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Choosing Rod Ends

The right ends to your means

In a companion article, we outline the various types of rod ends (a.k.a. spherical bearings). These bearings are used for a variety of industrial and aircraft applications as well as for certain automotive systems. Because rod ends are typically used in performance/competition vehicles, choosing the right end for the intended job is vital. Here are some of the practical considerations.

Misalignment

Without a bolt or fastener through the center of the rod end, the ball can rotate through a full circle, but 360-degree movement isn't possible when the end is installed. Given this fact, all rod ends' spheres can bind in their housings. This angle of misalignment is a critical factor when selecting rod ends. Exceeding the manufacturer's recommended maximum angles leads to premature rod-end wear at best and complete failure at worst.

To measure the angles necessary for the application, use a simple protractor to dimensionally check the geometry. Rod ends' angles of misalignment are listed in their specifications. Compare your measured angles to those in the end-manufacturer's catalog to select the right spherical bearing for the application. Never take the easy route and buy a rod end that simply looks big enough to handle the job.

Another thing to consider when choosing a rod end: Shanks sizes have different relationships to the bore's diameter. Some shanks (the threaded shaft parts) are the same diameter as the hole in the ball while others are one size larger than the bore (for example a 5/8-inch bore mated to a 3/4-inch shank). Larger-shank/smaller-bore configurations can withstand greater bending loads than corresponding same-diameter ends.

In construction, an oversize shank is generally made by installing an insert one size smaller in the body of the part with the larger shank. Because of this, a 5/8 x 3/4-inch rod end will exhibit higher load capacities than a 3/4 x 3/4-inch rod end (provided both are manufactured from similar materials because the insert has more body material around it). This asymmetrical rod-end style offers better "wrench-ability" on many suspension components.

One exception to the above oversize-shank construction philosophy: Some companies add a larger shank to a smaller body. While this configuration serves the same purpose as the oversize-shank scenario, it provides less meat around the ball (simply because there's less material surrounding the spherical bearing).

Ends Justify The Means

Once misalignment and dimension requirements are figured out, construction material is the next consideration. The balls usually are subjected to the highest loads, so they require the greatest hardness and strength. Some commercial rod-end balls are manufactured from bronze or even sintered steel. These materials are typically softer, but some sintered steels are up to the job. (BMW and Ford are two OE manufacturers that use sintered steel for high-stress applications such as connecting rods.) With a proper heat-treat, sintered steels can perform some tasks well.

On the other hand, high-quality rod ends normally incorporate heat-treated steel balls (typically chromoly, stainless and 52100 bearing steels). In order to remain round, the balls must be extremely hard—often the balls are chrome-plated to provide a smooth bearing surface. This is critical when the ball is placed under a load.

Given the heat-treat and hardness of the ball, the outer rod-end race has to be hard, but not as hard as the ball. Most three-piece rod ends use a race manufactured from through-hardened steel alloy or a stainless steel that can be hardened. In both of these instances, the outer races are heat-treated for wear-resistance and strength.

Commercial or economy rod ends usually have bodies constructed from low-carbon mild steels that can't be through-hardened. This material might work in a light-duty application, but rod-end bodies manufactured from chromoly or heat-treated stainless steel allow the physical size of the rod end to be reduced (simply because the material is stronger).

Another option is 7075-T6 aluminum, which has a tensile strength slightly greater than mild steel. However, aluminum is not as forgiving—it won't stretch or bend as far as mild steel before breaking. Heat-treated chromoly and stainless steel are almost twice as strong.

In a nutshell, unless you're absolutely positive about the loads encountered and have a good grasp of the limitations of aluminum rod ends, use high-quality heat-treated steel models. It's better to be safe than sorry.

Used Ends

Just like any other piece of hardware, a rod end has a limited mechanical life. It's tough to tell if a surplus or used rod end has reached its finite cycle or not. On a similar note, there is no safe way to repair or "tighten" a worn bearing. Any bearing (rod end or otherwise) that's dented or bent should be discarded. Likewise, any rod end that shows signs of stretching in the threads or the head should be 86'd. A rod end that's been in an accident should also be checked very closely. Replacing suspect bearings is cheap insurance.

How can you tell if a bearing is a high-quality piece or not? Inspect it like any other precision part. Look at the machine work. Is the surface on the race rough or smooth? Is the ball well-machined and smooth? If the rod end is a non-Teflon configuration, does the ball fit inside the body precisely or does it rattle or bind? If the end has a Teflon liner, are there gaps in the liner or areas where the liner is actually loose? (A Teflon-lined rod end should be one continuous tightly bonded piece.)

When all is said and done, the best rod ends are those that are designed and constructed through proper engineering and backed by a rigorous R&D program. Rod ends built with precision-ground steel balls and constructed with the highest-quality materials cost money. And at the end of this line, it's no place to be cheap.

Resources

Alinabal, www.alinabal.com

Aurora Bearing Company, www.aurorabearing.com

Baker Precision Products, www.bakerprecision.com

National Rod Ends, linkage.tuthill.com

QA-1 Precision Products, www.qa1.net

Rose Bearings, www.rosebearings.com