Clutch Fork Failure Analysis Prediction? [Archive]

QB93Z

12-20-2012, 11:11 AM

Here are some additional pictures of the broken clutch fork from a 1994 ZR-1 with greater than 130,000 miles.

The goal of our analysis was to determine if the failure could have been predicted by visual inspection prior to the failure.

The fork failure occurred during a routine shift from second to first gear while Yun was entering the parking space in the parking garage at his condo.

There were no prior indications of clutch operational degradation.

http://i407.photobucket.com/albums/pp152/QB93Z/2012%2012%20Dec%2016%20MVMD%20Broken%20Clutch%20Fo rk/DSC04931_zpsd7f5da2c.jpg

Here are some close up pictures of the fracture:

http://i407.photobucket.com/albums/pp152/QB93Z/2012%2012%20Dec%2016%20MVMD%20Broken%20Clutch%20Fo rk/DSC04937_zps8316ab26.jpg

http://i407.photobucket.com/albums/pp152/QB93Z/2012%2012%20Dec%2016%20MVMD%20Broken%20Clutch%20Fo rk/DSC04938_zps445ac8f2.jpg

http://i407.photobucket.com/albums/pp152/QB93Z/2012%2012%20Dec%2016%20MVMD%20Broken%20Clutch%20Fo rk/DSC04942_zpsebbca1a1.jpg

http://i407.photobucket.com/albums/pp152/QB93Z/2012%2012%20Dec%2016%20MVMD%20Broken%20Clutch%20Fo rk/DSC04941_zps44dca97d.jpg

It looks like the fork suffered fatigue failure under tensile stress during clutch opeartion. There is no indication of deformation (twisting or bending) occurring prior to the failure.

There is no indication of mechanical damage (strikes, marks, or scrapes) to the fork prior to the failure.

The failure crack probably started in the edges of the rivit hole and spread outward to the edges of the fork.

I suggest visual inspection of any clutch fork that is to be reinstalled. Examine the area around the rivit, looking for cracks.

I would be intersted in any other comments.

Jim

Scrrem

12-20-2012, 11:23 AM

Great write up Jim!!! :cheers: Makes me wish I had taken a better look at my fork before I put my new Bill B. transmission in. Oh well...not to fear...there always the Westminster Loaner Program :mrgreen:

secondchance

12-20-2012, 01:59 PM

Thanks for the write-up.
I agree with your assessment. Close inspection of rivet hole is about the best one can do.

Schrade

12-20-2012, 05:00 PM

...

I suggest visual inspection of any clutch fork that is to be reinstalled. Examine the area around the rivit, looking for cracks.

I would be intersted in any other comments.

Jim

Magnaflux.

XfireZ51

12-20-2012, 05:39 PM

Jim,

What's your thought of just doing a weld along that bend to add some cross section as a preventative measure?

Blue Flame Restorations

12-20-2012, 06:24 PM

You know what they say about big feet?......

Big broken clutch fork!!!!!!!!!

secondchance

12-20-2012, 06:45 PM

You know what they say about big feet?......

Big broken clutch fork!!!!!!!!!

Big feet?! I wear 8-1/2!

For it to be worthwhile reinforcing has to be on the open end on the break and around the begining of the crack - around the rivet. I don't know such small weld is doable. Maybe a plate with a hole for the rivet and weld over the hole/rivet and around the edge of the plate. A few holes on the plate filled with weld will be worthwhile. Again this is such a small area to work with... Not to mention clearance concern after adding the plate.

Blue Flame Restorations

12-20-2012, 07:21 PM

Big feet?! I wear 8-1/2!

For it to be worthwhile reinforcing has to be on the open end on the break and around the begining of the crack - around the rivet. I don't know such small weld is doable. Maybe a plate with a hole for the rivet and weld over the hole/rivet and around the edge of the plate. A few holes on the plate filled with weld will be worthwhile. Again this is such a small area to work with... Not to mention clearance concern after adding the plate.

Just remind me to NOT let you step on my foot at the Gathering in May. lol

On a serious note, I'm glad you got it fixed. The fracture is an anomoly, IMO. I don't think I'd ever bother reinforcing one prior to installation. Maybe if you drag race or do brutal shifting?

QB93Z

12-20-2012, 07:55 PM

Jim,

What's your thought of just doing a weld along that bend to add some cross section as a preventative measure?

The fork is forged steel. When you examine the fractured edge of the crack, the "skin" of the steel that was worked by the forging is visible. The working adds strength to a forged piece. I am afraid that welding without heat treatment would reduce the strength of the fork.

