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The unwanted loosening of threaded fasteners is a big pain for users and big business for those who offer pain relief. Here we are going to present one such solution that is not well known. First let’s define loosening of threaded fasteners in more detail. The tension in a threaded fastener can be lost in part or completely for one of three primary reasons:
The first two causes do not rely on relative movement and are not specific to threaded fasteners. For example, a rivet can yield in tension or become loose due to joint relaxation. But the last cause is unique to threaded fasteners. Under the heading “there is no such thing as a free lunch” the very characteristic that makes the threaded fastener so popular is also the cause of some unpredictable and undesirable behavior. The mating helix of the male and female components provides continuous axial movement as one component is rotated relative to the other. This allows the great flexibility of clamping over an infinite range of component thickness within the grip range of the fastener. It also provides a reusable and serviceable fastening system. Controlling this capability so that loosening only happens when it is desired is the purpose of the Nord-Lock® bolt securing system (www.nord-lock.com) shown in Fig1. This lock-washer prevents unwanted fastener rotation in a rather unique manner.
Figure 1 Nord-Lock washer pair
How it works
To understand how Nord-Lock® works and why it is different, a review of the cause of fastener loosening is in order. In most applications threaded fasteners secure components by clamping them together. This clamping action is created by putting the fastener into tension by elongating it a small amount as it is tightened. The fastener now behaves much like a coil spring that has been stretched. If it is allowed to, it will return to an unstressed state. What prevents this from happening is the friction between the mating threads and between the face of the fastener and the mating component. If the resisting force created by this friction can be overcome, the fastener will literally and figuratively return to a more relaxed state. Tests have shown that the most effective means of overcoming the friction force is when vibration or cyclical loading causes the clamped components to slide relative to one another. This in turn can cause intermittent periods were the securing force due to friction is reduced to the point where the energy contained within the fastener can be relieved through a reduction in its tension by loosening. The two most common methods of preventing this movement is to increase friction by interlocking the fastener with its mating component (toothed washers or serrations under bolt heads) or eliminating the clearance in the threads (thread locking compound, inserts, deformed threads, etc) which can also increase friction to varying degrees.
The Nord-Lock operating principle is a cam angle greater than the fastener’s helix angle. Counter-clockwise motion actually increases tension in the bolt. Source: Nord-Lock
Unique Aspect of Nord-Lock®
The unique aspect of Nord-Lock® is that it doesn’t attempt to prevent relative motion, but instead only allows this motion to tighten rather than loosen the bolt. They accomplish this with a paired set of washers whose inner surfaces act like cams with a greater ramp angle than the helix angle of the screw threads so if the bolt moves in the direction of loosening tension is actually increased (Fig 2).
John Gorenflo, Nord-Lock’s principal sales engineer, visited our lab and demonstrated their product with a portable Junker’s machine. Gerhard Junkers, a research engineer with the Unbrako Koblenz division of SPS, conducted studies of fastener loosening in the late 1960’s which are still considered some of the most influential in the field. Copies and variations of his original test setup are still the most widely accepted means of laboratory testing resistance to vibration loosening. The Nord-Lock® test setup converts the rotary motion of an electric drill to reciprocating linear motion through an eccentric. This causes an upper block to slide back and forth relative to a fixed lower block (Figs 3a, 3b). A nut is tightened against the upper block while the bolt is prevented from turning in the lower block through use of a hex key. A load cell incorporated into the stationary block measures clamp load as the nut is tightened and as the test is run.
Portable Junkers testing machine
Close-up of tester showing flexible member joining eccentric and moving upper block
Viewing the Junkers test of a standard nut/bolt combination should be required viewing of all engineers involved in joint design. No matter how many times one sees it; the sight of instantaneous tension loss is eye-opening. Also, the herd of split lock-washer users has needed thinning for some time. This test is a means to that end. Fig 4 shows a summary of the test John presented and is contained in Nord-Lock®’s literature and on their web site. It should be noted that for the Nord-Lock cams to do their job, the outer serrated surfaces must embed into the faces of the adjacent fastener and joint component and completely eliminate movement at those interfaces. As this is the same principle as a serrated flange nut or bolt, one could ask what additional benefit Nord-Lock® provides John acknowledges that serrated heads generally resist loosening better than the methods shown in Fig 4, but claims the absence of the counter-acting cams still allows movement and therefore significant loosening. In the second quarter of this year Archetype Joint will be conducting a comprehensive test of threaded fastener systems designed to be resistant to vibration loosening as part of our Comparison Test series. We will be including Nord-Lock® in that test.
Note that the washer pairs are glued together so that they cannot be installed in an incorrect orientation in the initial application. John did demonstrate how Nord-Lock® can be reused without affecting their performance. He noted that a rule-of-thumb is that in most applications the washers can be reused to the same extent as the fastener. In applications with high-strength bolts taken close to yield the faces will degrade more quickly. This brings up another application recommendation. Nord-Lock® recommends tensioning fasteners at 70% of yield, so that yield will not be exceeded during disassembly when the fastener is stretched as the moving washer rides over the cam of the fixed washer. Fasteners tensioned greater than 70% of the yield point are not recommended for reuse. Due to the increased friction associated with Nord-Lock® installation, it is recommended that a 20% increase in target torque over standard fasteners of the same finish be used as a starting point for testing.
Regarding current applications, as Nord-Lock® is based in Sweden there is greater penetration in Europe than North America. However John claims Nord-Lock is investing more resources in marketing within North America, resulting in a sales increase of 20% last year. Independent of where they are located, the majority of current use appears to be in heavy equipment like earthmoving, drilling, railways, power generation and the like.
Figure 4 Results of Junkers testing for Nord-Lock and other common fastener systems
Cost is a major driver in applications
Nord-Lock® is not cheap. In a standard yellow chromate finish, an M8 (5/16′) washer lists for approximately $0.25 per pair in small quantities. For an M16 (5/8″) fastener the price is about $0.75a pair. As these costs can be greater than the cost of the fastener, they will tend to be used in critical applications where “failure is not an option”. Following is a summary of what we see as the positive and negative aspects of Nord-Lock®.
Generally Negative or a Limitation When Compared to Alternatives
Generally Positive or an Advantage When Compared to Alternatives
Neutral or Roughly Equal Potential of Advantage or Disadvantage
In summary, Nord-Lock appears to be a very reliable solution to vibration loosening in hard joints under severe enough operating conditions that anti-locking strategies must be employed, and in industries where standard fasteners are the norm due to the service environment or economics. For example, the automobile industry has many of the severe applications conducive to Nord-Lock®, (which has been implemented to solve automotive production crises). However, the high production volumes, available engineering resources and advanced torque control equipment permits joints to be individually engineered to eliminate the use of Nord-Lock® at less cost than continuing to use it. This is not an option for most industries, and special fasteners create special service problems for products that the customer intents to service themselves. Finally, keep Nord-Lock® in mind as a fix for problem joints on products in production, as it’s easily implemented.
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