Theory of Rod Bolts and Other Prestressed Bolts
By Vance Jaqua
The connecting rod is one of the first areas of concern whenever one feels an engine has been overstressed. “It threw a rod” is a common cry after a disastrous engine failure. We seldom truly “throw” a rod; the usual failure is that the rod-to-crank bearing material breaks down from over stress or, more commonly, lack of lubrication. Accelerated wear then makes a large clearance, and the remaining soft bearing material is pounded out with large clanking noises.
The highest tensile force on the rod and bolts happens when the throttle is closed at high rpm. The rod and piston are being accelerated downward at close to 700g’s, and the crankcase pressure (ambient or greater) is pushing upward against inlet pressure that is near zero absolute from manifold vacuum. In an engine like the Lycoming O-360 this load is probably on the order of 6,000 pounds (I don’t have a good number for the weight of the piston or rod, but this is probably a pretty good estimate). The highest compressive load is near top dead center in a full throttle power stroke where the pressure can peak over 1,000 pounds per square inch (psi). The acceleration of the piston and rod compensate partly for this pressure loading, but it probably peaks at about 10,000 pounds compressive load.
Now the rod bolts on an engine like this are not particularly impressive, having about a 5/16-inch thread and a reduced section about 1/4-inch diameter (to match the thread root diameter). The load of 6,000 pounds spread between the two bolts would correspond to a stress level a bit over 61,000 psi. That is pretty scary, but isn’t that bolt good for over 150,000 psi? Well, yes and no. For steady state loading that would show a safety factor of over 2, but there is that little thing called “endurance limit” for cyclic loads. This is the stress level for repetitive loads that will let you last a million cycles (106 for you scientific notation types). For most steel alloys this value is about half the steady state value (or 150,000 is now down to 75,000). And on top of that, the stress concentration factor for a standard thread form is about 3-to-1 (now we’re down to 25,000 and in big trouble). It also might be worth mentioning that a million cycles at 2700 rpm is just a little over six hours.
Well, it obviously works better than that, so what is the secret? The answer is preload. If a bolted joint is preloaded by bolt pretension torque to a load greater than the cyclic load, it will not see any alternating load at all. For example if we preload the connecting rod joint to 10,000 pounds, the stress on the stretched bolt cannot increase until the tension load overcomes the locked-up compression in the joint and the surfaces start to separate. Until that point is reached, the forces will just reduce the effective compressive forces on the surface, and the bolt load stays the same. This is hard to visualize, but it is definitely true. For example if a 200-pound man stands on a box on the floor, and you try to pick up the box with 100 pounds worth of force, you have reduced how hard the box is pressing on the floor, but you have not increased the 200-pound force that his feet are putting on the box. Now replace that man with a clamped down spring with 200 pounds of force; the situation is the same. As you lift on the box you do not change the force until you have lifted the box off the floor (exceeding the 200 pounds of force) and are further compressing the spring. The bolts in the rod are really springs as you will see in a bit.
The force is set by the compression of the spring - only lifting the block and increasing compression of the spring will increase the stress in the bolt.
With a bolt it is very difficult to know how much preload you have applied. Just whip out your calibrated torque wrench, you say? Wrong! The torque wrench is directly dependent upon the friction in the threads, and on the thrust faces of the bolt or nut, and this friction is widely influenced by a whole bag full of things—surface finish, smooth or rough; lubrication, or lack of it; fit of the thread—you name it, and it can vary all over the map. To truly work the bolt to its maximum potential a preload as high as 80 percent of yield is required, and trying to work to that criteria with a torque wrench can easily lead to permanent stretching and even cracking in the thread roots.
The best method, and one that is quite common for aircraft engines, is torquing to an elastic elongation value. If you have access to both ends of the bolt, and the surfaces are flat, you can measure the length of the bolt before final tightening, and measure the few thousandths of an inch of stretching that indicates the desired amount of prestress. All steel bolts have virtually the same elastic modulus—30,000,000 pounds per inch per inch. If your bolt has a working length of 2 inches, a 100,000 psi prestress will lengthen it 0.0067 inches, a small number but easily read on a good micrometer. The bolts frequently have the major length reduced to roughly the same diameter as the thread root; it is just as strong (the weakest link is still the thread root diameter), and the amount of stretch is increased at the same load (because the stress is more evenly distributed along the length) for a better indication with the micrometer. If the bolt has been properly hardened, such that this load is below yield, it will act like the very stiff spring that it really is and relax back to its original length when unloaded, just like any other spring. If it is too soft (wrong material or improper heat treat), it will yield to this dimension and may have a crack, just as a spring made from a wire that is not properly hardened does not return to the same length if you stretch it too much.
Many fancy systems are used to properly prestress bolts and studs that are not so easily measured as a rod bolt. Properly designed bolts can be read for length by ultrasound in blind locations. Special load measuring washers are available where an outer ring is free to spin until the design load is reached. Special bolts are available with a pin indicator built into the head. However, in most cases the designer just doubles the bolt strength (goes to the next size up) and preloads with a torque wrench to a fraction of the available working load, and just accepts the scatter in actual strength.
In memory of our dear friend Vance Jaqua (1929-2006), who gave much of himself for the advancement of experimental aviation.