# Re: spring/shock absorber has "reactance"?

They say that the mechanical analogues of capacitors are springs, and
>of inductors are shock absorbers.
"They" are wrong.
Like shock absorbers, both capacitors and inductors can absorb "shock" (see
power supplies) and
like springs, both can store energy for later release (see resonant
circuits) .
In lay terms -
There are several elements in linear mechanical systems:
-mass (qty of "material"),
-spring (converts kinetic to potential), -damping (converts kinetic to heat and
deducts it from the mechanical sum),
-kinetic energy (energy from motion), and -potential energy (energy from
position).
There are several analogous elements in electromagnetics:
-charge (qty of "material"),
-capacitor and inductor, (converts kinetic to potential),
-resistor (converts kinetic to heat and deducts that energy from the electrical
sum)
-current (energy from motion of charge),
and
-voltage (defined as the potential energy between two positions in a field)
But do springs/shock absorbers have any kind of frequency-dependent >behaviors?
not as theoretical elements, nor within their "normal" operating range.
However, there are phenomena associated with springs and shocks that can
affect natural frequency over time. One classic is the automobile front
suspension, where loss of spring constant from fabrication-stress relaxation in
the coils (about 40% loss over two years) causes less-tight-handling, which can
be compensated back to near original by using a larger than original damping
constant (heavy duty shock).
(You can work it out from the equation fx^2 + cx + d, and there are a couple
of papers from the 70s on it if you are not familiar with frequency equations)

In the real world, as the frequency increases, the mass of the spring itself
enters into the system as a separate equation and creates tertiary resonance as
well as effecting the original theoretical spring-mass assumptions (f=ma).
There are a couple of charts I have seen listing corresponding elements for
linear mechancial, rotary mechanical, fluid power, air power, and
electromagnetic/electrical, and their equations.
Control engineers find them fairly useful.
hope it helps
Loading thread data ...
But both mechanical and thermal systems, translated to lumped electrical equivalents, can be dynamically modeled with Spice. If you're modeling something like an electronic servo system and its motor and loads, you may as well toss the whole system into the electrical domain; you're not going to have much fun moving all the electronic parts into a mechanical simulator.
Nonlinear isn't difficult. It's distributed or diffusion models that are harder to do with Spice.
John
Exactly. The mass and shape of the wheel determine the moment of inertia of the wheel. This can be analogous to inductance. The spring, analogous to capacitance. The combination forms a 'tuned' circuit that oscillates at a resonant frequency and requires only a little energy input to make up for frictional losses. Much like how a tuned circuit draws little real power from the supply to make up for resistive losses.
The escapement mechanically gets a 'nudge' from the pawl system that it controls as it swings through the center point. Electronically, this is similar to having a pulse of power applied as the current through the inductor approaches maximum.
daestrom
On Sat, 22 Jan 2005 11:18:16 +0000, Airy R.Bean top posted:
I think he was referring to transfer function, not mechanical alignment.
Anyway, what about lever-type shock absorbers?
John Larkin wrote in news: snipped-for-privacy@4ax.com:
Well, we can incorporate FEA models into our suspension design programs, so diffuse systems are easy. And I do have the antilock and stability control computers simulated in or interfaced with my suspension program. So that's the control side sorted out.
So, what is the electrical analogue of static and dynamic friction?
Cheers
Greg Locock
Dear Greg Locock:
...
I don't think electronics has a non-active analog. A diode has a threshold voltage, below which very little current flows. If the diode is formed of germanium, this is 0.3 volts. If silicon it is between 0.6 and 0.7 volts.
David A. Smith
Dear Greg Locock:
Fluorescent light fixtures require high voltage to fire, and lower voltage to sustain fluorescence once lit.
David A. Smith
As late as 1970-odd, most of the complex dynamic calculations were carried out using analogue computers. It is possible to build an electronic circuit that efectively simulates a friction system - I did a Master of Engineering degree based on an analogue computer model of a steam turbine and governor, looking specifically at nonlinear and frictin effects.
From memory, a LOT of the dynamic problems thrown up by the Saturn and similar early space missions were sorted out on electronic analogue computers ...
Bruce.
Draw the speed vs force diagrams and you will see that force is not proportional to speed. Dynamic friction is analogous to a zener diode producing a pd of V in series with an EMF of -1/2 V.
Franz

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