Six Shooter™ Fastening System Understanding Fastener Installation Factors www.partsmaster.com TORQUE/BOLT TENSION: TERMINOLOGY AND RELATIONSHIP If you have ever had to drill out a bolt or, worse, had to replace a piece of equipment due to a broken bolt, you know about the importance of fastener installa:on and failure preven:on. Understanding fastener installa:on factors significantly reduces the risk of bolted joint failures. In this document, we will discuss terminology, the torque/bolt tension rela:onship, the factors that influence that rela:onship, and remedies to correct those problema:c condi:ons. Also, you will learn how to decipher aspects of a torque chart and determine the bolt tension. Finally, we will discuss how you depend on quality bolts, such as the Partsmaster Six Shooter™ series, that takes all of this into account. The first thing you will want to understand is what torque and bolt tension are, how they are measured, and how are they related. Torque is a measurement of how much twisting force is applied when installing fasteners. This is measured in the form of force and length, stated in pound-feet (lb-ft), or Newton meters (Nm). Common measurement instruments include torque wrenches and multipliers. Clamping Force, also called Bolt Tension when specifically referring to fasteners, is the compressive force that a fastener exerts on the joint. This is measured in pounds (lbs) or newtons (N). Common measuring devices include load indicating bolts & washers, strain gages, tensioners, and ultrasonic controllers, among others. K-Factor is the relationship between the torque and clamping force (or preload). Commonly, it is also referred to as the “nut factor” and is mistakenly thought of as being the “coefficient of friction”; however, the K-Factor is really a general-purpose experimental constant. The K-Factor accounts for everything that affects the relationship between torque and preload – including friction, torsion, bending, plastic deformation of threads, and many other undesirable conditions that may or may not be anticipated. Unfortunately the K-Factor is determined experimentally, and wisdom preaches that K-Factors must be redetermined for each new application. In essence, the K-factor accounts for all the other factors that apply in a fastener assembly outside of the bolt and force exerted on it so it changes from application to application. There are many unintentional fastener conditions that affect the torque–clamping force (or torque tension) relationship of a fastener joint. These conditions could cause major clamping force variations and possibly lead to bolt or joint failure. The bolt having the highest bolt preload tension in an assembly incurs most of the joint’s load and stresses, and so is the most likely bolt to fail. Making an effort to reduce clamping force variations in all fasteners in an assembly helps evenly distribute the load and stress shared by each bolt to prevent joint failures. Just because a torque wrench indicates the bolt was installed properly, doesn’t make it true! Six Shooter™ Fastening System Partsmaster created this guide for training purposes only and assumes no responsibility for the end results of using this guide for installations. The user must determine torque-tension sustainability independently of this guide. 2 Engineers use clamping force when looking at the design strength of a fastened assembly. In a perfect world with perfect conditions – using a torque wrench to tighten a bolt to specific pound feet would deliver the mathematically predicted clamping force. Since conditions are not perfect, torque only gives us a general indication of the clamp force or tension. Measuring clamping force in finished assemblies is highly recommended on critical or high tension applications to make sure the conditions have not greatly skewed measurements. Skidmore-Wilhelm Bolt Tension Tester One common and easy-to-use instrument for measuring individual fastener clamping is a Skidmore-Wilhelm Bolt Tension Tester. Few people use it in the field and do not measure clamping force in the actual installation. The Skidmore Testers determine whether or not a specific fastener has a skewed torque-clamping force relationship before it is used in an assembly. Skidmore Testers are relatively quick and there are several videos online that can walk you through the process. Simply install the bolt into the Skidmore Tester and install the washer and nut. Use a torque wrench to tighten the nut to the suggested torque. As you apply torque you can read the measurement of the clamping force or alternatively, you can read the amount of torque needed to get the desired clamping force. Six Shooter™ Fastening System Partsmaster created this guide for training purposes only and assumes no responsibility for the end results of using this guide for installations. The user must determine torque-tension sustainability independently of this guide. 