NEED HELP WITH DETERMINING WAHT STAINLESS STEEL FASTENER TO USE

[swolsky]

Hey Everyone,

I am a new mechanical engineer. I am working on a redesigning an intake manifold and the PM wants to replace the factory class 8.8 intake manifold bolts with stainless steel. The upper intake and lower intake manifold are both plastic. The upper is bolted down to the lower intake using Class 8.8 SCREWS. Meaning the fastener is rated at class 8.8 but it utilizes screw threads to thread inside plastic bosses on the lower intake this leads to the plastic holes stripping out after removing the intake a couple of time. We want to replace the plastic bosses on the lower intake manifold with M6X1.0 brass inserts and use stainless steel M6X1.0 bolts to fasten the upper manifold to the lower so it can be removed several times without stripping out the hole. The torque spec on the stock upper to lower manifold is 9 N-m. We like to have a FOS of 1.5 so I want the stainless bolts to be able to handle a torque of 15 N-m without stripping out or shearing.

The head of the bolt is 8mm from flat to flat with that being said the moment arm from the center axis to where the torque is applied is 4mm. The Tensile area of an M6 fastener is 20.123 mm^2. The Torque is then equal to 15000 N-mm. Using the equation F=(T/r) I can find that force in newtons applied to the bolt is 4500N. Dividing that force of 4500N by the tensile area of the fastener (20.123mm^2) the Tensile Stress put onto the bolt equals 223.63 Mpa.

I want to use a 304 stainless due to its corrosion resistance but i cannot figure out what kind of OTS options there are that can withstand this force. MY fear is that stainless shears much easier that steel and the bolt will break upon torqueing or removal. I am not sure if the shank length of the bolt effects its strength at all but the longest bolt used to bolt the intake down has a shank length (bottom of the head to the end of the bolt) of 135mm.

Once of my co-workers said he used to design stainless bolts and said the length shouldn't have an effect but i am wary of his advice.

I really appreciate any sort of help or direction anyone can provide. The shigleys mechanical design textbook seems to not be helping me at all with this.

 

[mfgenggear]

OP
Run an actual test.
Buy the inserts and install in block in a vice.
Then tighten the screws by hand . Use feel to
Until the fastener feels is starting to twist.
Verify with a torque wrench and record.

Now with different fasteners a new test.
Install insert per manufacture recommendation.
Then install fastener with a torque wrench.
Record , then using a torque wrench but use a beam style torque wrench. Torque to failure and record.

 

[mfgenggear]

OP

I have installed thousands of fasteners of all types.
Low strength aluminum, plastics and other low tensile materials, look cross eyed and it will strip out. From over torque. And will vary from different mill runs because of tensile, and hardness , it will very. Also galling between fasteners will strip out dissimilar materials. Heat and corrosion will also cause stripping.

 

[Doug Hunter]

Call the screw supplier and ask for guidance. If you need one, Acument has been supplying in Automotive for a long time. Their Application Engineering can guide you through the process. They will have a good understanding and track record of which designs will satisfy the application. You'll need to consider statistical ranges of variation in the safety margins of your calculations. Also, I would use a flange head screw, not a hex-head. The flange will distribute the clamp force better and prevent digging from the hex corners. Let me know if you want a contact name.

 

[TugboatEng]

8.8 is a high strength faster, common stainless fasteners will not be a drop in replacement. A2-70 is common but does not achieve newr the strength of an 8.8 fastener. A2-80 or A4-80 are better but are still not the same strength. Proprietary fasteners such as Bumax 88 do get you into the same strength category. There are other manufacturers with parallel product lines such as Kolb. ASTM A286 or CRES fastenes are also stainless in the same strength category as 8.8. ARP produces stainless fasteners from Custom 450 which can equal 10.9 strength levels. 17-4PH is another available option.

 

[MintJulep]

The head of the bolt is 8mm from flat to flat with that being said the moment arm from the center axis to where the torque is applied is 4mm.

Huh? That's not relevant.

Anyways, how much clamping force do you need?

How to select fasteners:

1. Define the clamping force.
2. Decide the number of fasteners.
3. Decide material.
4. Decide diameter.
5. Decide if torque control is good enough.
6. If yes, calculate torque.

 

[BrianPetersen]

Unless this is a forced-induction engine (seems unlikely with the existing design using what I call "wood screws") the clamping force is irrelevant. The screws just need to hold the upper manifold in place (against vibration and the like). Torque limitations are more to protect from gorilla mechanics that aren't using a torque wrench to tighten the fasteners. It just needs enough torque for the screw to not come loose.

Any particular reason for wanting stainless-steel fasteners? I don't think I've ever seen one that used stainless fasteners in that application. Usually they're just regular 8.8 screws made from whatever such screws are usually made from. They might be plated, either galvanised or chrome-plated, if the appearance matters.

If corrosion is a big concern, you *could* use regular off the shelf stainless screws. They won't be "8.8" but it shouldn't matter in that application unless there's something unusual about it that you haven't told us, and they will be more expensive ...

 

[mfgenggear]

Agree or disagree, my opinion not with proper preload it screws will vibrate loose.

 

[3DDave]

Unless there are 10-30 diameters of engagement with the plastic parts the 8.8 fasteners are overkill, much as the Torx-recess screws that held thin plastic trim panels in place - pure decoration. They covered the Phillips recess flatheads that held the heavy bits, like the cantilevered radio housing, in place.

The plastic is far lower bulk stiffness over steel of any grade. Without knowing more about the amount of engagement and the actual geometry the best that can be done is go by pull-out strength of the base material.

 

[CWB1]

Not sure what the logic behind your analysis is, but you're redesigning a critical joint so should be completing both a BJA and working with a fastener supplier/lab for joint testing. Your inserts and fasteners need to be sized appropriately to balance clamp and bearing loads, and fastener stretch, otherwise gaskets leak, plastic breaks, and/or fasteners vibrate loose.