Pb of elementary mechanics on the springs

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Grelinette
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Pb of elementary mechanics on the springs




by Grelinette » 08/10/14, 14:45

Just to change the subject a little, I come back to my experiences of nags and submit a little mechanical problem on the springs.

(You will have understood my post has necessarily a link with the horse racing! Note however that the following reflection can in my opinion be interesting for all other traction systems tlq car, truck, tractor, bicycle, caddy, etc. ...)

explanations:

When I use a horse for work, I try to reduce the pulling jerks (collar hit), but also the braking jerks (when the horse brakes suddenly or stops dead).

In the first case, the most common, when a large pulling force is required of the horse (example moving a heavy log on the ground), it will gather its forces and give a violent blow of shoulder to put the load in movement then the tow in stride.
This jerk causes the wear and breakage of equipment, even injuries to the horse, not to mention the shaking felt in the carriage or on the trinqueballle.

Conversely, in the 2nd case, especially when driving and especially when going downhill, when the horse brakes or stops dead, the towed load pushes it under the effect of the kinetic energy acquired by the movement.


The idea is therefore to attenuate (cushion) the jolts in tension and in restraint (compression), while preserving the effectiveness of the gesture, by inserting a spring (pressure or traction, with or without hydraulics) between the tractor (the horse) and the load (carriage, tools or log) to make this traction link flexible and comfortable .
Image

So much for the explanations.


Now, to get to the heart of the matter, here is the mechanical problem posed:

What mechanical system or assembly using a spring either compression or traction, allows operation (amortization) both in Compression and in Traction ? (Am I clear?)

Other formulations of the problem:
How to use a compression spring Image to absorb both compression and traction,

and conversely, how to use a tension spring Image to absorb both pull ups and compressions? (Is that clearer ?)

And for the most insightful: which unique system can use either one or the other of the types of springs (pressure / traction) to absorb both compression and traction?


The first who offers the best system wins a sporty ride in trinqueballle ! : Mrgreen:
(nb: I already have 2 interesting proposals!)

PS: Does anyone know the difference in mechanical characteristics between a compression spring and a tension spring?
No one has yet been able to explain the mechanical differences to me with precision! Only a spring manufacturer told me that the compression springs were more suitable for forces in motion (e.g. car shock absorbers), whereas the traction springs were more suitable for forces that varied little or not at all (e.g. tension of a tarpaulin, weighing scale, etc.)… Curiously, certain springs for pool covers or for boat mooring are compression springs used in traction!…

Image
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by dirk pitt » 08/10/14, 16:09

first thing: a spring does not dampen! it "defers" the diffusion of energy.
in other words, it stores excess energy but releases it very quickly in general, causing oscillations.
a car mounted only on a spring is non-convertible.
so you have to amortize by dissipating a part of the energy.
it is the role of the shock absorber: generally with oil.
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by Ahmed » 08/10/14, 16:25

The springs working in compression are more resistant and more durable than those used in extension (if only because the first are limited in their stroke and do not risk exceeding their elastic limit).

A good assembly would consist in using two springs around the traction shaft, made integral with the latter by a fixed central support ring.
The shaft slides between a clevis secured to the towed tool, each cheek of which provides an external bearing face for each spring.
It is necessary to provide a slight pre-compression of the springs during assembly in order to eliminate the play (easy: it suffices to compress a few turns with a wire which will be cut, the assembly carried out).
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by Janic » 08/10/14, 16:26

good morning
dirk pitt has answered well. For slowing down or braking, you should mount a system that slows down or brakes that is found on caravans. For starting it is different, you would need a coil spring (braking) which would restore its energy at the start (A KERS) but it is rather expensive and a gas plant!
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by Did67 » 08/10/14, 19:01

1) Agree with Dirk. A spring does not solve anything. He "leaps", that's all. And indeed, without a shock absorber, it easily oscillates.

It is the friction which attenuates the oscillations, therefore which dissipates the energy ...

[I have fun, one day, take a ride with a 2CV without shock absorbers; but with the springs, of course! It's as crazy as riding a Mustang!]

2) A spring is never more than a torsion bar wound on itself to save space.

A torsion bar is, if you want, a big metallic spaghetti that you can twist. It is the periphery of the spaghetti that works (as elsewhere on a leaf spring).

