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Old Apr 4th 2018, 02:11 AM   #11
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Originally Posted by SpeedEC View Post
Hi benit13,
Yes. I have to use stretching mechanism in order to reduce the impact on the object from the Vehicle's Force. For example, providing extension springs in the Vehicle in order to give cushion to the object to protect it from the 1553 Kg-m/s sudden force while picking the Object. I need to calculate the time to provide the cushion using spring for that. Any idea?

Vehicle will continue its speed at 80KMPH approx (22.185 m/s) for few seconds. My ultimate requirement is to protect the Object from the force of the Vehicle using Stretching and Other Mechanisms.
Just to be specific, the quantity 1553 kg m/s is the momentum exchange (not the force). Because the momentum exchange occurs over one second, you also have a force of 1553 N.

The idea of increasing collision time is routine in the design of car safety features. Crumple zones, seat belts and airbags are all designed to increase the collision time during an impact and therefore reduce the impact force on individuals inside the car.

Unfortunately, these measures usually end up in a ruined car. Car features designed to protect the car (e.g. bull bars) are usually very dangerous for the target.

Collision time is usually very difficult to calculate because of the complicated nature of stress and strain, but if you have a situation where you're using springs, you could probably set up a problem using springs, calculate the force balance using Newton's laws and then calculate the time taken for the springs to compress. You'll need to look up Hooke's law
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Old Apr 4th 2018, 08:53 AM   #12
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Momentum exchanged on the Object = 1553 kg m/s

Force applied on the Object:

at 0.005 sec = 310600 N;
at 0.05 sec = 31060 N;
at 0.10 sec = 15530 N;
at 0.50 sec = 3106 N;
at 1 sec = 1553 N;

How can we save the Object from such a brutal force? Is it enough to distributing the force through out the Object's surface using Cushions like Airbag, etc? Or combining Spring action to reduce the impact?

Any suggestions?

thanks,
pmk
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Old Apr 4th 2018, 10:06 AM   #13
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Originally Posted by SpeedEC View Post
Momentum exchanged on the Object = 1553 kg m/s

Force applied on the Object:

at 0.005 sec = 310600 N;
at 0.05 sec = 31060 N;
at 0.10 sec = 15530 N;
at 0.50 sec = 3106 N;
at 1 sec = 1553 N;
Yes. You got the right idea!

How can we save the Object from such a brutal force? Is it enough to distributing the force through out the Object's surface using Cushions like Airbag, etc? Or combining Spring action to reduce the impact?
Crumple zones increase the duration of the collision so the momentum exchanged to the middle and rear of the car is less than the front of the car.

Air bags increase the collision time for the passengers because they are consistently slowed by the airbag as they lurch forward in their seat during the sudden deceleration.

Seat belts have a similar role as airbags because they are a little bit stretchy during collisions, but they are mainly designed to prevent passengers lurching forward too quickly and colliding with the seats or dashboard in front of them.

Yes, spreading out the force over a wide area is also a good idea since the actual damage sustained from a collision (strain) is based on the stress, which is like a pressure (force per unit area).

I assume springs aren't usually used because you don't want to them to spring back, but you could maybe have a very stiff spring which stores a lot of potential energy. I'm not sure whether it's a feasible prospect or not to be honest.

However, someone did post an interesting video recently of a heavy cart colliding with a glass bottle attached to a wall which didn't smash because the momentum was exchanged with a pin attached to an installed fly wheel in the vehicle. The pin pushes back and converts the linear kinetic energy to rotational kinetic energy in the flywheel over a long collision time (~1-2 seconds), reducing the force considerably.