3 INSTALLATION CLAMPING FORCE (BOLT TENSION) COMMON CONDITIONS THAT REDUCE BOLT TENSION (CLAMPING FORCE) AND HOW TO CORRECT THEM q Dinged Male Threads – This occurs in all grades of bolts, especially the large diameters and softer bolt grades, due to the jostling that occurs during transportation (when bolts are shipped from the manufacturer, to the plater, to the distributor, and finally to the end user). Remedy: Fully chase dinged threads with a die prior to installation. q Heavy Electroplated Zinc/Cadmium – This may occur at each end of electroplated bolts longer than 3” and inside electroplated nuts larger than 1/2" diameter (the result of an increase in the eddy current concentrations that occur during electroplating). Remedy: Fully chase the bolt threads with a die and/or the nuts with a tap prior to use. q Plated Fasteners vs. Non-Plated Fasteners – If both were installed using the same torque and all other conditions were equal, electroplated fasteners will produce about half the clamping force as non-plated fasteners. Remedy: Always take into account the fastener finish as a major factor when determining the installation torque. q Dirty Threads – This usually occurs from contact with dirty hands, dirty work areas, or dirty storage areas. Remedy: Inspect, clean, and dry threads before attempting to install. q Rusty Threads – This occurs over time when stored in damp environments. Do not use them or try to restore them by chasing the threads, as rust eats away the threaded surfaces and alters the thread dimensions. This will surely skew the targeted clamping force. Remedy: Fastener replacement is required. q Female Threads with Dried Thread Locker or Sealer – This usually occurs on equipment having threaded holes. Remedy: Fully chase the female threads with a tap, flush with solvent, and allow to dry. q Dirt or Debris Under the Fastener Heads – This will lower the true clamping force. Remedy: Ensure there is no debris of any kind under the bolt heads, washers, or nuts. q Crossed Threads – This happens when the threads of a bold do not match up with the threads of the nut. Remedy: If it’s a minor occurrence it can be fixed by fully chasing the bolt threads with a die and/or the nuts with a tap; however, it is wiser to replace the damaged fasteners if possible. q Stripped or Galled Threads – These are extremely unlikely to be fixed by re-cutting the damaged threads. Remedy: Fastener replacement is required. q Gross Misalignment of Parts – This may cause the sides of the bolts to rub against the insides of the through-holes of a misaligned component attachment. Remedy: Maintain hole alignment during the installation to ensure the bolts don’t rub the inside of the through-holes. Also, avoid inserting alignment punches in female threads. 4 Six Shooter™ Fastening System Partsmaster created this guide for training purposes only and assumes no responsibility for the end results of using this guide for installations. The user must determine torque-tension sustainability independently of this guide. 4 INSTALLATION CLAMPING FORCE (BOLT TENSION) COMMON CONDITIONS THAT REDUCE BOLT TENSION (CLAMPING FORCE) AND HOW TO CORRECT THEM q Dirty Through-Holes in the Mating Assembly – This may cause the sides of the bolts to rub against the dirt inside the through-holes of the component being attached. Remedy: Always pre-clean and dry mating assembly holes. q Warped or Unparalleled Joint Surfaces – This will grossly skew the torque and clamping force relationship. Remedy: The mating part must be re-machined or adjusted to make the component bearing surfaces parallel prior to fastening the joint assembly. q Tightening Sequence – This refers to the order or sequence in which each and every individual fastener in a multiple fastener assembly is tightened. Failure to recognize and execute the proper installation sequence will have similar affects as encountered with attaching Warped Mating Parts. q Tool Malfunctions – Torque wrenches and multipliers that are damaged or out of calibration could produce erroneous torque readings that could skew the torque and clamping force relationship. Remedy: Do not use tools that are suspected to be damaged or are out of calibration. q Improper Tool Use Prevention – Users require periodic training on how to use torque wrenches or multipliers. q Tightening the Bolt vs. Tightening the Nut – This will have a significant effect on the final clamping force. Remedy: Stay consistent and whenever possible, choose to torque the nut, not the bolt. q Soft Parts – Female threads tapped into soft metal or non-heat-treated steel will be problematic if the female thread strength is too much lower than the bolt. This could be a design issue, a result of too many reinstallations, or a result of over tightening in previous installations. q Oversized or Worn Female Threaded Holes – This can occur from over tightening the fastener system and/or from repeated fastener installation and removal. Remedy: If this situation is suspected, an engineer should be consulted for corrective action. q Under Head Friction – This refers to where the bearing surface of the nut or bolt being turned and tightened at the point where the fastener contacts the washer or component surface. Softer washers and surfaces create more friction hardened washers and surfaces and will result in lower clamping loads. Remedy: All washers in an assembly must be the same type. q Blind Holes Not Deep Enough or The Bolt is Too Long – This will create false torque readings. Remedy: Ensure the fastener length is appropriate to the blind-hole depth. Never drill & tap existing threaded holes to extend thread run-out. Use a shorter bolt. q Never Mix Fastener Grades in a Joint – Unequal or excessive clamping forces could cause failure. 