Understanding Today's Bearing Clearance Recipe - Engine Builder Magazine

Understanding Today’s Bearing Clearance Recipe

What’s needed to keep the rotating assembly rotating? It’s a recipe that includes three key ingredients: the bearing, the crankshaft and the oil. But understanding the part each plays as part of this recipe is what’s needed to keep your engines’ bearings from failing.

engine_bearingsWhat’s needed to keep the rotating assembly rotating? It’s a recipe that includes three key ingredients: the bearing, the crankshaft and the oil. But understanding the part each plays as part of this recipe is what’s needed to keep your engines’ bearings from failing.

When it comes to understanding what keeps a bearing from failing you must keep in mind that each ingredient, the bearing, the oil and the crankshaft all play a part in the recipe for long bearing life.

Changes in engine oil have caused manufacturers to make changes to bearings and crankshafts to make the recipe provide better bearing life than we have ever seen.

Let’s get a picture of how little clearance can be between a bearing and the crankshaft. The large circle represents what is supposed to be the diameter of a human hair. Note how small the .0001” circle is compared to a human hair.

Now let’s take a good look at the clearances between a crankshaft and bearings. Recently there has been a good awareness about the “hydrodynamic wedge” of oil that moves the crankshaft off center when the engine is running. That means that a bearing clearance that might be .002” could only be .0002” to .0003” between the bearing and crankshaft when the hydrodynamic wedge of oil moves the crankshaft off center. This is OK as long as this slight film of oil between the crank and bearing is not ruptured allowing crankshaft to bearing contact.

The quality of OE crankshaft surfaces has kept pace with the changes in oil. OE’s are requiring surface finish requirements that were almost pure fantasy years ago. Keep in mind that not only the surface finish is critical to keep from rupturing the oil film but the geometry of the journal is as critical. Out of roundness and taper used to have wide tolerances. It used to be common to allow a combination of .0005” out of round and taper. This tolerance did not create problems with heavier oil and larger bearing clearances, but today it is not acceptable.

High quality crankshaft manufacturing machines and the use of hard shoe polishers have changed the way OE’s and aftermarket suppliers can mass produce crankshafts that have the right ingredients to make the oil, bearing and crankshaft recipe work.

If your shop is lucky enough to have a profilometer you should be able to measure the surface finish of a journal. With a good quality micrometer and an experienced machinist the size, roundness and flatness can be measured. The problem some shops have is that their measuring equipment can also be experienced and might not be as accurate as it was when it was new.

Just be careful if you are using a belt style of crankshaft polisher that you don’t change the geometry. Flatness and taper are critical to keep from rupturing the very thin oil film between the crank and bearing. You can’t have good bearing life if the crankshaft is not good enough to be part of the recipe.

By Lyle Haley, The Shop Doc

You May Also Like

Shop Solutions May 2024

Those who submit Shop Solutions that are published are awarded a prepaid $100 Visa gift card. Submit your Shop Solution at [email protected].

Engine Builder and Engine Pro present Shop Solutions in each issue of Engine Builder Magazine and at enginebuildermag.com to provide machine shop owners and engine technicians the opportunity to share their knowledge to benefit the entire industry and their own shops. Those who submit Shop Solutions that are published are awarded a prepaid $100 Visa gift card. Submit your Shop Solution at [email protected]. You must include your name, shop name, shop address and shop telephone number. Submitted Shop Solutions not published will be kept on file and reevaluated for each month’s new entries.

Shop Solutions April 2024

Shop Solutions provide machine shop owners and engine technicians the opportunity to share their knowledge to benefit the entire industry and their own shops.

A Different Dyno Design

The dyno is a valuable tool, so it’s nice when an engine builder feels confident in the setup of it. Enter the shipping container engine dyno design.

Properties of Pistons

Pistons are perhaps one of the more sophisticated chunks of metal in the picture. Here’s what you should know.

A New Take on the Rotary Engine

What if we could design a new rotary engine that addresses certain limitations without violating the laws of physics? This is what LiquidPiston has been working at for over a decade.

Other Posts

Perfecting Ring Seal Soup

Using modern honing machines, surface finishes, crosshatch angles, ring materials, and coatings all combine to create a more efficient engine.

Connecting Rod Stress

Connecting rods are subject to constant stress through extreme tensile and compressive loads, each one tied to a different aspect of operation.

Balancing, A State of Equilibrium

The balance of a rotating assembly is critical in every aspect and for every engine.

Factors of Crankshaft Selection

From the high-performance powerplants propelling Top Fuel dragsters to the subdued engines found in family sedans and grocery getters, each crank must be tailored to, and appropriate for, its specific application.