Bronze Sleeve Bearings: Are you designing for lowest cost?

Bronze bearings are the most widely used sleeve bearing type, industry-wide. Even so, there seems to be a few misconceptions about these versatile products concerning their applicability, performance and most of all – their cost effectiveness. Let’s take a look at the facts. For example:

Bronze sleeve bearings are expensive because they depend on the price of copper, and copper is an expensive metal, right?

Don’t be misled by copper’s off-the-shelf cost alone The real raw material cost has to be figured on the basis of material bought minus the value of turning and machining scrap. Copper alloys, such as bearing bronzes, can typically command 50 to 70% (depending upon point of manufacture) of the price of initial bar or tubular stock when returned as scrap; other bearing metals have considerably lower values as scrap, and steel turnings have almost no value at all (you may have to pay to have them hauled away). Therefore whether you buy bearings or make them yourself, the net raw material cost is far more balanced than you might expect.

OK, but the cost of labor and other factors has been increasing steadily; doesn’t this offset those raw materials savings?

Wrong again. First, labor and overhead costs affect bearing types more or less uniformly, so these fact don’t have a unique influence on bronze bearing manufacturing costs. Second, and more important, bronze bearing alloys have an intrinsic economic feature working in their favor. They are naturally easy to machine, and the leaded grades have some of the highest machinability ratings (per ASTM E618) of engineering materials. Most bronzes don’t require heat treating and rarely need to be ground to hold finish and tolerance. They are, therefore, ideally suited to the new and highly productive CNC machining available today. As machine technology has grown, so have production rates and that translates into lower per piece costs.

But our design work calls for a lot of prototyping, including bearings. Designing and making one-of-a-kind bearings can be an expensive proposition.

Not if you specify bronze sleeve bearings. A bronze bearing is basically a simple product, and any machine shop can turn one out from standard tubular or bar stock in short order. Compare that with a rolling-element bearing, whose many components make very difficult – and extremely expensive – to produce in single units.

Does that hold for specials, too? Like spherical bearings, for example?

Absolutely. Every bearing designer knows that spherical bearings are among the most complex to design, and they’re certainly the most expensive to manufacture. In practical terms it is almost prohibitive to prototype a spherical rolling-element bearing because of the high costs involved. But making one, or two, or a hundred spherical bronze sleeve bearings is no more involved – or costly - than making a conventional bearing. So prototyping bronze sleeve bearings can be an economical part of the design process, and that alone gives the designer a lot more freedom.

All well and good, but the equipment we design calls for a wide variety of bearings running at high and low speeds and loads. We need the flexibility that rolling-element bearings are supposed to be noted for.

Look again. Sleeve bearings, and especially bronze sleeve bearings, operate efficiently over a wider range of PV values than any other type, with allowable PV values up to 3,000,000+ with the proper lubrication. At the low end of the PV scale, the generous load distribution and favorable fatigue characteristics inherent to sleeve bearings make them the natural choice for oscillating, intermittent-motion or less-than 360-degree rotation jobs. Their inherently low friction characteristics make them natural choices for mixedfilm or boundary lubrication conditions, and those are the modes most bearings operate in most of the time.

Give me an example.

That’s easy – take the huge carrier vehicle that transports the space shuttle and all our larger rockets from the vehicle assembly building to the launch pad at Cape Canaveral. It never moves faster than two miles per hour and its wheels turn on bronze sleeve bearings. For a more down-to-earth example, look at any piece of off-highway or earth-moving equipment; chances are that the major bearings outside the drive train are made from one of the bearing bronzes. On the other hand, did you know that the load-carrying capability of bronze oil lubricated sleeve bearings increase with rpm, while those of rolling-element bearings actually reach a limit and begin to fall off at high speeds, also resulting in shorter rolling element bearing life.

OK, sleeve bearings out-perform rolling elements, but we like the wide range of properties offered by a choice of materials such as aluminum, zinc or one of those new polymer materials. How can you ask me to limit my choice to bronze?

There is no such thing as an ideal bearing material, and all bearing materials have their place. The important point to remember is that the large family of alloys known as the bearing bronzes offer the widest choice of properties for the broadest possible range of bearing applications. If you want one word to describe the bearing bronzes, it would be versatility. There are soft and ductile bronzes for jobs that require conformability and the ability to embed occasional dirt particles; there are hard bronzes for maximum strength and load-carrying capacity. The bronzes are well known for their inherently low frictional coefficients; all have outstanding corrosion resistance and excellent shelf life, and they’re unsurpassed when it comes to handling shock loads or damping out noise and vibration. Bronze bearings operate at higher temperatures without losing ability to carry loads without creep and at sub-zero temperature without becoming hard and brittle. They have excellent heat dissipation for high loads and speeds.