[TR] Threads and Bolts

Michael Porter mdporter at dfn.com
Thu Nov 24 17:13:23 MST 2016

On 11/24/2016 11:51 AM, Dave wrote:
> Bolts with no slashes are grade two.  The grade of the bolt is 
> indicated by the number of slashes plus two.  I do not recommend grade 
> two for suspension duty.
> I've heard said that the trade-off with Grade 8 bolts is that they are 
> stronger but more brittle.  Since I am not a mechanical engineer I 
> cannot make a recommendation on substituting a grade 8 for a grade 5 
> but I would be concerned that the increased brittle nature may be a 
> detriment. Considering the seriousness of the consequences of a 
> suspension failure I would think again about making substitutions.  
> And just about anything you need is available next business day from 
> McMaster Carr (and other sources, certainly).  But you may have to buy 
> more than you need and pay shipping.
> I don't think fine thread vs coarse thread makes much difference.

Brittleness is relative.  The optimum condition for a strong bolt is 
toughness, the combination of tensile strength with resistance to both 
deformation and crack propagation.  It's just a function of metal 
crystalline behavior that higher tensile strengths are inversely 
proportional to rate of deformation.  The general perception is that 
bolts with very high tensile strength offer no protection from sudden 
breakage once the yield point is reached, but that's not exactly true.  
A lot depends upon the loads imposed.  If a grade 5 bolt works reliably 
in an application, breakage is very unlikely with a grade 8, because the 
grade 8 yield point is higher. This also allows for somewhat more 
clamping force, thus reducing the tendency of the fastened parts to move 
around under load.

The latter is important, because any bolt loaded in shear will fail at 
about 65% of UTS (ultimate tensile strength).  Even so, I've seen grade 
8 bolts that have deformed badly without shearing.  For example, on the 
buses we built, the rear axle was attached to a pair of spring beams 
(the item carrying the air springs that supported the weight of the bus 
and transmitted those loads to the axle). Each one was attached with 
four 3/4" grade 8 bolts in a box pattern, with a precision center pin to 
positively locate the axle.  Due to an engineering error (through holes 
too large), under big side loads (like the driver whacking a curb while 
in motion--much more common than one would think, especially in NYC), 
the spring beam would laterally rotate around the center pin, putting 
the bolts in direct shear.  Those dynamic loads, due to excessive 
movement, were much larger than the clamping load because movement adds 
kinetic energy. I saw one of those grade 8 bolts after such 
mistreatment, and it had an "S" curve in it, so that the centerline at 
the head of the bolt was offset from the centerline at the bottom of the 
bolt by nearly 1/2", and yet, the bolt had not cracked or broken.  
That's where the toughness of the bolt comes in--which depends upon a 
number of factors, such as the alloy, the kind of heat treatment, 
precipitation hardening, tempering and anti-corrosion coatings (to 
prevent corrosion cracking).

To my mind, a properly-made grade 8 bolt will be superior in performance 
to any grade 5 bolt, especially in suspension work. It's important to 
know the loads imposed, though.  In racing, a grade 5 will be fine in 
areas where the maximum load imposed will be sufficiently below the UTS 
of the bolt, and the clamped pieces are inspected regularly for 
stretched or deformed bolts, but the extra insurance of higher yield 
strength and higher clamping force make a grade 8 preferable.  I'd much 
rather have a bolt that never reaches its yield point under maximum load 
than one that yields and deforms before failure.



Michael Porter
Roswell, NM

Never let anyone drive you crazy when you know it's within walking distance....

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