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engineering behind choosing fastener (bolt) sizes [message #167921]

Sun, 29 April 2012 06:36

RF_Burns

Messages: 2277
Registered: June 2008
Location: S. Ontario, Canada

Karma: 3

Senior Member

I just finished changing to rear disk brakes and made this observation.

The rims are held on by 8 big b'jezus 1/2" fine thread bolts on about an 8" diameter. Makes sense, lots of weight on them.

But the whole wheel and brake assembly is fastened with 4 little coarse thread 3/8" bolts set no more than about 3" apart (very little mechanical advantage). Mind you they are grade 8 bolts and you could argue that after 35years the corrosion between the spindle and the bogie arm does most of the "fastening".

Same thing on the one-ton front end. The bearing assembly in that case is fastened with just 4 bolts on a much smaller diameter (however about 1/2", most likely metric equivalent).

So if four little 3/8" bolts on a 3-4" diameter can hold the whole wheel assembly safely, why is eight large bolts on a larger diameter needed to hold the rim and tire only?

Inquiring minds want to know!

Bruce Hislop
ON Canada
77PB, 455 Dick P. rebuilt, DynamicEFI EBL EFI & ESC.
1 ton front end
http://www.gmcmhphotos.com/photos/showphoto.php?photo=29001
My Staff says I never listen to them, or something like that

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Re: engineering behind choosing fastener (bolt) sizes [message #167926 is a reply to message #167921]

Sun, 29 April 2012 08:21

JohnL455

Messages: 4447
Registered: October 2006
Location: Woodstock, IL

Karma: 12

Senior Member

If you think that's odd, work on a Jeep product with the solid front axle. Three (3) 12 point bolts hold the hub assys to the knuckle. They have made millions of these and it doesn't seem to be an issuse with onroad, offroad and the huge mechanical advantage against them when guys put the big mudders and wrong offset wheels. I'm guessing as it is because it is a fixed atttachment point and the wheel is a rotating (flexing) assembly. If you recall the GMC prototype had 5 lug Toro wheels till they got stress cracks and they went to the 8 lug hubs before production. Probably not as much an issue with Alcoas and their thickness. But if you have ever had to troubleshoot someones car with a odd squeek that was traced down to the wheelcovers needing to be greased, then you realize that a steel wheel flexes (deforms) quite a bit on rotation with the loading on one side. Hence the need for the 8 large lugs and the flanged nuts to get some compression surface.

John Lebetski
Woodstock, IL
77 Eleganza II

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Re: engineering behind choosing fastener (bolt) sizes [message #167927 is a reply to message #167921]

Sun, 29 April 2012 08:57

Bob de Kruyff

Messages: 4260
Registered: January 2004
Location: Chandler, AZ

Karma: 1

Senior Member

rf_burns wrote on Sun, 29 April 2012 05:36

Good point Bruce. Wheels are a relatively thin gage compared to hubs and they also experience a serious amount of flex. The wheel lugs need to spread the load to minimize cracking--usually emanating from the lug holes.

Bob de Kruyff
78 Eleganza
Chandler, AZ

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Re: [GMCnet] engineering behind choosing fastener (bolt) sizes [message #167929 is a reply to message #167921]

Sun, 29 April 2012 09:07

USAussie

Messages: 15912
Registered: July 2007
Location: Sydney, Australia

Karma: 6

Senior Member

Bruce,

You kinda answered your own question. If you go MM X-7525 Page 4-12 Figure
25 you will see that there is a special tool to press the spindles out of
the bogie arms. That press fit transfers the forces on the wheel through the
spindle directly to the bogie. The four 3/8" bolts don't carry much load
from the wheels if any.

The bolts keep the brake backing plate from rotating. That puts them in
shear. I did a Google search and found this table:

http://nucor-fastener.com/Files/PDFs/TechDataSheets/TDS_013_Shear_Strength.p
df

It notes that a 3/8" Grade 8 bolt has a minimum shear strength of 9,939
pounds. If I have interpreted that chart properly it would take a twisting
force of 39,758 pounds to shear those four bolts.

No one has reported shearing these bolts in the 5 years I've been hanging
around here so it is safe to assume that braking does not generate that much
twisting force.

