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  • What is the term that when two parts sticks together?

    Discussion in 'The main mechanical design forum' started by KevinC, Jun 24, 2011.

    1. KevinC

      KevinC Well-Known Member

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      I remember when two similar metal, when rubbing against each other when in motion, especially at a heated environment (doesn't have to be high, let's say 30-40c), one will stick to another pieces, what is that term called? Gouling?

      I wanted to read more about that, so that's why I want to know the exact name of the term. Thank you for reading my message!
       
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    3. PWASS

      PWASS Well-Known Member

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      Re: What is the term that when two parts sticks together

      This process is called galling and is most apparent when using stainless steel nuts and bolts.

      Stainless Steel by nature galls, however you can reduce the likeliness of the parts galling, by not subjecting the parts to heat. Heat not only in application, but also in installation (i.e. high speed torque wrenches, air guns etc), it is best to apply parts by hand, and do not over tighten.

      You a can also have a light wax coating, this helps lubricate the parts during installation.

      Another proven way to reduce the galling is to mix differnet grades of stainless. For example, if you are using a bolt and nut, use a 304 Stainless Nut & a 316 Stainless Bolt. They chemical composition differences help to reduce the level of gauling.

      If all else fails have the parts vaccum hardened, which also helps but it is not as economical.
       
    4. KevinC

      KevinC Well-Known Member

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      Re: What is the term that when two parts sticks together

      PWASS:

      Thank you SOOO much! This is some really awesome information. You not only given me meaning, as well you have gave me some awesome suggestion!
       
    5. Harshad

      Harshad Member

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      Re: What is the term that when two parts sticks together

      Dear Engineers,

      This is question of Physics....

      We are always able to separate out two parts, because of air. It is air trapped between two surfaces, keep both the surface apart always. When we rub two metal, upper level of surface is getting heated because of friction. If we observe minutely, it is process of rubbing two surfaces, surface quality get polished due to rubbing action, this will reduce small pit depth on the surface, which can not visible through naked eyes. This pits are accommodating a very small amount of atmospheric air. as we rub two surfaces, this pocket size get reduced, and hence, air quality reduces. At a stage when u get quite finish surface, quantity of air is not enough capable to apart masses from each other, and they stick with each other. We can see this practically as highly polished metal block is very difficult to lift from similar highly polished surface. This process results, closeness of molecules of one material with molecules of another material. It shows a very small amount of acceptability and attraction between molecules of different material.

      We can see the anti frictional effect between surfaces. Friction is depends on roughness of surface. Highly finished surface offers higher frictional resistance. There are mainly two terms plays major role

      1) Frictional resistants.
      2) Molecular Attraction force.

      when surface is rough, it offers less resistance to friction, and high friction is taking place and liberates heat. At this stage molecular attraction is very less.

      when surface is enough smooth, it offers high resistance to friction, and zero friction taking place when no heat appears, At this stage molecular attraction is very very high. this attraction is responsible to stick two surfaces together.

      We may term this as " Molecular Sickness".

      Regards,
      Harshad.
       
    6. PWASS

      PWASS Well-Known Member

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      Re: What is the term that when two parts sticks together

      Harshad

      What you describe is called Wringing and is a totally different process to Galling.

      Wringing is commonly used to attach Slip Gauges together.
       
    7. KevinC

      KevinC Well-Known Member

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      Re: What is the term that when two parts sticks together

      Thanks guys. I've just learn that theory not too long ago. Quite interesting that this is being brought up as well. Very informative. Thank you!
       
    8. Dave B

      Dave B Active Member

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      Galling what is it?

      Harshad
      I am a bit late in this discussion on galling , I just wanted to correct you a bit more on this subject.
      Two materials of similar hardness, under pressure with a sliding motion will exhibit a tendency to "gall" in which a transfer of material from each surface occurs to the other on a molecular scale.
      This has the affect of welding but by a different mechanism.
      This is enhanced with smoother surface finish as the two surfaces become more intimate contact with each other.

      There are certain conditions in which this is more pronounced. Nickle alloys for example will gall easily together.
      Thus all of the 300 series stainless materials can gall with each other. Aluminum will gall with itself or stainless as well.
      Nickle plated parts also can gall.
      This risk can be reduced by having one surface harder than the other, insuring their is contamination in the joint grease or other debris, plating with a different material such as silver.
      In stainless compression fittings, the nuts some times are plated in silver in the thread area such as in some of the Swagelok products for example.
      Plastic materials will wear quickly do to galling for example if say a gear and axle are molded from the same material.

      This is a significant material effect that every designer of part should be aware of it.
      They should study what conditions will reduce it and what to avoid that would enhance it;s likely hood.

      I once worked on a product that had a thin stainless steel sheet with light contact pressure sliding over the top of another stainless sheet in a carriage to a printer. The carriage would lock up if it stopped while both sheets were in contact. To separate would require a screw driver to pry the small spots (where actual welding had occurred) apart from each other.
      This was random since the swathing was image specific the carriage would usually travel past the point of contact when stopped to reverse direction.

      The burnishing of the two surfaces from wear-in ,also makes the two surfaces closer in contact as you suggest which will enhance this effect as the likelihood of contaminates keeping the two surfaces apart is reduced.

      A common galling example is nut and bolts or bolts or screws in threaded holes. Often the gall occurs during tightening when the thread tension is maximum near the end of torquing the the last bit of compression in the joint. When the screw or bolt is removed it wither rotates a small amount and freezes up or it doesn't move at all. The torque to remove exceeds the shank strength and the fastener head is twist off . The screw or bolt is now permanently in the threaded hole. It must be drilled out to be remove.




       

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