Designing for assembly with adhesives

Adhesives have found widespread design acceptance in many industrial manufacturing applications, providing solutions for structural bonding, cylindrical assembly, threadlocking, flange and thread sealing, thermal management, wire bonding and harnessing, and a range of other design challenges. The ability of adhesives to bond dissimilar materials quickly, efficiently, and cost-effectively has enabled the actual production of many designs not possible using mechanical fastening methods alone.

Adhesives offer unique advantages over mechanical and thermal fastening methods. Rather than concentrating stress at a single point, adhesives distribute stress load over a broader area, resulting in a more even distribution. A joint bonded with adhesive resists flex and vibration stresses better than, for example, a riveted joint. Adhesives form a seal as well as a bond, eliminating corrosion, which often occurs in a mechanically fastened joint. They join irregularly shaped surfaces more easily, impact the weight of an assembly negligibly, create virtually no change in part dimensions or geometry, and quickly and easily bond dissimilar substrates and heat-sensitive materials.

Limitations of adhesives include setting and curing time (the amount of time it takes for the adhesive to fixture and strengthen fully), surface preparation requirements, and the potential need for joint disassembly.

Mechanical fasteners and adhesives can work together to form a stronger bond than either method alone -- two cases, threadlocking and gasketing, are good examples. Design engineers who want to improve the safety and quality of an assembly will use a mechanical fastener with a torque setting in tandem with a threadlocking adhesive. The anaerobic thread- locker guarantees that the assembly will not fail or loosen, and that corrosion will not shorten the life of the fastener. Liquid form-in-place gasketing materials are used to dress conventional rubber, paper, and cork gasketing materials and can often completely replace cut gaskets. These liquid anaerobic gasket dressings fill surface imperfections in the mating flanges and extend the life of the gasket.

Adhesive joint design basics. Stress plays a significant role in the success or failure of a joint bonded with adhesives. Engineers must have a solid understanding of how stress is distributed across two mating substrates in order to design the strongest possible joint.

As shown in the accompanying figure, five types of stresses commonly affect assemblies bonded with adhesives. Most adhesives offer excellent resistance to tensile, shear, and compressive stresses, but are very weak in cleavage and peel strength. Therefore, in order to design the strongest possible adhesive joints, engineers should eliminate cleavage and peel forces from the basic joint design. The best joint designs allow for maximum possible bond area, and rely upon both mechanical locking methods and the strength of the adhesive bond for long-term success. Since the ends of a bond resist a greater amount of stress than the middle, joint width is more important than substrate overlap to successful joint design. By increasing the width of a bond, the bond area at each end increases and the overall joint is made stronger.

There are five types of joints commonly found in assemblies bonded with adhesives. The simple lap joint illustrated in Figure 1 can be improved in a number of ways. By simply increasing the joint's width, the bond area at the ends of the joint is increased, strengthening the adhesive bond. The joint can be redesigned to become a single or double lap shear with a larger bond face for increased strength. The joint can become a joggle lap joint with perpendicular bonded faces. In all cases, peel and cleavage forces on the end of the joint are reduced.

The growing variety of adhesives available in the marketplace makes selecting the proper adhesive a challenging experience. Adhesive selection should start by answering questions about the parts to be bonded and the end use application. One of the most important considerations is the environment in which the device will operate.

The most frequent reasons for joint failure do not involve adhesive strength. Typically, the failure of an adhesive joint is due to poor design, inadequate surface preparation, or improper adhesive selection for the substrates and the operating environment. Assemblies should always be thoroughly tested during the design phase to ensure that bonding will be successful during manufacturing and over the life of the device.

Answers from 'Ask the Expert' at www.designnews.com

Bolted Joints expert, Bill Eccles, Technical Director, Bolt Science Ltd. offers the following advice:

Q: I am writing an engineering specification for the tightening of threaded fasteners in a thermal printer. What torque specifications and standards would assist me.

A: Almost every company has its own torque specifications. The fundamental problem is that the bolt clamp load is controlled via a tightening torque. Typically, 90% of the torque is used to overcome friction, with only 10% to provide preload. Significant scatter can occur. Also, vibrational loosening, joint embedding, and gasket creep reduce a fastener's preload. There is normally no easy way to check for the correct preload in the fastener. Most fastener tool manufacturers will provide a list of tightening torques but they must be used with caution. Also have a look at the SAE J174 standard Torque-Tension Test Procedure for Steel Threaded Fasteners.