Necessary Properties of Bearings and Bushings

The choice of lubricant gets all the attention but the proper selection of bearing and bushing material is critical. Few materials satisfy the requirements listed below.

Bearings and bushings must have the following characteristics:

it must be strong enough to carry the load - high compressive strength.
it must have high fatigue strength - durable.
it must have low friction.
it must resist welding and seizing.
it should have high thermal conductivity to remove friction heat from the surface.
the ability to absorb and discard small contaminant particles to keep them from scoring the shaft.
capable of slight adjustments to compensate for shaft roughness and misalignment - this is the ability of the bearing to "wear-in".
it must be corrosion resistant.
it must be relatively easy to machine or form to shape-reasonable cost.

Bronze alloy bearings and bushings have all of these characteristics . Shown here is an aluminum bronze bushing. Aluminum bronze is commonly used as a valve guide material.
Selection of the correct material for the conditions is critical. The picture at the left shows a poor aluminum bronze surface finish. Poor material selection and poor finish resulted in a fatal aircraft accident (Cessna 404 Vh-ANV) when the shaft seized to this bearing. This is a photograph of the suspect part.

Picture to the left is of a connecting rod bearing. Notice the white "spider" web marks on the left side. These are early fatigue cracks from overload. Now look at the right side of the bearing. Notice the white area. This is where the shaft did not contact.

Misalignment caused the shaft to press against the left side of the bearing, overloading the bearing material and cracking it. Eventually, small pieces of bearing material will break out. This will release oil pressure from between the shaft and bearing. Complete destruction of the bearing will then occur. This will allow the connecting rod to pound against the bearing shell. Next the pounding will break a connecting rod bolt. The rod cap will peal away from the rod and allow the rod to fling itself out through the crankcase.

Its always a good idea to take a good look at old bearings and bushings upon disassembly. You can quickly see if the shaft and bearing "like" each other. In this example, something is not in alignment and must be fixed. Replacement of wear parts would not solve the alignment problem. Plain bearings will adapt to some mis-alignment without failing.

Notice the piece of bearing material missing from this Lycoming con rod. Also notice that the missing piece extends to the edge of the bearing. The edge is important because when the cavity reach the edge, then the oil can escape from between the bearing and shaft. This ruptures the oil film and quickly causes failure. If you are increasing engine horsepower beyond the original design then your concerns are:

Are the bearings strong enough to take the extra load?, and
for how long?

In the recent past, Lycoming want through several generations of bearings to meet these concerns on their engines. What generation bearing will your "hopped-up" engine use?

Piston skirt showing scuffing damage (metal to metal rubbing). Aluminum conducts heat better than most other metals.

The piston skirt is also a bearing surface. All of the engine torque - the force turning the propeller occurs because the piston presses against the cylinder wall. The more torque, the more the piston skirt presses. Its part of Newton's Law where every action has a reaction. In an engine the action of the turning propeller is very visible, the reaction is hidden away inside the engine and gets forgotten.

There were a several engine failures in Continental IO-520 engines in the 1990's when the piston design reduced the piston bearing surface (thrust face) to an area about the size of a quarter. Cylinder wall cracking and at least one fatal aircraft accident resulted before the piston was quietly redesigned. You can inspect used pistons and see the thrust face area on the piston.

Connecting rod bearing showing poor shaft alignment. Contact occurs only on right hand side and bottom of bearing. The bearing "took" the misalignment in this case without immediately failing. The soft babbit material will make some adjustment for misalignment.

The soft babbit material - white metals are often used for babbit. These include lead, tin, antimony, zinc, aluminum. For extra strength - silver. Lead is being used less often. However, during the process of making non-lead babbit, weaker bearings have caused failures, including several fatal aircraft accidents. Bearings have a fatigue life and are impossible to inspect without engine disassembly.

To the left is a picture of a magneto gear and block assembly. The steel shaft rides in a bronze "self-lubricating" olite bushing. Olite bushings are made from particles that are squeezed (sintered) together under high-pressure. Small pores in the material absorb oil. The white felt ring is used as an oil reservoir to provide additional lubrication. Once this part is assembled, it may operate for 20 years or longer with little wear.

The white spots on this connecting rod bearing are aluminum particles that have embed into the soft bearing surface. The ability of particles to 'sink into" the bearing prevents the particles from sticking out and scoring the shaft. The connecting rod bearing can be considered a fine oil filter. Particles enter the bearing through the oil channel and cannot pass out because of the very thin film of oil that separates the surfaces. Particles are trapped and pounded into the bearing.

There is a limit as to the amount of material that the bearing can capture before loss of oil clearance causes the bearing to contact the shaft. When this happens the babbit heats up and melts. Only the bearing shell remains. The connecting rod then pounds against the shaft, eventually breaking one of the rod bolts.

Bronze gear (large one) and worm. This is a picture of the starter reduction gearing in a Continental IO-520 starter adapter. Worm gear sufaces slide across one another as the gear rotates. They require a gear material that will not wear or gall when the two surfaces rub.

Bronze and its various alloys better meet all of the necessary properties than any other material. This is why bronze is used in many bushing and bearing applications. Some bronze alloys, such as aluminum bronze, require full lubrication, while other bronze alloys do better in conditions of partial lubrication