Can a Torqued Bolt Become Loose?

The answer to this question is, "Yes it can." But we need to qualify this answer. First lets eliminate the possibility of a nut rotating, thus loosening the torque. So now we are left with “how else can a torqued bolt become loose?”. OK here is one way that a bolt, after it has been properly torqued, can become loose. This phenomenon is call “embedment relaxation”.

Embedment relaxation is a phenomenon in mechanical engineering in which the surfaces between mechanical members of a loaded joint embed. It can lead to failure by fatigue and is of particular concern when considering the design of critical fastener joints such as in the through-bolts in the crankcase supporting the engine main bearings.

In critical fastener joints, embedment can mean loss of preload or clamping force. Flattening of a surface allows the strain (or preload) of a bolt to relax, which in turn correlates with a loss in tension and thus preload. Therefore, embedment can lead directly to loosening of a fastener joint and subsequent failure.
In bolted joints, most of the embedment occurs during torquing. Only embedment that occurs after installation can cause a loss of preload, and values of up to 0.0005 inches can be seen at each surface mate, as reported by SAE.

As each surface is pressed together, the high spots are crushed and deformed to form a surface capable of supporting the load.

Surface Finish:
There are multiple surfaces in the joint:

1. between the head of the bolt and the washer,
2. between the bolt washer and faying surface (faying surface is the surface of the object being fastened together),
3. between the two faying surfaces,
4. between the faying surface and nut washer, and
5. between the nut washer and the nut face.

Each of these surfaces squeeze together. Any paint, sealants, nicks, or alignment errors are gradually crushed down to support the load. Bolt threads also embed. Threads are pulled in shear, slightly increasing the thread pitch. Nut threads are compressed and lose a little pitch. As embedment occurs, the surfaces press further together and reduce the bolt's clamping force (preload). Note that relaxation occurs without any off-rotation of the nut.

Under optimum joint conditions in a lab one can expect between 1 and 11 percent. Obviously then, in the field, more clamping force can be lost. Lockheed did a lab test, Report No. LR 25049 where they tested 1 inch by 12 UNJF thread size L-1101 engine pylon bolts. They used lab conditions, with hardened steel bushings. In a static joint with no load fluctuations, between 1 and 11% preload was lost. The greatest loss occurred in the first eight hours after installation.

I have also read that the SAE did a study on this subject and they determined that as much as 20% of the preload was lost due to embedment. Some aircraft engine overhaul shops recommend that all the critical bolts on the engine be re-torqued after the first 25 hours of operations. This is most likely when the embedment has settled into a position where the surfaces are supporting themselves.

This may explain why some aircraft engine manufactures recommend that a complete 100 hour inspection be performed after the first 25 hours of operations after an overhaul or new engine is installed.

Checking the Torque on Critcal Engine Through-Bolts. How Important is This?

When you investigate aircraft accidents you get to learn some really important lessons. Usually by then it is a bit late. I was involved, as an expert witness, in a legal case where a TCM IO-520 engine had a catastrophic engine failure in flight. The crankshaft fractured and broke near the rear section and the engine instantly stopped producing power, which is what you would expect. Luckily no one was seriously injured.

The post accident investigation of the engine components revealed that the cause of the crankshaft failure was due to oil starvation to the main bearing journal. The oil starvation was caused by the main bearing shell rotating about 30 degrees within the main bearing crankcase saddle. The mating surfaces (parting surfaces) of the left and right crankcase saddles had signs of severe fretting. There were no other signs of fretting anywhere else in the case mating surfaces. Our determination as to the originating cause of the failure was that the through-bolts had inadequate torque. It had been about one year since the engine was overhauled and 440 hours. This was being maintained under Part 135 and was receiving 100 hour inspection. The records showed that no one had ever checked the torque of these bolts during their routine maintenance.

In this engine there are 5 main bearing saddles and each one has two through-bolts, one above and one below the main bearing saddle. This allows for a total of 10 through-bolts. These through-bolts are extremely important in clamping the entire case halves together and supporting the crankshaft. It was our determination that the clamping force was inadequate which allowed the bearing shell to rotate due to its loss of crush. It is this crush that keeps the bearing shell in place, not those two small tangs. This clamping force on each of the 5 main bearing saddles is supplied by the two through-bolts at each saddle.

The torque of these through-bolts is what sets the pre-load or stretch, which in turn applies the clamping force. It is this clamping force that resists movement between the two case halves. Even the smallest amount of movement between these surfaces will cause fretting. Fretting is a type of wear that occurs when two or more surfaces are in contact with each other under a load. Now if there is any movement between these surfaces, even at the microscopic level, fretting will happen. This fretting is wear, so now the surfaces are losing material which means that the clamping force will weaken as more material is lost, which in turn allows for more movement. Now you have a self-sustaining cycle which will progress rapidly.

The FAR's under Part 43 Appendix D contains a checklist of required items that is to be inspected for each 100 hour/Annual inspection. Below is a quote from Appendix D

(d) Each person performing an annual or 100-hour inspection shall inspect (where

applicable) components of the engine and nacelle group as follows:

(1) Engine section—for visual evidence of excessive oil, fuel, or hydraulic leaks,

and sources of such leaks.

(2) Studs and nuts—for improper torquing and obvious defects.

As you can see in (d) (2) it calls out for inspecting nuts for improper torque as part of the engine inspection. This is pretty vague as it does not specify exactly which studs and nuts to check. It is up to the mechanic to decide which ones they are going to check as it is impractical to check every one. Personally I would be looking at those that are critical verses non-critical. These through-bolts are the few that I would classify as “critical”.

So now you are asking, “if a nut is torqued why would I ever need to recheck it?” Well that is a good question and it deserves to be addressed in another topic so you will have to read my blog post on that subject here. Go here for that explanation.

The bottom line to this blog is that if the operator of this aircraft had spent the time to check the torque on these through-bolts, and had re-torqued them as required, then this failure might have been prevented.