MadSci Network: Engineering

Re: What causes metal gears to wear down over time?

Date: Wed Mar 12 20:43:45 2008
Posted By: James Griepenburg, , Chemical consultant, Chemmet Services
Area of science: Engineering
ID: 1204357891.Eg


Any material will wear But metals have some idiosyncratic mechanisms.  
This website outlines them:

The properties of metals that commonly cause wear are their softness, 
corrosion especially reaction with oxygen, and metal to metal bonding.
The common mechanisms are:

Abrasion-an abrasive material comes in contact with the metal and wears 
the metal away an example is sandpaper or a file. This is solved by 
cleanliness or by using a harder metal and sometimes by lubrication or 
coatings but coatings can also wear and cause their own problems.

Removal and reformation of a surface oxide layer, fretting corrosion- 
This is common in aluminum and stainless steels which form a hard 
protective oxide layer that can break from the surface exposing metal 
that oxidizes etc. The removed oxide can then act as an abrasive. This 
can be reduced by lowering stress, lubrication and removing oxygen and 
using dissimilar metals. This is sometimes controlled by surface 
treatment such as nitriding or case hardening.

Metal to metal bonding-

Metal bonds are such that pure metals when forced into contact will bond 
together.  Throw in some corrosion and fretting and havoc ensues. These 
bonds form and break and eventually form wear particles.  This can be 
reduced by good lubrication, reduced pressures, coatings, dissimilar 
metal surfaces and good bearing design and surface finish.

Lubricants must not be forced from the interfaces by pressure.  That's 
why certain slippery materials such as silicones and WD-40 can be 
ineffective under stress. The choice of a grease or lubrication system 
can be critical. 

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