Bolted joints exposed to dynamic loads or pure vibrations are
very likely to loosen if movements under the head/nut occur. This
problem forces designers to invent different kinds of bolt securing
methods. Mentioning all known securing systems would lead to a very
long list. The main principles for bolt securing are: *
Design the joints with a large ratio between clamping
length/diameter. The bolt will be relatively slim which makes it
more flexible and the surface contact under the head/ nut will be
maintained. Furthermore the bolt has to rotate longer before the
clamping force will be lost. This method is an expensive design.
Extension sockets and of course long bolts have to be used. *
Castle nut or the head/nut secured with a tab washer or locking
wire. These securing systems involve relatively complicated assembly
and disassembly. Unfortunately they cannot provide firm locking and
part of the bolt force is lost. * Increase of friction under the
head/nut or in the thread. Locking nuts with increased friction in
the thread have the advantage that they cannot completely loosen by
themselves. Unfortunately, the increase of friction is not effective
at high bolt tension. Non-free-spinning threads makes assembly
laborious. * Friction towards the material is increased by
providing heads/nuts with serrated surface, often in combination
with a flange in order to increase the surface contact and the
friction force. This method usually requires a considerably higher
torque. Upon assembly and mainly disassembly a strong scratching
towards the material will occur. However, a large serrated surface
will prevent the bolt/nut from loosening. * Friction washers with
serrations work as described above, however, provide easier storage
of fasteners. * To use adhesives in the threads has become a
very common method. It requires minimum storage of fasteners but
does have some other requirements. In order to obtain an effective
locking effect the surfaces need to be cleaned before assembly.
Service and maintenance can be troublesome since disassembly is not
easy and cleaning of the surfaces is a must before reassembly. Using
adhesives results in a considerable increase in friction with great
variation in clamp force. Applying adhesives in the threads is an
ineffective method when service is required. Furthermore, a curing
time is required to provide an effective locking effect. At an
emergency repair the adhesive may not be available.
Make the bolt self-locking
Nord-Lock is a pair of washers with wedge locking action
according to DIN 25 201. This method uses tension instead of
friction. On one side the washers have cams with a greater rise than
the pitch of the bolt. In addition, there are radial teeth on
the opposite side. The washers come pre-assembled in pairs cam face
on cam face. When the bolt and/or nut is tightened, the teeth grip
and seat the mating surfaces. The washer is locked in place,
allowing movement only across the face of the cams. The tension
created makes the bolt/nut self-locking. Vibration tests using
the Junker method and test results from the German TUV confirm the
practical experiences that Nord-Lockâ does provide a secure locking
method for bolted joints exposed to dynamic loads and vibrations. A
bolt, which has to increase tension in order to loosen, cannot
possibly loosen itself. Nord-Lock AB has been producing the
washers since 1982. The company perform a customer service by
checking torque - load ratios for most kind of joints and run Junker
tests in the range of M3 - M27 bolt size. The joint consists of two
plates with a needle bearing in between. The bolt tension is
measured during the vibration test. When opening a bolt locked with
Nord-Lockâ there is a small increase of tension during the cam
overriding. Even at very moderate preloads the Nord-Lockâ washers
provide a secure and vibration proof locking. The test diagram
(Fig. 2) shows the bolt tension during vibration testing of some
common fasteners. The tension will drop completely for non-secured
nuts and most of the tension will be lost for nuts with added
friction to the thread or with spring washers. Some of these may
hold better but it is all depending on the friction. A lubricated
thread makes the nut turn loose quicker than when the surfaces are
dry. All vibration tests show a positive result when using
Nord-Lock. Some of the initial tension gets lost due to settlements
since the test is rather brutal with an amplitude of ±0.3mm. The
locking function proves itself when the nut is being un-tightened
with a wrench. A clear increase of tension is registered. Vibration
tests with the same bolt and mating material as an actual case
provide a good proof of how safe the joint will be even under worst
circumstances (vibrations, dynamic and shock loads). A test on
UNF 5/8 bolts with and amplitude of ±1mm is so brutal that tab
washers will open due anti-clockwise turning of the bolts and there
is a complete loss of clamp load. The same test with Nord-Lockâ
washers shows only a minor loss of clamp load caused by settlements
in the thread but there is no off-turning motion. The Nord-Lock
washers can be used on long holes as well. The edges of the long
hole can be deformed by using the standard dimension of washers with
a regular nut. This combination of fasteners leads to slightly
greater settlements than if a washer with larger outer diameter is
used. Therefore it's recommended to use the special washers with a
larger outer diameter in combination with a flanged bolt/nut. To
assemble a bolt on powder painted surfaces has become common. Even
here, Nord-Lock secures safely. Comparative Junker vibration tests
with painted and unpainted surfaces show a limited difference of
settlement. Using a standard dimension washer with a regular nut
on a painted surface the preload in the joint will decrease slightly
due to settlements not only in the thread but also in the layer of
paint, however, the locking effect remains positive. Using a washer
with a larger outer diameter in combination with a flanged bolt/nut
will minimise the settlements.
