Aug. 17th. 2008
MECHANICS 3.0: TORQUE AND FASTENERS
Torque and Fasteners
"Tight's tight, too tight's broke." A saying that's probably been around since beginners held a half-inch driver. Seriously, have you ever wondered about how tight you should snug that nut? Of course, you could look in the manual which usually (but not always) cites torques.The following article is a short introduction to the physics of torque and some rules of thumb when doing precision shop work with fasteners.
Moment-of-force, known to engineers and mechanics as torque (from the Latin torquere, to twist, wind, wrench) is the turning effect produced by a force applied to a moment-arm. Specifically, the moment-arm or lever is the perpendicular distance drawn from the pivot or fulcrum to the line-of-action of the force. The moment-arm can be thought of as the effective radius, if this radius is zero, the torque is zero regardless of the applied force. Alternatively, a relatively small force can produce a large torque if the effective radius is large. Torque also has a sense, that is either clockwise or counterclockwise. The dimensions of torque are force times distance: in SI units, newton-meter, or N-m; in British Engineering, pound-foot, or lb-ft..
The torque wrench is a measuring tool that allows precise tightening of critical fasteners. Over-tightening results in stripped threads, air-gap changes in spark plugs, and other wedge-type phenomena. Under-tightening causes fretting which is fastener erosion due to vibration. Modern engine repair require torque plates bolted top and bottom to cylinders then tightened to specifications of actual assembly prior to bore measurements, over-boring, and honing. Torque wrenches are the adjustable click-type with +/ - 3% to +/ - 6% accuracy. Good direct read dial-types have accuracies ranging from +/ -1% to +/ - 2%. Accuracy being a function of whether the wrench operates clockwise only or both clockwise and counterclockwise. Less seen are torque screwdrivers, torque angle gauges, and electronic torque wrenches. In race and areospace applicatons these expensive tools are habitually calibrated against digital torque testers or horizontal bench testers to within 1% accuracy. It is highly unlikely that a competent mechanic would use any torque wrench to loosen a fastener.
In general use light oil or white grease for assembly unless torque specs call for dry, clean fasteners. Use molybdenum-base antiseize (unless Loctite is specified) on steel and stainless steel bolts in aluminum or cast iron. Lubrication allows for more accurate torque readings and increases ease of disassembly. Follow service manual recommendations and diagrams. En lieu of these, when assembling parts that require multiple fasteners, such as oil pans, or cylinder heads, tighten the part in a crisscross pattern from the inside to the outside. To loosen the same part, crisscross from the outer to the innermost fastener. In addition to pattern, tighteniing to final torque should be incremental; for example, after snugging, 5 lb-ft increases are safe to finish at 50 lb-ft..
Unless specified differently, torque values given in tables are for dry, clean fasteners. Plating, lubrication, grade, fastener type, threads per inch are some factors that need consideration. Dry cadmium, zinc, and chrome plated nuts and bolts with diameters 1/ 4 in. (6.35 mm) or lager multiply the given torque by 0.80, 0.85, and 1.0 respectively. Light oil requires a torque multiplier of 0.90, as do dry cadmium plated socket-head cap screws. Studs can be torqued to cap screw values of equivalent grade. SAE bolt grade is commonly shown by marks pointing inward form the corner of the flats of hex head types. The following marks/ grade are in increasing order of minimum tensile strength: no marks/ usually grade 2, 3 marks/ grade 5, 5 marks/ grade 7, 6 marks/ grade 8. More threads per inch result in greater torque for bolts of equal diameter. For example, a dry 1/ 2 inch (13 mm) bolt with 20 threads per inch can be torqued to 52 lb-ft (70.6 N-m) compared to one with 13 threads per inch at 38 lb-ft (51.6 N-m).