Caliper Bolts - Are they really one time use?


Indy GT said:
Although I certainly understand why an OE would want to specify a one-time use only fastener be it for the locktite issue mentioned earlier or some perceived "safety" concern, has anyone ever seen anywhere a technical description of WHY this recommendation is made (other than to maybe sell additional fasteners although probably not a big revenue stream)? Likely not.

You may imply from a one-time-use bolt that "yielding" of the bolt material is somehow harmful to the bolt and thus its structural integrity is somehow compromised by this event; thus requiring you to throw the bolt away. I do not believe this is true.

In a non-fatigue environment (i.e. one in which the bolt sees constant, non-varying load which the connection designer wants to strive for and can design for), the bolt is just fine being loaded into the material yield portion of the stress-strain diagram. Typical "critical" bolted connections (ones where particular attention to detail is warranted) use high strength fasteners (good, controlled and specified material properties) which have good elongation properties which enables loads into the yield portion of the stress/strain curve.

Without getting too technical and posting load/unload diagrams for bolt stresses below and above the material yield strength, the bolt will work just fine even after being loaded above the yield strength (stress) of the material. Good bolted joint engineering design practice is to torque a bolt to 90% of material “proof strength” which is roughly equivalent to 90% of material yield strength. (Ref. Shigley, ‘Mechanical Engineering Design’ for those who care). Thus “normal” (or reusable) bolts in a properly designed joint using engine oil for assembly, are already very close to loads approaching the bolt material yield strength.

Thread and head collar friction loads (which consume a large part of this initial torque before the torque can result in clamping load) are very large variables in bolt torque calculations. If the installer puts the nut on dry, with engine oil or uses “never-seize” during assembly, very large swings in the amount of actual load the initial torque produces in the fastener can and do occur. This is because the thread lubricants can significantly alter the thread friction which consumes torque before producing bolt clamp load. Thus, in reality a properly designed bolted assembly using never-seize (when torquing guidance does not specifically call out use of this assembly lubrication) torqued to specification can load a regular bolt into material yield easily without the assembler evening knowing this fact.

To be safe, one should follow the replacement instructions dictated by the OE, but it would be interesting to have the application designer state, why.
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This is exactly what I was about to post, but indy beat me to it. Oh well......
 
:lol
 
I like to sit next to Bill at parties… it makes people think I am smarter than I actually am. However, I try not to speak after him as it has the opposite effect.
 
sahlman said:
Some background for you guys on the chassis bolts that corresponds to many of your comments:

None of the shock, control arms or anti-rollbar bolts are torque to yield/angle or single use bolts. I am not positive on why the manual said to replace the upper shock bolts. (I had design and release on the shocks and chassis bolt connections to the frame) If this statement changed in later manuals then I think it was an error or else FCSD decided it was unnecessary. The only reason I can think of that FCSD might have originally spec’d upper shock bolt replacement was due to threading into the aluminum casting (the control arm bolts then should have the same direction of replacement) in case the bolt coating was compromised in install or removal allowing galvanic reaction and the potential for the bolt to seize in the casting. I have not heard of any incidence of this issue in service and we reused bolts constantly in development. And I do not recall discussion on the need for replacement in service.

The caliper bolts were also standard torque spec and bolts (not torque to yield/angle) and I think the caliper bolt replacement direction in the manual may be a standard Ford statement for caliper bolts, an error or the same FCSD threading into aluminum concern. I verified it is a standard bolt with the original FGT brake design engineer.

I actually reviewed going to torque to yield for all of these bolts because it provides the most consistent torque setting and can allow for smaller bolts for weight and package. However, my detailed bolt calculations called out smaller bolts than people were used to seeing already, torque to yield is not standard practice for chassis bolts (they are for engine assembly) and this specific discussion and potential confusion is an example of why we stuck with standard torque specs and bolts.

Note- ALL chassis bolt torque specs were for dry joints, no lube, no anti-seize. As Indy GT states, any type of lubrication will change the bolt preload due to changing the thread friction when torqued to our dry spec and has a good chance of yielding the bolt.

