Undesired bonding between two parts under pressure 4

[cicciazzo]

Is anybody out there expert on unwanted bonding between surfaces of parts coupled under force or pressure?
I'd like to find out all major parameters, for example surface finishing, geometry, etc.
I'm also interested in mechanical literature about.
Thanks

 

[EnglishMuffin]

When you actually want to do this, it's called "diffusion bonding". Using this as a search term, there is a large amount of literature out there on the subject which should help you.

 

[unclesyd]

Adding to EnglishMuffin's post, make sure you search NASA Technical literature.

 

[jbel]

'diffusion bonding'

Is this what happens when you wring gage blocks together????

 

[EnglishMuffin]

Yes, but only if you leave them together too long. (Definitely don't leave them wrung overnight).

 

[diamondjim]

I would think that the finer
the surface finish the greater
the bonding. Just a guess.

 

[EnglishMuffin]

Well, yes, but I think flatness is important too, especially at low pressures. A somewhat separate yet illuminating issue is the following : Until well into the twentieth century, there was still some argument about what causes friction. Was it caused perhaps by rough interlocking surfaces leading to energy loss because of induced microscopic motion in the normal direction, or was it just interatomic force ? If it were due to interatomic force, the argument went, why was there no noticeable force of attraction in the normal direction between the two surfaces in question ? Well, it turns out that there is a normal force of attraction, but you don't usually notice it because when you separate the two surfaces, the force drops almost to zero at the instant that the surfaces truly separate. The force of attraction is dependent on the actual surface area in contact, which is a very small percentage of the apparent contact area, and is actually a collection of very small lands whose area drops to zero as the surfaces are pulled apart. But if you can make the lands large enough, the force does not drop off as much and the parts stay stuck together, as with gage blocks.

 

[ChrisatEastAg]

You may find this site interesting; a discussion of the mathematics of cold-welding. The company responsible makes a friction-enhancer to increase the strength of press fit joints. Apparently the mathematics of interacting surfaces in relative motion is known as 'tribology'.

http://www.tribtech.com/app.1.htm

http://www.tribology-abc.com/sub1.htm

 

[cicciazzo]

I don't think it concerns tribology.
The case I'm studying now regards still parts, both made of steel, coupled on a conical surface, altough it could be affected by vibrations.
Temperature is about 50 C, more or less (probably more).
Think of a brake, working on a conical surface and not on a disc.

 

[EnglishMuffin]

Since a common definition of Tribology is "the subject that deals with the design, friction, wear, and lubrication of interacting surfaces in relative motion", you are technically correct. However, since some of the physics is closely related, whether the surfaces are in relative motion or not, I wouldn't necessarily rule it out in your search. I do know that there can be problems with cold welding of sliding surfaces in outer space, because of the elimination of natural oxidation, so NASA are undoubtedly knowledgeable on the subject as unclesyd says.

 

[nbucska]

My prior employer experienced a horror story by stainless
steel screws spontaneously welding solid...


<nbucska@pcperipherals DOT com> subj: eng-tips
read FAQ240-1032

 

[EnglishMuffin]

I do know that stainless steel sliding on stainless steel is often avoided because of its known tendency to "pick up". It would be interesting to know why it is particularly prone to this - it might be simply because it oxidizes less readily than many other metals.

 

[cicciazzo]

What about surface finishing parameters?
Parts processed with shot peening proved to be better, from this specific point of view.
Following the theory of cold welding, it could be due to a decrease of areas in contact, I think.

 

[CoryPad]

EnglishMuffin,

The "pick up" to which you refer is the same as galling for threaded fasteners. It is due to small-scale fracture and re-welding of the surface.

cicciazzo,

Shot peening improves performance by increasing the surface hardness, thereby making it less prone to deformation.

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[jbel]

This condition frequently occurs on machine tool spindles. Many machine tool spindles have CAT 40 tapers which is, I believe, a 7/24 taper. When the tool holder and spindle nose mate, 'ideal' contact along the entire length of the taper does not take place. This may be due to slight differences in taper, temperature differential of the mating parts, dirty mating surfaces, or other unaccounted for reasons.

As the spindle is spun up to speed, often up to 20000 RPM, the nose of the machine tool taper begins to deform outward. As this occurs, there is a constant force along the axis of the toolholder that retains it in the spindle.

The combination of deformed spindle nose, the axial force for toolholder retention, and less than 'ideal' mating between the two tapers, often makes the toolholder 'stick' or wedge into the mating taper surface. The stick is one or both of the surfaces deforming and then being forced into the other taper. This can be observed by running at high speeds for an extended period and then removing the tool from the spindle. A distinctive 'POP' can be heard as the tool is removed from the spindle.

For this reason CAT 40 or CAT 50 tapers are not usually used on spindles that run at greater than 20000 RPM. HSK style toolholders perform much better at these higher speeds.

 

[EnglishMuffin]

CoryPad: OK. But why is Stainless Steel in particular so prone to this ? Don't you think that low oxidation rates might have something to do with it as well ? What about Gold for example ? Doesn't it too behave in this way ?

 

[NickE]

EnglishMuffin-

The reason galling occurs is due to the thickness of the oxide film on SS. The film is so thin that as two parts are placed into contact and moved the film fractures exposing surfaces of unoxidized steel. These then weld together. This also happens with Al alloys and Ti. I guess you could say that it has something to do with the "stainless" properties. Mainly since the film is generally so thin that its easy to break.

nick

 

[CoryPad]

I concur with NickE - a thin oxide layer on stainless, Al, and Ti produces a tendency towards galling. I have no experience with gold in engineering applications, but it could behave similarly for the same reason.

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.