WINTER 2014

162 THE DISTRIBUTOR’S

162 THE DISTRIBUTOR’S LINK HOW TO BUILD A SAFE AND RELIABLE BOLTED/SCREWED JOINT continued from page 8 If the external force is also dynamic/cyclic the threaded fastener is a very vulnerable machine element with high stress concentrations at the thread roots. A dynamic load can easily cause fatigue failures in the fastener if a larger portion of that load hits the fastener in each load cycle. A fatigue failure in a threaded fastener is usually located in the first engaged thread between bolt/nut or screw/blind hole. This is where about one third of the total load is concentrated. Fatigue failures can, of course, also happen in the head/shaft junction if the bearing area on the head if is not perpendicular to the fastener axis. It does not take a large angle to break off the bolt/screw head in cyclic loading if the fastener is short. With longer fasteners, clamping on more material, we are (sort of) mostly forgiven, since the bending stress in the fillet radius under the head becomes lower with longer fasteners due to their willingness to flex. External Forces This is a tricky area, since external forces are not always easy to calculate. The word “estimate” is a more likely substitute. A static load, acting parallel to the fastener axis, is the least problematic scenario particularly if we see no bending in the fastener. A stiff joint, with plenty of material to be clamped, will by itself limit bending stresses. If external forces are dynamic and applied eccentrically to the fastener it will require more clamping force on the joint. We will have to use a larger or stronger fastener to add more clamping. When we have also side loads or combinations of tension and side loads, the fastener must clamp even harder. The tension component will unload the joint and the remaining clamp force must now be able to prevent the joint from slipping sideways. This is, by far, the most difficult joint type to deal with, and also demands very high preload forces with stronger or larger fasteners. If we must clamp harder, we also have to watch for the higher surface pressures on the joint parts that could cause embedment and result in load losses. Materials We must always try to combine joint and fastener materials that will perform well together in terms of strength, corrosion resistance and thermal behavior. Use a fastener strength that is sufficient for the joint and the forces acting on it. To use stronger fasteners than needed for the joint not only add cost, but can also make the fasteners vulnerable to stress corrosion and some other negatives. Most fasteners and joint materials are subjected to some environmental influences like corrosion. It is important to choose the kind of protection that will prevent corrosion, but also to avoid process induced hydrogen when electro plating, or zinc brittleness when hot dip galvanizing higher strength fasteners. If we use carbon or alloy steel fasteners or stainless steel fasteners for the assembly of aluminium part, we must not only consider the anodic/cathodic relationship, but also the difference in thermal expansion rates when joint temperatures go up or down. Temperatures A reasonable “normal” temperature range for joints of steel with steel fasteners could, in my opinion, be +120°C (250°F) down to -40°C (-40°F). You may find other suggestions like up to 150°C (300°F) from the automobile industries. Since we typically assemble our equipment at room temperature, any higher or lower temperatures for the joint will alter the elastic modulus (aka Young’s Modulus). With increased temperature the E-modulus goes down, with lower temperatures it goes up. Within the temperature ranges mentioned above, the changes are considered marginal and we don’t have to adjust in our joint calculations. If, however, we go above the temperatures indicated we should use the E-modulus for these higher temperatures if the joint have to live with these during service conditions. With lower we must do the same. To use “standard” steel fasteners for these outside temperature conditions may not be acceptable and we should consider other materials like stainless or special alloys made for higher or lower temperatures. Assembly To calculate tightening torque values based on assembly tools with high accuracy can be problematic if we don’t have the same tools for repair and service. A very good (but expensive) DC electric tool will result in a tension scatter of +/- 10% which is excellent. If a repair person is using a noisy tool like an impact wrench with a tension scatter of +/-60%, he will likely break the fastener or not produce a sufficient preload (bad in both cases). It is, therefore, important to base a design on realistic tool figures and also inform service people about the tightening tool to be used. Just because a tool makes a lot of noise does not mean it is a good tool. I firmly believe that impact wrenches should be banned for use with quality joints or reworked for left-hand direction only (great for breaking loose rusty nuts). please turn to page 164

Indianapolis based Würth Service Supply has brought on several experienced industry leaders to support its core business as it builds towards its goal to double sales over the next few years. Jason Talley is appointed as Vice President of Sales. Talley has more than 18 years’ experience in the industrial distribution industry, the majority of which have been spent with Würth Service Supply. Talley joined Würth in 1998 as a sales representative and moved on to become Special Projects Manager, Regional General Manager, and Director of Sales before his appointment as V.P. of Sales. In his new role, Talley will oversee the sales organization, engineering, marketing, product management, and quoting for the company. From outside of the organization, Würth has brought on Ken Misch as Vice President of Operations. Misch will be responsible for the Operations organization within the company including the Central Distribution Center in Indianapolis, Regional Operations Managers throughout the U.S., the CPS® implementation team, logistics, and the Indianapolis Distribution Center warehouse team. JoAnn Hussion is now responsible for procurement as the Purchasing Manager, and David Miller will head the western regional quotes team as the Quotes Manager. Hussion brings nearly 20 years’ experience in industry quoting and sourcing, including more than a decade of experience with Würth, previously. Miller brings over 10 years of global sourcing experience. Additions to the sales team include Therresa Rust, Jeff Diller, and August Elliott. Therresa Rust is a Key Account Manager, focusing on the energy sector. Rust previously managed a sales territory for Würth and has returned to focus on large accounts. Jeff Diller has taken on a role new to the company as a New Business Development Manager. His role will focus on bringing in new business opportunities for the Key Account team to manage. Diller brings experience in sales and new business development. August Elliott is responsible for the southern sales territory as a Regional Sales Manager. Elliott’s background in sales, territory management, and new business development will be helpful taking on this territory. For more information on Würth Service, visit their website at www.wurthservice.com. THE DISTRIBUTOR’S LINK 163