[For those who knew the 4L, it had two long crossbars, under the chassis, placed just in front of the rear wheels; these bars replaced the springs found on other cars; suddenly, on one side, the wheel was slightly more forward than the wheel on the other side because the two bars were parallel, but offset! You can verify !]

It is therefore based on the principle of a long metal bar that is twisted, within the limits of elasticity (which means that when you let go, you return to the initial position). If one twists more, one crosses the elastic limit, and the bar does not return any more to its initial position.

A spring can only work on compression if the turns are not contiguous ... If it is done correctly, the turns touch before the elastic limit is reached! So this spring is "protected" against too much deformation.

A spring works in traction, but here you have to be careful not to exceed the elasticity limit (some springs are embedded in a support that "blocks" the traction at a certain level, to avoid irreversible damage).

But apart from this question of contiguous turns or not, on the physical principle, no difference! To my knowledge.
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by Did67 » 08/10/14, 19:05

So, I forgot my answer: you don't need a spring, but a shock absorber.

Find a sufficiently strong shock absorber! [look for the cmions!)
a
At startup, it will stretch to its maximum limit, and will not be butted. During a slowdown, it will go to opposite stop while dampening the movement. AND rebeliotte at the next pull.

But apart from absorbing the jolts, it will dissipate energy, not supply it!
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by Flytox » 08/10/14, 19:35

A spring with variable pitch (Deliberately nonlinear behavior) will help you to better pass the shoulder or butt blow. Its "range of use" is greater between small and large efforts (before hitting the stop). But in any case you will not be able to free yourself from a shock absorber.

http://fr.wikibooks.org/wiki/Les_ressor ... mportement

For the depreciation, you can find in the scrapyard. For cars, they strongly dampen on rebound and little on compression. You would need 2 identical mounted head to tail and halfway to rest, since you have to amortize in both directions.
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by Ahmed » 08/10/14, 20:09

The damping function is unavoidable for high frequencies which are likely, indeed to generate phenomena of resonance, however, in the particular case which interests us here, I would be less affirmative.
It is simply a matter of clipping the blows of the towed assembly: in normal times this device would not intervene and its operation would remain punctual, as a kind of security.

The only spring with a certain degree of damping, by construction, is the leaf spring (some modern models are equipped with ferrodo), which is why it has long been used on horse-drawn vehicles.
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by Grelinette » 08/10/14, 21:27

Thank you for your quick answers.

Certainly, as Dirk Pitt points out, a spring restores the energy it has stored during compression or stretching, but as Ahmed says, the phenomenon of resonance is unlikely in this case (in my opinion given the slowness of the movement).

A priori, a simple spring would already attenuate violent jolts ("hard shock") and should therefore not cause resonance.

To stay in animal traction, in the past we used systems with springs like this:
Image
Note that there is a spring in the center and 2 at the ends of the lifter (the lifter having the role of distributing the successive forces of the shoulders in motion at the central traction point).

or the more current one:
Image

The use of springs in animal traction, mainly for agricultural work (eg skidding) is debated in the community, but the users of springs are firmly convinced of their effectiveness and their usefulness.

I looked carefully if there were measurements and data (comparative curves) to compare the traction with or without springs, but I found nothing.

On the other hand, I saw that some bicycle trailer attachments had a small shock-absorbing spring on the drawbar, probably to improve traction comfort or the comfort of the passenger (child).
Image

... and comfort level (without resonance), there are also these springs:
Image

I also found a car tow bar with spring, but I have the impression that it only works in one direction: compression, so when braking:
Image

That said, if a system allows the operation of the spring in compression AND in traction (Ahmed also offers a functional solution), nothing prevents using the system with an oil damper type mountain bike shock absorber.

The first question is whether this "damped link" is beneficial for traction.

I would tend to answer YES already for the comfort of the horse who does not feel a "stop" when starting,

then again YES for the traction efficiency at start, because this damped connection allows the horse to start moving even before the towed mass moves: it is probably not much, but the horse being in motion, it acquires already makes additional kinetic energy favorable to traction.

To give an image: if I want to move my broken down car, I think I will be more comfortable and more efficient by pulling on a relatively flexible rope (not an elastic band), rather than a rigid chain.

What do you think ?
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by moinsdewatt » 08/10/14, 21:54

Ask Hic,

after being infatuated with magnets which produce energy it will perhaps be recycled in the subject of the nuts which also produce energy.

: Lol:
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