Thursday, September 27, 2012

A Professional Approach to Threadlocking

An essential precondition for ensuring the long service life of machines is that the components installed will withstand stresses such as vibrations, thermal expansion of materials and major temperature variations. For secure locking of threaded connections, the Loctite brand from Henkel offers high-performance liquid threadlockers that are easy to use and prevent threads from loosening.

Every mechanical engineer knows the challenge of finding ways to prevent nuts and bolts from working loose. Machine components have to be highly durable and withstand even micro-movements caused, for example, by the different thermal expansion rates of materials such as steel and aluminum. A variety of means exist to secure threaded connections against loosening when exposed to external factors such as vibration or corrosion of the thread. Mechanical methods, however, can fail rapidly. Anaerobic threadlockers from Loctite – whether medium-strength or high-strength – can be relied on to assure a durably secure connection every time.

When a threaded connection fails
Regardless of its thread diameter, a screw or bolt is always stretched axially (along its length) like a spring when it is tightened. It builds up a preload or clamping force by means of friction in the threads due to pressure between the mating flanks and the surface pressure acting on the screw head. The connection thus achieved can, however, fail and the screw may work loose. In gearboxes and engine blocks, for example, this can ultimately lead to leakage.

If salt water or moisture penetrates a thread, the surfaces can become corroded. A screw may shear off or threads become stripped when parts are being dismantled. Care should also be taken when using different types of material, as galvanic corrosion may cause them to rust. This can happen when different metallic materials come into contact with an electrolyte like water. The less noble of the two metals will then be destroyed.

Mechanical aids for retaining screws and bolts, such as serrated lock washers or screws with ribbed or sawtooth heads, will not always guarantee a lasting connection either. Even when they have been properly inserted and tightened, the connection may still fail due to expansion and contraction of the materials caused by high temperature differences in the machine.

Chemical threadlockers: resistant to vibration and temperature
A reliable way of achieving durable threaded connections is to use anaerobic adhesives. When properly applied, chemical thread retention – by means of anaerobic adhesive bonding – offers many advantages over mechanical retention methods. To begin with, anaerobic adhesives make the connection resistant to vibration and allow controlled preloading of the fastener. In addition, the threadlocker cures only in the absence of air and in contact with metal. When the products have been properly applied, they will spread evenly over the thread surfaces, thus filling the clearance between mating threads. A good choice for medium-strength and medium-viscosity threadlocking is Loctite 243. It ensures that the threaded connection is resistant to vibration, media and temperature, while still allowing disassembly. It acts both as a retainer and as a sealant and prevents corrosion in the thread. Two further products, the medium-strength threadlocker Loctite 2400 and its high-strength counterpart Loctite 2700, contain no harmful substances and so do not require hazard labeling. They thus make an important contribution to user health and safety. The adhesives are easy to use and economical. They also reduce complexity, since a single threadlocking product can be used for all thread sizes. All of these aspects result in significant cost advantages over mechanical retaining elements.

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