All of the suggestions are good, but the fork is not a very expensive piece. I was hoping to get a quick visual Go-No-Go inspection that could be used to decide if a replacement fork is prudent when the clutch is apart for some other reason.

Jim

nelson007

12-20-2012, 08:04 PM

Hello Jim,
Could heat have any effect on the part to cause it to break ?

Nelson007

cvette98pacecar

12-20-2012, 08:09 PM

Hello Jim,
Could heat have any effect on the part to cause it to break ?

Nelson007

Heat could effect the tensile strength, However where the stress fracture is I highly doubt that portion of the fork got that hot.

I am sure glad it did not break when I was driving her. Although Dave and I were driving extremely slow and careful.

scottfab

12-20-2012, 08:43 PM

seems to me some weld beads along the indicated lines for strength could be a preemptive counter measure. (both top and bottom side)

http://zr1.net/forum/picture.php?albumid=199&pictureid=1759

secondchance

12-21-2012, 08:20 AM

I am going to say nothing silly and wait for Dr. Phil to chime in. Also, Dave Bright's opinion would be interesting. Does Dave come on the forum?

QB93Z

12-21-2012, 09:16 AM

Hello Jim,
Could heat have any effect on the part to cause it to break ?

Nelson007

I was referring to heat treatment of forged metal parts. The temperatures involved would be in the 1000 to 1500 degree range. The temperatures involved in normal operation are no where near that.

Jim

scottfab

12-21-2012, 10:57 AM

Big feet?! I wear 8-1/2!

For it to be worthwhile reinforcing has to be on the open end on the break and around the begining of the crack - around the rivet. I don't know such small weld is doable. Maybe a plate with a hole for the rivet and weld over the hole/rivet and around the edge of the plate. A few holes on the plate filled with weld will be worthwhile. Again this is such a small area to work with... Not to mention clearance concern after adding the plate.

I'm not a welding expert (have welder will make a mess) but have seen skilled welders lay down a bead of high strength steel (E1203X) that is low profile. (profile set by the welder). Such steel is MUCH stronger than the original steel. (120,000 psi).
Not sure this idea would work on a broken fork though. I'd have to be a preemptive move.

Anyone know a source for replacement forks?
This is a part I want to have on hand AND have it strengthened before installation.

Schrade

12-21-2012, 07:48 PM

efnfast

12-21-2012, 08:08 PM

Is this part NLA?????

secondchance

12-21-2012, 09:04 PM

Is this part NLA?????

You can still find them.

scottfab

12-21-2012, 09:55 PM

You can still find them.

Maybe Bill B. ?
Or where ?

cvette98pacecar

12-21-2012, 10:35 PM

Maybe Bill B. ?
Or where ?

http://www.powertorquesystems.com/Corvette/CorvetteCF.htm

efnfast

12-22-2012, 07:58 AM

So I assume this is not the same fork used in all C4's?
Is this ZF unique to our ZR-1's?

QB93Z

12-22-2012, 09:18 AM

Kind of confused here about 'welding' / 'preventative' measures...

OP asked about Failure Analysis Prediction, and I think the discussion would have ended when someone mentioned Magnaflux (besides visual inspection)...

I agree that magna flux inspection would be an excellent test to identify potential failure. But, it is not a test I can do in the garage while I have a clutch apart for some other reason.

My conclusion is that the fork is not a part that "wears" down like a brake rotor. It is either a good clutch fork, or it has failed and you can't operate the clutch. I guess there could be a noticeable bend before it fails.

Jim

WVZR-1

12-22-2012, 01:04 PM

Good question? :dontknow:

I have a 90 (L98) with ZF S6 40 trans also and I expect that the clutch fork is identical but am not positive.
I would never try to weld or stiffen a clutch fork (the failure is an anomaly) and the heat treatment of the original is unknown.
I have seen clutch forks on Ebay as well as complete bell housings with clutch forks installed.

Clutch forks do have three wear points (fingers at the throwout bearing, pivot, slave cylinder plunger) all of which should be greased a tad upon installation.

All C4 ZF's use the same part #'d clutch fork! ALL!!

cvette98pacecar

12-22-2012, 03:19 PM

I'm not a welding expert (have welder will make a mess) but have seen skilled welders lay down a bead of high strength steel (E1203X) that is low profile. (profile set by the welder). Such steel is MUCH stronger than the original steel. (120,000 psi).
Not sure this idea would work on a broken fork though. I'd have to be a preemptive move.

Anyone know a source for replacement forks?
This is a part I want to have on hand AND have it strengthened before installation.

