Coatings and Torque Tension
In an automated installation, the most positive way to insure proper clamp is with machines that read torque rate of change and angle curve. This reads both the mechanical properties of the bolt and the properties of the joint, assuring no broken bolts with correct clamp and yield. These machines even sense variations in bolt grade. The next best level of assurance, would be in torque control measurement devices. These latter are good but cannot read variations in joint resiliency, coefficients of friction or define bolt grade.
A common means of production fastener installation is the use of stall guns. This is a good method, but the stall or clamp force must be matched to the anticipated running torque to get proper preload. Finishes with a high or broad torque band requirement must have the stall gun sized to the maximum limit of that range, otherwise, finishes with a broad torque range can lead to a high percentage of bolts with low clamp. This means bolts at the lower end of the installation torque band may see too much clamp load causing yield beyond the plastic limit of the bolt or “cam out” of the head. If torque variation is not taken into account on the factory floor when a running change is made, from a broad torque band finish to a finish with a lower narrow band torque, problems may result.
Low volume or field installation usually employs hand torque wrenches or “turn of the nut” techniques. These methods can be very effective in the hands of a good mechanic where it’s insured the male and female threads are in good condition, clean and lubricated. In plant or field situations where variables, including labor quality, may not be well controlled, dry lubricant films offer advantages of torque consistency and narrow range, enhancing hand torque wrench or turn of the nut results. Resin bonded dry films offer some level of corrosion protection. With hostile environments, the graphite / moly anti-seize features assist in breakaway of badly corroded joints. When applied over platings or phosphate for hostile environments, dry films can offer multiples of the level of protection afforded by any of the basic protective feature.
Below is torque data extracted from many tests at different facilities run for General Motors and Ford Motor Company on different types of finishes used on M10-1.5 prevailing torque nuts and various size engine head bolts over many years. These tests were run on a variety of dry film formulation variations and plating/conversion coating finishes. The originals were a large amount of data, some of which is proprietary to the testing sources. For this reason, we have extracted selective segments of the data and extrapolated/interpolated for various finishes on head bolts and prevailing torque nuts only as a basic information tool to illustrate trends. View this data in that perspective.
Finish Effects on Fastener Torque
Torque Band Variables:
This short overview does not cover potentials introduced by standards tolerance within a thread class or a bolt grade or the manufacturing capability and inherent quality control variables. This discussion will cover only some of the variables introduced by various finishes utilized by the product engineer. Occasionally, finishes are selected for a single specific reason such as corrosion protection or anti-seize. Often the engineer, while knowledgeable on thread flank design, strength of materials, etc., is more a generalists regarding specifics of metal finishing. They rely on material departments or outside vendors for the specifics of the particular finish. Effective communication is often lacking between groups.
Plain Oiled Finish:
This is the cheapest of the lubricated protective finishes. Lubrication properties depend on the type of oil used. Corrosion resistance depends on the amount of the oil on the surface. The more enduring oils tend to be those which are thick, allowing them to stay on the surface longer in the wet condition. These often employ wax or soap to add to the weight of oil retained at the surface. These dry out by evaporation or volatilization via temperature and time. They usually are of value for first installation only. See typical relative torque comparisons on the following charts: Head Bolts#1, Head Bolts#2 and Prevailing Torque on Nuts.
Phosphate and Oil:
The phosphate crystal structure is used as a mechanism to hold oil at the surface of the treated product, thereby increasing the duration of oil protection. Heavy phosphate crystals, when sealed with chromic acid, have very limited corrosion resistance without oil. Heavy phosphate crystals, with oil, can provide up to 180 hours of 5% N.S.S. Protection decreases with aging. Heavy crystal structures adversely effect torque. Lighter crystal structure, while having less deleterious effects on torque, provides less corrosion protection. The manganese crystal, being friable, has the least adverse effects on torque but more limited corrosion protection. See typical relative torque comparisons on the following charts: Head Bolts#1, Head Bolts#2and Prevailing Torque on Nuts.
Zinc Plate and Chromate:
Zinc’s position on the electromotive scale provides corrosion protection for steel. Zinc does not have a positive benefit on torque. Chromate seal provides additional corrosion resistance but will oxidize with time as part of their protective mechanism. This oxidation normally has an adverse effective on torque. Chromate deteriorates, cracks and sometimes spall’s off above 325° F. Waxes and oil improve torque and corrosion resistance but deteriorate with time and environment. See chart on Prevailing Torque on Nuts.
Cadmium Plate and Chromate:
Cadmium plate has excellent torque results when waxed. The corrosion resistance of cadmium plate is enhanced by the application of chromate and lubricity is enhanced by wax/oil. The deficiencies of wax and oil on zinc plate apply to use on cadmium. See chart on Prevailing Torque on Nuts.
Dry Film Lubrication:
Graphite, molybdenum disulfide, tungsten disulfide or various fluorocarbons are often used as fastener lubricants. Each has specific properties of value for particular needs. For added properties they are often used in a resin binder to economically hold them where wanted, enhance their protective properties and matrix them and other materials such as metallic’s for a desired degree for performance. Blends retain narrow torque bands but can vary in absolute torque levels.
A “rule of thumb” would be, graphite has the highest temperature capability with good high load and high-speed coefficient of friction values. Moly and tungsten disulfide have better high load/speed coefficient of friction with about ½ the high temperature capability in degrees Fahrenheit. Fluorocarbons have good low speed/medium load coefficient of friction but has limited performance at elevated temperature and load. Blends of these materials, in a resin binder, have proved best for many specific needs. See typical relative torque comparisons on the following charts: Head Bolts#1, Head Bolts#2 and Prevailing Torque on Nuts. These coating formulas give their best coefficient of friction results on hard substrates. These coatings are applied over platings or phosphate to achieve improved corrosion protection. Depending on many factors, this can range from 48 hours in 5% N.S.S. to 1000 hours in a 5% N.S.S.