A smart fastening system can be a cost savings approach to your fastening needs by allowing the joints to be fastened to their best level of clamp load for maximum life in the field. As a result of proper tightening at the manufacturing and assembly level the warranty cost for after sale repairs can be kept low due to the higher level of quality assemblies in the beginning. A poorly fastened joint does not look any different than a quality fastened joint to the naked eye and therefore does not get the attention it should, consequently the joint gets into the customer's hands and eventually comes back as a warranty problem and an extra expense that could have been avoided.
Unfortunately quality is not always as tangible and visible as leather seats and total quality has to be introduced at every step of the design, manufacture and assembly process. Even the smallest of screws and the way they are fastened can have a large effect on the perceived quality of an assembled product. Consider the trim screws or the mounting screws for the i.p. of an automobile, if they are not assembled properly you can end up with a squeak or rattle. That alone reduces the perceived quality by a big factor.
Now consider the really important joints such as the safety critical or the 'walk home critical'. These can include the suspension , steering and wheels as well as the engine and transmission assemblies. Probably the most important of the joints in the engine is the connecting rod joint. Of course all the joints in the engine are critical but the connecting rod joints must be fastened correctly or else the engine can fly apart, literally!! The connecting rods are usually manufactured in a two step process; the caps are sawed from the rod and the caps and rod ends are machined then fastened together and the bearing hole is bored and honed. The fasteners are removed and the bearing shells are inserted and the rod assembly is fastened to the crankshaft. If the fastening is not adequate at either of these stations the chances for a non uniform oil thickness or the chances of an egg shaped hole is highly probable. These conditions alone can lead to a scored crank, spun bearing or even worse - a blown engine. Internal engine repairs and replacements are very expensive, both in monetary value and in customer satisfaction value.
The joints in the engine assembly that demand particular care are 1) the connecting rod to crankshaft; 2) the main caps to the block or the main girdle to block depending on the design; 3) the flywheel to the crankshaft; 4) the camshaft bearing strap joint, again depending on the design; 5) the head to block joint which is probably the most interactive and sensitive joint in the engine; 6) the cam driving sprocket mounting; 7) the front damper or hub to crankshaft joint which in some engines is also the oil pump driver; 8) the manual clutch to flywheel mounting. These joints can be considered as both safety critical and walk home critical. I don't have to illustrate what could be the result of a blown engine due to a loose connecting rod bolt in rush hour traffic in Detroit or any other large city.
The other critical joints that demand proper fastening are the suspension and steering components. All the fasteners that hold engine cradles to the body, the engine mounting, the axle mounting and wheel mounting all deserve the best type of fastening that can be afforded at the initial assembly point. I will repeat from the beginning that quality is not always as easy to see as leather seats but a trouble free car assembly and subsequent 100,000 mile satisfied customer is definitely a reward for the built in quality through quality fastening.
There is no doubt that sophisticated fastening equipment is more expensive than the old style air driven reaction torque nutrunners, but the times have changed drastically and so have the customers expectations for the most value and quality for their money, so investment must be made to deliver the best possible product to the customer. Assembly is the final stage of manufacture and this is where the quality of all the parts have to come together in the final product build. The engine was assembled the best way and the body and chassis were assembled the best way now all the components come together and the final step is to marry everything together using the best fastening that can help give that build the quality the customer looks for and deserves for his money. This is why it is economically feasible to invest in smart fastening systems.
It is economic suicide to not build in quality, the systems are available now and are affordable. The fastening strategies are the state of the art and joint analysis is available to determine the fastening strategy that will best give quality fastening to your joints.
George Lorimer, Retired.
G.M. Powertrain Fastener Lab.