Back to basics: What's the difference between a dashpot and a snubber?

Dashpots and snubbers are two types of pneumatic device used for controlling the movement of a load — typically for the purpose of controlled deceleration or motion damping. Although they can be used to control rotary motion, the more common uses for dashpots and snubbers in industrial applications involve the control of linear motion mechanisms, such as solenoids or spring-loaded devices.

Both dashpots and snubbers contain two primary parts: a glass cylinder with a polished bore and a precision piston, often made of a low-friction material such as graphite. And both devices operate by forcing ambient air through an adjustable orifice at a controlled rate. But despite these similarities, dashpots and snubbers are designed for different applications. Case in point — a dashpot is best for applications that require accurately controlled force or velocity, whereas a snubber is best used for end-of-stroke damping where accurate control of impact is required.

A dashpot has a connecting rod that joins the load to the piston and provides control throughout the stroke, either by extending the connecting rod and piston (in pull mode) or by causing the connecting rod and piston to retract into the cylinder — in push mode. Dashpots can also control motion in both directions.

Both operating modes — push and pull — rely on the change in air pressure inside the cylinder. In push mode, as the piston moves farther into the cylinder, the air inside the cylinder is compressed, causing the pressure to rise. In pull mode, as the piston retracts out of the cylinder, the pressure inside the cylinder falls and creates a partial vacuum.

Dashpots in pull mode work best when the movement needs to be controlled for the entire stroke. This is because the air column is short when the motion begins and the damping force increases quickly, providing controlled motion after just a short amount of travel. It’s also important that a dashpot used in pull mode is returned to the starting position — otherwise, if the piston isn’t fully retracted, the air column inside the cylinder will be relatively long and the damping force will be slow to increase.

A dashpot in push mode works best for applications that require a reduced impact at the end of travel. In this mode, the pressure inside the cylinder rises as the column of air becomes shorter.

With push damping, the damping effect is lower at the beginning of the movement. This is because the column of air is relatively long and requires some amount of movement to create sufficient pressure to provide damping. With push damping, there can be a noticeable effect of the load bouncing on the air column (sometimes referred to as an air spring) midway through the stroke, as the pressure rises and begins to dampen the movement.

Also referred to as pneumatic shock absorbers, snubbers differ from dashpots in two ways: The piston is not attached to the load being controlled, and damping occurs in compression only — as for push damping.

When the load contacts the piston rod of a snubber, the force of the load causes the piston to move, compressing the air inside the cylinder. This compression of air provides a controlled deceleration, with the amount of deceleration depending on the magnitude and speed of the load and the adjustment of the orifice.

If the damping is too high, the load will bounce on the air column (air spring effect) or bounce on landing. If the damping is too low, the load will land with too much force, causing damage to it or to other equipment. Although snubbers provide only compression damping, their accuracy in controlling movement and reducing shock is better than that of dashpots.

OVERVIEW OF VIBRATION MITIGATION IN INDUSTRY

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 negative-stiffness 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 wirerope 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. These also include oowered 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.

Image courtesy of ACE Controls

Passive vibration mitigation in fluidpower 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 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.

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 hundredthousand 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 pushin 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.