Edited by Stephen J. Mraz
Experimental Analysis of Thread Movement in Bolted Connections Due to Vibration, Auburn University report prepared for NASA research project NAS8-39131, 1995
New Criteria of Self-Loosening of Fasteners Under Vibration, Gerhard H. Junker, SAE Paper No. 69005 1969
Loosening of Fasteners by Vibration, R. C. Baubles, G.J. McCormick, C.C. Faromi, 1996
An Introduction to the Design and Behavior of Bolted Joints, John G. Bickford, 1995
Threaded-fastener technology has been around since screws were used at the Hanging Gardens of Babylon in the 7th century B.C. And many of the nuts and bolts used today were originally designed a century or more ago. Still, bolted joints remain one of the most common elements of construction and machine design. But many organizations and people still struggle with the problem of nuts and bolts loosening, which lead to problems ranging from inconveniences to catastrophes.
Many bolt failures are caused by the joint losing clamp load due to vibration and high dynamic loads. This can loosen bolts, the result of relative movement eventually overcoming the friction between threads. Loosening can also make bolts shear or fall out completely.
Bolt are often overstressed during installation, which can create uncontrolled clamp loads. Omitting lubrication or using poor-quality lubricant also creates some uncontrolled loads. Such loads weaken bolts and even lead some to fail. Many people, even engineers, then jump to the conclusion that if a bolted joint fails, it must be a bad bolt. Often, they simply retrofit the failed bolt with a more capable and more-expensive replacement. So they end up using a Grade 8 bolt rather than the failed Grade 5 bolt. In reality, the original bolt may have been perfectly fine but improper assembly or a poorly designed fastener set up conditions that led to the bolted joint’s failure.
Lubricants are critical to healthy bolted joints. When fasteners aren’t properly lubricated, elevated friction levels can reduce clamp loads while increasing clamp-load deviation. More friction adds resistance during tightening, making bolts harder to install. More friction also increases the torque needed for a joint load because installers must first overcome this additional friction before getting a good clamping load. (Meeting proper torque specifications is a prerequisite for a good clamp load.) Many engineers and designers do not realize that approximately 90% of the torque applied to a bolt goes to overcoming friction. Only 10% goes to clamp load. In light of this, the last thing you want to do is add friction to the thread’s mating surfaces.
While crankcase oil from a car may be good for engine bearings, it is not very effective on bolt threads. The latest research indicates graphite or moly-based (molybdenum disulfide) lubricants are best at reducing friction and limiting torsional stress. This stress arises from bolts deformed by a torque or twisting movements during tightening. Depending on the application, experts generally recommend graphite lubricants, as opposed to petroleum-based solvents which cause galling with stainless steels.
Although adhesives sometimes prevent bolts from loosening, they have several disadvantages. For example, many of them introduce friction which can create a torsion load on the bolt.
Other disadvantages of adhesives besides potential increased friction include:
Difficult to disassemble once cured. High and low temperatures can create inconsistent results. Limited storage life of the adhesive (needs to be replaced over time). Potential health hazards, including (allergic reactions and skin rashes).
Our solution to loosening bolts
The basic principle is that the angle on the cam exceeds the angle on the pitch of the nut or bolt’s threads. When the bolt is tightened, the serrations bite into the mating surfaces and the cams interlock. This means the washers must be made of a harder material than the mating surfaces.
Once locked in place, the washers only permit movement across the faces of the cams. If the nut or bolt tries to loosen, the washer halves separate under the head of the bolt or nut by riding up the cam’s ramp. The separation creates an axial load on the shaft, causing the bolt to remain in tension or “stretch” and thereby maintains the clamp load. Cam surfaces are smooth, so there is little friction to overcome and bolts can be untightened with nearly the same force as the original preload torque. The washers only require the most common nut and bolt hardware, and their locking function is not lost when nut or bolt threads are lubricated, plus they are reusable.
Most fasteners use one of four designs: Friction-based fasteners are inexpensive and easy to install, although friction is not representative of clamp load. These designs include:
Prevailing-torque nuts typically have a deformed shape or distorted threads and are sometimes referred to as top lock nuts.
Nylon insert nuts come with a nylon ring inside the threads and are available in a variety of types and colors.
Jam nuts are double nuts consisting of a jam nut and a regular nut.
Spring-based fasteners are inexpensive but ineffective against vibrations. They include splitring washers or cupped spring washers, sometimes referred to as Belleville washers.
Interference-fit versions are effective although extremely time consuming to install and, therefore, not cost efficient. They include different assembly techniques such as safety wiring, cotter pins, peening, welding and tabs, in which bent metal is forced against the bolt to keep it from turning.
Tension-based, two-piece washer systems rely on cams and serrated friction surfaces. They are simple, effective, and reusable.
• Thoroughly clean bolt holes and threads with a solvent to eliminate residual adhesives, lubricants, and particulate matter.
• Make sure an effective moly-based or graphite-based lubricant is applied to thread-mating surfaces.
• Torque to proper bolt specifcations.
• Make sure new bolts of the correct grade are used in critical applications. (Used bolts may be damaged and visual inspections are not reliable.)
• Consult the fastener manufacturer when special conditions such as high temperatures are present.
• Ensure the service and calibration on your torque wrench is current.
• Tighten flange bolts in torque steps and in a star pattern or other pattern to exert a uniform clamp load where applicable.
Most modern equipment uses one or more of the above designs. Many of these fastening designs are acceptable in light-duty applications where joints are not critical or undergoing high vibrations. But when vibrations are strong enough or dynamic loads high enough, many fasteners aren’t up to the task.
During vibrations, localized slip at the bolt/nut contact surfaces can loosen threaded fasteners. Friction-based, spring-based, and interference-fit fasteners do not handle these conditions well because of their natural tendency to follow the threads and make the bolt move away from the joint. A nut has a preferred direction of rotation when subjected to vibrations and thread friction has been overcome. And only a small amount of vibration-induced fastener movement can significantly reduce clamp load. Once some clamp load is lost, the bolt moves even more, creating more clearance in the shaft, and weakening bolts can shear due to wear and tear.
These designs face other problems as well, Many fasteners such as split washers and top-lock or stover nuts are not reusable. And fasteners such as nylon insert nuts are temperature sensitive. At high temperatures, nylon inserts melt or degrade. Interference-fit designs can be difficult, expensive, and take twice as long to install due to the extra effort needed to thread the length of the bolt with resistance. Additional skill may be required for wiring, which is also time consuming to install and remove.
Tests can determine a fastener’s resistance to vibration and loosening. One, the Junker Vibration test (DIN 65151), measures clamp load in joints while they vibrate. It introduces vibrations that cause inertial forces in the tightened bolted joint to be overcome friction in the threads and cause loosening. With most fasteners, a significant or complete loss of clamp load is evident within a few seconds. NASM-1312-7 (MIL-STD1312-7) is the U.S. standard for vibration testing.