Mechanical, fluid-power, and elastomeric vibration dampers

Shock and vibration-damping technologies safely decelerate loads in motion systems. Pictured left to right are a heavy industrial shock absorber, TUBUS profile damper, PET shock absorber, MAGNUM industrial shock absorber, and FYN series rotary damper — all from ACE Controls.

Vibration in industrial machinery often originates from imbalances inherent to motors, gearboxes, and other turning component that excites natural frequencies. Dampers in this context are components that counteract and often isolate the vibration source from the rest of the design or machine — to prevent its propagation of mechanical oscillation.

Passive vibration mitigation includes passive isolators — such as closed-cell foam slabs, metal coil or wave springs, wire rope isolators (common in military applications) and rubber machine mounts. These reduce system natural frequency to less than that of excitation frequency… though to be clear, traditional springs and rubber have near-zero damping capabilities.

Precision passive isolators include negativestiffness isolators, which decrease system natural frequency upon loading via a kinematic linkage of beam columns and springs … typically stacked in series for tilt, horizontal, and vertical motion.

In fact, many isolators do impart a small amount of damping. Some (including wire-rope isolators) also attenuate shock impact at sudden move ends, hard stops, and collisions without recoil — on conveyor e-brakes or axis stroke ends, for example.

In contrast with isolators, passive dampers (such as material slabs and mechanical linkages) change the kinetic energy of vibration into heat. Passive damping usually employs viscous fluids, viscoelastic materials, piezo elements, or simple magnetics.

Active dampers include an array of electronically controlled force cancellers that employ a power source, sensors, and actuation to counteract vibration with disruptive interference … out of phase with the source vibration. These include:

• Shocks containing magnetorheological fluid that stiffen to solid with controllable yield strength under an applied magnetic field in milliseconds

• Powered piezoelectric elements affixed to the design frame at critical points to execute active countermotions and address vibration (usually detected by acceleration sensors) and under the control of a DSP.

Regarding passive vibration damping ... those in mechanical formats include tuned mass dampers that affix to machinery or structures to damp narrow frequency bands of vibration. Other options for passive mechanical damping are friction and piezoelectric action prompted by simple resistive shunt.

Passive vibration mitigation in fluid-power formats include hydraulic mounts, air springs, and bladders to isolate and damp industrial machines and other moving equipment. Passive air tables are another iteration, though increasingly displaced by other technologies. Gas springs (of the piston-type plunger design) act as kinematic holds.

Many pneumatic dampers have a similar structure — with a cylindrical chamber containing a piston and compressed air behind it — though work on moving axes. Oil dashpots employ a contained volume of fluid to resist motion with viscous friction to damp (though not isolate) vibration.

Now consider passive vibration mitigation in elastomeric material formats — which offer simplicity in format and application. Foam slabs isolate well though lack durability. Rubber and neoprene excel at isolation but not damping. In contrast, another option is viscoelastic material from Sorbothane Inc. that excels at vibration and shock control — absorbing up to 94.7% of the latter.

The material is classified as viscoelastic, as it exhibits both viscous and elastic characteristics when subject to deformation. A high delta tangent — also called the loss factor or damping coefficient — means an out-of-phase time relationship between shock impact or vibration and force transmission. The viscoelastic material absorbs more than half the energies at 1 to 30,000 Hertz for powerful vibration damping … shedding the energy as heat (through hysteresis) and directing remaining energy perpendicularly — 90° out of phase from the vibration or shock source.

The Sentinel 1 onboard monitoring system for shock absorbers imparts IIoT functionality to energy-absorption technologies. From ITT Enidine, the new microelectronic solution lets plant operators get real-time energy management and performance monitoring on critical machine sections. The Sentinel 1 uses energy harvesting for power to allow wireless operation … and has a reach of 2,000 ft to communicate with PLCs and other automation hardware in the facility.

Special case of gas springs

Gas springs, also called gas dampers, tension springs, or gas-pressure springs depending on the setup and context, are compressed-air or oil cylinders that install in motion designs to damp forces and return kinematic linkages (and more complicated assemblies) to default positions. Gas springs work through a piston on the end of a rod that protrudes from a steel cylinder body; usually compressed gas (often nitrogen) within the cylinder exerts force on this piston to reassume and maintain set positions. Nitrogen is common here because it’s inert and nonflammable. In such designs, oil or grease between the piston and other contacting parts minimize friction.

In fact, the small amount of oil in these gas springs serves another function — to further damp and gently decelerate gas springs during full extension or compression. Some setups even include a fine hole in the piston for damping that’s still slower than with other designs; such slow-damper springs are common on safety gates and doors.

In contrast, extended-reach gas springs usually leverage telescoping mechanisms pairing multiple cylinders on one rod; then the smaller cylinder extends from within the larger cylinder. Consider one particularly long-stroke application: Passive heave compensators — systems on ships or offshore oil-rig systems that reduce the effect of waves on engineered structures — use gas springs with strokes to many meters long.

Still other gas-spring applications include those for medical beds and hoists; industrial equipment such as machine-tool presses; off-highway and automotive equipment for hatches, hoods, and covers; office equipment and furniture; and general strut and support applications. Fast-acting gas springs find use in weaponry and aerospace design. Specific variations include gas springs with standard or fixed-height cylinders; spindle-only designs; and cable, return, adjustable autoreturn, nonrotating, stage, and multi-mode cylinders.

No matter the iteration, gas-spring extension force — a value that usually ranges from 1 to 5,000 N — depends on piston-rod cross-section multiplied by fill pressure. Manufacturers commonly express extension force with two values — for rod extension and rod retraction — at normal ambient temperature and with the piston rod pointing downward. (Note that typical ranges are only those most common; some gas-spring applications in heavy industries use gas springs delivering several hundred-thousand Newtons cases.) Other gas-spring definitions include two pull-in forces — at rod extension and rod retraction — and overall friction force. These values depend on the gas spring’s gas and damping-oil volumes. Various nozzle orifices and oil quantity allow control of push-out and push-in speed.

If design parameters are unknown, look for manufacturers capable of prototyping — especially for designs requiring an exact force that’s hard to pre-estimate — as in lifting a frame in a set time, for example. Here, some manufacturers sell prefilled cylinders sporting bleed valves. Then installers can bleed gas from the cylinders after system setup to get the correct forceacceleration actuation profile.

The only caveat here is that if too much gas is bled, the assembly will need a new spring. That’s why OEM-level quantities of gas springs justify pre-engineered cylinders with preset pressurization. Or gas springs can offer full in-design adjustability via bleed valves and movable-endstop pressurization mechanisms, Bowden cables, knobs, and more. Some emergency-use gas springs also employ gas-generator cartridges that resemble those in airbags.