Engine Mounts - Drivetrain Misalignment 1

[Autotecheng]

Hello, I stumbled across this odd note from GM's engineers in their service manual:

"Notice: Broken or deteriorated mounts can cause misalignment and destruction of certain drive train components. When a single mount breaks, the remaining mounts are subjected to abnormally high stresses."

Conventional wisdom says that the engine and transmission are aligned by the dowels and secured together with a number of bolts.

But transmission casings are fairly flexible and could allow some alignment changes if not supported properly... they often weigh around 200 pounds in service.

Flex plate failures are common on GM trucks so it begs the question -

WHICH drive train components are they referring to, and how are they affected?

I am wondering if the torque converter is the culprit. The housing of the torque converter is bolted to the crankshaft, but the turbine and oil pump gear are in fixed relationship to the transmission casing.

What do you think?

P.S. I like how strong the language regarding the effect is - "destruction" and how vague the language regarding the cause is - "certain components"

 

[pontiacjack]

I'm fairly certain the "driveline" referred to in the manual begins at the transmission output shaft.

 

[patprimmer]

Jack, whilst I agree one would think the universal joints or CVs would accomodate any movement.

Regards
Pat
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[MikeHalloran]

When a motor mount is broken, the transmission output torque goes into rotating the entire engine and transmission about an axis through the two surviving mounts, for the few cm until it hits something, or until the vehicle moves. Then the engine/transmission drops, and the cycle restarts. At the same time, the u-joints are at different angles, which introduces another torque variation that isn't ordinarily present.

If the terrain is flat like here in SoFla, and you drive super gently (not always possible in SoFla), you might drive around with a broken motor mount for months without noticing. More likely, the extra vibration on accel/decel will kill your u-joints pretty quickly.

I.e., the reference is certainly to u-joints.

Flex plates fail from flexing. Yeah, duh, but bear with me. Some flex is normal from torque transmission, when the plate wrinkles a little. Other possible causes include angular misalignment, e.g. a little dirt on the crank flange face or the engine flange face, radial misalignment, e.g. holes and pilots off center a little, or maybe a bad stackup so the converter face is too close to or too far from ( axially ) the crank flange. You might also have bad stackups when the crank flange face moves axially, as in when the crank bearing with the thrust faces wears out from never changing the oil, etc.

Oh. And the torque converter, which can expand axially from internal pressure. There isn't supposed to be very much pressure, but I think there are transmission faults than can inflate a converter.





Mike Halloran
Pembroke Pines, FL, USA

 

[Tmoose]

Hi Autotecheng,

what is the title and publishing date of that Gm service manual?

thanks

Dan T

 

[MiketheEngineer]

Like the Internet - if GM says it is true - then it is??!!

 

[patprimmer]

I thought on anything even remotely modern in design, like since the 1950s had engine mounts that if the rubber failed, there was a steel backup that limited movement to an inch or so. The engine jumped around and made serious noise well before it moved enough to damage anything else.

Regards
Pat
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[Autotecheng]

>I'm fairly certain the "driveline" referred to in the manual begins at the transmission output shaft.

IE, the drive shaft angles.

>Jack, whilst I agree one would think the universal joints or CVs would accomodate any movement.

CV inner joints (tripod style) can operate at angles up to 25 degrees normally, I believe. Outer joints are even more flexible. Universal joints are far less forgiving, the torque and the speed of a drive shaft is far greater than a CV joint and the angle is much more limited. From what I've read from Dana and other sources drive shaft angles need to be maintained to within a couple of degrees of each other and not to exceed 3 degrees total.

I have a head ache so I can't be more specific, but drive shaft alignment is certainly a concern with failing powertrain mounts.

>When a motor mount is broken, the transmission output torque goes into rotating the entire engine and transmission about an axis through the two surviving mounts, for the few cm until it hits something, or until the vehicle moves. Then the engine/transmission drops, and the cycle restarts. At the same time, the u-joints are at different angles, which introduces another torque variation that isn't ordinarily present.

Yeah, I know what you mean. I have trouble imagining how much the torque changes. But what I think is that with a typical engine that rotates clockwise as seen from the driver's point of view, the thrust forces are going to be down on the right side as the pistons are pushed down and up on the left side. In other words I think the engine produces an angular momentum or torque which will cause it to try to spin.

>Oh. And the torque converter, which can expand axially from internal pressure. There isn't supposed to be very much pressure, but I think there are transmission faults than can inflate a converter.

I've come across that issue in my research as well, if the torque converter charge circuit is restricted on the outlet side the pressure can build inside the torque converter to the point that its stamped steel housing expands and deforms. Which can cause it to bottom out into the transmission and damage the trans oil pump. It also causes the flywheel and crankshaft to be pulled forward destroying the thrust bearing.

