Among the various coatings applied to fasteners is a class of compounds known as torque modifiers. These substances are added after the plating process to modify the torque-tension properties of the fastener as it is installed. As discussed in an earlier issue of Products Finishing (April 1998), tension is the property that is desired in an attachment. The tension, or clamping load, is the force that causes the parts of the joint to be pulled together. When it is greater than the forces acting upon it to pull it apart, the joint does not come loose. Since tension is not measurable in production environments, torque is used as a measure. Unfortunately, about 90% of the torque force is lost to friction of rubbing surfaces, leaving only about 10% to "pull up" the parts to clamp load. Small changes to the friction side of the equation make large variations to the clamping side. Often the assembly plants think the parts go together too hard and dip one in oil. A loss of only 5% in friction equals a gain of 50% in clamp, which leads to stripped threads, failed parts, yielded material, etc.
Often there is a conscientious desire to modify the torque. Perhaps the surrounding areas have a lower torque, and the desire is to use a single installation tool range setting. Often modifiers are used to control scatter. Scatter is the spread between the highest load and the lowest at a particular torque. While the average torque may be the same, wide scatters can cause a joint to fail either by being too loose at one side or too high at the other.
While it is known that different platings, sealers and even thread-locking adhesives affect the torque/tension relationship as much as 100%, the substances we are discussing are added specifically for the torque modifying property. The most common one is oil, used by mechanics and backyard handymen. Spray lubricants and oils reduce the friction on parts, but inconsistently. The results are rarely reproducible, due particularly to the different compound formulations. They are popular because they are easy to apply, cheap, act as a rust preventive and are generally acceptable for most hand assembled and non-critical attachments. When applied over a phosphate base, they offer 24-96 hours neutral salt-spray corrosion protection.
Waxes are the second most popular type of additive to plated surfaces. The major use of wax is in all metal crimped locknuts. Originally over cadmium, and more recently over zinc-based organic coatings, waxes control the scatter in the torque curves. Since many locknuts are used in areas of critical attachments, i.e., steering and suspension, accurate andnarrow scatter bands are a prime requisite. Wax also is frequently used with thread forming bolts (seat belt bolts), for ergonomic reasons, to control drive effort on the part of the installer. Recent improvements in wax formulations have lead to reductions in M12 seat belt bolt installation specifications which were lowered from about the 65Nm range to the area around 20Nm. With the advantages of thread forming fasteners (those that roll a thread rather than cut one) coming into more and more engineering designs, the waxing of the bolts has become a new and profitable sideline for many plating shops. Several proprietary compounds are on the market, and companies are looking for applicators.
Serious torque modifications are the specialty of MacDermid Corp. It manufactures a complete line of compounds to apply to fasteners to produce the modifications required for each application. With the decrease and eventual elimination of cadmium, the lubricating properties of cadmium in the joint attachment process needed to be duplicated. Various "new" platings have surfaced, but most have a common flaw. They are dip/spin heavy zinc or metallic-based paints. These cause thread fill, washer sticking, erratic gauging and jammed fasteners at the assembly plants.
Torque fluids can be easily applied (usually by dip/spin), and their consistency can be compared to water. They are applied over almost all other coatings and can be air-dried for maximum energy efficiency. Since the active compound is encapsulated in the suspension, shelf life is long and unaffected by storage conditions. Other application methods are by spray, dip/drain for larger parts, mixed in the topcoat or sealant for a reduction in the number of processing steps or even mixed with organic lacquers for a glossy topcoat appearance. The fluids are usually used once and have limited reuses, providing for one time maximum use. Serviceable parts with numerous off and on requirements are not candidates for torque fluids.
Other fluids on the market have the same properties as the ones discussed above. Their availability and use haven't been explored in as much depth by the author as has the MacDermid products.