Friday, February 20, 2015

HOW TO :: Selecting your Bushing Material

In a perfect world selecting your bushing material could be so easy.  Just imagine....the journal surfaces are smooth and always separated by a full film lubricant.   The materials never experience elastic or thermal distortions and the lubricants have no abrasive particles.  In this perfect scenario, you would only need to consider selecting a suitable material by it's strength qualities and take no other factors into consideration.

Sounds great, except, that scenario does not exist.  The reality is - a bushing can distort, journal & bushing surfaces can not be made perfectly smooth, and the bushing may be occasionally "starved" of lubricant. With the real factors in mind, the choice of good bushing materials in conjunction with bushing design & an appropriate lubricant are essential in how to select your bushing material. 

Where do you start - what are the desired material qualities?
After years of testing and trials, experience has shown us that good bushing material has certain general desired qualities:
  • Score Resistance - contains antiweld / antiseizure characteristics
  • High Compressive Strength - can carry a load without disintegration
  • High Fatigue Strength - can provide a decent service life under stress
  • Deformable - the material will permit to yield without causing failure
  • Corrosion Resistant - reduces or eliminates wearing away caused by various acids
  • Low Shear Strength - resists movement of one layer of the material in respect to the other
  • Structurally uniform - a sound, non-porous surface that maintains full-film lubrication
  • Low Cost & Availability - this one is self explanatory!
Since no single material comprises all of these desired qualities, you will most likely have to compromise on one or more of the above qualities. For example, one material might exhibit great fatigue strength but have poor deformability. 

How about Cast Bronze?
In general Bronze offers a great compromise with respect to the desired qualities for a good bushing material.   Take a look:

Cast Bronzes (example: C93200, C83600, C90300, C93700, etc)

Bronze bushings are suitable for a wide range of applications.  They have a great number of benefits that a great number of alloys don't offer.  They have great strength and can be economically produced in a variety of shapes and sizes.  There are extremely reliable in the sense that they can made as a single, solid unit with no bond or liner to fail.  The high thermal conductivity of cast bronzes is an advantage in taking heat from the bushing surface, which tends to prevent "hot spots.  However, despite the benefits of cast bronzes as a whole, it is extremely important to select the right alloy for the job.

There are many Cast Bronzes - which one do I choose?
This is the #1 question our sales group will typically receive from our customers.  But the choice always boils down to one question -- What is the bushing being used for?   When you take four popular bushing alloys and line the characteristics up against one another you see how one alloy might work better than the other.  Take a look below at the characteristic box:

Now the above box is by no means comprehensive and does not include other material features such as wear resistance or machinability rating, etc.  However, it is helpful to see the popular bronze alloys side by side showing how one alloy might be preferable to your application over another alloy.