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Rod End Designs

All spherical bearings aren't created equal

Rod ends are common components in vehicles that are modified for performance. These ends are found on four-link suspension setups and ladder bars as well as on track locators, carburetor linkages, steering components, clutch linkages and more. People tend to take these vital links for granted until they break.

Rod ends are the end of the line, so to speak, for some considerable stresses. And some rod ends are exceptional while others are plain junk. Unless you know what you're looking for, they almost all look the same.

The Basics

By design, a rod end basically consists of a spherical ball that's designed to rotate inside a housing. This ball is the bearing, and the housing it's contained in is the race. The spherical ball is machined flat on each side, and this modified "sphere" is bored with a hole through the center.

Rod ends are manufactured in a variety of ways. "Commercial" or economy one-piece models are the cheapest, but fully swaged, two-piece units are the only ones that should be considered for safety-related automotive systems such as steering linkage and multilink suspensions. On these two-piece ends, the body is formed (or "swaged") around the ball so that the race is actually part of the body. When looking at affordable rod ends, this is the only type that offers decent radial or pull strength coupled with good axial strength (resistance to the ball being pushed out of the body).

Racing rod ends (often called "aircraft" ends) use a three-piece precision design. Here, the race is formed around the ball, then this insert is staked into the body. Benefits are a much closer fit along with a much higher degree of precision between the ball and the race. This design also allows different materials to be incorporated during the construction process so that the rod end can be manufactured to best match the application. Races can be made of mild steel, alloy or stainless steel (brass or aluminum bronze is sometimes used, but should be avoided because of its low strength), and bodies are made in mild steel, alloy, stainless steels, aluminum or even titanium.

Teflon Liners

Another optional feature on rod ends is Teflon liners, which allow the rod end to be self-lubricating. This is important, since greasing a rod end once it's installed on a car is usually difficult if not impossible. More importantly, grease fittings can physically weaken the rod-end housing. (That's why universal joints with grease fittings are more fragile than solid models.) Furthermore, grease on the rod-end ball attracts grit, which can work its way between the ball and race and accelerate wear.

A Teflon liner eliminates all of these predicaments. By design, a Teflon liner consists of a carrier component, usually a fabric (which provides compressive strength), a Teflon component for lubricity along with various bonding resins. The liner is bonded to the race so that the ball actually rides on the liner. The movement of the ball rubs Teflon on the ball, which in turn provides lubrication. Typically, two- and three-piece rod ends are available with Teflon liners. When looking at Teflon-lined ends, beware of inexpensive examples with virgin Teflon, which has a relatively soft compressive strength (approximately 10,000-psi). In contrast, a high-quality composite Teflon liner has a compressive strength of 40,000-60,000 psi. Good Teflon liners also eliminate clearance between the ball and race, making for a tighter fit.

But simply adding a Teflon liner to a rod end doesn't ensure precision or performance. Liner failure is described as the rod end "beating out," which can happen two ways. For one, some "econo" rod ends have races that are constructed with molded plastic (occasionally mixed with fiberglass filler). Teflon may also be added for lubricity to yield low-strength (no more than 15,000 psi) self-lubricating liners. The upshot is that the race can deform long before the body sees any damage.

The other type of liner "beating out" failure involves the physical bond between the liner and the race. As you might expect, the self-lubricating nature of the liner makes it difficult to bond to the race. If the manufacturer can't make this bond sufficiently strong, then the liner will become detached from the race. Each time the rod end becomes misaligned, pieces of the liner disintegrate and clearances eventually become excessive. The government has MIL specs for Teflon-lined bearings, but there are no standards for liner bond strength. Of course, a rod-end manufacturer that makes a line of bearings for military use has the capability of producing high-quality Teflon-lined race bearings to this same manufacturing criteria.

Those are rod-end basics in a nutshell. In an accompanying article, we discuss other end notes, including how to choose the best spherical bearing for the intended job.

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