Washer (hardware)#Spring and locking washers

Under screws securing measures are understood to be the unintentional loosening or loosening a screw connection to external influences (eg vibration (vibration), corrosion, setting the connection ), and thus the loss of prestressing force to prevent the working principle of the screw. This is realized by additional (and mostly mitverspannte ) components, special shape elements or adhesives.

Overview

Especially in large-scale mean Threadlockers additional amount of material, logistics, assembly and the risk of non- installation or defectiveness of the screw lock itself or the assembly results. Therefore, the goal of any bolted connection to waive additional security measures as possible. Critical bolted joints, which must meet high safety standards, are exposed to bad or no cost or unfavorable loads are constructively dimensioned taking into account the ambient conditions.

Use

If you are uncertain or non-compliance with the required minimum grip length of bolt connections, and uncertain environments screw fuses are still common and appropriate. An example of a non-professional and properly executed screw connection is the attachment of ( aluminum ) fenders on (steel ) frame of bicycles. Here, the screw clamp force is removed over time by the flow of aluminum and corrosive electrochemical influences. The screw comes loose and the hole in the Aluschutzblech is knocked out by the vibrations while driving.

For dynamic loads, especially perpendicular to the screw axis ( lateral load ), tilt screw connections to redeem himself. A remedy against this component failure primarily provides the correct interpretation and construction of the screw connection. The basic principle in engineering applies:

The components to be connected to give as little as possible (large cross sections, no creep, high modulus of elasticity - if structurally feasible), so be stiff
The corresponding screw connection, however, must be compliant as possible ( eg by use of expansion bolts )
Bolted connections with high strength bolts and nuts. These are only useful if they can be tightened accordingly ( extensive use of bolt strength) and on the tense parts no plastic deformation occurs ( interfacial pressure ).

In places where such measures are not possible ( safety wire ) can be backed low-strength bolted joints with adhesive, specially shaped nuts with a nylon insert ( self-locking nuts), by castellated nuts with cotter pin or through the wiring. The securing of screws in aluminum parts can be realized eg by using self -locking threaded inserts or special thread. When using stainless steel screws to these missions, it may come to eat. An ornate exception is the method to execute the thread of the fixing nut as right-hand thread, the associated thread the lock nut on the bolt on the other hand as a left-hand thread. This method is common in the mining industry, as well as on old bicycles with threaded sprockets on the rear wheel hub. In the application of castellated nut and cotter pin and nut are secured to each other. For high-strength bolted connections only a few possibilities of an additional backup offer if the use of bolt strength and the associated large prestressing force does not lead to success:

Lock washers ( single or double)
Sticking ( liquid or microencapsulated adhesive)

Calculation of screw

In the design, as well as the calculation at the detection of screw connections determining the biasing force as follows, the procedure is in principle:

Determining the maximum and minimum axial operating load
Identification and selection of so-called partial or joints texture and material parameters on the basis of constructive set installation situation
Determining or estimating the coefficients of friction occurring through lubrication of the screw before tightening to determine the so-called tightening torque of the screw.
Identify and define the way how or with which the screw is tightened
Taking into account the material properties of the screw on the planned temperature range
Proof of fatigue strength under dynamic or changing load applications

In the first bill can be determined so-called draft überschläglich the size or the cross section of the screw on the biasing force and the coefficient of friction on the basis of the axial operating load, and the required torque when tightening the screw connection.

In addition, the so-called set or creep is taken into account. Under the setting is defined as the " squashing " or flattening of " peaks " of the existing surface roughness or compliance of the strained materials through initiated when tightening the screw joint preload force. This effect can be especially for soft materials such as gaskets, repealing bias and thus the effectiveness of the screw lead after a short period of operation. This is also the so-called loosening moment and the force for releasing ( untwisting ) of the screw is required, included. Then the behavior of the screw in the planned temperature range the materials used will be determined by the installation situation. In this case, the different behavior of the materials used, such as the thermal expansion is taken into account.

Finally, it is typically the detection of the fatigue strength of the screw, including the components to be attached to lead. Here, the strength of the components used on the basis of the materials used, the Verbausituation geometric design of the components, as well as by means of the number and type of load changes in the dynamic ( oscillatory ) strain is detected. It is permissible in some industries, that this evidence can be given taking into account the temperature behavior of the components by means of a FEM calculation.

