure from gas springs mounted near the hinges will break the plastic. To prevent this, mount lower-force gas springs away from the hinges. And, as added insurance, always use large mounting brackets on flimsy material. Other design parameters include determining the number of gas springs required (typically two for wide covers), handle loads, operating temperatures and rigidity of the cover.
Mounting
Designers should mount gas springs rod-down when covers are in closed positions. This keeps oil on the seal and provides maximum damping at the end of the stroke. End fittings should rotate freely to avoid misalignment, binding, and side loading, all of which impair operation and durability. Ball-and-socket end fittings are preferred because they direct spring forces through centerlines even when they are offset mounted. Because gas springs should move freely, they should not be used for secondary mounting of wiring harnesses, lights, and other accessories.
Springs should be positioned to permit enough overtravel to prevent overcompression when covers close. Springs should be mounted so doors or lids are fully open when the rods reach internal stops. To avoid large tensile forces in springs, operators should not lean on covers or try to open them further when springs are fully extended. Nor should springs be used as hand-holds.
Installation should not be designed to balance at room temperature because more force is required at lower temperatures. (The operating range of gas springs is typically –40 to 180°C.) Cycle rates should not exceed 5 cycles/min since vibration from flexing or loose latches accelerates seal wear. Avoid nicking, scratching, bending, clamping and painting rods, as these damage seals. Gas springs should not be used in highly corrosive atmospheres, and commercial lubricants attack rubber seals. Gas springs are self-lubricating and require no maintenance after installation.
Gas spring selection Selecting the right gas spring for each application is relatively pain-free. One rule of thumb is to select the largest spring that fits within size constraints, simply to guarantee performance. But, suppose engineers must limit the size of a gas spring while ensuring the spring will hold open a car’s hatchback. The most important equation for determining the gas spring’s size is: Fs,o = Mw,oX fw/Ms,o This equation yields the force required from the gas spring to counterbalance the weight of the hatchback. Designers should add 30 to 50% to Fs,o for a force reserve. This will ensure that the gas spring functions, even at low temperatures, throughout its useful life. Next check spring performance in the closed position using the following equation: Fh = ((–Fw X Mm,o) + (Fc X Ms,c))/Mh When Fs,c > Mw,c X fw/Ms,c, the cover will rise when closed. When the design calls for the cover to hold closed, engineers should readjust mounting points. This can be done by decreasing Ms,c, increasing Ms,o, or both. The cover counterbalances when: Fs,c = Mw,c X fw/Ms,c The cover holds closed when: Fs,c< Mw,c X fw/Ms,c When the design calls for the cover to rise automatically, readjust mounting points. This can be done by either increasing Ms,c, decreasing Ms,o, or both. At any position during rotation of the cover, the spring force necessary to counterbalance the cover weight may be calculated using the general equation: F = Mw X Fw/Ms |