I could be wrong and my suppositions way off base! ;-)

Regards,
Rob M.

-----Original Message-----
From: Bruce Hislop

I just finished changing to rear disk brakes and made this observation.

The rims are held on by 8 big b'jezus 1/2" fine thread bolts on about an 8"
diameter. Makes sense, lots of weight on them.

But the whole wheel and brake assembly is fastened with 4 little coarse
thread 3/8" bolts set no more than about 3" apart (very little mechanical
advantage). Mind you they are grade 8 bolts and you could argue that after
35years the corrosion between the spindle and the bogie arm does most of the
"fastening".

Same thing on the one-ton front end. The bearing assembly in that case is
fastened with just 4 bolts on a much smaller diameter (however about 1/2",
most likely metric equivalent).

So if four little 3/8" bolts on a 3-4" diameter can hold the whole wheel
assembly safely, why is eight large bolts on a larger diameter needed to
hold the rim and tire only?

Inquiring minds want to know!

Bruce

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Regards, Rob M. (USAussie) The Pedantic Mechanic Sydney, Australia '75 Avion - AUS - The Blue Streak TZE365V100428 '75 Avion - USA - Double Trouble TZE365V100426

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Re: [GMCnet] engineering behind choosing fastener (bolt) sizes [message #167932 is a reply to message #167929]

Sun, 29 April 2012 09:27

emerystora

Messages: 4442
Registered: January 2004

Karma: 13

Senior Member

Rob

I used to think that they were a press fit also but Wes Coughlan told me years ago that they were not a press fit. They are a close fit but a new one will easily slide into the arm.

The reason that they have to be pressed out is that after being in a while the joint corrodes (rusts) and that makes them very difficult to remove.

I recall taking one out for Kerry Tandy at a Cody, WY GMCWS rally. I used a sledge hammer to pound it back and forth to loosen it. When the hole was cleaned up the new one slid right into place using one hand. A tool was not needed to reassembly it as would have been required if it were a press fit.

Emery Stora
77 Kingsley
Santa Fe, NM / Frederick,CO

On Apr 29, 2012, at 8:07 AM, Rob Mueller <robmueller@iinet.net.au> wrote:

> Bruce,
>
> You kinda answered your own question. If you go MM X-7525 Page 4-12 Figure
> 25 you will see that there is a special tool to press the spindles out of
> the bogie arms. That press fit transfers the forces on the wheel through the
> spindle directly to the bogie. The four 3/8" bolts don't carry much load
> from the wheels if any.
>
> The bolts keep the brake backing plate from rotating. That puts them in
> shear. I did a Google search and found this table:
>
> http://nucor-fastener.com/Files/PDFs/TechDataSheets/TDS_013_Shear_Strength.p
> df
>
> It notes that a 3/8" Grade 8 bolt has a minimum shear strength of 9,939
> pounds. If I have interpreted that chart properly it would take a twisting
> force of 39,758 pounds to shear those four bolts.
>
> No one has reported shearing these bolts in the 5 years I've been hanging
> around here so it is safe to assume that braking does not generate that much
> twisting force.
>
> I could be wrong and my suppositions way off base! ;-)
>
> Regards,
> Rob M.
>
> -----Original Message-----
> From: Bruce Hislop
>
> I just finished changing to rear disk brakes and made this observation.
>
> The rims are held on by 8 big b'jezus 1/2" fine thread bolts on about an 8"
> diameter. Makes sense, lots of weight on them.
>
> But the whole wheel and brake assembly is fastened with 4 little coarse
> thread 3/8" bolts set no more than about 3" apart (very little mechanical
> advantage). Mind you they are grade 8 bolts and you could argue that after
> 35years the corrosion between the spindle and the bogie arm does most of the
> "fastening".
>
> Same thing on the one-ton front end. The bearing assembly in that case is
> fastened with just 4 bolts on a much smaller diameter (however about 1/2",
> most likely metric equivalent).
>
> So if four little 3/8" bolts on a 3-4" diameter can hold the whole wheel
> assembly safely, why is eight large bolts on a larger diameter needed to
> hold the rim and tire only?
>
> Inquiring minds want to know!
>
> Bruce
>
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
> http://temp.gmcnet.org/cgi-bin/mailman/listinfo/gmclist
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Re: engineering behind choosing fastener (bolt) sizes [message #167939 is a reply to message #167921]

