94 THE DISTRIBUTOR’S LINK BENGT BLENDULF A LOGICAL APPROACH TO A BOLTED JOINT DESIGN from page 8 FIGURE 2 accuracy, lubrication, speed and others. We now have a tool from IFI that can help us if we are using Torque-Tension method to get the right preload in the fastener. It is called “Torque Book for Fasteners”, a product developed by Joe Greenslade. From this document we can find nominal friction values for the vast majority of lubricants and for various surface conditions. This help would take us much closer to develop a reasonable accurate torque value. This would, combined with the rightly chosen tightening tool, give a solid start for the joint design. To verify the joint we need to calculate how all those variables in geometry, forces, materials, temperatures, tightening, fastener choices, etc. will fit together. In Figure 3 below, we can see some of the mathematics from SR1 (VDI 2230) on the joint. This is a great tool for joint calculations and it showed that the starting point in Figure 1 gave us a too high surface pressure under bolt head and nut. So, the “design force” now goes up from using a M6 fastener to a M12 in the 8.8 column or a M10 in the 10.9 column. This type of estimation of bolt size will be right in most cases, but must also be verified. So, we are off to a good start, but need more information to judge the joint. First of all, are the MATERIALS in the joint compatible? If all joint parts are made of steel (any type) and we are using steel fasteners we should have no problems. If, however, the joint is made of aluminium we must make allowances for that. Both for the normal elasticities at room temperature, and any TEMPERATURE (high or low) influences on elasticities. Any combination of materials in the joint can have a great impact on its success. TIGHTENING will also play a major role in the joint performance (good or bad). This is an area where we have great problems with many variables with tool FIGURE 3 CONTINUED ON PAGE 95
THE DISTRIBUTOR’S LINK 95 BENGT BLENDULF A LOGICAL APPROACH TO A BOLTED JOINT DESIGN from page 94 By adding hard washers (at least 200 HV) for 8.8 I could get the design OK and avoid embedment and load losses. We can also do this manually, but it would add a few pages to this excellent magazine. The purpose of this article is to identify the steps to take for making a good joint. We need to define 1. Geometry of joint 2. Forces/loads (preload and external) 3. Materials in joint and fasteners 4. Temperatures (outside of -40°C to +120°C) 5. Tightening method 6. Fastener choices 7. Other influences If we follow that path, we will reduce joint failures and unnecessary recalls. If you are not in the joint design area, but are involved otherwise due to joint problems, the list above may help you to understand why things can go bad. For More Information Very good information can also be found at: IFI Industrial Fasteners Institute 6363 Oak Tree Blvd. Independence OH 44131 Seminars for Engineers info@SeminarsforEngineers.com Fastener Training Institute info@FastenerTraining.org EduPro US, Inc. bengt@edupro.us David Miller (for SR1), PCB Load and Torque 24350 Indoplex Circle, Farmington Hills, MI 48335 BENGT BLENDULF
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