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rotate
11-09-2007, 01:31 PM
This is probably a simple question. If torque is applied to a shaft, then there must be some measurable twisting of the shaft. Is this correct?

More generally, is it correct to say that any ridgid body where force is applied must physical deform?

I'm trying to wrap my mind around the fact that rigid objects like steel even under very low pressure actually do deform albeit very small and perhaps even below the level that we can measure.

Asquith
11-09-2007, 01:59 PM
Yes, everything that has a force applied to it does deflect. For example, the apparently miraculous ability of a concrete floor to exert an exactly equal and opposite reaction to your body weight results simply from it deflecting minutely, in proportion to your weight.

As for torsion, it has long been the practice on ships' propeller shafts to measure the amount of twist to determine the torque transmitted. Knowing the speed, the horsepower can then be calculated.

brucepts
11-09-2007, 02:12 PM
Do a web search on "strain gage" this should get you some info to read . . .

Forrest Addy
11-09-2007, 02:24 PM
Consider the lowly allen wrench. The long versions can be felt to deflect under hand effort alone when torquing a screw. Even a steel railroad bridge deflects to some degree when a mosquito lands on it. Our engieering sciences is sufficiently sophisticated that an investigator armed with the bridge's engineering data can tell you how much deflection occurs depending on the weight of the mosquito.

Thirere's an old saying in engineering: "Everything is made out of rubber." And its true. Everthing deflects in proportion to its load, configiration, and material stiffness characteristics.

ptjw7uk
11-09-2007, 02:37 PM
I think there will always be an amount of twist in a shaft usually called torsion. Torsion can also be used as a suspension unit as per some old British Leyland and if memory serves a Porsce had it gives a compact means of springing.

Peter

bigmac
11-09-2007, 02:45 PM
Quote "Everything is made out of rubber."

The way I heard it is that 'Everything is a spring' but what's the diff right.

jesse557
11-09-2007, 03:21 PM
It is called "angle of twist". For a circular shaft the angle (phi) is:

phi = (T*L) / (J*G)

where

T = applied torque
L = length of the shaft
J = polar moment of inertia
G = Modulus of Rigidity (material property)

Lew Hartswick
11-09-2007, 06:03 PM
I think there will always be an amount of twist in a shaft usually called torsion. Torsion can also be used as a suspension unit as per some old British Leyland and if memory serves a Porsce had it gives a compact means of springing.

Peter
Yes and even a few American cars used torsion bar suspension.
And not too many years ago either. :-)
...lew...

kendall
11-09-2007, 06:44 PM
newer rangers have trorsion bar front suspension, favorable point for many off roaders as they can get a couple inches of lift with a wrench..

The most common US user was Chrysler, nice smooth ride and ease of tuning are the nice points of it.

ken.

dicks42000
11-10-2007, 12:35 PM
I think it's strange that people have difficulty accepting that materials deflect under load....but I guess I was raised on it. Even bricks, maountains and the Tower of Babel deflect under load.

Given the western persons distaste for mathematics, I suppose that's also behind our disdain for materials science since there is a lot of analysis involved. (Very boring math....) Eastern Europe, Japan, Korea and India have lots going on it the field, though. Canada barely produces a return on the radar.

As Forrest alluded, think of the humble Allen wrench, preferably an Unbrako...good illustration of torsion. It happens in all structures subjected to a turning moment. Even coil springs are an example of torsion....
Shafts, gear boxes, crankshafts, clutches, engines and compressors are all devices that use or produce torque and due to elasticity and the nature of their construction also induce or suffer from tosional vibration. This can be damaging to sister components in a drive line. (Eg. engine-gear box set up.) Torsional vibration can beat gear sets and bearings to a premature death. Hence the whole interesting field of vibration analysis, flexible couplings, dampers, tuning of drive lines etc.....

At least it interested me. I'd still like to get back into it. How though ?

Rick

John Stevenson
11-10-2007, 12:46 PM
At least it interested me. I'd still like to get back into it. How though ?

Rick

Bondage ...................:rolleyes:

.