Advantages
For machinery, which requires regular service and maintenance,
Nord-Lockâ is the optimum solution of bolt securing. Bolted joints
secured with Nord-Lockâ are easy to untighten with general tools and
mounting elements can easily be lubricated before reassembly.
Normally lubricated bolts loosen easier than if the friction is
higher. Lubricants in combination with Nord-Lock do not jeopardise
the security. The bolt is not only safely locked but also easy to
open and the fasteners are re-usable. When securing a stud bolt
it is normally locked in the taped hole with an adhesive and the nut
is often of jamming type. This method is complicated. Difficult
repair work has to be done if the bolt should break from fatigue. A
stud bolt with the same pitch in both ends will not turn by itself
and the nut can simply and safely be secured to the material with
Nord-Lock and any repair job will be easy. Thanks to the fact
that Nord-Lock washers do not rely on friction they will not loose
their locking effect due to lubrication. Lubrication is always
important to reduce friction upon assembly. With less friction, less
torque is required to obtain the desired preload. Besides that the
spread of the preload will be minimised by more controlled
friction. The washers are available in through hardened steel
either yellow zinc chromated, Dacromet or Delta coated and also in
stainless steel 316. The stainless steel washers can be used at
temperatures up to 500ûC without any reduction in performance. This
is an important factor for many applications such as turbines and
engines. The washers come in pre-assembled pairs, which makes
assembly easy and safe. The standard dimension range for steel
washers is M3 to M130 and for stainless steel M3 to M80. They lock
standard and high grade bolts. Ultra Dynamics in the United
Kingdom is a manufacturer of rail retarder systems used in
marshalling yards worldwide. These hydraulic retarders are at set
distances along the track to bring the train to the correct speed
for that part of the marshalling yard. Due to the fact that the
retarders are exposed to high/low frequency vibrations, shock loads,
pressure loads and daily temperature changes, Nord-Lock washers were
chosen as the standard fastener. Friction style fasteners were ruled
out, as the dynamic loads cause transverse movements in the stud
thread and under the nut leading to severe loss of friction making
the risk of slackening too high. In order to achieve an optimum
of clamping force, lubrication of the threads and nuts is necessary.
With measures for lower friction, the locking methods based on
increased friction will loose their effect. Using Nord-Lock, which
locks by tension, in combination with lubricants will provide a safe
joint with high tension. Another Nord-Lock user is VAE railway
technology in Zeltweg, Austria, a leader in the construction of high
speed switch points. Stable and precisely set tracks allow high
speeds up to 250km/h. The tracks are being equipped with high
speed switch points. One location for Nord-Lock is the switch driver
responsible for the changing of points and the quick positioning of
the switch tong. This tong absorbs extreme dynamic loads, transverse
movements in the bolt head and/or nut leading to a severe loss of
friction. Nord-Locks wedge-locking effect safely and economically
solves the problem. The geometric design of the fasteners used
properly in a joint guarantees self-locking regardless of friction.
The bolt securing system was chosen after stringent tests were
carried out by different engineering bodies for use in the cab, the
chassis and the passenger compartments. Alstom DDF in France is
installing Nord-Lock washer to secure bolts on structural and
traction connections in order to achieve maximum safety. The washers
have proven to solve loosening problems on rails, switches and
signal systems in Europe and in the US. |