Sun, 29 April 2012 10:59

Matt Colie

Messages: 8547
Registered: March 2007
Location: S.E. Michigan

Karma: 7

Senior Member

rf_burns wrote on Sun, 29 April 2012 07:36

Bruce,

I am going to work this down as best I can, like Albert Eistein was quoted "If you can't explain it to your grandmother, you don't understand it."

First, the lug nuts are 9/16" and that does not make any difference at all. The bolt size, count and circle were all determined by the wheels that the design team got to use. Yes, flexing is an issue (says the guy the broke a wheel), but it is not really the issue here for fastener size and location.

Except as already noted (employing existing parts) fasteners selected and placed largely on the maximum load anticipated in that joint. Sometimes this is done with complex mathematical modeling studies, but in the period our coaches were designed that stuff was not capable even though it did exist (in crude forms). Most likely some engineer got out his books and calculated the load that a part was likely to experience. He would have all the static load data that the design provided and he may have had data from dynamic tests done on vehicle bucks or mules. Then he would have looked over the design to see how many (few) fasteners it would take to to carry that load and still have a significant safety margin. In many places in a vehicle, the fasteners are only there to keep the parts from falling off if the expected load is reversed during service or some unanticipated situation. Many suspension parts are this way.

Safety margins in all of automotive design where a traditional nightmare. Since the mid-50's crash situations have been a design consideration (not always the highest, but there) and this has often lead to problematic assembly and service issues. Bargaining unit work rules and assembly quality also come into play in the early design phase. It is well understood that a difficult assembly will probably have quality issues in the plant.

In the case of a GMC rear spindle, the two components are completely engaged and though not a press fit, the fasteners will never see much load. People (most people) don't tend to drive a coach so hard that the lateral (side) load is any issue because all the dishes would end up on the floor. There is almost no chance (with the original single bag) of there ever being any significant shock loading on that joint. As someone else noted, probably the worst load is in severe braking and that little 11*2 Bendix can't product that much retarding torque. Even your new disks can't do that much before the tires skid, and if you put on the neat reaction arm setup - now they won't see any retarding torque at all.

If that didn't answer all your questions, or even started some new ones, I'm here.

If you are of a mood, and we have time at Amana, I can go over some fastener stories that will amuse you.

Matt

Matt & Mary Colie - Chaumière -'73 Glacier 23 - Members GMCMI, GMCGL, GMCES
Electronically Controlled Quiet Engine Cooling Fan with OE Rear Drum Brakes with Applied Control Arms
SE Michigan - Near DTW - Twixt A2 and Detroit

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Re: [GMCnet] engineering behind choosing fastener (bolt) sizes [message #167965 is a reply to message #167932]

Sun, 29 April 2012 17:24

USAussie

Messages: 15912
Registered: July 2007
Location: Sydney, Australia

Karma: 6

Senior Member

Emery,

I stand corrected, thanks!

However, from what you note below what Bruce stated appears to be correct
and corrosion DOES turn the spindle and bogie arm into one piece! ;-)

Regards,
Rob M.

-----Original Message-----
From: Emery Stora

Rob

I used to think that they were a press fit also but Wes Coughlan told me
years ago that they were not a press fit. They are a close fit but a new one
will easily slide into the arm.

The reason that they have to be pressed out is that after being in a while
the joint corrodes (rusts) and that makes them very difficult to remove.

I recall taking one out for Kerry Tandy at a Cody, WY GMCWS rally. I used a
sledge hammer to pound it back and forth to loosen it. When the hole was
cleaned up the new one slid right into place using one hand. A tool was not
needed to reassembly it as would have been required if it were a press fit.

Emery

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Regards, Rob M. (USAussie) The Pedantic Mechanic Sydney, Australia '75 Avion - AUS - The Blue Streak TZE365V100428 '75 Avion - USA - Double Trouble TZE365V100426

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