Crank Walk, is it modes lining up?

[bjones18]

I was reading and subsequently lost a thread from an experienced non DSM engine builder asking about the differences between 6 bolt and 7 bolt blocks. I wrote this reply but had trouble posting it, so I started a new thread.

Good question. I am assuming it is in relation to crankwalk (CW). I am unfortunate as to have purchaced a vehicle with CW I have read theories on "mystery" tolerances and oil squirters. To most of us the OEMs seem to be stupid, but they aren't. Generally, tolerances can be tightened up relatively quickly in production, (especially if warranty $$ come in). The oil squirter theory has been poo-pooed by several builders.

CW is not new, it has been around for decades. I have read many articles on thrust bearing failure and attempts by bearing mfg's to resolve this (direct oiling by chamfering the bearing split, and sinusoidal oil wedging). Also, I have read that DSM went to a 3 piece thrust bearing on later 4G63s to alleviate CW. Why would DSM do this? Why not just do for the 7bolt what they did in the 6 bolt to fix CW Maybe they were "tooled" into the problem.

The fundamental difference I see is the 7 bolt girdle support vs the "free standing" main cap used for the thrust bearing on the 6 bolt. Girdles are better...right? They are stiffer.

I have seen engine block finite element anlysis results on V-8 main-cap girdles, and the effects the girdle stiffness has on block vibration modes and frequencies. Girdles make engines quiet, light weight, and stiff. The majority of force on a crank is from the rods trying to rip the crank out of the block. CW is a failure due to axial forces.

I can only think of two sources to create axial displacement:
1. clutch forces reacted by thrust bearing.
2. having a natural frequency of crank/rods (axial vibration mode) overlapping the natural frequency (axial) of the thrust bearing support.
2a. (same as 2 stated differently) 6 bolt thrust main has lower axial stiffness; higher crank mass.

Clearly, the oil film barrier between the crank thrust surface and thrust bearing surface is being eliminated under "some" circumstance(s) which promotes thrust bearing wear. How could this happen? The thrust force applied over the thrust surface area creates enough pressure to overcome the viscous barrier. What...DSM should know enough to calculate the required bearing surface area right? Unexpected vibration modes are every mechanical engineers nightmare (Tacoma Narrows bridge). Now, if we do have an axial vibration mode of the crank with a natural frequency close to an axial block mode, it is very difficult ($$$) to fix. Very very precise tolerances would be required to minimise the crank displacment (match mount bearing/block/crank to axial stack), to minimise the mass of the crank from "pounding" the thrust bearing. We'll leave the pounding up to the ACT 2600's

If this is root cause for CW maybe a reduced axial stiffness of the 6 bolt thrust cap allows the film barrier to remain intact; or maybe combined with increased mass of the 6 bolt lower end, the natural frequencies don't line up.

Hey.... I'm looking for a reason my car was laid up just after I bought it...AND why am I looking to put a 6 bolt in.

Bob
'97 Talon with a maintenance/seat time ratio similar to that of an F-16

 

[pneumo]

Maybe the axial frequencies in the 7 bolt block don't compliment the frequencies of the crank. Did you ever tap your knuckle on the side of a bare cranks counterweight? It rings with a clear metallic tone. Maybe the unbridged thrust girdle of the 6 bolt block lets it move with the cranks frequencies, instead of resisting them.

A crankshaft could easily get an axial vibration started when the piston is at TDC or BDC. I can picture the rod pushing straight up (or down) on the crank as it reaches TDC and BDC, and the adjacent counterweights being pulled toward each other.

What thread was your reply supposed to go into?

 

[Defiant]

I can only think of two sources to create axial displacement:
1. clutch forces reacted by thrust bearing.
2. having a natural frequency of crank/rods (axial vibration mode) overlapping the natural frequency (axial) of the thrust bearing support.
2a. (same as 2 stated differently) 6 bolt thrust main has lower axial stiffness; higher crank mass.

3. Cornering loads in a transversely-mounted engine.

 

[highpsi93tsi]

Tight tolerances (done properly) will allow an engine to run for many many thousands of miles. Loose tolerances are for racing..racing engines dont last.
Timing may be a factor. If the piston heading up meets a power stroke and too advanced
ignition timing (spark, air, fuel) before its at TDC its forced back down when it was only half way up. This is similar accurance with nitrous engines that arent properly retarded for each pill.
retarding the timing on a nitrous V8 is common practice but its rarely heard of on banger motors.
Iam saying the problem may be an old and tired ECU... AND mangled tolerances....AND a MBC. {suppost to retard for each psi just like spray..how many of you have retaded one degree for each 2-LBs boost.??..exactly)