In a typical design institute or department, a lot of time and effort is invested in the functionality of mechanical designs. The fasteners incorporated in the designs normally receive scant attention from engineers.
The amount of planning, capital and investment that goes into projects can easily make the little things seem insignificant.
Bolts and nuts are looked upon as trivial hardware but if the fasteners allow a joint to fail, serious consequences may follow. This is especially true when the joints are subjected to dynamic loads.
Traditional bolt securing methods are generally based on friction, which means that the bolt/nut is prevented from turning loose by friction in the bolt thread and the joint. This will work in static conditions but not when there are dynamic forces involved.
In such a case the fasteners will turn loose leading to severe loss of tension and eventually fatigue failure in the joint. Expensive warranty claims, down time and maintenance costs may result.
Reliable locking function
Nord-Lock has developed a securing system that makes fasteners self-locking. The method is based on a pair of washers that provide a wedge-locking action meeting DIN 25201 - a method using tension instead of friction to secure a bolted joint.
On one side the washers have cams with an angle [g1] greater than the rise of the bolt thread [g26].
In addition, there are radial teeth on the opposing washer sides.
The washers come pre-assembled in pairs cam face to cam face. When the bolt and/or nut is tightened, the teeth grip and seat the mating surfaces.
The washer pair is locked in place, allowing movement only across the face of the cams. Any attempt from the bolt/nut to rotate loose is blocked by the wedge effect of the cams.
Nord-Lock has, over the past two decades, developed into a global product, used widely for solving bolt securing problems in industries such as railway, transportation, heavy machinery, offshore, marine, power generation and forestry.
Since using Nord-Lock will ensure quick assembly/disassembly, maintenance time can be reduced to a minimum.
Nord-Lock has outstanding quality and consequently it normally can be reused. The result is reduced life cycle cost (LCC).
Lubricate the bolt
Nord-Lock provides safe locking in both dry and lubricated conditions. However, lubricating a bolted joint will increase the durability and safety of the application.
Lubricating bolted joints is an effective method of reducing friction, which in turn reduces torsion stress and preload deviation.
Minimising friction will significantly improve the joint's performance characteristics and thereby making control over the clamp load possible. It is especially beneficial for applications with large and/or long bolts. An appropriate lubricant can also minimise the effect of galling in stainless steel bolts. Since Nord-Lock washers utilise tension instead of friction to secure the fasteners, the locking function will not be compromised by lubrication.
he following illustrates the importance of lubricating bolted joints. One customer manufacturing hedge trimmers had experienced bolt failure when four high-grade M12 bolts holding a hydraulic motor broke from fatigue. The engineers thought they had taken all safety measures available by using a nylon inserted nut, thread adhesive and Nord-Lock washers.
Looking at the torque figure, it was suspected that the clamp load was too low so the torque/load ratio was measured.
Using the specified torque value supplied by the customer, it was determined that only 25 per cent of the yield strength was utilised in the bolts due to too much friction from the nut and the adhesive - the preload was too low resulting in unwanted movement in the joint. As the applied torque could not be tripled, the design was revised.
By replacing the adhesive with thread grease in combination with Nord-Lock, the problem was solved.
The company's new recommendations for torque have been set from laboratory tests done by Nord-Lock using calibrated torque transmitters and load cells.
In-house testing
Nord-Lock's in-house testing laboratories perform a wide range of tests every year either
for internal studies or on behalf of the company's customers. It has been found that Nord-Lock's lubricant, GTP600, provides all the benefits you would expect from a good lubricant even during long-term use in harsh environments.
GTP600 is a graphite paste of high purity which outperforms any conventional lubricant in laboratory tests.
When tightening bolts and nuts, using GTP600 will ensure low and uniform friction conditions, making it possible to utilise the bolts' full capacity. GTP600 helps optimise the performance and increases the cost efficiency of bolted joints.
The compound is environmentally friendly, complying with EINECS (EU) and TSCA (US) regulations.
Joints lubricated with GTP600 can be easily dismantled after years of service in corrosive environments.
Thread friction
Minimising thread friction with lubrication and at the same time safely secure a joint may seem like an impossible equation for design engineers who are used to traditional locking methods.
Systems that rely on friction in their attempts to secure bolted joints do not manage well in harsh applications when faced with vibration and dynamic loads.
The use of these systems is the single most common reason why the full capacity of the bolted joints is not utilised. It is also one of the causes for bolt fatigue failures in demanding applications.
The resulting lower friction in lubricated joints secured with Nord-Lock will ensure safely locked fasteners at the highest possible preload level.
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Harlen Seow is with Nord-Lock International AB, Malmö, Sweden. www.nord-lock.com
