<VV> oil filter and alternator mount bolts...

[Kirby Smith]

One ME at work remembered a rule of thumb that 90% of the torque was 
taken up by three threads, but could only guess that he read it in 
Shigley (presumably "Mechanical Engineering Design" edition unknown). 
The guess was it was due to tolerances in thread forming.

My quick scan of one edition of Shigley showed the following on page 
320: "Three full threads are all that are required to develop full bolt 
strength."  This is a somewhat different point than the above, and 
suggests that if more threads are engaged, the bolt is the weak element. 
  Of course, then one has to ask why threads are ever stripped in the 
mating hole.

On the subject of strength: Shigley reports the following SAE minimums

Grade 2 55k psi yield
Grade 5 85k psi yield
Grade 8 120k psi yield

For static conditions, a low safety factor of 1.1 was recommended, but 
for dynamic conditions (that I think was the justification for higher 
bolt grades in an earlier message here) the required safety factors are

Grade 2: 2.2X
Grades 5 and 8: 3X

Thus, the tensile limits that should be applied are

Grade 2 25k psi
Grade 5 28k psi
Grade 8 40k psi

kirby


Kirby Smith wrote:
> I hope no one misunderstood me; I was not promoting cheap nor do I have 
> grade 2 CRES bolts holding down my alternator.  In fact, I'm a fan of 
> overkill.  I'm reasonably sure (without looking) that my alternator is 
> using the OEM bolts.  However, I was in doubt about grade 2 bolts 
> breaking in that application.
> 
> The three threads point I made was not to assert that more threads are 
> not needed, but that with steel on steel most of the force is taken up 
> by the area of only three threads, no matter how many are engaged, or so 
> I have inferred in the past from the MEs I interact with at work.  I 
> understood that to be due to standard tolerances in thread forming, but 
> I will ask.  I don't believe they intended to imply that the engaged 
> surface area was continuous so that only three threads had to be 
> engaged, but I will ask about that also.
> 
> kirby
> 
> 
> JVHRoberts at aol.com wrote:
> 
>> Well, the aluminum is also lower in modulus than steel, so load 
>> spreading by all the threads happens with little or no permanent 
>> deformation of the aluminum part. So, the bolt is the weakest link. 
>> Grade 5 is what GM uses for all automotive fasteners as a minimum. 
>> Going less than that is flirting with failure. GRade 2 is FAR less 
>> fatigue resistant than Grade 5, Grade 2 is FAR softer than Grade 5, 
>> and I just won't use anything less for ANY automotive fasteners. Add 
>> to that the thermal cycling, and that aluminum has nearly 3 times the 
>> CTE of steel, and you could slowly stretch bolts that way. Given that 
>> the oil filter mount carries the load of the alternator (which, if 
>> you'll notice, uses Grade 8 bolts from the factory, no doubt due to 
>> the high bending moment on those!) and the idler pulley, it's just not 
>> worth it to go cheap.
>>  
>> John
>>