Crankshaft Thrust Bearing Failure

For years both transmission and engine rebuilders have struggled at times to determine the cause of crankshaft thrust bearing failure.

So, In most instances, all of the facts concerning the situation; are not revealed at the onset of the crankshaft thrust bearing failure.

Crankshaft Thrust Bearing Failure Background

Although thrust bearing run on a thin film of oil; just like radial journal (connecting rod and main) bearings; they cannot support nearly as much load. While radial bearings can carry loads measured in thousands of pounds per square inch of projected bearing area; thrust bearings can only support loads of a few hundred pounds per square inch.

Radial journal bearings develop their higher load capacity; from the way the curved surfaces of the bearing and journal meet to form a wedge. Furthermore, Shaft rotation pulls oil into this wedge shaped area of the clearance space; to create an oil film which actually supports the shaft. Thrust bearings typically consist of two flat mating surfaces; with no natural wedge shape in the clearance space; to promote the formation of an oil film to support the load.

For this reason, many heavy-duty diesel engines use separate thrust washers; with a contoured face to enable them to support higher thrust loads. These thrust washers either have multiple tapered ramps and relatively small flat pads; or they have curved surfaces that follow a sine-wave contour around their circumference.

Crankshaft Thrust Bearing Failure Cause Of Failure

So, There are only four common factors which generally cause thrust bearing failures. They are:

Poor crankshaft surface finish
Misalignment
Overloading
Surface finish

Crankshaft thrust faces are difficult to grind; because they are done; using the side of the grinding wheel. Consequently, Grinding marks left on the crankshaft face; produce a visual swirl or sunburst pattern with scratches; sometimes criss crossing one another; in a cross-hatch pattern similar to hone marks on a cylinder wall. So, If these grinding marks are not completely removed by polishing; they will remove the oil film from the surface of the thrust bearing. A properly finished crankshaft thrust face should only have very fine polishing marks; that go around the thrust surface in a circumferential pattern. Finally, Shot peening can dramatically increase a Crankshaft’s life; so it may be a good idea to look into this.

Alignment:

So, A grinding wheel that does not cut cleanly may create hot spots on the workpiece; leading to a wavy, out-of-flat surface. The side of the wheel must also be dressed at exactly 90° to its outside diameter. This will produce a thrust face that is square to the axis of the main bearing journal. So the crankshaft grinding wheel must be fed into the thrust face very slowly; and also allowed to “spark out” completely. The machinist should be very careful to only remove minimal stock for a “clean-up” of the crankshaft surface.

In most instances a remanufactured crankshaft does not require grinding of the thrust face(s); so the grinding wheel will not even contact them. Oversize thrust bearings do exist.

Grinding crankshaft thrust faces requires detailed attention during the procedure. Also, repeated wheel dressings may be required. Maintaining sufficient coolant between the grinding wheel and thrust surface must be attained; to prevent stone loading and “burn” spots on the thrust surface. All thrust surface grinding should end in a complete “spark out”; before the grinding wheel is moved away from the area being ground. Following the above procedures with care should also maintain a thrust surface that is 90° to the crankshaft centerline.

When Assembling Thrust Bearings:

Tighten main cap bolts to approximately 10 to 15 ft.lb. to seat bearings, then loosen.

Tap main cap toward rear of engine with a soft faced hammer.
Tighten main cap bolts, finger tight.
Using a bar, force the crankshaft as far forward in the block as possible; to align the bearing rear thrust faces.
While holding shaft in forward position, tighten main cap bolts to 10 to 15 ft.lbs.
Complete tightening main cap bolts to specifications in 2 or 3 equal steps.

The above procedure should align the bearing thrust faces with the crankshaft; to maximize the amount of bearing area in contact for load carrying.

Loading:

A number of factors may contribute to wear and overloading of a thrust bearing, such as:

1. Poor crankshaft surface finish.

2. Poor crankshaft surface geometry.

3. External overloading due to.

a) Excessive Torque converter pressure.

b) Improper throw out bearing adjustment.

c) Riding the clutch pedal.

d) Excessive rearward crankshaft load pressure due to a malfunctioning front mounted accessory drive.

