Bearing Design Guide: Chapter Three: SELECTING THE BRONZE SLEEVE BEARING ALLOY

             Let me just tell you that when I was in high school my least favorite class to go to was Chemistry.  Now don't get me wrong, the Lab days were my favorite and I couldn't wait to have one of them every other week!  Fast forward 20 years (holy cow...has it really been that long?) and would you look at that, I'm using what I learned in Chemistry class almost everyday.  Before working here I would look at a piece of solid or cored bar and say "its just a piece of metal" and have no clue what the actual material was. Well, now I know and who would've thought that when they tell you back in school that you will need this one day, you actually do.  So lets do this...

                 Chapter Three:  SELECTING THE BRONZE SLEEVE BEARING ALLOY

While the bronze alloys are the most common and versatile of sleeve bearing materials, the alloy selected must contain various features as load carrying ability without deformation; low coefficient of friction; wear  resistance; compatibility with shaft materials; finishes; and hardnesses. It also must have a resistance to pounding loads or impacts when encountered,  corrosion resistance and other properties.

               These properties are derived from the addition of various elements to the bronze alloy at    casting.  Since there is no bronze alloy that possesses all of the desired properties, the alloy will always be a compromise.

Bronze is an alloy of copper and tin. Several of the following elements can be added to obtain the most important, desired effects.

Getting Comfortable With the Elements

 

Lead (Pb), even in amounts as small as 1% or less, improves the working qualities and machine-ability of the alloy.

Because lead does not alloy with bronze over 3%, it retains its characteristics and melting point. The lead separates in form of globules and are trapped in the copper-tin dendrites. These globules act as a natural lubricant because lead has a low affinity for steel, reducing the tendency to seize or weld to the shaft, even when lubrication fails. In lead content exceeding 10%, the load-carrying ability with impacts, or pounding loads are reduced in proportion  to the copper-tin ratio. The lead content permits the alloy to operate at high surface speeds and the ability is increased with increased lead content.

 

 Tin (Sn), in any amount, hardens copper and is a measure of the alloys ability to carry the load. As the tin content increases, its load-carrying ability  increases.

Bronze alloys with less than 4% tin content are generally unsuited for bearing purposes because the alloy has a low resistance to deformation and exhibits the tendency to wear rapidly in the absence of lubrication.  Increasing the tin content to 5 to 11% improves the hardness and the tensile strength; above 11%, however, it has the effect of reducing the ductility and increasing the brittleness.

 

 Zinc (Zn), when added to bronze in amounts of 3% or less, acts as a deoxidizer, a hardener and  increases the fluidity of the metal during casting.  Zinc additions increase corrosion resistance of copper in environments such as natural gas or, where sulfur attack is involved.  Above 16% zinc content, the alloy becomes subject to dezincification but with  small amounts of  arsenic, tin or phosphorous, this process is inhibited.

                                        

 

 Phosphorus (P) also is used as a deoxidizer and a hardener and is preferred over zinc when the alloy is used for bearing purposes.

 

Nickel (Ni) increases the density and hardness by grain refinement which also helps to lower the wear rate. The addition  of this element  also improves the corrosion resistance of the alloy.

 

 

Manganese (Mn) also is used as a deoxidizer  and strengthens the alloy by grain refinement.  It is  limited to aluminum bronzes and manganese bronzes  .

                Iron (Fe) is used as a hardener and grain refiner in copper zinc and aluminum bronze alloys.

 

Aluminum (Al) is used as a flux and deoxidizers in yellow brass where fluidity is essential in running thin sectioned castings. It increases the yield  strength and hardness in aluminum bronzes.  Increased aluminum content reduces the elongation and ductibility of the aluminum bronzes but permits heat treatment  above 10% content. Aluminum is an undesirable element in the leaded bronze alloys as the lead content is to the aluminum bronze alloys.

So there you have it. The elements above will help you create the perfect recipe, if you will, to produce the perfect bearing for your application. I hope you enjoyed your trip with me down my high school memory lane.  I say goodbye for now.  Until next time my metal loving friends...

Next Up:  Week Four, Chapter 4: Load Speed Categories and Usages