Tuo on seikkaperäinen (sivulla 7 tässä pitkäksi venähtäneessä ketjussakin) vikavaihtoehtolista.
Lisäksi Cleviten dokumentin
http://www.cleviteengineparts.ca/public ... 1-1208.pdf mukaan sivulta 27
Tiedosto
http://www.mustangclub.fi/board/downloa ... p?id=14796
Premature Thrust Bearing Failure
BACKGROUND
Although thrust bearings 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 psi. Radial journal bearings develop their higher load capacity from the way the curved surfaces of the bearing and journal meet to form a wedge. 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.
Conventional thrust bearings are made by incorporating flanges at the ends of a radial journal bearing. This provides ease in assembly and this design has been used successfully for many years. Either tear-drop or through grooves on the flange faces and wedge shaped ramps at each parting line allow oil to enter between the shaft and bearing surfaces. However, the vast majority of the bearing surfaces and the entire shaft surface are flat making it much harder to create and maintain an oil film. If you have ever taken two gauge blocks and wiped them perfectly clean and pressed them together with a twisting action you know that they will stick together. This is very much like what happens as a thrust load applied to the end of a crankshaft squeezes the oil out from between the shaft and bearing surfaces. If the load is too great, the oil film collapses and the surfaces want to stick together, resulting in a wiping failure.
For many years some heavy-duty diesel engines have used separate thrust washers with a profiled face to enable them to support higher thrust loads. These thrust washers either have multiple tapered ramps and relatively small flat pads or have curved surfaces that follow a sine wave contour around their circumference.
CAUSES OF FAILURE
Aside from the obvious causes, such as dirt contamination and misassembly, there are only three common factors that generally cause thrust bearing failures. These are:
• Poor crankshaft surface finish
• Misalignment
• External overloading
SURFACE FINISH
Crankshaft thrust faces are difficult to grind because they are done using the side of the grinding wheel. Grinding marks left on the crankshaft face produce a visual swirl or sunburst pattern with scratches sometimes crisscrossing one another in a crosshatch pattern similar to hone marks on a cylinder wall. If these grinding marks are not completely removed by polishing, they will remove the oil film from the surface of the thrust bearing much like multiple windshield wiper blades.
A properly finished crankshaft thrust face should only have very fine polishing marks that go around the thrust surface in a circumferential pattern. A surface finish of 15 Ra or less is recommended (10 Ra or less for heavy duty or highly loaded engines).
ALIGNMENT
The grinding wheel side face must be dressed periodically to provide a clean, sharp cutting surface. A grinding wheel that does not cut cleanly may create hot spots on the work piece leading to a wavy, out-of-flat surface. The side of the wheel must also be dressed at exactly 90° to its OD to produce a thrust face that is square to the axis of the main bearing journal.
When assembling thrust bearings:
1. Tighten main cap bolts to approximately 10 to 15 ft. lbs. to seat bearings then loosen.
2. Tap main cap toward rear of engine with a soft-faced hammer.
3. Tighten main cap bolts, finger tight.
4. Using a bar, force the crankshaft as far forward in the block as possible to align bearing rear thrust faces.
5. While holding shaft in forward position, tighten main cap bolts to 10 to 15 ft lbs.
6. 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 (in no specific order):
• Excessive pump pressure
• Torque converter expansion
• Torque converter internal wear
• Pump drive gear installed backwards
• Wrong torque converter
• Wrong flex plate
• Wrong flywheel bolts
• Misalignment of the engine and the major transmission components
• Improper throw out bearing adjustment
• Riding the clutch pedal
DIAGNOSING PROBLEMS
By the time a thrust bearing failure becomes evident, the parts have usually been so severely damaged that there is little if any evidence of the cause. 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:
• Is there evidence of distress anywhere else in the engine that would indicate a lubrication problem or foreign particle contamination?
• Were the proper bearing shells 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 un-failed thrust face on the crankshaft for surface finish and geometry. This may give an indication of the original quality of the failed face.
• Once you are satisfied that all potential internal sources have been eliminated; ask about potential external sources of either overloading or misalignment.
• Did the vehicle have a prior thrust bearing failure?
• What external parts were replaced?
• Was the correct transmission installed?
Was the correct torque converter installed?