5 Six Shooter™ Fastening System Partsmaster created this guide for training purposes only and assumes no responsibility for the end results of using this guide for installations. The user must determine torque-tension sustainability independently of this guide. 5 INSTALLATION CLAMPING FORCE (BOLT TENSION) COMMON CONDITIONS THAT INCREASE BOLT TENSION (CLAMPING FORCE) & COMMON CORRECTIVE ACTIONS / OBSERVATIONS Oil, Wax, and Anti-Seize on the threaded surface area increases the clamping force and must be factored in when choosing the installation torque to prevent seriously overstressing the bolt. Oil Coatings which are intentionally applied by the fastener manufacturers, usually to prevent corrosion in storage, include: 1) Phos-Oil coating, which is usually applied to structural hex bolts, and is of medium-dark gray to black appearance, and 2) Black-Ox coating, typically applied to hex socket-driven fasteners, and is of dark gray to black appearance. Black-Ox coating should not have an oil coating but some manufacturers apply oil to prevent corrosion during overseas shipping. Wax Coatings are intentionally applied by the fastener manufacturer to ease installation of difficult-toinstall fastener nuts and is generally regarded as desirable. Wax is very difficult to remove, so removal should not be attempted. Some examples of where wax is commonly applied to nuts are: § Larger diameter (usually ≥ 1/2" diameter) nuts which are stronger than Gr. 8 and could have either Zinc or Cadmium Electroplating. § Some Stainless Steel Nuts to prevent galling or cold welding of the mating threads. § Some All-Metal or Nylon Insert Locking Nuts. Six Shooter™ Fastening System Partsmaster created this guide for training purposes only and assumes no responsibility for the end results of using this guide for installations. The user must determine torque-tension sustainability independently of this guide. 6 Anti-Seize - The purpose of Anti-Seize is to lower installation torque and to reduce clamping force variations or scatter. It also prevents galling, which is especially important when installing or removing stainless steel fasteners. It is also great at reducing thread corrosion rate and adds lubricity to prevent fastener seizing during future removal activities. Unintended Oil Exposure can occur during fastener installation. Oil can transfer onto threads from oily or greasy hands and/or work surfaces, or as a result of unintended lubricant overspray. Also, threads that are chased will usually come in contact with cutting and tapping fluids. A great and effective product from Partsmaster is AS-201 FG. AS-201 FG provides a K-Factor of 0.16 and has a service temperature range of -10ºF/-32ºC to 2300ºF/1260ºC. It can be used in food plants where no food contact will occur. The AS-201 FG is a white colored product that is stain resistant. TORQUE CHART DETERMINING BOLT TENSION The primary goal of fastener assembly is to assemble using the correct clamping force. The best fastener installation scenario occurs when the manufacturer provides specific fastener assembly instructions, i.e., as found in an included service or maintenance manual. Fastener assembly instructions are usually provided when critical fastener joints are installed or will require routine or periodic maintenance. Manufacturer’s technical specifications, service manuals, or drawings always take precedence over generalized torque charts and general installation guides. When specs, manuals or drawings are not available, torque charts provide a general guide towards achieving the anticipated clamping force or bolt tension. Variables such as bolt grade, bolt coating, prior service history of the bolted joint, gasket or ungasket joint, service temperatures, joint design (single shear, double shear, compression, etc.), joint load (static or dynamic), joint vibration, corrosion, and how critical the component is in service must be considered when determining the preload target ranges, typically as a percentage of the yield strength of the bolts. Six Shooter™ Fastening System Partsmaster created this guide for training purposes only and assumes no responsibility for the end results of using this guide for installations. The user must determine torque-tension sustainability independently of this guide. 