Rod Ends and Sphericals: How To

Bearing Selection:

When selecting a rod end or spherical bearing for your application it is crucial to understand the different types of liners and materials. The most common type of rod end you’ll find in an FSAE paddock has a hardened alloy steel ball and race (both of which are usually chrome plated) with an incredibly thin PTFE liner between the two. These spherical bearings have an excellent balance of ultimate load and low friction which make them great choices for motorsports. 

Choosing the same bearing without the PTFE liner can offer greater ultimate loads, but without any lubrication these bearings will wear much more quickly. Once the ball and race are worn the axial load limit of the bearing can be greatly reduced, and the free movement in the joint will lead to shock loading in the system.

Browsing through a bearing catalog may show that a bearing with a nylon race offers a higher static radial load capacity, but they do so at the cost of stiffness, and when a nylon race fails they fracture into many pieces leaving the ball and race completely disconnected.

There are a variety of rod end housing materials available with widely ranging yield strengths, and there are also housings available with thread sizes which do not match the bore diameter. These can be used to match the tensile strength of the housing thread to the shear strength of the bolt through the bore of the bearing. Careful analysis of housing material (steel or aluminum) as well as ball, race, and liner materials should always be performed to ensure the needs of your specific application are met.

Spherical bearings are also available with different preloads, which defines how tightly the bearing race is crimped down onto the ball at assembly. Using a bearing with increased preload will give you more stiction when the joint is new, but can increase the service life of the joint by preventing play in the joint as the bearing faces wear. For the lifecycle of the average FSAE car with a properly chosen bearing this is not something to worry about.

Bearing Orientation:

Spherical bearings have vastly different load ratings for axial forces and radial forces, with the ultimate axial load often being 5-10% of the ultimate radial load. When designing the joints on your car you should take care to arrange the bearings so that you minimize the axial load, or choose your bearing based on the axial load. For most FSAE cars this means keeping the center plane of the bearing aligned to the plane of your A-arm so that lateral and longitudinal forces on the tire are reacted radially by the bearing.

Spherical bearings have limited misalignment from their midplane, and it is critical that the components don’t collide when the parts articulate. It is best practice to align linkages with their mounts (or vice versa) so that at the middle of the required articulation range the flat face of the ball and housing of the rod end are parallel. Most rod ends can only allow a small amount of misalignment when mounted between two tabs, this is listed on the spec sheet for your rod end. 

Misalignment limited by a narrow mount that is only as wide as the spherical ball.

If you exceed this allowable misalignment you'll quickly bend the shank on the rod end, or bend the mount, and either will quickly lead to component failure. To increase the allowable misalignment for your rod end you need a bushing on each side of the rod end, and there are several ways to do this.

The simplest way is to turn two spacers which have an outer diameter that matches the diameter of the flat face on your rod end and an inner diameter that matches the inner diameter of your rod end. This will have a very small wall thickness, so if made from aluminum it may deform when the bolt is tightened. These simple straight spacers are easy to make, but are annoying to install because they need to be held in place while the bolt is installed. To make assembly easier, make these spacers a permanent part of the mount. Some teams will make these spacers into a “top hat” shaped bushing which is welded into the mount. This also serves to massively increase the bearing surface of your mount, strengthening it substantially.

Correctly engineered, top hat bushings allow the maximum rated misalignment.

If you need more misalignment for your bearing you can use high-misalignment “super swivel” rod ends, with bushings incorporated into the ball; or make your own removable bushings like these. Because these bushings locate inside of your spherical bearing you will need to increase the rod end bore to maintain the same bolt diameter. These bushings are almost never available for the common rod end sizes found in FSAE, but you can easily make them on a CNC lathe. Some bushings have a round profile so that the bushing effectively becomes one with the ball of your rod end. This prevents the rod end from operating with a small sliver of contact area between the bearing race and the ball when it is articulated past its maximum designed articulation, but the dimensional tolerances and surface finish requirements on this surface must be correct to prevent shaving the PTFE liner out of your rod end or leaving the bearing race unsupported. With these bushings you can easily achieve 40+ degrees of misalignment.

You can make your own high-misalignment top hats, but detail design is even more important.

Some teams will use rod end safety washers for similar purposes. These are fine if made to appropriate tolerances, but because they are primarily intended for single shear applications the commercially available versions are often not accurate enough for use in double shear mounts.

Installation Design and Practice:

Installing a spherical bearing is not a trivial matter and it must be done correctly. One common way of retaining a spherical bearing is by using a housing with a flange at one end and a retaining ring at the other, which offers the ability to remove and replace the bearing as needed. This requires a housing which is wider than the ball, so removable misalignment washers or removable top hats will be required. A lighter method is to permanently retain the bearing into the part by staking or swaging. Both staking and swaging create a flange on either side of the bearing which lock it in place.

Staking is the process of deforming the housing around the bearing. This requires a spherical bearing with a chamfer on either side of the housing, which is the most commonly offered design. Staking tools are relatively inexpensive and the process can be done in most FSAE shops.

Swaging a bearing deforms the outer race of the bearing to create two flanges which lock it into the housing; this is how most rod ends are assembled. This requires very accurately machined chamfers on the housing. Swaging requires tools which are about 5x more expensive than staking tools.

No matter which method you use, correctly machining the housing bore before the install is imperative. For the ¼” spherical bearings often seen in FSAE the correct housing bore tolerance is usually about +0.000 -0.0005”. If you’ve welded the bearing housing when fabricating it is almost certainly now outside of tolerance, so it's best practice to ream the housing to final size after welding. If a spherical bearing is installed into a housing which is too small it will apply a constant radial load, which will increase the stiction in the joint.

Retaining a spherical bearing into a housing using a bearing retaining compound is a dangerous proposition, as the shear strength on the adhesive can often be the weakest link in the system. This becomes an issue when the bearing sees an axial load and the bearing retaining compound fails in shear. In addition, as the bearing retaining compound cures it expands, which exerts a radial preload on the bearing causing stiction. Lastly, should any bearing retaining compound make its way inside the joint it will immediately render it useless.

Looking after your bearings:

Chrome plated or stainless steel bearings do not require much maintenance, especially if used with a PTFE liner. It is best practice to blow any water out of the bearings any time they get wet, and keep them clear of dirt and debris. In my experience racing in the desert, rod end boots and seals cause more problems than they solve by trapping water and debris inside the joint instead of allowing it to fall away.

PTFE lined bearings do not require any extra lubrication, but unlined bearings will benefit from an extreme-pressure grease being applied, consult your bearing manufacturer’s documentation for specifics. Bent rod ends cannot be unbent, and any time a rod end bends you should replace it immediately and rectify the issue which caused it to bend! Similarly, any time a spherical bearing or rod end develops slop it is best practice to replace it and rectify the cause.