Strain Energy
Strain Energy
(OP)
Hi
Help an old fart here, I have an impact situation where a mass M travelling at X m/s collides with a spring mounted block, I went for the usual energy transfer ie:-
0.5mV^2 = 0.5 Kx^2
Then I started to consider what effect spring preload would have if any?
My final thoughts was that it would not have any effect because we have a certain amount of kinetic energy and that energy needs to be absorbed, yet if the mass hits the spring loaded stop and the spring has a higher preload than before surely this must have some effect.
All the googling I have tried seems only to deal with springs not preloaded or springs releasing energy.
slightly confused
desertfox
Help an old fart here, I have an impact situation where a mass M travelling at X m/s collides with a spring mounted block, I went for the usual energy transfer ie:-
0.5mV^2 = 0.5 Kx^2
Then I started to consider what effect spring preload would have if any?
My final thoughts was that it would not have any effect because we have a certain amount of kinetic energy and that energy needs to be absorbed, yet if the mass hits the spring loaded stop and the spring has a higher preload than before surely this must have some effect.
All the googling I have tried seems only to deal with springs not preloaded or springs releasing energy.
slightly confused
desertfox





RE: Strain Energy
My motto: Learn something new every day
Also: There's usually a good reason why everyone does it that way
RE: Strain Energy
i think spring preload (due to the weight of the block ?) doesn't affect the spring loads.
Quando Omni Flunkus Moritati
RE: Strain Energy
Thanks for the responses
Assume the springs are in line with the colliding mass, what I am trying to workout is will altering the spring preload have an effect on a moving mass travelling towards it and finally hitting it.
If I consider 0.5 x k x x^2 this takes no account of the spring preload, so if I work this energy transfer out it seems to me that the spring is in a free state prior to the impact which is not the case in practice.
RE: Strain Energy
x_final = x_init+sqrt(m/k)*X
Quando Omni Flunkus Moritati
RE: Strain Energy
That was my thought at one point also so in that case:-
0.5 k x^2 (intial) + 0.5 k x^2 (new) = 0.5 m V^2 (moving mass)
do we agree ?
Regards
desertfox
RE: Strain Energy
Quando Omni Flunkus Moritati
RE: Strain Energy
So the initial spring load won't help slow the impact down, at least not the way you've written it.
I was thinking if I have higher potential energy in the spring prior to impact that it would help reduce the impact energy but looking at how you've written it, it adds to the impact.
RE: Strain Energy
but that's not what i posted above !!?? 'cause i think i just did 1/2*k*(x_final^2-x_init^2) = 1/2*m*X^2
and this says x_final = x_init+dx (a fixed amount, =sqrt(m/k)*X)
but i think it's wrong ...
x_final^2-x_init^2 = m/k*X^2
x_final=x_init+delta_x
2*x_init*delta_x + delta_x^2 = m/k*X^2
so that the change in spring compression depends on the initial compression
Quando Omni Flunkus Moritati
RE: Strain Energy
My thinking was preload the spring so it has potential energy, when the mass hits its ressisted by whatever potential energy is in the spring, so I know the kinetic energy of the moving mass and the initial potential energy of the spring, now if I subtract the potential energy of the spring from the kinetic energy of the moving mass the difference would be how much more the spring needs to compress.
I'm not sure what your last post is trying to say, so I put what I think in words in the above paragraph.
regards
desertfox
RE: Strain Energy
1/2*k*(xfinal-xinit)^2 = 1/2m*X^2
Quando Omni Flunkus Moritati
RE: Strain Energy
So back to the preload does not help reduce impact. if I rearrange your formula then the potential energy in the spring adds to the impact energy.
RE: Strain Energy
and it shouldn't've been ...
1/2*k*(xfinal-xinit)^2 = 1/2m*X^2 ... instead
1/2*k*(xfinal^2-xinit^2) = 1/2m*X^2
math fails, see sketch ...
the two hashed areas are the same size, the same energy, but one is narrower than the other ... the preload reduced the compression due to the impact.
Quando Omni Flunkus Moritati
RE: Strain Energy
Mike McCann
MMC Engineering
RE: Strain Energy
Quando Omni Flunkus Moritati
RE: Strain Energy
Mike McCann
MMC Engineering
RE: Strain Energy
Quando Omni Flunkus Moritati
RE: Strain Energy
Mike McCann
MMC Engineering
RE: Strain Energy
Thanks for the responses!
Msquared48 yes I totally agree, that the moving mass needs enough force to overcome the spring preload otherwise it stops.
That's why used the potential energy of the preload spring and subtracted It from the moving mass kinetic energy, then whatever kinetic energy was left had to be absorbed by the spring.
Rb1957 looks like we agree now!
Regards
Desertfox