johnc Posted September 21, 2005 Share Posted September 21, 2005 When tightening a fastener, the fastener is subject to two types of stress: tension and shear. Tension Tension stress comes from the stretching of the fastener after the bearing face has contacted the work face. This is the most important stress and is what keeps the parts together, keeps the fastener from loosening, and keeps the fastener from failing in use. Tension stress remains in the fastener the entire time its fastened. Shear Shear stress comes from installing the fastener. It is caused by the friction of the threads and the friction of the bearing surface when it contacts the work face. Shear stress drops to zero the instant tightening stops but reachs a very high value just before tightening stops. Shear stress makes it difficult to impart the correct amount of tension stress in the fastener because shear stress affects torque readings. On a dry fastener, about 50% of the torque applied comes from the friction between the bearing surface and the work face, 40% from the thread friction, and only about 10% of the torque is actually used to apply tension. The single biggest reduction in shear stress is the proper use of thread lubricants (anti-sieze). It reduces the amount of torque needed to achieve the desired level of tension stress in the fastener. The seconds best way to reduce the effects of shear stress is to tighten the faster in several steps. Quote Link to comment Share on other sites More sharing options...
2126 Posted September 21, 2005 Share Posted September 21, 2005 Hey John, just a question on the bolt or stud shear. Isn't the load actually a torsional shear as opposed to a purely shear load? Additionally, I believe the advantage of a stud vs a bolt (talking about securing the head) is that you can physically measure the stretch on a stud, which is not posiible with a bolt in a blind hole. Quote Link to comment Share on other sites More sharing options...
johnc Posted September 21, 2005 Share Posted September 21, 2005 No such thing as "torsional shear." Torsion load comes from the wrench and shear is the frictional resistance to the torsion load. If the fastener bottoms in a hole before the bearing surface engages the work face while you're still turning the wrench, you'll still have a torsion load even though the shear load is minimal. Quote Link to comment Share on other sites More sharing options...
johnc Posted September 21, 2005 Share Posted September 21, 2005 Although I might be mistaken, I think you have to get at both ends of a fastener to measure stretch. Easy for rod bolts, hard for cylinder head bolts. Quote Link to comment Share on other sites More sharing options...
SleeperZ Posted September 21, 2005 Share Posted September 21, 2005 Jeez, I need to go through this whole dang thread. Your machine shop techer is wrong; an insert that depends on the strength of the parent metal to remain in place is not any stronger then the parent metal itself.But the insert is larger diameter, and has more metal surrounding it. Does that not increase the strength of the fastener anchor? Quote Link to comment Share on other sites More sharing options...
2126 Posted September 22, 2005 Share Posted September 22, 2005 No such thing as "torsional shear." Torsion load comes from the wrench and shear is the frictional resistance to the torsion load. If the fastener bottoms in a hole before the bearing surface engages the work face while you're still turning the wrench, you'll still have a torsion load even though the shear load is minimal. Just did a search on "torsional shear" and as I've been schooled, there is such a thing! There are a number of stresses that can cause shear....torsional shear being of of them! Check it out. I certainly respect your knowledge and experience and I'm sure we are talking about the same thing here but, in this case I think you are incorrect with your nomenclature. Just trying to pass along a little engineering knowledge. Quote Link to comment Share on other sites More sharing options...
Tony D Posted September 22, 2005 Share Posted September 22, 2005 But the insert is larger diameter, and has more metal surrounding it. Does that not increase the strength of the fastener anchor? Oftentimes the thread pitch and diameter of the outer portion of the helicoil is different than the inner pitch and diameter, leading to that statement. KeenSerts arguably are stronger, but those aren't Helicoils. Also, Helicoils are normally used in non-ferrous metals, so because of the outer diameter and pitch, it would be harder for Godzilla The Torquemonster to pull the helicoiled threads out as the former raw alloy threads.... Quote Link to comment Share on other sites More sharing options...
pparaska Posted September 22, 2005 Share Posted September 22, 2005 I think what John is talking about is the frictional force at the threads. That cause a localized stress in the thread area of the fastener and whatever it's threading into that can be thought of as having a shearing component that's parallel to the thread helix direction. There's also a bending and a tensile stress. Torsional shear stress is the component of stress in the bolt or stud shank that is, yes, torsional, or directed around the circumferential direction. The other major stress in the bolt or stud is tensile, that is, what comes about from the stretching of the bolt as the threaded end is drawn down into the engine block, while the head remains at the same height above the block deck (ignoring the smaller amount that the head is compressing locally around the head area and below it). The threaded region of the fastener has some pretty complex stress gradients, whereas the shank has generally just tensile and torsional stress while it's being torqued. After the torqueing is done, it's largely tensile only, with the friction at each end keeping a low torque in the shank. That might nearly disappear once the engine heat cycles. Bolts can have bending stresses in them as well. But you'd better hope their a good bit lower in magnitude than the tensile stress, or the bolt may fail if taken to the torque spec. Stress is really a tensor quantity that can be explained in several coordinate directions. In this case, it's convenient to talk about a coordinate system that's cylindrical, with the axis along the bolt's centerline. You can also describe the stress in principal stress components, but that's not really helpful here, even though it is important in determining the strength of the bolt and the torque spec if a below-yield bolt is being designed. Quote Link to comment Share on other sites More sharing options...
