Tips for using threadlockers and retaining compounds on machines

Adhesive technology continues to advance, with Henkel typically introducing a dozen or more new products every year. Nevertheless, threadlocking adhesives and retaining compounds, which are the focus for this present '10 tips' technical article, are well and truly proven in the field, having been in use since the first Loctite products were launched in the 1950s. Today they are used in almost every branch of engineering including aerospace, automotive, medical, and oil and gas - as well as machine building, of course. Threadlocking adhesives and retaining compounds are fairly similar anaerobic adhesive-based products, with the former being used for threaded fasteners and the latter for non-threaded cylindrical assemblies such as bearings.

What follows are ten tips that will help machine builders make the best use of these products to save time and cost during manufacture and assembly, and improve reliability once the machines are in operation. The tips will be helpful for engineers who have little or no experience with threadlockers and retaining compounds, and even engineers who are familiar with such products are likely to find something of interest.

1. Use threadlocking adhesives to improve reliability

Threadlocking adhesives are excellent at resisting vibration loosening because they fill the voids between the male and female threads. Unless the operational temperature exceeds the limit of the high-temperature products (which can survive 232degC), threadlocking adhesives outperform other fastener locking techniques such as special washers. By using threadlocking adhesives, machine builders ensure fasteners will not vibrate loose, which can improve machine reliability significantly.

2. Use threadlocking adhesives to both lock and seal

Because threadlocking adhesives fill the void between the threads, they can double as a threadlocker and sealant. Machine builders can therefore speed assembly and save cost because there is no need for separate sealing components, and through-holes can be used instead of blind holes to simplify thread tapping.

3. Use retaining compounds to simplify bearing arrangements and save cost

Traditional techniques for retaining cylindrical components include circlips, clamp plates and shrink-fitting using heat. Circlips and clap plates require additional components and machining operations, while shrink fitting requires tight tolerances and precise heating. By using retaining compounds, the design of the bearing arrangement can be simplified, parts count reduced, and assembly time shortened - all of which saves cost for the machine builder.

4. Use retaining compounds for easier bearing assembly

Bearings are manufactured to very tight tolerances, and traditional fitting techniques require relatively tight tolerances to be achieved in order to avoid bearings moving in their housings or on their shafts. Typically this will mean that bearings have to be assembled using presses and/or heat. However, by specifying a retaining compound with the correct gap-filling properties, the housing or shaft can be specified as a slip fit so that assembly can be by hand, which simplifies assembly and saves time and cost.

5. Don't forget thread sealants

In many ways thread sealants are similar to threadlocking adhesives but they are designed specifically to provide a fluid-tight seal on metals and, with some grades of sealant, plastics. Most products are liquids or gels, but there is also a cord-type product that is wound around the thread in a similar manner to PTFE tape, though the thread sealing cord (or yarn) has the advantage that joints can be adjusted by rotating in either direction without adversely affecting the seal. Some thread sealants achieve a pressure-tight seal instantly while others require time to cure.

6. Always use the optimum products

Within Henkel's Loctite range there are many different grades of threadlocking adhesive and retaining compound. The products vary in viscosity, strength, temperature resistance, gap-filling ability, suitability for use on particular surfaces, cure time and resistance to oils, for example. Selecting the optimum grade for a particular application will ensure that it performs as required and is also easy and convenient to apply.

s{7. Use appropriate dispensing equipment

Machine builders working on one-offs may simply apply threadlocking adhesives or retaining compounds by hand, directly from the tube or bottle (or stick in the case of the wax-like, semi-solid Threadlocker Sticks supplied in Pritt Stick-style packaging). However, there can be situations where a degree of automation is more appropriate, particularly if machines are being assembled in higher volumes: readily reprogrammable 2D or 3D robotic systems are very effective and efficient for complex profiles, even with short production runs; volumetric dispense-on-demand equipment for assisted manual assembly will ensure consistent dispensing for adequate joint strength while minimising waste and the need to wipe away excess product; and spinning Rotorspray dispensers apply an even bead of retaining compound to internal cylindrical surfaces. Henkel can advise on appropriate dispensing equipment and, if required, provide turnkey automated dispensing systems.

8. No adverse impact on maintenance

Engineers are sometimes reluctant to use threadlocking adhesives or retaining compounds because they are concerned that these products could make maintenance more difficult. However, different strengths of products are available, so lower-strength grades can be specified if regular maintenance is necessary. And even if high-strength products are specified, it is still possible to unscrew threaded joints by using sufficient torque. Retaining compounds can be overcome by applying force, heat or a combination of the two. After disassembly, old adhesives can be simply cleaned off threads or bearing housings before components are reassembled; absolute cleanliness prior to reapplication of threadlocking adhesive or retaining compound is not essential. Any slight inconvenience associated with disassembly or cleaning is easily outweighed by the benefits gained during the initial manufacture and assembly stages,

9. Understand the need for cleaning (or not)

Engineering components are likely to have an amount of surface contamination from cutting fluids, lubricating oils or corrosion inhibitors. Do not assume that components have to be spotlessly clean before applying a threadlocking adhesive or retaining compound. Many of the products available today are tolerant of some surface contamination, so it is important to understand which components need to be cleaned, and to what degree. This depends on the contaminants and the products being used, but advice is available in the product literature, or the manufacturer should be contacted with specific questions. Remember that under-preparation may compromise the joint strength, while over-preparation is time-consuming and wasteful of resources.

10. If in doubt, ask

It is simple to buy a threadlocking adhesive or retaining compound from a distributor, and often there are benefits to be enjoyed without considering the application in much depth. However, investing a little time in selecting the optimum product, joint design and assembly method can make a significant difference to a machine builder's costs and the machine's reliability. Henkel has some of the world's leading experts in adhesive technologies, and the application engineers are extremely knowledgeable; if they have not encountered a similar application previously, they are capable of creating an innovative solution to a customer's particular problems. Henkel Ltd has facilities in Hemel Hempstead where customers can trial different products, and the experts are always willing to help determine the ultimate joint design, assembly method and grade of threadlocking adhesive or retaining compound. Application engineers can also undertake site visits if that is more convenient.

Henkel's Loctite Success Secured campaign includes a series of free webinars that will be of benefit to machine builders considering using threadlocking adhesives and retaining compounds. The microsite also features a number of success stories and solutions guides, plus there is an online enquiry form for customers with specific questions.