Screw locking
Under bolt securing measures to be understood that the unintentional loosening or slackening of a screw connection prevent. Loosening occurs through oscillation (vibration), strong or repeatedly changing transverse, longitudinal or rotational forces, through corrosion or setting of the connection . The pretensioning force as the operating principle of the screw connection is reduced . This is prevented by adhesives or additional special form elements. The prerequisite is that the pre-tensioning force (tightening torque) and the strength of the materials used for the screws match the material of the parts to be screwed. Spring washers and spring washers are no longer recommended (standards have been withdrawn).
overview
Especially in mass production, screw locking means additional expenditure in terms of material, logistics, assembly, processing and also the risk of non-installation or a deficiency in the screw locking itself or the assembly result. Therefore, the aim of every screw connection is to avoid additional safety measures as far as possible. Critical screw connections that have to meet high safety standards, are difficult to see or cannot be seen at all or are exposed to unfavorable loads, are constructively dimensioned taking the environmental conditions into account.
commitment
In the event of uncertainties or non-compliance with the necessary minimum clamping lengths of screw connections or unclear environmental conditions, screw locking devices are still common or appropriate. An example of a screw connection that has not been carried out professionally and properly is the attachment of aluminum fenders to the steel frames of bicycles. Here the screw clamping force is canceled over time by the flow of aluminum and by corrosive electrochemical influences. The screw connection loosens and the hole in the aluminum protective plate is knocked out by the vibrations while driving.
With dynamic loads, especially perpendicular to the screw axis (transverse load), screw connections tend to loosen themselves. A remedy for this component failure is primarily the correct design and construction of the screw connection. The following is a principle in mechanical engineering:
- The components to be connected should yield as little as possible (large cross-sections, no creep , high modulus of elasticity - if feasible in terms of design), i.e. they should be stiff
- The associated screw connection, on the other hand, must be as flexible as possible (e.g. by using expansion screws )
- Screw connections with high-strength bolts and nuts. These are only useful if they can be tightened accordingly (extensive use of the screw strength) and no plastic deformations occur on the clamped parts (boundary surface pressure).
In places where such measures are not possible, low- strength screw connections can be secured with adhesive , by specially shaped nuts with plastic inserts (self-locking nuts), by castle nuts with cotter pins or by wiring (wire locking) . Securing screws in aluminum parts can e.g. B. be realized by self-locking thread inserts or special threads. The use of stainless steel screws in these inserts can result in seizure. A complex exception is the method that the thread of the fastening nut as right-hand threads to execute, the associated thread of the lock nut on the bolt on the other hand as a left hand thread . This method is common in mining , as well as on old bicycles with screwed sprockets on the rear wheel hub .
When using castle nut and split pin, bolt and nut are secured to one another. For high-strength screw connections there are only a few options for additional securing if the use of the screw strength and the associated high pre-tensioning force do not lead to success:
- Ratchet screws and nuts
- Lock washers (single or double)
- Gluing (liquid or microencapsulated adhesive)
Calculation of screw connections
When designing, calculating and verifying screw connections, the following principle is used to determine the preload force:
- Determination of the maximum and minimum axial operating load
- Determination and selection of the so-called partial or parting joint properties and material parameters based on the constructional specified installation situation
- Determination or estimation of the occurring coefficients of friction by lubricating the screw connection before tightening to determine the so-called tightening torque of the screw connection.
- Determination and definition of the way and with what the screw connection is tightened
- Consideration of the material properties of the screw connections over the planned temperature range
- Proof of fatigue strength in the case of dynamic or alternating force applications
In the first so-called draft calculation, the size or cross-section of the screw connection can be roughly determined using the preload force and the coefficient of friction based on the axial operating load and the required torque when tightening the screw connection.
The so-called settling or creeping behavior is also taken into account. Setting is understood as the “pressing” or flattening of “mountain peaks” of the existing surface roughness or flexibility of the tensioned materials due to the pretensioning force introduced when the screw connection is tightened. This effect can be particularly effective with soft materials, e.g. B. seals, already after a short period of operation lead to the removal of the preload and thus the effectiveness of the screw connection. The so-called loosening torque and the force required to loosen (unscrew) the screw connection are also taken into account. The behavior of the screw connection in the planned temperature range for the materials used is then checked based on the installation situation. The different behavior of the materials used, e.g. B. the thermal expansion taken into account.
Finally, as a rule, proof of the fatigue strength of the screw connection, including the components to be fastened, must be provided. Here, the strength of the components used is verified on the basis of the materials used, the installation situation, the geometrical design of the components and the number and type of load changes during dynamic (oscillating) loads. In some industries it is permissible that this verification can be carried out with the help of an FEM calculation , taking into account the temperature behavior of the components.
The following boundary conditions generally apply to the mathematical design:
- Bolted connections are only preloaded, occurring transverse forces (forces that cause the bolted connection to shift or shear off) must also be taken into account as operating load via the coefficient of friction.
- The repeated tightening and loosening of a screw connection must usually be taken into account by a further proof calculation. In the case of highly stressed screw connections, it is generally assumed that all screws and securing elements must be replaced with new, unused components.
- The tightening torque applied to the screw connection is constant after assembly.
- There is no over-tightening or the specified tightening torque is deliberately exceeded during assembly.
- The assembly is carried out using standardized tools. The use of extensions or universal joints is generally not permitted.
- The applied tightening torque can be determined during assembly.
