What Are Rod Bearings?by Lyle Burwell
In automotive mechanics a rod bearing is a plain bearing (as opposed to a ball bearing) used to hold spinning shafts in place where support is required to keep the rod straight over the course of its run. The phrase is also used as shorthand for connecting rod bearings, which are quite a different matter. If it is suggested to you that a problem with your car is caused by worn or faulty rod bearings, it is vital that you clarify which type of bearing is being identified.
A rod bearing is a two-piece (top and bottom) band of smooth metal. In the case of driveshaft rod bearings, for example, the top half of the bearing is a semi-circle that is mounted to the undercarriage of the vehicle. The bottom half of the bearing is bolted to the top half once the driveshaft is in place. A pinhole through the top half of the bearing allows lubrication to reach the inside. This permits the shaft to spin within the bearing without generating excessive heat.
Connecting Rod Bearings
Connecting rod bearings (also called rod-end bearings) are a component in the piston-to-driveshaft portion of the powertrain. This spherical articulating joint is also known as a heim joint in the United States and a rose joint in the United Kingdom. The device was invented in Nazi Germany and was discovered by the Allies in a German plane shot down over England in 1940. Patent rights were given to the Rose Bearing Company in the UK and the Heim Company for North America. Rod-end bearings connect the piston rod (also called the gudgeon or wrist pin) to the bearing journal. In this context "journal" refers to the section of the piston rod that contacts and turns the driveshaft. Replacing the connecting rod bearings is a standard step in an engine rebuild.
If you are told that your rod bearing is spun, it means a breakdown in the rod bearing lubrication system occurred and that left the rod bearing is not lubricated. The shaft, spinning inside a dry bearing, heats up very quickly. The shaft expands under the heat to the point where it grabs and detaches the bearing from its fastenings. The bearing is then essentially welded to the rod and spins with it. Driving a car with a spun rod for any length of time will heat up the rod-end bearing, resulting in throwing a piston rod, which will destroy the engine beyond repair
Recognizing Bearing Problems
Failure to recognize and correct rod bearing problems will lead to catastrophic engine failure. Most modern passenger vehicles have sealed transmissions. This allows the driveshaft to spin while submerged in transmission fluid and makes rod bearing failure very rare. Unfortunately, sealed units eliminate the need for a transmission fluid reservoir, which was the earliest warning system for transmission fluid leaks. Bearing problems should be suspected when a persistent ticking that increases with rpm is audible.
- photo_camera Thinkstock/Comstock/Getty Images