[TR] Threads and Bolts
dave1massey at cs.com
Thu Nov 24 20:04:14 MST 2016
Thanks for filling in the gaps in my response. On the other hand,I've extracted a grade 8 bolt that had snapped on a rockershaft pedestal, not exactly a high stress application. I attributed it to the differences in thermal expansion between the steel bolt and the aluminum pedestal. But, perhaps it was a faulty bolt. There was a rash of counterfeit grade 8 hardware in distribution a couple decades ago and perhaps those have found their way into your local hardware stores where inventory tends to move slowly.
From: Michael Porter <mdporter at dfn.com>
To: Dave <dave1massey at cs.com>; levilevi <levilevi at comcast.net>; triumphs <triumphs at autox.team.net>
Sent: Thu, Nov 24, 2016 6:13 pm
Subject: Re: [TR] Threads and Bolts
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.
Never let anyone drive you crazy when you know it's within walking distance....
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