johnc Posted September 22, 2005 Share Posted September 22, 2005 People can put together words to come up with whatever terms they want (isn't that how the German language works?) As far as loads are concerned there are just a few basic ones: Static Dymanic (interetia) Internal Resistance (pressure) Shear Torsion Tension Compresion Screw threads are ramps and ramps generate shear. The wrench on the end of the bolt generates torsion. I separate those two in my mind because if the threads are removed you can still generate torsion with the wrench. But, I'm not an engineer so maybe I've got it wrong. Quote Link to comment Share on other sites More sharing options...
johnc Posted September 22, 2005 Share Posted September 22, 2005 But the insert is larger diameter, and has more metal surrounding it. Does that not increase the strength of the fastener anchor? Whatever the insert, it can't be any stronger then the yield strength of the metal the insert is installed in. Now, it "might" be "stronger" because it grabs a little bit more of the parent metal becuase of a slightly large OD, but so much depends on the quality of the insert, how it was installed, and how much parent metal was lost int he hole enlargement that I don't think the OD difference matters. Quote Link to comment Share on other sites More sharing options...
pparaska Posted September 22, 2005 Share Posted September 22, 2005 People can put together words to come up with whatever terms they want (isn't that how the German language works?) That's what I was getting at - the name doesn't matter if you are talking about the correct stresses, etc. I agree with what you wrote, but was trying to connect it with what 2126 posted. I think you were talking about the first thing below (ramps and shear) and 2126 was talking about the second thing below (torsion in the shank). Of course, one is related to the other. Screw threads are ramps and ramps generate shear. The wrench on the end of the bolt generates torsion. I separate those two in my mind because if the threads are removed you can still generate torsion with the wrench. But, I'm not an engineer so maybe I've got it wrong. No, I think you have it right and I is an engineer (or I play one) - mechanical engineer. I just think you and 2126 were talking about different parts of the same structural problem of head bolt stresses. Quote Link to comment Share on other sites More sharing options...
pparaska Posted September 22, 2005 Share Posted September 22, 2005 Whatever the insert, it can't be any stronger then the yield strength of the metal the insert is installed in. Now, it "might" be "stronger" because it grabs a little bit more of the parent metal becuase of a slightly large OD, but so much depends on the quality of the insert, how it was installed, and how much parent metal was lost int he hole enlargement that I don't think the OD difference matters. If there's enough parent material around the insert to handle the loads imparted to the insert, and the install was good (threads for OD of insert not too large in diameter, etc.), I believe you are correct that it can actually be a better connection than when the original threads in the block were in good condition. That is, if the insert material is stronger (ought to be, were' talking good quality machined steels versus cast iron), and it spreads the load out to a larger area of the block. I had a main cap bolt strip out of a 327 Chevy. The machine shop said they'd done plenty of Helicoil inserts for such problems and never had a come back. It worked fine in my 327 too. Quote Link to comment Share on other sites More sharing options...
2126 Posted September 22, 2005 Share Posted September 22, 2005 That's what I was getting at - the name doesn't matter if you are talking about the correct stresses' date=' etc. I agree with what you wrote, but was trying to connect it with what 2126 posted. I think you were talking about the first thing below (ramps and shear) and 2126 was talking about the second thing below (torsion in the shank). Of course, one is related to the other. No, I think you have it right and I is an engineer (or I play one) - mechanical engineer. I just think you and 2126 were talking about different parts of the same structural problem of head bolt stresses.[/quote'] Thanks Pete! I think we all know what takes place when securing a fastener. I guess the point I was trying make (and I have a smile on my face) is....I've been in so many technical discusion where the two parties are explaining the same thing but using different or misused terms to discribe it and the discusion turns into a disagreement. So, I always advocate using the proper nomenclature for a given science...that way it is much easier to understand the details. John, no harm intended...I always like to read what you have to say, as I have learned many things I did not know. Quote Link to comment Share on other sites More sharing options...
Kazuya1274 Posted September 23, 2005 Share Posted September 23, 2005 The advantage of a stud/nut combo is clear over that of a regular bolt. Much of the torque of a bolt is lost due to the friction of turning the entire fastener (more thread surface in block) and therefore less clamping force is translated holding the two pieces together. Both setups are equally strong' date=' just the stud/nut combo clamps the parts tighter together.-jeremy-[/quote'] zguy36, I went through a couple stock head gaskets on my turbo, and on the last one I switched over to ARP studs. I installed them exactly per the instructions, and the head gasket ended up leaking coolant out the side of the block. I'm not placing any blame, but I never had that problem with the stock turbo head bolts. If I were to do it all again, I would use the stock turbo bolts with a better head gasket. So my question is, if there is more clamping force with stud/nut combo, then how could my head gasket have been dumping coolant out with more clamping force? Quote Link to comment Share on other sites More sharing options...
srgunz Posted September 23, 2005 Share Posted September 23, 2005 So are the Datsun torque values dry values? Some people use lubricant, therefore I suspect they are overtourqing the bolts. What about anti-sieze on the threads. How does that affect the torque value? It seems to me after a few uses I would throw the bolts away and get new. What say you? Quote Link to comment Share on other sites More sharing options...
johnc Posted September 24, 2005 Share Posted September 24, 2005 It seems to me after a few uses I would throw the bolts away and get new. I think of head and flywheel bolts as single use items. Both go through countless cyclical loads over their installation life. Quote Link to comment Share on other sites More sharing options...
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