A.K. Boomer
11-10-2007, 01:06 PM
Yes even the typical coil spring is nothing but a rolled up torsion bar and in fact is the way it functions, I know some auto manuf. used to paint a line on thier drive axles so you could see if they got "pushed to far" if the straight line turned into a spiral it was time to get another, In general, Iv tightend so many fasteners in my life that Its actually safer practice for me not to use a torque wrench, I still do on things like head bolts and con-rod and mains and stuff --- but generally I just look at the bolt size length hardness and pitch and what its going into and then go by the elasticity of the feel, Iv saved allot of work at stopping short before something not too critical would have stripped out, Iv also gone beyond in certain aplications that allow to were I would not feel comfy with the recomended torque...

Asquith
11-10-2007, 01:11 PM
I can highly recommend this book:-

http://www.amazon.com/Structures-Things-Dont-Fall-Paperback/dp/0306801515

Very readable and entertaining book about structures, strength of components, deflection, etc, etc. It is not a textbook, but if you want the basic formulas, they’re there. Fascinating for everyone from the layperson (my wife enjoyed it) to the professional engineer.

sch
11-10-2007, 04:51 PM
One of the side affects of having even a cheap machinist's level
is seeing how much you can deflect it by pushing on part of the
machine. IIRC I could get a deflection off level in my CI 12x36
lathe bed by pushing down on the bed with my hand. Probably
not more than a few tenths, but it was detectable. Now maybe
it was the heavy sheet metal stands that were moving or maybe
the concrete but something moved. My Escort '91 wagon had a
big C shaped torsion spring under the rear wheels, discovered it
when one of the rubber mounts to the axle worked loose and I
got a big creak turning hard one way. Axle part was ~3' wide
with 15" arms going forward to clamp to the frame on either side.

JeffG
11-10-2007, 07:51 PM
The most distrubing example of torsional deflection I ever experienced was while riding a Great Lakes ore freighter - a sister ship of the Edmund Fitzgerald - from Duluth to Chicago in February in the mid '70s. The trip was part of a research project to see if winter navigation was possible on the lakes. We got into a storm on Lake Superior with about 50-knot winds, temp was around -30. Ships are basically big box girders, and with large waves at about 45 degrees on the bow, you could watch the ship twist slowly back and forth.

oldtiffie
11-10-2007, 08:38 PM
Originally Posted by dicks42000
At least it interested me. I'd still like to get back into it. How though ?

Rick



Bondage ...................:rolleyes:

.

Twisted people?

And when it varies or oscillates it sometimes goes "Cheep, cheep, cheep".

ipso facto: talk's cheap.

John Stevenson
11-11-2007, 05:19 AM
Twisted people?

And when it varies or oscillates it sometimes goes "Cheep, cheep, cheep".

ipso facto: talk's cheap.

Two things in this world go "Cheep, cheep, cheep." one is a budgerigar and the other is a Chinese prostitute.

.

Evan
11-11-2007, 08:46 AM
Absolutely nothing in existence is perfectly rigid. Why do you think Viagra sells so well?

The closest thing to a perfectly rigid substance it thought to be neutronium, what neutron stars are made from. It is hypothesized that a neutron star is a giant crystal composed of neutrons packed as densely as is possible without disappearing down it's own arsehole because of gravity. If struck the entire object is believed to ring like a bell. There is evidence for this in observations of these objects.

But even that isn't a perfectly rigid body. Gravity wins no matter what and can compress anything and everything.

As well, the speed of light limits how fast any deflection of any part of anything can be communicated to any other part of something. If some portion of an object is twisted that twist cannot be communicated faster than the speed of light to other parts. This may seem of purely academic interest but devices in computer chips routinely deal with the limits imposed by the speed of light.

Matter as we deal with it is composed of electrical fields, that is what we interact with in all solid objects. The other two nuclear forces don't play a sensible part in macroscopic interactions with matter.

This brings to mind a simple thought experiment. What happens to a flywheel that is somehow actually physically infinitely rigid if we rotate it about the center quickly enough that the tip speed should exceed the speed of light?

[edit]

Side note: While there is no such thing as perfect rigidity there is perfect elasticity. Glass is considered to be perfectly elastic. It exhibits zero plastic deformation (until it fails) under stress when below the plastic transition temperature of the molecules.

John Stevenson
11-11-2007, 08:54 AM
This bring to mind a simple thought experiment. What happens to a flywheel that is somehow actually physically infinitely rigid if we rotate it about the center quickly enough that the tip speed should exceed the speed of light?


This one is easy as Lucas did experimentation work on this in the early 1950's.

As the rim speed exceeds the speed of light it turns into a black hole and the Lucas headlight was born..........

.