In the design calculation, the following boundary conditions usually apply:

Screw only be loaded to bias occurring lateral forces (forces that cause a displacement or shearing of the screw ) are to be considered on the coefficient of friction as the operating load.
The multiple tightening and loosening a screw must be considered as a rule by a further proof of invoice. For this purpose, it is assumed in highly stressed bolted joints usually assume that all screws and securing elements are to be replaced by new, unused components.
The introduced torque of the screw is constant after assembly.
There is no coating or a deliberate transgression of the specified tightening torque during assembly.
The installation is done with standardized tools. The use of extensions or universal joints is not usually permitted.
The introduced tightening torque can be determined during installation.

As typical examples for the calculation of bolted bolted flange connections are often used with a dished bottom.

Classification

Screw locks are usually non-positively (eg, spring washers, spring washers ), form fit ( eg safety plates with flaps, dials ) or cohesive (eg screw locking varnish, glue ).

Among the non-positive backups include self-locking nuts, such as hex nuts with clamping part.

Screw locks are designed to prevent biasing force losses due to Replace and creep ( loosening ) and dynamic loading (risk of partial or complete Losdrehens ) or at least keep small.

Screw locks can be classified into the following classes:

Unscrew lock: Prevents the prestressing force decreases (eg, glue, rib screws).
Captive: Prevents solve elements of the screw and the connection falls apart. The biasing force of the screw as an effective force can not be maintained in certain circumstances.

In practice, be distinguished in Threadlockers also in the following categories:

Self-or foreign -resistant screw locks to maintain the preload: Outside serrated lock washer (center)
Disc spring
Wave washer
Schnorr washer

Acting as a captive screws ineffective backups: sapwood,
Circlip
Snap ring
Special washers
Self-locking nut

Ineffective bolts fuses according to DIN

The following equipment items are identified as ineffective screw locks, because they have proved, under certain circumstances, to maintain the pretension of a screw as ineffective ( all specified standards have now withdrawn):

Spring washers ( DIN 127, DIN 128 and DIN 6905 )
Spring washers (DIN 137 and DIN 6904 )
Toothed washers (DIN 6797)
Serrated lock washers (DIN 6798)
Locking plates ( DIN 93, DIN 432 and DIN 463 )
Sicherungsnäpfe (DIN 526)
Lock nuts (DIN 7967)
Castellated nut ( DIN 937) with sap

However, these components can often still used as so-called " tether ".

One reason for this is that a properly executed screws compound has much higher biasing forces than the above mentioned elements can build up on spring force. So a spring ring is according to DIN 127 already at 5 % of the nominal preload of screws of property class 8.8 block and acts only as a washer.

The counter-attack with another mother is only useful when the force between the nuts is significantly greater than the biasing force in the connection to be established. In addition, this connection will not be able to tolerate without loss of constant vibration as well as the biasing force of releasing of the connection.

Security elements and standards

Clamp nuts with plastic insert EN ISO 7040, 7043, 10511, 10512th (Alt: DIN 982, 985, 6924 )
Hexagon nuts with metallic clamping part EN ISO 7042, 7044, 10513th (Alt: DIN 980, 6925 )

Non-standard safety elements ( varying degrees of effectiveness )

There are a number of different, partly of proprietary elements:

Wedge lock washer pair
Lock washer
Liquid adhesive
Microencapsulated adhesive ( see also DIN 267-27 )
Special thread profiles (eg trilobular )
Self-locking threaded inserts
Stain nylon coating ( see also DIN 267-28 )
Limes ring

Safety wiring for aircraft

Since the vibration of aircraft are very high, all screws must be secured. An addition to other types of backups frequently used type of backup is the safety wire. In this case, a wire is drawn through a securing hole in the screw head, and the two wire ends of the same length verzwirbelt and pulled at a fixed point or a second bolt head by a securing hole and turn verzwirbelt at the ends. The backup must be performed according to certain rules in which the wire guide, the twist direction, number of wire strikes are determined depending on the wire diameter and construction of the drill ends with at least 3 and usually not more than 6 strokes. The total length of a wire fuse must not exceed 24 inches ( 609.6 mm). To either twist to use a backup tongs.

Screw locks with electrical connections

For electrical connections conventional screw locks from the engineering can not be used. The task of creating an electrically safe point of contact is not possible with a conventional screw and a screw locking over a long period of time.

Often protective earth screw connections are made too short. Therefore, usually a resilient element is provided. There are so-called smooth and self-locking spring rings for electrical connections. The latter have hooked ends and damage while solving the surfaces. For electrical connections, they shrink by violating the oxide electrical contact resistance, whereas in non- gas-tight contact points running the long-term stability of the contact resistance remains questionable.

For electrical connections (eg ground terminals ), the use of adhesive to secure the mounting screw is not useful because the adhesive can pull in the joints and acts as an insulator there.

Aluminium Thread-locking fluid Interchangeable parts

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