Last edited by benit13; Apr 4th 2018 at 10:10 AM.
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Old Apr 4th 2018, 03:59 PM   #14
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When a force is applied suddenly to one side of an object
it takes time for the stresses caused by that force to even out through the object.
So you have very high stresses close to the point of application of the force,
and little or no stress away from the point of application.
This stress gradient can be more damaging than the stress itself.
Applying the force more gradually allows the stresses to distribute more evenly through the object.
This is where the "jerk" (rate of change of acceleration) comes in, allowing the acceleration to build to a maximum and then reduce down again,
thereby allowing the stresses to be distributed evenly through the object.
Springs and airbags etc. help to reduce the jerk.
It is the jerk that generally causes whiplash injuries in crashes, rather than the actual acceleration (deceleration).
Have you any estimate of the sort of force your object can withstand?
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Old Apr 5th 2018, 10:07 PM   #15
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Have you any estimate of the sort of force your object can withstand?
Object is nothing but Human Body as I said earlier. From the google, it can withstand against up to 4000 N (I believe, unit area of force).

but you could maybe have a very stiff spring which stores a lot of potential energy. I'm not sure whether it's a feasible prospect or not to be honest.
If we use stiff spring, then we can save potential energy but definitely NOT the Object. I am using spring (Elongation - Extension spring) for reducing the impact on the Object gradually.

you don't want to them to spring back,
We don't need to bother about whether Object come back to initial position or not. Our primary focus is to save the Object from the frontal impact only.

from the Google, I found the time taken for deflating the airbag after hitting by the Driver head:
The time it takes for the airbag to finish deploying is approximately 0.03 seconds. That deployed airbag will deflate and the total time it takes is about 0.1 seconds.
I hope, Airbags Cloths (nylon) used in cars have holes in it. When head collides on it, it starts release the pressure thru' these holes. But, I am thinking to use ordinary airbag without any holes and won't deflate after collision as cars Airbag do. But, my airbag will act like a cushion simply.

I have planned to use both Airbags (1st stage protection) and Extension Spring (2nd stage protection).

Suggestions welcome.

thanks,
pmk

Last edited by SpeedEC; Apr 5th 2018 at 10:30 PM.
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Old Apr 6th 2018, 02:37 AM   #16
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A (healthy) human can probably withstand up to 10g with nothing worse than some temporary modest discomfort.
Up to 30g with some major discomfort (e.g. bruising of the internal organs).

I would suggest that, for ease of use, 10g is near enough 100m/s/s for this estimate.

The main issue is to stop the limbs and head flailing about.
As long as the entire body moves as a single unit it is surprisingly robust.
Spread the load as evenly as possible over the entire body.

Coming back to the Jerk, the muscles in the human body will automatically adjust to compensate for an acceleration.
However they take time to adjust,
which is why a sudden onset of acceleration will cause more injuries than the same maximum acceleration bought on gradually.
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Old Apr 6th 2018, 03:35 AM   #17
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As far as my requirements concerned, I got 2 answers out of 4.

Code:
1. Human/Object withstand  capability against the g-force.
2. The Force acting on Human/Object when pick/hit by the Vehicle
So, It is time to calculate the time requirement to reduce the force using air(bag) suspension and Spring stretching using Extension Spring.

Code:
Case 1: air(bag) suspension

Time required to extend the force = 1 second;

Airbag size : 3.25’ x 2’ x 2’;
Airbag Volume = 13 cubic foot;
                    = 28.3169 litres / cubic foot;
                    = 13 x 28.3169 = 368 litres;
We should release the air volume when hitting the Object/Human = 184 litres in 1 second through air valve. Keep the remaining air in the Airbag for securing Object/Human.

Code:
Case 2: Stretching Mechanism using Extension Spring
Time required to extend the force = 1 second;  // same as air(bag) suspension
[So, Impact could be reduced to half of actual impact]
As long as the entire body moves as a single unit it is surprisingly robust.
Spread the load as evenly as possible over the entire body.
Yes. The proposed design like this.

Seat belts have a similar role as airbags because they are a little bit stretchy during collisions, but they are mainly designed to prevent passengers lurching forward too quickly and colliding with the seats or dashboard in front of them.
Yes. the proposed spring stretching mechanism is like this.

If you have any idea about my proposed system, please let me know.

thank you both Woody & benit13 for providing timely help.

pmk

Last edited by SpeedEC; Apr 6th 2018 at 03:40 AM.
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