Lew Hartswick
11-11-2007, 09:22 AM
Side note: While there is no such thing as perfect rigidity there is perfect elasticity. Glass is considered to be perfectly elastic. It exhibits zero plastic deformation (until it fails) under stress when below the plastic transition temperature of the molecules.

Wait a minute: I was taught (admitdly a long time ago) that glass
was a super cooled liquid. That was the explanation of the thicker
glass in the bottom of real old windows that had been in a vertical
position for centuries. Say it's not so Evan.
...lew...

Evan
11-11-2007, 09:38 AM
It's not so. The glass was made and installed that way. Glass is not a supercooled fluid as has been taught in the past. It is an amorphous solid and does not flow. While not crystalline it does exhibit a definite change in properties when it cools. Below a certain temperature the atoms are locked in place as they do not possess sufficient kinetic energy to overcome the normal electronic binding forces that hold matter together. Crystallinity is not a requirement for a substance to be a solid. Amorphous or "glassy" iron is used to make transformers.

wierdscience
11-11-2007, 08:37 PM
Wait a minute: I was taught (admitdly a long time ago) that glass
was a super cooled liquid. That was the explanation of the thicker
glass in the bottom of real old windows that had been in a vertical
position for centuries. Say it's not so Evan.
...lew...

Ah,something I learned once.Back way back when if you wanted a flat piece of glass for your cathedral you had it gathered up out of the furnace and rolled out flat between two water soaked wooden rollers.Wood being what it is and heat doing what it does the wood got warmer and the glass got cooler as it ran through the rollers and viola the tapered glass.Since it's basic logic to put the heavy side down when assembling things verticaly the thicker sections ended up on the bottom.

Later if you wanted a flat pane for your church or school you had a cylinder blown and then cut the ends of said cylinder off making a tube.One side of the tube was scored and broken and then was re-heated and allowed to unroll and layout flat making a flat ,but distorted panel.

Fast forward to modern day,now it's poured out onto a pool of molten tin which yields a nearly perfectly flat panel of any desired length.

barts
11-11-2007, 09:39 PM
But even that isn't a perfectly rigid body. Gravity wins no matter what and can compress anything and everything.

As well, the speed of light limits how fast any deflection of any part of anything can be communicated to any other part of something. ...

Shouldn't this be the speed of sound? Load & deflection are propagated as a pressure wave, not an electromagnetic one.

- Bart

Todd Tolhurst
11-11-2007, 09:44 PM
The speed of light is the ultimate limit to the speed of sound in any medium.

rotate
11-11-2007, 11:00 PM
Thanks for all the great answers.

Let me see if I understand all this correctly.

Really there's no such thing as mechanical force and much of what we consider to be mechanical force is either gravitation force or electromagnetic force. Since we can't really produce gravitation force at will or if we do, then it's very small (e.g. waving your hand), most of what humans consider mechanical force is really electromagnetic.

So, does this mean that all mechanical forces that we describe in machining is really electromagnetic in nature? As an example when you strike an anvel with a hammer, do the atoms on the surface of the anvil and the hammer compress or temporarily reposition themselve (assuming elastic deformation). In other words, does this cause temporary increase in the density of steel around the impact? I'm trying to undertand how a spring works in a microscopic level. I would also think that there's very low frequency radio wave emitted by sudden acceleration in the charge (ie. of the atoms relative to the nucleus).

Evan
11-12-2007, 12:37 AM
So, does this mean that all mechanical forces that we describe in machining is really electromagnetic in nature?

There are four basic forces that we know about. Those are gravity, electromagnetic, the strong force and the weak force. Gravity stands separately from the other three at this time as we don't yet know how it fits in. The strong force operates only within the atom and is responsible for holding it together. The weak force mediates nuclear decay. Neither has any influence over the interactions between atoms. The elctromagnetic force is a product of the electrons that surround the nucleus of an atom. They are responsible for all of the normal interactions we see with matter. Of course the main exceptions are those reactions that involve the fission/fusing of atoms, nuclear weapons being an example.

Other than that all the interactions of matter that we deal with (other than gravity) are mediated by the electromagnetic force. It is electromagnetism that prevents you from falling through the floor. It is electromagnetism that holds the atoms of steel together in a crystal lattice. The elastic bending of a piece of spring steel stretches those bonds. If stretched too far the bonds are broken and dislocations and slippage of the crystals occur, resulting in plastic deformation.