7 TORQUE CHART Yield strength refers to the specific point where the amount of an applied tension load causes permanent fastener deformation. The following example scenarios can help you determine the amount of bolt tension needed: q 25% of Bolt Yield Strength § Unimportant non-gasketed joints exposed to static loads. § § Foundation and anchor bolts under static load. Joints where fasteners are exposed to serious stress corrosion cracking problems. q 40% of Bolt Yield Strength § Gasketed joints in routine service, including those covered by the ASTM Code, which have not given any past problems. q 50 – 60% of Bolt Yield Strength § Average non-gasketed joints with normal safety or performance concerns, where past experience does not suggest higher or lower preloads. § § A good place to start a search for the optimum preload when some trial and error is acceptable. This is typically the maximum preload for gasketed joints designed to the ASME Code rules. There will be a few exceptions. q 70 – 75% of Bolt Yield Strength § Upper limit for non-gasketed joints with which past preloads were too low (leaks, self-loosening, joint slip, joint separation under load or fatigue). § Torque control measures must be understood and applied. q 85 – 95% of Bolt Yield Strength § § Joints having consistent “low preload problems in the past.” Where the need to avoid failure dictates the use of special techniques such as stretch or ultrasonic control. § Where service loads (or ignorance of service loads) makes it unwise to take the fasteners any closer to the yield point. § May require using specialty tools, techniques and load indicating fasteners. q 100% of Bolt Yield Strength § § Structural steel bolts tightened by turn-of-nut procedure. High-performance or problem joints facing self-loosening, fatigue, or other problems where service loads can be predicted with sufficient accuracy to that the fasteners will not be ratcheted to rupture later in service. 8 § Sometimes used in gasketed joints in automobile applications. § § Requires specialty tools and assembly techniques. May require using specialty load indicating fasteners. Six Shooter™ Fastening System Partsmaster created this guide for training purposes only and assumes no responsibility for the end results of using this guide for installations. The user must determine torque-tension sustainability independently of this guide. 8 PARTSMASTER™ Six Shooter™ FASTENING SYSTEM The Partsmaster Six Shooter™ Fastening System reduces bolt failure by providing an enhanced Grade 8 product. They are made in the USA for better craftsmanship and quality. Imported fasteners are known to have a higher rate of variance from specifications. Knowing where your bolts are coming from better insures conformance with quality standards. All Six Shooter™ cap screws are completely traceable with lot numbers and can be traced back to the heat and batch. Six Shooter™s are tougher and less brittle due to being alloyed with chrominum, nickel, and molybdenum and baked to relieve hydrogen embrittlement not typically performed by all manufactures but seals the safety and quality. Dy-Cro-Sist plating for ultimate corrosion resistance reduces the chance of inadvertently reduced clamping force. Cold rolled, not cut, threads ensure that the threads are just as strong as the head and neck of the cap screw. To give even more confidence in Partsmaster’s assurance of quality, they have registered with the US Patent and Trademark office to prevent counterfeiting and has a multi million insurance policy. Partsmaster has been providing products and solutions to service the Maintenance, Repair, and Operations industry for over 40 years. We solve problems for our customers and help them reduce the total cost of ownership. Partsmaster, a premier MRO solutions company dedicated to making its customers’ work easier, was established in 1968 as a division of NCH Corporation, a leading provider of industrial goods since 1919. Partsmaster offers intelligent solutions to solve problems. Our exclusive brands include products you cannot get anywhere else. The Partsmaster brands include Cryotool® cutting tools; Dyna Systems® fastener line; Mega Metal® polymer repair compounds; Trust-X® high productivity abrasives; X-Ergon® welding products; and Partsmaster Services. Combine our products with sales and service commitment, and Partsmaster is your knowledgeable, strategic partner that understands your business needs and always takes care of you. 9 Six Shooter™ Fastening System Partsmaster created this guide for training purposes only and assumes no responsibility for the end results of using this guide for installations. The user must determine torque-tension sustainability independently of this guide. 9 www.partsmaster.com PARTSMASTER™, A Division of NCH Corp. P.O. Box 655326 • Dallas, TX 75265-5326 1-800-336-0450 1M504 • © 2015 NCH Corporation References: 1. John H. Bickford, An Introduction to the Design and Behavior of Bolted Joints, 2nd Edition, Revised and Expanded, by, Copyright 1990, Marcel Dekker Inc. NY, NY. 2. SAE Handbook, Volume 1 10
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