Scott
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Thanks, truly appreciate your taking the time to give us some insight into the subject. I no longer have to feel guilty re-using perfectly good caliper bolts.
 
sahlman said:
Some background for you guys on the chassis bolts that corresponds to many of your comments:

None of the shock, control arms or anti-rollbar bolts are torque to yield/angle or single use bolts. I am not positive on why the manual said to replace the upper shock bolts. (I had design and release on the shocks and chassis bolt connections to the frame) If this statement changed in later manuals then I think it was an error or else FCSD decided it was unnecessary. The only reason I can think of that FCSD might have originally spec’d upper shock bolt replacement was due to threading into the aluminum casting (the control arm bolts then should have the same direction of replacement) in case the bolt coating was compromised in install or removal allowing galvanic reaction and the potential for the bolt to seize in the casting. I have not heard of any incidence of this issue in service and we reused bolts constantly in development. And I do not recall discussion on the need for replacement in service.

The caliper bolts were also standard torque spec and bolts (not torque to yield/angle) and I think the caliper bolt replacement direction in the manual may be a standard Ford statement for caliper bolts, an error or the same FCSD threading into aluminum concern. I verified it is a standard bolt with the original FGT brake design engineer.

I actually reviewed going to torque to yield for all of these bolts because it provides the most consistent torque setting and can allow for smaller bolts for weight and package. However, my detailed bolt calculations called out smaller bolts than people were used to seeing already, torque to yield is not standard practice for chassis bolts (they are for engine assembly) and this specific discussion and potential confusion is an example of why we stuck with standard torque specs and bolts.

Note- ALL chassis bolt torque specs were for dry joints, no lube, no anti-seize. As Indy GT states, any type of lubrication will change the bolt preload due to changing the thread friction when torqued to our dry spec and has a good chance of yielding the bolt.

Scott
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Scott,
Thank you for taking the time to reply. Another great example as to the usefulness of this forum ..:thumbsup
 
Thank you to everyone that contributed to this thread. A majority of you have validated my instincts ..... I will reuse the caliper bolts!
 
Scott, I too appreciate your posting as well! Good to have second opinions, especially from the actual application designers.

Bolts seem so simple and and uncomplicated but actually there is a significant amount of engineering to safe, reliable bolted connections. Like those of our axle bolts (which needed to be replaced early on due to hydrogen embrittlement), rod/main bolts/studs, flywheel bolts, etc. I always took an interest in trying to better understand bolts in schooling and the fastener section of my text book is well worn with pages detached from the binding spine.

FENZO said:
I saw this one Sunday morning on Horsepower TV, thought it was a decent demonstration of how different lubes can affect torque.

http://youtu.be/6-XXVk_vwKw
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Fenzo, this is a very good demo on preload variability due to thread lubrication. Thanks for posting for others to see. Over the years I have collected and read many ARP catalogues as they contain a great deal of good information. They make first class fasteners and really know the mechanics of how fasteners work and function in various environments. Good stuff and would recommend their products without reservation.

And Mark, you (and/or your brother) can sit by me any time! Not sure about the "smartness" transfer, you and your friends seem to be building one heck of a first-class engine. Can't wait to hear how it runs and your impressions of all your efforts.
 
Good info here - thank you. I wondered why my manual stated to replace the shock bolts (I re-used) when I changed the springs.

Sorta along this thread:

Remember to always return your click-type torque wrench to its lowest setting after use and calibrate occasionally.
 
Scott & Bill -

Excellent information

Fenzo -

Thanks for the info on the ARP Ultra Lube - I plan to put it on my wheel studs to prevent overtorquing the wheel nuts. Will also be used in many other places on many vehicles where I've been using their Assembly Lube (obviously not in the engine assembly where assembly lube is needed, though).
 
Great to get the "inside" scoop.

Now a days many things are done for other reasons than technical.

