By Richard Lang of Abonnel Precision Engineering
This quarter we would talk a little about bronzes, their properties and applications. One of our readers said they occasionally hear people refer to a brass bush when in fact they want a bronze bush and could we write something on this. Lets also look at the range of materials one may come across:
- Leaded Gun Metal (LG2)
- Aluminium Bronze
- High tensile brass
- Phosphor Bronze (PB1)
- Manganese bronze
The factors that will effect performance in your application of any plain bearing material will be:
- compatibility or the anti-weld and anti-scoring
characteristics. A good bearing material will have a low propensity for the microscopic high spots to weld to the shaft material.
- conformability is the ability to compensate for misalignment and manufacturing imperfections. A soft relatively pliable material (low modulus of elasticity) is good.
- embeddability or the ability to absorb dirt and metal particles without scoring and wear. In metals this is the same as conformability but not so in plastics.
- fatigue resistance is of note when the load changes direction- look for cracks at right angles to the surface, flaking surface. A soft material over say steel can be used to counter this.
- corrosion resistance is important in salt water, acids and oxidised oils.
- load capacity is a function of tensile strength and hardness. Higher hardness and strength give greater load carrying capacity but give poorer anti-scoring, conformability and embeddability properties. Where high loads dictate hard bearings, make sure that; the shaft is harder; both have high finish; free of dirt e.g. oil filtration; constant oil film (oil pump interlocked at start up); high standards of alignment (bearing location design and checking procedures). Where there are high loads the bearing material may need to be backed with say steel, which can prevent deflection within the allowable misalignment limits for the bearing.
- high thermal conductivity is required to remove heat and the expansion characteristics should match the shaft and surrounding components.
Lets now focus on a range of brasses and bronzes which are available in Australia. The first thing to realise is that there are wrought standards AS2738.2 and cast standards AS2738.3. Wrought products are factory produced by rolling and mechanical working while bearing bronze castings are usually by one of the following techniques:
- sand cast
- continuous cast
In the case of bronze the continuous cast material has all of the advantages of the wrought products in other materials i.e. uniformity, greater density, better mechanical properties and economy of mass production in limited complexity of shapes such as round and hollow bar.
There are numerous copper alloys but there are four groups:
- copper-lead (20-40% lead)
- leaded bronze (4-25% lead, 4-10% tin)
- tin bronze (8-20% tin)
- aluminium bronze
Brasses have a high zinc component while in the bronze range zinc is low and tin is the major addition to the copper. Copper- lead and leaded bronze have the best bearing properties such as compatibility, conformability and embeddability. The addition of tin, aluminium and iron increase the mechanical properties. Manganese increases strength and corrosion resistance while silicon increases strength, hardness and machinability. Zinc and lead improve machinability. For an acid resistant zinc-free bronze the zinc needs to be less than 0.05% and phosphorous up to 0.15% as in AS 2738.3 C90250 or C90710 is also close.
Copper- lead alloys
They are the simplest metallurgical and act like white metal. Hardness is similar to white metal at room temperature but the copper- lead alloys are harder when the operating temperature is around 1500C. They are used in a sandwich construction with steel backing and white metal lining. The lead does not dissolve fully in the copper and can act as a smearing lubricant. Used in combustion engines for main and connecting rod bearings and for moderate loads and speeds in electric motors, and turbines. Good fatigue strength and high load capacity.
Have better compatibility than tin bronzes as the lead particles smear over the bearing to help when lubrication is poor. Good general application for intermediate loads and speeds such as machine tools, farm machinery, pumps and rolling mills.
The small amount of lead in these is for machinability. Zinc and nickel for strength. Shafts should have a minimum brinell hardness of 300 – 400 to handle the lower compatibility. Used in high load, low speed applications such as stacker bogies, earth moving equipment, rolling mills, connecting rod bushes and valve guides.
Very high strength used for their excellent shock and wear resistance properties at elevated temperatures (over 250 0 C) in heavy duty low speed applications with good lubrication such as crushers. Better than manganese bronze or high tensile brass at elevated temperatures
Mike Sondalini – Maintenance Engineer
We (Accendo Reliability) published this article with the kind permission of Feed Forward Publishing, a subsidiary of BIN95.com
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