Intermediate Shaft Bearing

Porsche 996/997/986/987 Intermediate Shaft Bearing Background

Background

Porsche’s early liquid cooled sixes have suffered a checkered past.  An infamous problem being the troublesome intermediate shaft (IMS) bearing in the original  M96/M97 liquid cooled engines.  That design used a double or single row ball bearing that has an expensive and too-frequent record of premature failure with dire consequences for both engine and checkbook.

The current “fixes” range from additional oilers to ball bearings with ceramic balls in place of the original steel ones, i.e., the hybrid ceramic that is popular today, the plain bushing style and cylindrical roller bearings.

False Premises

One claim made is that the original bearings’ oil seal fails and engine oil “washes” the lubricating grease out, causing the failure.  Interestingly enough, grease is motor oil mixed with an agent that liquefies under normal operation and becomes close to the base oil anyway.   https://en.wikipedia.org/wiki/Grease_(lubricant)   One of the recommended ‘fixes’ is to remove the bearings’ oil seal to allow oil in.  This contradicts the “oil washing the grease out” theory.  

Engine oil will lubricate the bearings properly.


The premise of inadequate lubrication misses the real cause of failure.  Ball bearings can run with minimal lubrication i.e. ‘drip lubrication’.  Watch the last two minutes of this bearing removal video:  https://www.youtube.com/watch?v=JKbsNpjvYDw&t=128s.   You can see oil pouring out of the IMS tube upon bearing removal.

 

Oil starvation is not an issue here.  

Additional Oilers are Not Needed

The other false premise is, “an overloaded bearing can live if enough oil is provided.”  This is blatantly false.  Were it true, large and very expensive bearings (i.e. railcar, heavy trucks) wouldn’t be needed as inexpensive small bearings could be over-lubricated to take their place. Consequently the ‘oiler kits’ aren’t the answer.  The worn/bad IMS bearings evidence spalling (flaking/chipping) of the balls and raceways.


                                     This is caused  by overloading.

 

A lubrication fault would be evidenced by bluing (overheating) or metallic smearing/welding of the balls and races.These failure modes are not present in this  application.

Self-lubrication

Elastohydrodynamic (EHL) lubrication, simply put, is the tiny wedge-shaped body of oil that is in front of and between the roller/raceway interface.  EHL provides a self-pumping action that can raise the contact point oil pressure to several thousand psi.  This is the real lubrication pressure,everything else is just a function of delivery.  An added bonus is that oil viscosity also increases.

Roller bearings enjoy more benefit from EHL as the flanges of the races  better contain the oil wedge. A ball bearing acts like the nose of a ship and pushes the oil out past the sides of the races where it does no good. (https://en.wikipedia.org/wiki/Lubrication)

Thrust Control

The importance of THRUST CONTROL is completely over-stated.  This is a non-starter as the IMS sprocket and chain system is loaded radially, any axial load (thrust) is minimal and well within the  thrust capacity of all bearings offered i.e. ball or cylindrical. (see table below)

The bearing manufacturers state axial load capacities to be from 15% to 50% of radial load ratings for CONTINUOUS OPERATION. The 996/997/986/987  IMS bearings experience ‘a GLIDING or GLANCING LOAD’ only, so thrust loading is much less significant.

Ceramic hybrid bearings have a much lower predicted life than conventional all-steel bearings.

The idea that ceramic/hybrid bearings are an upgrade is contradicted by this recent NASA report on the subject of  ceramic/hybrid bearings vs. conventional all steel bearings (oem).

 

To summarize the NASA report  ceramic/hybrid bearings have ½ the predicted life (fatigue life) of a conventional all-steel bearing.  Ceramic balls are very hard and overload the ‘softer’ steel races.  In a conventional ‘all steel’ bearing all components ‘share’ the imposed load thus wear longer.   https://ntrs.nasa.gov/search.jsp?R=20050175860

Bearing Load Capacity

The fundamental problem is that the original bearings were overloaded as proven by the higher failure rate of single row bearings compared to double row bearings. No amount of extra lubrication can correct the problem of an overloaded bearing .  The key to long bearing life (L/10) is the bearing’s rated load capacity (C or Co).  All things being equal, when a bearings’ load capacity is doubled, the bearing’s life is extended four times or more.