Number of engineers in the USA 2008, 1.6 million.

http://www.bls.gov/oco/ocos027.htm

Number of lawyers in the USA in 2007, 1.14 million.

http://wiki.answers.com/Q/How_many_lawyers_are_in_the_US

I wonder what the ratios of these two professions are in other countries.
 
Glad to help out guys. Great question to start the thread and comments by you guys on an important topic. Good stuff.

I would just watch for damage to the bolt coatings and replace them if the coating appears compromised because it may seize in the aluminum if it is left in there for a few years. However, we spec'd the very best coatings available at the time so they should do very well.

Indy GT- agreed for sure- bolted joints are definitely far more complicated than they appear...especially on safety critical parts like these. I checked my calculations for the FGT literally 5 times over and had others check my work. I designed those specific tight counterbores with long tapers leading to the threads on the control arm and shock mounts at the frame castings to minimize cross threading and allow a lot more bolt stretch for a much better joint than standard. We'll have to "geek out" over joint design sometime!

Scott
 
BlackICE said:
Great to get the "inside" scoop.

Now a days many things are done for other reasons than technical.

Number of engineers in the USA 2008, 1.6 million.

http://www.bls.gov/oco/ocos027.htm

Number of lawyers in the USA in 2007, 1.14 million.

http://wiki.answers.com/Q/How_many_lawyers_are_in_the_US

I wonder what the ratios of these two professions are in other countries.
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How many Engineers that are also Lawyers? I have been trying get data on that for a while.

If my Son's plans go well, he will become an Engineer first (May be Physicist) and then off to Law School. He is gunning for Intellectual Property Law and Stanford.
I also made him a deal. I will match cash $-for-$ for every scholarship/grant he can get.


sahlman said:
Glad to help out guys. Great question to start the thread and comments by you guys on an important topic. Good stuff.

I would just watch for damage to the bolt coatings and replace them if the coating appears compromised because it may seize in the aluminum if it is left in there for a few years. However, we spec'd the very best coatings available at the time so they should do very well.

Scott
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Good stuff Scott.
 
HHGT said:
How many Engineers that are also Lawyers? I have been trying get data on that for a while.

If my Son's plans go well, he will become an Engineer first (May be Physicist) and then off to Law School. He is gunning for Intellectual Property Law and Stanford.
I also made him a deal. I will match cash $-for-$ for every scholarship/grant he can get.




Good stuff Scott.
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I would be glad if my kids could just give me the opportunity to pay for a top notch school.
 
Scott-

Though relagated to "geek" status by some, the caliper of owners on this GT Forum is top notch (nod of thanks to DBK for his sandbox rules) and most have an appreciation for and inquisitive of the technical aspects Ford designed into our cars. I have found technical topic development is welcomed by the owners who have a thirst for understanding. (Well Frank is maybe an exception but he is busy with his microscope.)

Please continue to contribute, and I look forward to getting together at one of the rallys to geek out!!
 
Indy GT said:
Scott-

Though relagated to "geek" status by some, the caliper of owners on this GT Forum is top notch (nod of thanks to DBK for his sandbox rules) and most have an appreciation for and inquisitive of the technical aspects Ford designed into our cars. I have found technical topic development is welcomed by the owners who have a thirst for understanding. (Well Frank is maybe an exception but he is busy with his microscope.)

Please continue to contribute, and I look forward to getting together at one of the rallys to geek out!!
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Indy GT,

For sure use of the terms relating to "geeks" here by me is primarily tongue in cheek. I catch a lot of flak from my buddies for being a hard core engineer even though I have been pretty active in athletics and a gearhead since I was a little punk and don't consider myself a "geek" either. We'll for sure catch up at one of the next rallys.

I have been very impressed with the level of knowledge on this forum in general and the passion for the FGT and all things mechanical...especially cars. Very cool.

I will continue to check back for chassis related threads that make sense for me to comment on...almost commented on Danica thread since I was engineer for her one season at Rahal, but decided to leave it alone being the FGT Forum as DBK noted on the thread!

Scott
 
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