Tech question on shock absorbers

[bertieg]

Technically im talking about aircraft undercarriages here, but the principle is the same

i have a practice question for an exam tomorrow. the question asks what the function of the oil in a shock absorber is. is it to damp the rebound, or absorb the landing shock?

the mock exam answer sheet is saying it is to absorb the landing shock, but is that not the job of a spring/air? the oil is there to slow down the movement of the shock?

anyone knowledgeable enough to give an opinion?



cheers

 

[Chip-mk1]

In the context of cars, the shock effects travel in both directions, the spring is a component too, but the oil resisting moving through the valving inside the shock also provides a resistance to movement.

 

[bertieg]

so you would agree in that the oil's job is to slow down the movement of the shock absorber, not to absorb the initial impact? of course the oil doesnt just damp the rebound, it would damp the initial compression too

 

[Chip-mk1]

so you would agree in that the oil's job is to slow down the movement of the shock absorber, not to absorb the initial impact? of course the oil doesnt just damp the rebound, it would damp the initial compression too

Yes totally, the oil slows the movement, even if you completely removed the spring there would still be compression resistance, but its more the RATE of change that it controls.

What I mean by that is if you turn into a corner and then hold a constant radius (laps of a roundabout for example), the spring will dictate how far the car rolls (total spring rate inc roll bar if fitted) but the shock effects how quickly it allows it to move to this point.

 

[bertieg]

Yes totally, the oil slows the movement, even if you completely removed the spring there would still be compression resistance, but its more the RATE of change that it controls.

What I mean by that is if you turn into a corner and then hold a constant radius (laps of a roundabout for example), the spring will dictate how far the car rolls (total spring rate inc roll bar if fitted) but the shock effects how quickly it allows it to move to this point.

thats pretty much confirmed my thoughts, and proved the exam paper wrong!


cheers

 

[Chip-mk1]

Im not sure how transferrable the car example is to landing gear on a plane though TBH, maybe the oil on that functions more like a spring on a car as it isnt relying on a spring to keep constnatly lifting it up and down etc..

 

[bertieg]

Im not sure how transferrable the car example is to landing gear on a plane though TBH, maybe the oil on that functions more like a spring on a car as it isnt relying on a spring to keep constnatly lifting it up and down etc..

the question is quite vague, and doesn't specify the exact type of undercarriage leg, but a lot of the principles are the same. compressed air would be used in place of a spring to absorb impact

 

[Fred@BTM]

any use? http://www.fzt.haw-hamburg.de/pers/Scholz/dglr/hh/text_2008_06_05_LandingGear.pdf

 

[Cup_Phil]

You can't transfer how a car suspension works to a plane

AFAIK planes don't use springs on the suspension - it's just a giant piston with oil and the friction of moving through the oil absorbs the impact

I would have thought a spring is a very bad idea when landing a plane on perfectly smooth tarmac as it would just bounce the plane back up. You just want it to absorb the impact.
Springs on cars are used to absorb uneven road surfaces and the spring pushes the wheel back down to the surface after the bump.

 

[James28]

As Phil said, Planes don't use springs though do they? Therefore only the damper can absorb the impact.

 

[Chip-mk1]

From what he is saying they use compressed gas in place of the spring?

 

[sprintcup]

I don't know if this helps but we use hydraulic buffers in the lift industry for lifts traveling at over 1meter per second, the buffer is a fail safe point to bring the lift car fully loaded to a controlled stop for it's rated speed, the oil is used to absorb the landing shock and stop the lift at under 1g, but has a minimum and maximum operating load.

Regards Russ......

 

[Spee]

Aircraft landing gear uses oil and nitrogen, no springs involved. Although some smaller stuff, puddle jumpers, rotary and older stuff use a spring in the process.

 

[Sam@NP]

Whats the bolt spacing on a 777? Got some H&Rs that might fit.