Here's a short video showing it happening:

http://www.youtube.com/watch?v=YEfqKoilLkI

>what is the title and publishing date of that Gm service manual?

"Engine Mount Inspection" Publishing is current as of today and goes back over a decade.

>I thought on anything even remotely modern in design, like since the 1950s had engine mounts that if the rubber failed, there was a steel backup that limited movement to an inch or so. The engine jumped around and made serious noise well before it moved enough to damage anything else.

As far as I can tell a great many bushings are used in automotive engineering, and they serve multiple purposes, like isolating and absorbing vibrations and also maintaining component alignment. They're usually mechanically reinforced so that the components wont separate in case of a total failure, and this is what I see with most engine mounts.

As they age and wear their ability to maintain correct alignment under load decreases, then their ability to isolate vibrations, and eventually you get banging noises as the parts collide and smash into each other. Like control arm bushings or engine mounts.

Anecdotally one of my first cars broke its engine mounts and a few months later the trans went out. Front wheel drive too.

The oil pump and torque converter alignment and the input shaft and torque converter turbine alignment have to be pretty good...

What do you guys think? The warning in the service manual is vague but full of ill portent.

 

[GregLocock]

UJs (hookes joints) can handle anything up to 10 degrees misalignment with ease, in fact for durability you are recommended to run at least 2 degrees at all times (you don't on cars for various reasons). BUT if they are operated at significant angles they will generate mechanically significant torques at a 2,4, 8 etc multipliers of the shaft speed. By mechanically significant I mean around sin(misalignenment)*input torque.

These torques will be present right through the driveline and will be taken out one way or the other at the flex plate or clutch at one end, and something complex at the other.

I doubt these torques would damage the gearbox casing bellhousing blocks, they will subject the internals of the gearbox to some nasty high frequency high aplitude torques that they were not designed for.

Incidentally, I woudn't rely on a consumer vehicle having any protection against broken engine mounts. The old GM ice cream slabs had some sort of interlocking metal plates, and I have seen some cables, but by and large a modern mount seems to have no mechanical redundancy.

Typically you would lose your downpipe before anything else.




Cheers

Greg Locock


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[Autotecheng]

Greg - do you know if there is a comparable effect with CV axle angles?

Also, if the drivetrain is subjected to the high amplitude high frequency torque variations you describe I would expect it to take a heavy toll on the clutches of the transmission and the lockup clutch of the converter. And as those clutches wear it accelerates all the other transmission failures related to clogging, plugging and excessive heat.

But if the drive shaft alignment was excessively bad I would expect the driver to notice it, do you think it would become noticeable before it was damaging the driveline or long after?

 

[patprimmer]

I imagine it really matters a lot where the thrust from the vibrations are taken. Since helical gears are used, the thrust from them must be controlled close to the source. In a manual transmission, obviously at the main shaft and lay shaft bearings. I am not familiar enough with automatics, but I would think aft of the oil pump or torque converter. I'm not sure about the clutch packs, but it should go to increasing clamping force if it gets that far. More likely if it gets that far it transmits straight through the shaft on which the clutch elements float to the bearings that support that shaft.

I suppose someone who actually knows might correct me.

Regards
Pat
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[GregLocock]

The flex plate would be the primary elastic element once the LU clutch was engaged.

Tripod and rzeppa joints do generate tiny torque fluctations if they are operating within their envelope, i wouldn't worry about that as they are designed to run at significant angles continuously.

Obviously once they are outside their envelope all hell breaks loose, although you can actually damage a tripod in particular by wrecking one ball bearing, and the thing will still work. An insensitive driver won't notice.

I don't know about Rzeppas, never wrecked one!

Cheers

Greg Locock


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[Autotecheng]

Pat -

Most automatic transmissions use a planetary gearset where the gears are on shafts without any bearings at all, there are some thrust shims in some cases and thrust bearings throughout the transmission but the thrust loads are essentially locked down at the carrier and are not transmitted throughout the rest of the transmission AFAIK.

Take a look at this gearset and you'll see what I mean:

http://i44.photobucket.com/albums/f23/vasa58/ZF/zf13.jpg

That's why I am concerned with the varying torque loads - the clutches have to be applied firmly enough to prevent slipping under those torque spikes.

In a manual transmission I agree that the shaft bearings will take the brunt of the thrust loads. Additionally the input shaft bearing will suffer from misalignment of the input shaft and crankshaft, usually after the crankshaft bushing/bearing has failed first.

