During endurance testing of a new product, some screws were found to loosen and break. When I examined the broken screws, I found that they had laps or folds under the head. Laps or folds are defects, which can be introduced during the heading process. The fatigue cracks in the screws had started from these defects.
As soon as it was discovered that the screws were defective, the vendor went to work finding a new source and providing replacements. They were able to get us new, fully conforming screws in record time.
Meanwhile, I continued to investigate the failure. It's one thing to identify a non-conformance, and another thing to identify the root cause of a failure. Yes, the screws were defective, but this still didn't fully explain why they failed. Screws don't tend to experience bending fatigue unless they are loose. Why were the screws coming loose in the first place?
While I was pondering this question, an assembly line worker came to me with a new problem. When installing the new screws, the internal threads on the mating part were stripping out. What now?
The aluminum from the mating part lodged in the threads of the screw showed me that there were only a few threads of engagement. These threads had to support the entire clamp load of the joint. This load, over the surface area of these threads, exceeded the shear strength of the aluminum, causing the threads to strip out.
Why didn't this happen with the old screws? As it turns out, the old screws were about half a thread longer than the new screws. (Both were within the acceptable length tolerance.) This extra bit of engagement was just enough to ensure that the internal threads wouldn't be ripped out of the aluminum when the screw was installed. However, the stress was still high enough for the internal threads to yield. This caused a reduction in clamp load, which allowed the screws to loosen and fatigue.
The root cause for both problems was the same: a joint design which didn't allow for enough thread engagement. How much thread engagement is enough?
Dave, thanks for letting us get inside the head of a failure analysis specialist. Much like detective work, it's fun to get in on the thought process as you follow the trail to the end resolution. And with this example, it shows you really do have to sweat the small stuff.
When one is tightening bolts on a car, there are torque specs in most cases. Thus one can use a torque wrench to ensure that you're meeting the spec and not putting undue stress on the bolt-plus-nut assembly (and also not cracking the metal parts that bolt and nut are clamping together). Of course we all know that in many cases, in repairs and particularly with home mechanics, bolts are just tightened and the "spec" is just done by eyeballing it (i.e., no torque wrench used). So my question is, is there any analogy for screws? In other words, how to you ensure a screw is tightened properly but not overtightened?
@Alexander: Yes, the situation is absolutely analagous. There are torque specifications for screws, just like there are for bolts and nuts. In either case, you are applying a torque in order to create a specific clamp load. This is the force which keeps the joint from coming apart.
Whether the fastener is a screw or a bolt, a big part of the torque which you apply goes into overcoming underhead friction. If there is a washer in the assembly, this is friction between the head of the fastener and the washer. If there is no washer in the assembly, it is the friction between the head of the fastener and the mating surface.
Another big part of the torque goes into overcoming thread friction. For a bolt, this is the friction between the threads of the bolt and the nut; for a screw, it is the friction between the screw threads and the mating part.
The smallest, but most important, part of the torque goes into stretching the screw or bolt. This part is responsible for the clamp load. The more the fastener is stretched, the greater the clamp load. Of course, if the fastener is stretched too much, it may yield or break. (As I discussed in my article, the mating threads also need to withstand the clamp load -- and if it is spread over too few threads, they may yield or break).
This brief discussion of torque-tension relationships shows why proper thread lubrication is important. By minimizing thread friction, proper lubrication allows you to produce more clamp load with the same amount of torque. It also shows why it is often advantageous to use a washer. Using a washer helps to make the underhead friction predictable and consistent.
Clearly, the torque-tension relationship for a given joint is highly dependent on the materials and finishes used. For example, if you replace a zinc-plated steel screw with a stainless steel screw, you shouldn't expect to be able to achieve the same clamp load with the same torque, because the friction coefficients will be different.
In order to come up with a torque specification for a specific joint, it's often a good idea to do a torque-tension study. Generic torque charts can be misleading. It's important to understand how much variation can be expected in terms of underhead friction and thread friction in your application. It's also important to understand how much variation in torque you can expect from your torque wrench or driver. When you have a good understanding of all of the variables involved in the torque-tension relationship, you can specify an installation torque which will provide the clamp load your application needs.
@wb8nbs: I would use that rule of thumb with a lot of caution. For one thing, it doesn't take into account the strength of the material you're threading into. You need more thread engagement if you are threading into a soft material than you would need if you were threading into a hard material. It also doesn't take the clamp load into account. If you need to withstand a higher clamp load, you need to engage more threads.
Based on the approach I described in the column, I calculated that you need a minimum of 7 threads of engagement when threading a Grade 5 3/8"-16 screw with a 6600 pound proof load into an aluminum casting with a 36,500 psi tensile strength. In this case, three threads would definitely not be enough.
The 1.5D rule of thumb which I mentioned would dictate 9 threads of engagement for a 3/8"-16 screw, regardless of the clamp load or the internal thread material. This is likely to be conservative in nearly every situation, but it's still just a rule of thumb and your mileage may vary. The best bet is to go through the calculation.
Many companies have internal standards used for thread engagement required for end product use. These are basicly charts with diameters and loads that dictate the required engagement for a tight / lasting joint.
Once at a previous job, we had issues with threaded holes stirpping out when molds were hung on the aluminum platens of a molding machine. We could not understand this as we had a regulation on the maximum torque allowed on the screws. Observation of the process showed that we had one tech on one shift that would tighten until the torque wrench clicked then would tighten and additional half turn. When asked, he said that he just wanted to make sure that it was tight enough. We explained the reasoning behind the maximum torque and we really did not see the issue again. Someitmes you can have too much of a good thing.
@Tim: "Too much of a good thing" is a common problem. When I worked in a foundry, there was a melt operator who consistently poured steel as much as 100°F above the recommended temperature. When I asked him why, he said that it was because other operators who poured at temperatures below the recommended temperature had too many defects. This was true, but he was getting plenty of defects, too. (In fact, the finishing department had asked me to talk to him, because they were tired of welding up the defective castings he poured). I asked why he didn't pour at the recommended temperature. This thought didn't seem to have occurred to him. I guess nobody ever read him the story of Goldilocks when he was a kid.
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