• Was the correct flex plate used? At installation there should be a minimum of 1/16” (1/8” preferred) clearance between the flex plate and converter to allow for converter expansion.
• Were the correct flex plate mounting bolts used?
• Is there evidence of the converter hitting the flex plate mounting bolts?
• Was the transmission properly aligned to the engine?
• Were all dowel pins in place?
• Check condition of pilot bearing.
• If a used torque converter was re-used, is it worn internally?
• If a rebuilt transmission was installed, did the torque converter engage the pump drive spline properly? An improperly installed pump drive gear may prevent full engagement of the converter.
• Was the transmission pump pressure checked and found to be within specification?
• Check external transmission cooling lines and heat exchanger for restrictions that will increase pump pressure.
• If a manual transmission was installed, was the throw out bearing properly adjusted?
• What condition was the throw out bearing found to be in? A properly adjusted throw out bearing that is worn or overheated may indicate the operator was “Riding the Clutch”.
HELP FOR THE THRUST BEARING
When a problem application is encountered, every effort should be made to 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 help in problem applications. 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 chamfer on the ID edge of the bearing parting line from the oil groove to the rear thrust face only. (See diagram.) This enlarged ID chamfer will allow pressurized oil from the bearing oil groove to reach the loaded thrust face without passing through the bearings clearance space first. Since there is a load against the rear thrust face, the load should restrict oil flow and there should not be a noticeable loss in oil pressure. Although this modification is not a guaranteed cure-all it should help if all other conditions, such as surface finish, alignment, cleanliness and loading are within reasonable limits.
RECENT DEVELOPMENTS
In the past few years some new automotive engine designs have begun using various methods to enable them to carry higher thrust loads imposed by some of the newer automatic transmissions. The use of thrust washers exclusively, a combination of thrust washers and flange bearings, radial grooving (a series of 3 - 5 straight grooves on the flange face), flange bearings with profiled (ramp and pad design) flange faces, or assembled flanges (separate thrust washers which are mechanically attached to the bearing). It is important to keep in mind that most of these developments are application specific and may not be available or even feasible for older applications.
SEVERE USE RECOMMENDATIONS
Crankshaft surface finish and shape are key factors affecting the performance of all bearings. These factors become even more critical for thrust surfaces. As in any bearing, increased loading reduces oil film thickness between shaft and bearing surfaces. This is a much more critical situation in thrust bearings due to their flat faces which make formation of an oil film extremely difficult. Radial bearings (those which carry loads in a radial direction like rod and main bearings) form a natural wedge where shaft and bearing surfaces come together in the clearance space. Shaft rotation pulls a wedge of oil into the loaded area of the bearing and forms an oil film that supports the load.
Thrust faces, on the other hand, are made up of two flat surfaces that do not form a natural wedge where they meet. In order to help form an oil film, artificial wedge shaped areas are machined into the bearing surfaces at the ends and sometimes adjacent to the grooves. In spite of all the common design efforts, thrust bearings still run on a much thinner film of oil that makes crankshaft surface finish critical in the successful performance of these bearings.
Recent samples of thrust face surface finish on crankshafts from blown fuel “Hemi” engines have confirmed that better finishes resulted in a reduced rate of bearing distress. The study also showed that when no damage occurred, the crankshaft surface finish was improved after running. The surface finishes of 12 crankshafts were measured (7 new and 5 used). The new shafts ranged from a high of 30 Ra to a low of 5 Ra. The used shafts had a very similar range from 31Ra to 4 Ra. Although this represents only a small sampling, it does demonstrate a correlation between surface finish and performance when the condition of mating bearing surfaces was evaluated. Prior to these measurements, race experience had shown no problems on a crankshaft with a thrust-face Ra of 6 and DID show problems on crankshafts when the Ra was over 20!
Obtaining a good finish on the thrust face of a crankshaft is difficult to do because it uses side-wheel grinding. Side grinding causes marks that spiral outward toward the OD of the thrust face and may also cause crosshatch marks resembling honing patterns. Both patterns are detrimental to the formation of an oil film because they work like wipers as the shaft rotates. Grinding marks must be removed by polishing. Only a circular pattern should remain. Surface finish should be checked in a tangential direction and must be held to 10 Ra max. The thrust surface should be flat within .0002” max.