Note: There Are Other, Commonly-Thought Issues Such As:

Torque converter ballooning
The wrong flexplate bolts
The wrong torque converter
Pump gears being installed backward

All of these problems will cause undue force on the crankshaft thrust surface. It will also cause the same undue force on the pump gears; since all of these problems result in the pump gear pressing on the crankshaft via the torque converter. The result is serious pump damage, in a very short period of time (within minutes or hours). So, To help prevent any further damage; you should make sure to look after your torque converter as well.

Crankshaft Thrust Bearing Failure Diagnosing The Problem

So, By the time a Crankshaft Thrust Bearing Failure becomes evident; the parts have usually been so severely damaged that there is little if any evidence of the cause. As a result, the bearing is generally worn; into the steel backing which has; severely worn the crankshaft thrust face as well. So how do you tell what happened? Start by looking for the most obvious internal sources.

Engine related problems

Is there evidence of distress anywhere else in the engine that would indicate; a lubrication problem or foreign particle contamination:

Were the correct bearing shells installed; and were they installed correctly?
If the thrust bearing is in an end position; was the adjacent oil seal correctly installed? An incorrectly installed rope seal can cause sufficient heat to disrupt bearing lubrication.
Examine the front thrust face on the crankshaft for surface finish and geometry. This may give an indication of the original quality of the failed face.

Crankshaft Thrust Bearing Failure Transmission Related Problems:

Did the engine have a prior Crankshaft Thrust Bearing Failure:

So, What external parts were replaced?
And, Were there any performance modifications made to the transmission?
Also, Was an additional cooler for the transmission installed?
Correct flexplate used?
Was the transmission property aligned to the engine?
Were all dowel pins in place?
Was the transmission-to-cooler pressure checked; and found to be excessive? If the return line has very low pressure compared to the transmission-to-cooler pressure line, check for a restricted cooler or cooler lines.
What condition was the throw out bearing in?

How Does The Torque Converter Exert Force On The Crankshaft

So, There are many theories on this subject, ranging from converter ballooning to spline lock. But, Most of these theories have little real bases and rely little on fact. The force on the crankshaft from the torque converter is simple. It is the same principle as a servo piston or any other hydraulic component: Pressure, multiplied by area, equals force. The pressure part is easy; it’s simply the internal torque converter pressure.

The area is a little trickier. The area that is part of this equation; is the difference between the area of the front half of the converter and the rear half. The oil pressure does exert a force that tries to expand the converter like a balloon. However, it is the fact that the front of the converter has more surface area than; the rear that causes the forward force on the crankshaft.

Causes For Excessive Torque Converter Pressure

So, There are two main causes for excessive torque converter pressure; restrictions in the cooler circuit and modifications or malfunctions that result in high line pressure. One step for combating restrictions in the cooler circuit is to run larger cooler lines. Another, is to install any additional cooler in parallel as opposed to in series.

Modifications that can result in higher than normal converter pressure; include using an overly-heavy pressure regulator spring; or excessive cross-drilling into the cooler charge circuit. Control problems such as a missing vacuum line or stuck modulator valve can also cause high pressure.

Always find the cause of distress and correct it before completing repairs, or you risk a repeat failure.

A simple modification to the upper thrust bearing may be beneficial in some engines. Install the upper thrust bearing in the block; to determine which thrust face; is toward the rear of the engine. Using a small, fine tooth, flat file, increase the amount of chamfer to approximately .040″ (1 mm); on the inside diameter edge of the bearing parting line. Finally, Carefully file at the centrally located oil groove; and stroke the file at an angle toward the rear thrust face only.

So, It is very important not to contact the bearing surface with the end of the file. The resulting enlarged ID chamfer will allow pressurized engine oil from the pre-existing groove to reach the loaded thrust face. As a result, This additional source of oiling will reach the loaded thrust face without passing through the bearing clearance first (direct oiling).

Special Thanks to Lance Wiggins and ATRA for Great Information

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