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Technology, Oil-Free Air Compressors

A Tale of Two Metals


Piston compressors are the oldest and most common type of industrial air compressors, relying on the principle of displacement to increase the pressure of the enclosed volume of gas or air. They can be found in DIYers' garages, hobby workshops, and full-service auto body and mechanic shops. They’re versatile – available in a variety of configurations, including single-acting, double-acting, oil-lubricated or oil-free – economical, and sometimes portable – the utility infielder of the working world.

The principal mechanics of a piston compressor is typically constructed of one of two materials – aluminum or cast iron. We’re going to give a quick breakdown of the differences between the two. First, let’s look at the characteristics of these two common and versatile materials.

  1. Aluminum – The properties of aluminum include: low density and therefore low weight, high strength, superior malleability, easy machining, excellent corrosion resistance and good thermal and electrical conductivity are among aluminum’s most important properties. Aluminum is also very easy to recycle.
  1. Cast iron – Cast iron has good compressive strength, or ability to withstand loads that would reduce its size,but relatively poor tensile strength, or flexibility. Because of the impurities in cast iron and its crystalline structure, although it is a strong material in compression, it is weak in tension and is very brittle. (Side note: wrought iron, most commonly found used in outdoor furniture and garden fixtures, is essentially reprocessed cast iron; this involves puddling, or reheating cast iron and manually mixing air in with the molten mass.)

With a piston air compressor, the type of material involved refers to the metal used in the pump or shaft that surrounds the piston. Solid cast iron is considered more durable. But it’s also much heavier. For small oil-free compressors, cylinders formed from aluminum tubing are often used. Its excellent heat transfer capability and high-tensile strength make it especially suitable in a compressor because of its high performance/low weight ratio. Aluminum also is not susceptible to rust formation. Cast iron cylinders with low-speed pistons have traditionally been recognized for their durability. Aluminum compressors also cool faster and are lighter, but performance is pretty equal.

Which one is better? Ultimately, performance and durability are what matters the most. The number and size of cylinders used is dependent on the capacity required and the number of stages. Multiple cylinders may be used in single stage compressors. Usually all the cylinders will be the same size. In two-stage compressors, the size of each successive stage cylinder is reduced as the air passed from cylinder to cylinder requires less space at each stage.

Mobility and ease of use are the primary issues to consider. If the compressor, for your needs, will be frequently moved from location to location, then aluminum is probably the best bet. If the compressor will be relatively stationary, cast iron is equally suitable.

What are your experiences with cast iron versus aluminum compressors? What do you prefer, and why? Let us know in the comment section below.


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