since 1940

Cylinder stud failure. Broken stud

Broken cylinder hold-down stud from continental engine. Notice the clean break with no bending or distortion to the metal. Notice also the black deposit on the flange next to the stud. This is fretting damage caused by movement of the cylinder on the crankcase. Clean break and signs of joint movement indicate a fatigue failure caused by cyclic stress from a loss of stud preload.

If the studs break straight with very little plastic deformation (bending of material), the absence of plastic deformation indicates that the studs failed at a stress level below their yield point. The studs, therefore, were not overloaded. If they were, they would have deformed. This type of break is a fatigue fracture. Fatigue is defined as the cracking, flaking or spalling of a surface due to stresses beyond the endurance limit of the material

Fatigue fractures are flat and smooth except the last portion near the edge that twists off as the crack propagates across the part. This last portion, called a "shear lip," fails at a 45 degree angle. Since the shear lip is the last portion to fail, the point opposite to the shear lip is the origin point. Fatigue fractures propagate across the part. At each stress cycle the crack grows a small bit more. This start and stop crack growth leaves lines radiating away from the crack called "beach marks." Crack growth accelerates and beach marks spread apart as crack progression weakens the part. The crack origin is where the beach marks are the closest together. The broken cylinder studs exhibit these fatigue characteristics; little plastic deformation, shear lips, and beach marks. The failure was not from detonation, pilot error, or other conditions that might overload the studs. The studs broke from fatigue.

What causes fatigue failure?

Fatigue requires cyclic stress. Cyclic stress results from cyclic strain. By strain we mean a change in length. Strain gauges, to illustrate, measure very small changes in length. By measuring strain we apply a mathematically formula called "Hook's Law" to derive stress. The point here is that cyclic stress requires cyclic changes in length of the stud. Fatigue failure is evidence that the stud experienced cyclic changes in length. Cyclic strain loading of the studs doesn't occur if the cylinders are adequately clamped together. Only when the surfaces separate, however slightly, does the stud cyclically strain. This means the joint was loose.

What caused the loose joint?

Preload is the clamping force resulting from torque applied to the studs. A fancy way of saying the joint was loose is to say the joint had inadequate preload. Embedment is the coming together of two surfaces as the contact points deform. Embedment relaxes preload and allows the parting surfaces to separate. The stud elongates (strains) and feels tension stress as combustion forces separate the parting surfaces. Fatigue failure then is a sign of inadequate preload. Why was preload inadequate? Either, (1) the studs were improperly torqued, or, (2) the clamped surfaces moved closer together and loosened the joint. Clamping to clean, metal-to-metal surfaces prevents embedment relaxation. Sealants on the cylinder hold-down flange extrude out from between the surfaces and allows the surfaces to come together, which relaxes preload. A thick layer of paint under the nut relaxes preload as the paint wears away and loosens the nut. There are no signs of sealants or an excessively thick paint layer under the heads of these nuts. We can conclude that preload loss was not due to embedment of the surfaces.

 

Lycoming cylinder flange showing how only a thin layer of primer is used on the hold down flange. Consult Lycoming Overhaul Manual for specific painting instructions. There should not be a thick layer of paint under the hold down nuts.	Continental O-470 cylinder flange showing improper paint film around hold-down boss

 Were the studs improperly torqued?

 Clamping force from torque is no more accurate than the predicted value of thread friction in the fastener. If we apply 40 foot pounds of torque to a stud, without thread friction, torque energy stretches the stud in tension and tightens the joint. Threads that are dirty, painted, or damaged have lots of friction. Torque energy twists instead of stretches the stud. Very little energy goes to stretching the stud and the joint is loose. The engineer who specifies the amount of torque assumes a friction value for the threads. To make sure this value exists at the time of torque he may specify the type of thread lubricant to use while torquing the stud. To get the correct preload the threads should be clean, undamaged, and lubricated with the manufacturer's specified thread lubricant. Among lubricants that Lycoming specifies is 90% SAE 50W engine oil and 10% STP. Continental, for example, has in the past specified Oil Grade 50, MHS27 for cylinder studs and through-bolts.

Could excessive thread friction have caused inadequate stud preload? Examination of the broken studs shows no signs of paint or damaged threads that would have prevented an accurate preload from the specified torque value. Are there other signs of improper torque? The crankcase column supports are fretted. Fretting shows that the parting surfaces were loose enough to rub. This means that the through-bolts were also loose. Since both the studs and through-bolts were loose the failure of the cylinder hold-down studs probably resulted from applying torque improperly to the hold-down nuts.

 An inspection for crankcase fretting may be necessary if the cylinder hold-down studs fail. This can be done by pull-testing the propeller with a spring scale. With the spark plugs removed place a scale on the propeller and pull the propeller with the spring scale and record the reading. After repair and with the hold-down nuts properly torqued perform the pull-test again. If the reading is higher it shows that fretting of the crankcase has reduced main bearing clearance causing crank journal to rub the bearing. The engine should be disassembled and the crankcase repaired. If one cylinder hold-down stud breaks, all the studs on that cylinder pad must be replaced.

In determining the proper torque value for a given stud, the elastic limit and operating pressures are taken into consideration and the resulting torque value approaches the elastic limit of the material of the stud and the preload produced by the proper torque is greater than the operating load. Consequently, if properly torqued, a stud cannot fail in fatigue since it can experience not stress. If a stud is loose, it is subject to fatigue failure and the adjacent studs are immediately placed under a greater operating pressure and likely to be stressed beyond their elastic limit. All of the hold-down studs must be replaced.

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