The problem with an metal when you heat it you are going to soften the metal, to add strength to this part you are going to have to utilize a driving arc which will allow the stick to dig into the fork, once you are completed in adding the two welds the fork will be excessively hot. At this point you would need a secondary compound to cool the fork fast enough to reforge the fork, water would not add sufficient quenching to re-forge the material.

HAWAIIZR-1

12-22-2012, 03:40 PM

Hello Jim/Yun,

What are your folks thought on the condition of the hydraulic clutch system being the most important factor? I don't know the details before the failure, but was it hard to engage at all or just felt normal? The reason I as is when my slave cylinder was failing due to defect of that reverse seal problem it worked normal at first then later got hard to depress and then nothing at all. I remember when Rich (no Screm, the other one) had a problem and bent fork it was difficult to depress and continued pressure ended up bending the fork, but not breaking it like this. So if there were not issues with the clutch system or the pressure plate then I agree too that just a defect in the part that eventually gave way.

HAWAIIZR-1

12-22-2012, 04:09 PM

scottfab

12-22-2012, 04:13 PM

http://www.powertorquesystems.com/Corvette/CorvetteCF.htm

:icon_thum:icon_boun

HAWAIIZR-1

12-22-2012, 04:18 PM

:icon_thum:icon_boun

Scott, you're right on about Jim and he is first class when it comes to service and products. :thumbsup:

secondchance

12-22-2012, 05:43 PM

Okay, now I see Yun's original post of the problem and it appears no issue with the clutch system at all to contribute to the failure. Wow, I wonder what Bill B thinks of this issue? I did buy a spare fork from Jim Jandik earlier this year and I guess it is good to have one on the shelf just in case. Thanks for sharing this.

Hi Craig,

Hope all is well. As you noticed no issue whatsoever before the failure. I was lucky that it happened just as I was parking.
I agree that if my slave was failing and I kept putting excessive force the fork would have bent (warping of side flanges). Fact that it ripped makes me suspect defect around the rivet hole.

HAWAIIZR-1

12-22-2012, 05:58 PM

Hi Craig,

Hope all is well. As you noticed no issue whatsoever before the failure. I was lucky that it happened just as I was parking.
I agree that if my slave was failing and I kept putting excessive force the fork would have bent (warping of side flanges). Fact that it ripped makes me suspect defect around the rivet hole.

Hey Yun,
All is well, thanks and hoping the same with you and family. I have been buried with work/school and inactive lately. Yes, timing was good for failure as a positive point like when mine (clutch slave) failed and I was at the traffic signal by my home; had to grind a few gears in the process. That rivet area does seem like it could be a weak point and defect as you stated. These forks seems to be a pretty stout item and I wonder where originally made and what quality of metal used? I guess the stuff from Jim Jandik is the best we can get being OEM stuff. Take care and thanks again for sharing.

scottfab

12-22-2012, 06:39 PM

... snip...
. Fact that it ripped makes me suspect defect around the rivet hole.

Sounds like we should all take a look at the rivet hole next time we're in there. (can't believer I just said that)

Maybe a dental mirror and a few double jointed fingers?

HAWAIIZR-1

12-22-2012, 06:54 PM

Sounds like we should all take a look at the rivet hole next time we're in there. (can't believer I just said that)

Maybe a dental mirror and a few double jointed fingers?

G spot?:censored:

BlackWidow#2

12-24-2012, 02:15 PM

I had a similar problem back in 98 or 99. The problem was traced back to a CenterForce pressure plate. The weights and fingers had worn somewhat, causing a bind when engaging and disengaging.
George

Marc Haibeck

12-27-2012, 02:40 AM

secondchance

12-27-2012, 08:39 AM

Maybe high pedal force can break the fork? We encountered a car that had a problem with excessive force needed to depress the clutch pedal. It felt like the car had a racing clutch. The owner ignored the high pedal force. After about 10k miles the fork broke. When we disassembled the clutch we found an OE pressure plate. The fork pivot ball was totally dry, and had a lot of red fretting corrosion. I wonder if someone replaced the clutch and cleaned the bell housing and then did not re-lubricate the pivot ball. We installed a new fork with moly disulfide lubricant on the pivot ball. We put the clutch back into service because it was in good condition. Then the pedal force was normal.