The Porsche IMS has a “package size” that can’t be easily changed, so a higher load capacity bearing to fit the existing dimensions is needed.  The highest capacity bearing design for the given package size is a cylindrical roller bearing.

 

Cylindrical bearings have heavy-duty rollers instead of balls.  This creates a much stronger bearing at the expense of only slightly higher friction.  These bearings cost more than ball bearings, but their load capacity and thus their life greatly exceed steel ball or hybrid ceramic ball bearing designs. 

To further explain  the issue, all bearings have two load ratings specified, CO, a stationary rating, and C, a dynamic or rolling rating.  The dynamic rating, C, is always higher.  When the rolling elements, balls or rollers spin, the load is better distributed within the bearing.

This is the reason for the “low mileage” failures that have occurred in ‘Sunday’ cars.

The CO  or static rating for the ball bearings is too low and the balls dent the bearing’s races when the engine is stopped as the chain tensioners still push on the chain for a period of time after engine shut-down, micro-denting the races.  This wear snowballs. Cylindrical rollers have much higher CO capacity, so this  static denting is minimal.

Misconceptions about load/life rating

There is an industry myth floating around about bearing fatigue life.  The specification L10 is the number of hours that 90% of the bearings are expected to run and show minimal wear.  L/10 life is not the failure point;  even the 10% showing wear can continue to operate safely but with more noise/vibration.

 http://www.ntnamericas.com/en/website/documents/brochures-and-literature/catalogs/ntn_a-1500-iii_low_res_d1_20091218.pdf   (see page 4  or 11)

 

The Pacomar life/load calculator shown in the link below was used for all calculations.  www.pacamor.com/technical/lifeloadcalculator.php


Keep in mind that these are “Ideal L10” ratings and subject to installation specific conditions.  However, the data does provide relevant comparative bearing lives.

 

All data below is for bearings that will replace the stock IMS bearings without machining. 

 

All  (C) and (Co) bearing loads are in pounds and fatigue life (L10) is in hours. 

 

SKF Bearing Corp. data was used for load capacities.  Co = Static    C = Rotating

IMS Bearing Load Parameters

Cylindrical rollers, by design, have much higher load capacities than same sized ball bearings.

 

During installation, the cylindrical roller bearing is installed in two separate pieces, thus there is no worry about installation forces damaging the rollers or raceways.

 

In one case, a cylindrical IMS roller bearing was still in service three years after replacing a nearly failed OEM bearing which had resulted in heavy metal flakes in the IMS tube. This, so far, has been a successful repair application where installation of hybrid ceramic bearings is not permitted by their vendor.

 

Summary

a) It is not lack of oil.

b) A repair using 'Hybrid Ceramic bearings' is questionable as evidenced by the NASA report linked below.  NASA says “A hybrid ceramic bearing” has approximately half the life than a standard ‘all steel’ bearing. https://ntrs.nasa.gov/search.jsp?R=20050175860

c) It is IMPROVED by the use of a cylindrical roller bearing.

d) Factory oil change intervals should be reduced to no more than 7500 miles or one year.

All oils certified for the engine will protect the cylindrical roller IMS bearing.

References 

NASA... Hybrid vs. All Steel Bearing Life, Summary:  “Hybrid ceramic bearings exhibit half the life of a regular all-steel bearing.”
https://ntrs.nasa.gov/search.jsp?R=20050175860
 

Bearing Life Calculator: www.pacamor.com/technical/lifeloadcalculator.php

Bearing load ratings: Bearings, Seals, Lubrication, Mechatronics & Services | SKF Group

Respectfully submitted this date: 03/28/18

Robert Hearst
Precision Automotive & Electric
8516 N.E. Bothell Way Bothell, Wa.  98011
425.486.5400

www.por-ims.com

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