Dampener/Buffer

[1337engie]

Hi all,

I am currently designing a dampening/buffer device to stop a horizontally moving mass of 35kg travelling between 30km/hr and 100km/hr.

I have been asked to explore the possibility of using thick hollow tube with a solid tube of similar diameter fitted inside of it. The solid tube is to be sealed in using o rings or similar such that the moving object will contact the solid rod and move into the hollow rod - creating pressure (like a bike pump). A pressure relief valve is to be fitted into the hollow rod to vent the pressure created to allow the solid rod to move.

Does this sound feasible? Are there other options to recommend? I have calculated the pressure created inside the cylinder, should the relief valve vent at a pressure well below this? Are there other issues to be considered?

Thanking you

 

[btrueblood]

First things first - is this a homework problem?

Does this sound feasible?

Yes. Similar devices are used in crash barriers and bumpers all over the place, or at least were when 5-mph bumpers were mandated for US cars. They are also used in the suspensions of most automotive vehicles, under the improper term "shock absorber".

Are there other options to recommend?

Depends. Must the device be re-usable/resettable? Typical highway crash barrier "decelerators" nowadays are just big plastic drums full of water. Cheap to replace and refill. Truck runaway ramps are big pits full of gravel, or porous/frangible concrete. A stack of foam blocks might be enough to cushion the object if the deceleration rate is not that critical. Human stunt people are decelerated using air bags with blowout panels. Aircraft are decelerated on aircraft carriers by snagging a cable that is tied to a piston-and-cylinder arrangement much like what you describe. As a young lad in university, we decelerated projectiles travelling at Mach 1 to 5 using metal turnings from the lathe contained in what was basically a 55-gallon drum (very little would typically be left of the projectile, but I believe we never punched a hole thru the concrete walls).

I have calculated the pressure created inside the cylinder, should the relief valve vent at a pressure well below this?

Well, the valve needs to vent to prevent blowout of the o-rings, and/or rupture of the container. Simulataneously, the venting rate will control the de-acceleration rate of the moving object, which presumably must be controlled to some limit. The venting rate of a relief valve depends on the pressure...which in your case is also the controlled variable...sounds like an optimization problem, no?

Are there other issues to be considered?

Keeping the two rods aligned if hit by an off-angle impact would be one. Compressibility of the fluid, deformation of the container, dynamic water-hammer effects on the relief valves. Whether seals will work repeatably at the velocity/pressure you intend to use them at.

 

[gruntguru]

This type of problem (very similar to a FSAE impact attenuator) usually requires maximum energy dissipation with minimum peak-acceleration and minimum displacement. To achieve this, the force will ideally be constant throughout the deceleration. With a compressible fluid like air, achieving constant force will mean pre-charging the cylinder to the relief-pressure, so that pressure relief occurs from the beginning of the impact. Be aware that any residual volume will facilitate rebound. Use of a single stage cylinder will mean a damper at least twice as long as it's stroke. This can be improved with multi-staging or using an air-bag instead of a cylinder.

Engineering is the art of creating things you need, from things you can get.

 

[kingnero]

Why are you designing this - as it already exists in commercially available units, in much sorts and variations that you are bound to find one that meets your criteria?

Although its fluidum is usually oil, (think of a standard shock absorber), they do exist in air models as well, eg. for hygienic reasons.