Bolted flange connections with a dished bottom are often used as a typical example for calculating screw connections .
Secured by gluing
Thread adhesives and screw locking varnish serve as mechanical protection against unintentional loosening. In addition, they are also used to seal threads in pneumatics, high pressure gas technology and hydraulics.
- 2-component adhesive , hardens in a certain time
- 1-component adhesive, activated by certain metals, hardens in the absence of air (anaerobic)
- Microspheres encapsulated, burst when screwed on
Classification
Screw locks are usually positively (z. B. spring washers , spring washers ), form-fitting (z. B. locking plate with lobes, dials) or cohesively (z. B. thread lock , adhesive ).
The force-fit securing also includes self-locking nuts, e.g. B. Hex nuts with clamping part.
Screw locking devices should prevent or at least keep pretensioning force losses as a result of setting and creeping (loosening) and dynamic loading (risk of partial or complete loosening) to a minimum.
Screw locks can be classified into the following classes:
- Anti-rotation lock
- Prevents the pre-tensioning force from dropping (e.g. adhesive, ribbed screws).
- Loss protection
- Prevents elements of the screw connection from loosening and the connection from falling apart. The pretensioning force of the screw connection as an effective force may not be able to be maintained.
In practice, screw locks are also divided into the following categories:
- Screw locking
- self- locking or externally locking , to maintain the preload:
- Toothed pulley on the outside (center)
- Toothed pulley internally
- Disc spring
- Schnorr washer
- Loss protection
- ineffective as a screw lock:
- Sapwood ,
- Locking ring
- Snap ring
- special washers
- Self-locking mother
Screw locks whose standard has been withdrawn
The following machine elements have proven to be ineffective under certain circumstances in maintaining the preload of a screw connection. All specified standards have been withdrawn:
| Spring washers | DIN 127, DIN 128 and DIN 6905 |
| Spring washers, wave washers | DIN 137 and DIN 6904 |
| Tooth lock washers | DIN 6797 |
| Serrated lock washers | DIN 6798 |
| Lock washers | DIN 93, DIN 432 and DIN 463 |
| Safety catches | DIN 526 |
| Lock nuts | DIN 7967 |
| Castle nut | DIN 937 with split pin |
An example of ineffectiveness is a screw connection of strength class 8.8 that is tightened with sufficient torque. Significantly higher preload forces act than a spring washer can build up. A spring washer according to DIN 127 is already at 5% of the nominal preload force of such a screw connection and only acts like a washer.
In some cases, however, these components can serve as protection against loss.
Lock nut
Countering with another nut (counter nut ) only makes sense if the force between the nuts is significantly greater than the pretensioning force in the connection to be created, for example with a screw as an articulated connection between two components.
Security elements and standards
- Clamp nuts with plastic insert EN ISO 7040, 7043, 10511, 10512 (old: DIN 982, 985, 6924)
- Hexagon nuts with metallic clamping part EN ISO 7042, 7044, 10513 (old: DIN 980, 6925)
Non-standardized security elements (different effectiveness)
There are a number of different elements, some of which are protected by patents:
- Screws and nuts with shaped elements on the support side (ribbed, toothed or locking edge profile )
- Pair of wedge lock washers
- Lock washer
- Liquid adhesives
- microencapsulated adhesives (see also DIN 267-27)
- special thread profiles (e.g. trilobular )
- self-locking thread inserts
- Nylon patch coating (see also DIN 267-28)
- Limes ring
Wire fuse for aircraft
Since the vibrations on aircraft are very strong, all screws must be secured. A type of fuse that is frequently used in addition to other types of fuse is the wire fuse. A wire is pulled through a locking hole in the screw head and the two wire ends of the same length are twisted and pulled through a locking hole at a fixed point or a second screw head and then twisted again at the ends. The securing must be carried out according to certain regulations, in which the wire guide, the twist direction, number of wire turns depending on the wire diameter and design of the twist ends with at least three and usually a maximum of six turns are specified. The total length of a wire fuse must not exceed 24 inches (609.6 mm). To twist the safety wire, wire twisting pliers are used, commonly simply called twisting pliers or safety pliers .
Screw locking for electrical connections
For electrical connections, conventional screw locks from mechanical engineering cannot be used. The task of creating an electrically safe contact point is not possible with a conventional screw connection and screw locking device over a long period of time.
Often the protective earth screw connections are too short. A resilient element is therefore usually provided. There are so-called smooth and self-locking spring washers for electrical connections. The latter have hook-shaped ends and damage the surfaces when loosened. In the case of electrical connections, they reduce the electrical contact resistance by damaging the oxide layer, with the long-term stability of the contact resistance remaining questionable in the case of non-gas-tight contact points.
In the case of electrical connections (e.g. ground connections), the use of adhesive to secure the fastening screw does not make sense because the adhesive can pull itself into the joints and has an insulating effect there.
Web links
General overviews of the types of screw locking, without considering the effectiveness
- Some screw locking standards
- Construction atlas: screw locking
Individual evidence
- ↑ Hubert Hinzen: Machine elements 1. Oldenbourg Wissenschaftsverlag, 2007, ISBN 3-486-58081-7 , p. 370.
- ↑ Oliver Hartmann: Lufthansa B1 course lesson transcript. (PDF; 378 kB) Module M6 Materials & Hardware. March 9, 2005, p. 17 , accessed January 6, 2013 .