Greg -

>Tripod and rzeppa joints do generate tiny torque fluctations if they are operating within their envelope, i wouldn't worry about that as they are designed to run at significant angles continuously.

I know the CV axles can handle a wide rangle of motion, but they do have limits. Check out this article from Cardone

http://my.cardone.com/techdocs/PT 60-0001.pdf

They blame the engine mounts as well:

"Drivetrain alignment out of specifications caused by:
• Defective or maladjusted engine mounts
• Missing or loose mounting hardware
• Front end damage"


And these vibration engineer guys say CV axle misalignment causes excessive vibrations and related problems, they point the finger at ride height and engine mounts

http://www.vibratesoftware.com/html_help/2011/Diagnosis/Propshaft/Third_Order_Propshaft.htm

"Improper angles of propshafts equipped with Constant Velocity (CV) Joints. Check the vehicle trim height. Look for any suspension or body height modifications which could affect the angle of the CV-joint. Inspect powertrain mounting for looseness also."

Drive shaft alignment concerns me in a few different ways. Over time the ride height will change (not necessarily uniformly either), and this will affect the distance between the wheel and the body, which will affect the angle of the shaft. If the engine mounts sag then the angle of the shaft on the inboard side will change. Under harsh acceleration the whole engine/transaxle assembly will be torqued in the direction of the engine's rotation. Under harsh deceleration the wheels will be slowing down while the engine will have forward inertia - the engine mounts - torque mounts - are what will keep the engine from flying forward.

So on one hand I know that axle joints can accomodate a fairly wide range of motion but on the other hand I feel like if the engine mounts aren't maintained along with the suspension geometry (ride height), there will be significant static and dynamic changes.

Enough apparently to cause the axles to pull out.

In theory the effect of the misaligned drive axles/shafts is to create excessive vibrations and fluctuating torque loads, which would destroy the bearings and clutches.

I'm not an expert.

 

[GregLocock]

What did you think "operating within their envelope, " meant? It means operating within limits.

"So on one hand I know that axle joints can accomodate a fairly wide range of motion but on the other hand I feel like if the engine mounts aren't maintained along with the suspension geometry (ride height), there will be significant static and dynamic changes.

Enough apparently to cause the axles to pull out."

Yes possibly. I doubt a ride height change will unless the extreme position in jounce and rebound are increased. Usually your car can cope with wheel-free - on a two post hoist, without breaking the joint. Similarly in jounce the upper limit of suspension compression under X g loading doesn't change much when people change ride heights with springs. Often the maximum travel in each direction is limited by the shock absorber.

A broken diff or engine mount could certainly allow enough motion for a tripod joint to over-articulate. As you might imagine the manufacturer is keen for this not to happen, so when we design a suspension we work out how much the tripod joints artiiculate at the extremes of jounce rebound, engine roll (etc) and steering, and make sure it doesn't exceed the limits specified by the manufacturer which are a combination of plunge and angle.

Cheers

Greg Locock


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[patprimmer]

My experience with CV joints is in off road racing. We had 24" travel and no angle related problems other than the short life you expect with the extreme loads from free reving in the air to sudden pull back to road speed as the car hits the ground and goes from full jounce to full compression in a very short time and the traction generated by that loading.

I really don't see a problem with any CV in normal road car suspension travel range. Of course I discount high lift kits but not lowered springs even with a bit of extra compression from modified bump stops.

I have personally experienced a seized bearing on one leg of a tripot joint on my DD and it shook so bad I thought the fillings would shake out of my teeth. It's impossible to imagine all but the most stupid inattentive drivers could not notice something is seriously wrong. Yes I know we should not underestimate the destructive talents of idiots or imagine they are in short supply.

Regards
Pat
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[Peter7307]

"It's impossible to imagine all but the most stupid inattentive drivers could not notice something is seriously wrong. Yes I know we should not underestimate the destructive talents of idiots or imagine they are in short supply.

Regards
Pat"

Quite possibly the most accurate statement ever to grace this forum!

Thanks Pat , it gave me a good laugh in an otherwise ordinary day.

Peter.

 

[evelrod]

Pat, in reference to your post about motor mounts being captive should the rubber/elastomer mount fail...All I can say, since I have not examined all cars, obviously, is my '50 Rambler, my '77 Dodge one ton PU, a variety of 60's era Anglia, Cortina, Hillman, and other cars of UK origin...personal experience here...
When they fail they fail completely leaving the engine to do and move wherever it damn well pleases.
I did have a friend with a late 50's Chebby loose a left front motor mount during a left turn that locked the steering in the left turn mode...very disconcerting...then again, I'm not a Chebby person and since no harm was done, I thought it rather amusing from the passenger seat.

Rod