Here is a picture of the broken fork.
http://www.zr1specialist.com/HAT%20Web/facts%20and%20failures/clutch%20fork.htm

Hi Marc,

I think my clutch may have gone thru a similar situation as you are describing.
About three months ago I felt a driveline slop, suspected overly loose flywheel (original unit w/ 134,000 miles) and disassembled to replace the flywheel. Prior to this clutch felt a little odd - sort of like pivot point was changing as the pedal was pushed down - not progressive.
Upon disassembly we found the pivot stud was worn badly where the fork was riding. However, the fork look ok visually and was not replaced. Jim V and I came to a similar conclusion that the fork may have been over-stressed while being operated w/ wornout pivot stud.
It's interesting the failed fork you came across failed closer to the slave cylinder side. Whereas mine failed right at the rivet hole. It may be the fork in my car had a defect at the rivet hole and relatively small stress caused by the pivot stud was all it took to fail.
And I did lubricated the pivot stud and fork ends that slip over the throwout bearing w/ high temp bearing grease. Whether it has molybdenum disulfide additive or not - I have no idea.

secondchance

12-27-2012, 09:02 AM

Here is a pic of the pivot stud - don't let this happen to you!
http://i658.photobucket.com/albums/uu303/byongyun/photo-4_zps86b7f75b.jpg

LancePearson

12-27-2012, 10:12 AM

I'm no metalurgist but that fracture is not a crack propagating from stress. It looks very much to me like a failure of the steel to be heat treated correctly and was too brittle and not elastic enough to handle the load as a result. If parts were trackable all the way back I'd bet it was not heat treated before forming correctly.

Lance

secondchance

12-27-2012, 10:28 AM

scottfab

12-27-2012, 10:51 AM

Here is a pic of the pivot stud - don't let this happen to you!
http://i658.photobucket.com/albums/uu303/byongyun/photo-4_zps86b7f75b.jpg

You have to wonder about the geometry of the design. I'd expect to see wear on the top rounded surface. Looks to be a lot of sideways wear. Simple lack of lubrication should still wear on the designed pivot surface. Not good.

secondchance

12-27-2012, 11:06 AM

You have to wonder about the geometry of the design. I'd expect to see wear on the top rounded surface. Looks to be a lot of sideways wear. Simple lack of lubrication should still wear on the designed pivot surface. Not good.

Scott,

Where it wore is the pivot surface.

A26B

12-27-2012, 11:21 AM

It's a pull-release clutch, not a push-release, hence the "ball" surface on the underside of the pivot, where the load is.

LancePearson

12-27-2012, 12:47 PM

Lance,

You may be right.

We, the WAZOO gang, need to meet you. You are not that far away.
We meet occasionally at Westminster, MD or at Mt. Vernon, VA.
Gotta see your new ride.
My first Z was a red w/ black interior-may have been a twin to yours if you have black interior.
Let me know next time you meet in Mt. Vernon...it's about 110 miles each way for me and I have to make arrangements for my two greyhound girls to have a dog sitter. These are such great cars. There are photos of my car in one of my albums.....

Lance...red/red 1991 0682

WARP TEN

12-27-2012, 01:07 PM

I was looking at Marc Haibeck's website and saw he has a clutch fork failure mentioned, but it broke at a different point due to lack of lubrication. So I am not sure what will show when we are looking at a fork to anticipate possible failure. Here is what he showed and said :

http://www.zr1specialist.com/HAT%20Web/facts%20and%20failures/DCP_1265a.jpg

This fork broke because it was subjected to high load during many clutch release cycles. It was used with an OE type Valeo pressure plate.
The root cause was lack of lubrication on the pivot ball. The pivot ball was dry with no lubricant. The pedal force required to release the clutch was about twice the normal amount.
With a new fork and a lubricated pivot ball the pedal force to release the clutch returned to normal. The correct pivot ball lubricant has a molybdenum disulfide high pressure anti-friction additive. The pivot ball should be freshly lubricated when a new clutch is installed. The tips that engage with the throw out bearing should also be lubricated.

LancePearson

12-27-2012, 01:11 PM

Interesting that it broke along what may be a forming score line.....

secondchance

12-27-2012, 01:16 PM

Let me know next time you meet in Mt. Vernon...it's about 110 miles each way for me and I have to make arrangements for my two greyhound girls to have a dog sitter. These are such great cars. There are photos of my car in one of my albums.....

Lance...red/red 1991 0682

Looks like you have black interior AND Borla exhaust just like I had.
May I sit in your car and reminise? I'll be back in 1991!

A26B

12-27-2012, 02:40 PM

Virtually all fatigue breaks occur at stress risers, i.e. nicks, scratches, sharp corners, tooling marks, etc. That's why parts designed to shear at specific axial load pressures are polished.

This get me to thinking about "weak links." The cause of fork failure sure points to excessive pivot friction. if the clutch fork was strengthened and excessive pedal pressure ignored, eventually the pivot would break or wear to the point the fork would come off the pivot. I'm not sure what exactly might happen, but it could be much more catastrophic if the fork got tangled up in there.

The moral of the story is check the pivot, check fork, replace if worn and lubricate always.