<VV> Suspension Fasteners

[Sethracer at aol.com]

Frank wrote:
Do you mean if a grade 8 bolt/nut is used in place of a grade 5 that one  
must use a higher torque spec even if the grade 5 torque spec was more than  
adequate for the clamping force required? I do realize if grade 5 is used in  
place of grade 8 then the torque would have to be lowered, otherwise fastener  
failure would be possible. But of couse I would not use less rated fasteners, it  
is just an example.

So Seth writes:




Frank - I can only reply with a story. I worked at a manufacturing facility  
building tracked vehicles for the logging industry. The basic tracks and drive 
 system was based on the Army M113 personnel carrier, but much of the vehicle 
was  redesigned for the forest environment. The Military had specified Grade 
8  hardware for portions of the drive area and the part numbers were carried  
over. New part areas, much of the vehicle, were designed for Grade 5 hardware, 
 since the lower stress allowed plenty of strength overhead for a Grade 5. On 
the  production line, fasteners were constantly being mistakenly installed, 
sometimes  Grade 8 instead of Grade 5, also Grade 5 instead of Grade 8.  
Finally, the  engineers threw up their pencils and changed all the hardware 
installation  drawings to specify Grade 8 only. Great! - Except that they had 
forgotten to  change the assembly drawings to reflect the torque needed to retain the 
Grade 8  bolt. Remember that the higher call out for the torque value on the 
(stronger)  Grade 8 fastener is used to stretch the bolt, with that clamping 
force on the  threads keeping the nut or bolt from backing out. Thread locking 
compounds help  this feature. After the logging vehicles reached the field, 
parts kept falling  off as the under-torqued grade 8 fasteners backed out!  We 
had to do a  full drawing review and change all the torquing instructions to the 
higher  values. That cost a bunch of money! (Not counting the field service 
costs of  visiting all the customer sites!) Grade 8 fasteners must be torqued 
to the  higher called out values to provide enough bolt stretch to retain the 
bolt.  Locking compounds will help. - Seth Emerson 



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