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What kind of bolts and nuts never loosen?
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What kind of bolts and nuts never loosen?

Views: 29     Author: Site Editor     Publish Time: 2018-11-16      Origin: www.fuchun-casting.com

Many of the traditional thread locking devices are unable to maintain bolt tension and clamping forces resulting in premature failure and potentially, costly downtime.Bolts and nuts are the most common and common fastening parts in all kinds of equipment, but they are very critical in some situations. Once loosened and dropped, they may cause fatal damage to the equipment.


Why assemblies fail

Threaded assemblies generally fail because of a loss of bolt tension, the main causes of which are relaxation and self-loosening.

Relaxation results in a change in bolt tension which leads to reduced clamping forces. This effect is mostly triggered by settling – pressure-induced smoothing of the surface roughness – or creeping, the time-dependent yielding of substrates when loads exceed the compressive strength of the material; clamping gaskets are a typical example.

If the elastic capacity of the assembly is inadequate, for example, if bolts are too stiff or the wrong length to diameter ratio is selected, there can be no compensation for loss of bolt tension.

Self-loosening is caused by any type of dynamic load, such as vibration or changes in temperature, insufficient clamp load and poorly fitting parts, allowing relative movements to increase the risk of self-loosening.

These load changes lead to short-term frictionless situations where the bolt unwinds from the nut.

The sum of these very small movements ultimately results in the loosening of the threaded assembly.

bolts and nuts

How to prevent the loosening of bolts and nuts is a problem we often face. Ningbo Fuchun Precision Casting Co., Ltd. operates various fasteners all the year round, and summarizes some methods to prevent bolts and nuts from loosening.


Common methods of bolt loosening prevention

Friction anti-loosening, Mechanical anti-loosening, and other methods.


Friction anti-loosening

It mainly includes twin nuts, spring washers, self-locking nuts (one end of the nut is made into a non-circular closure or radial closure after peak opening, the closure is opened after the nut is tightened, the elastic force of the closure is used to make the screw thread tighten), thread glue and other methods.


Mechanical anti-loosening

It mainly includes round nuts, slot nuts with finned washers, open pins, stop gaskets, anti-loosening nuts, etc.

The working principle of anti-loosening nut: it is permanent attachment of special engineering plastics to the thread, making the internal and external threads in the process of tightening, engineering plastics are extruded to produce strong reaction force, which greatly increases the friction between the internal and external threads, and provides absolute resistance to vibration. This resistance is completely distributed in the whole meshing section of the internal and external threads, and the friction force has nothing to do with the tightening pressure between the internal and external threads. Different from the previous stopping method, it relies on the friction force produced by the pressure after the screw tightening to stop the retreat. Once the screw is a little loose and the pressure decreases, the friction disappears rapidly and loses the stopping effect, resulting in the loosening of the screw. However, the shortcoming of plastic loosening prevention is that it can not meet the requirements of high temperature and severe cold because of the environmental impact.


Other methods

Mainly includes end riveting anti-loosening, punching point anti-loosening, spot welding anti-loosening, bonding anti-loosening, series steel wire anti-loosening, anti-loosening rubber anti-loosening.

Anti-loosening and anti-loosening glue is a common anti-loosening method, which can be divided into pre-coated chemical glue and dispensing glue according to its usage; anti-loosening glue, sealing glue and adjusting glue according to its usage; nylon glue and chemical glue according to reaction type. Loctite, ThreeBond, 3M, Nylok and so on are well-known foreign brands in the pine-proof glue industry.


Common methods of nut loosening prevention:

In order to prevent loosening of screw nuts, three mature methods of loosening prevention have been gradually formed after opening various brain holes: friction loosening prevention, mechanical loosening prevention and permanent loosening prevention.

Common mechanical anti-loosening methods include stop washer, open pin and so on. Common anti-loosening friction is generally used gaskets, self-locking, twin nuts and so.



  • What is 'multiple certification'?

    This is where a batch of steel meets more than one specification or grade. It is a way of allowing melting shops to produce stainless steel more efficiently by restricting the number of different types of steel. The chemical composition and mechanical properties of the steel can meet more than one grade within the same standard or across a number of standards. This also allows stockholders to minimise stock levels.

    For example, it is common for 1.4401 and 1.4404 (316 and 316L) to be dual certified - that is the carbon content is less than 0.030%. Steel certified to both European and US standards is also common.

  • What surface finishes are available on stainless steels?

    There are many different types of surface finish on stainless steel. Some of these originate from the mill but many are applied later during processing, for example polished, brushed, blasted, etched and coloured finishes.

    The importance of surface finish in determining the corrosion resistance of the stainless steel surface cannot be overemphasised. A rough surface finish can effectively lower the corrosion resistance to that of a lower grade of stainless steel.

  • Can I use stainless steel at high temperatures?

    Various types of stainless steel are used across the whole temperature range from ambient to 1100 deg C. The choice of grade depends on several factors:

    1. Maximum temperature of operation
    2. Time at temperature, cyclic nature of process
    3. Type of atmosphere, oxidising , reducing, sulphidising, carburising.
    4. Strength requirement

    In the European standards, a distinction is made between stainless steels and heat-resisting steels. However, this distinction is often blurred and it is useful to consider them as one range of steels.

    Increasing amounts of Chromium and silicon impart greater oxidation resistance. Increasing amounts of Nickel impart greater carburisation resistance.

  • Can I use stainless steel at low temperatures?

    Austenitic stainless steels are extensively used for service down to as low as liquid helium temperature (-269 deg C). This is largely due to the lack of a clearly defined transition from ductile to brittle fracture in impact toughness testing.

    Toughness is measured by impacting a small sample with a swinging hammer. The distance which the hammer swings after impact is a measure of the toughness. The shorter the distance, the tougher the steel as the energy of the hammer is absorbed by the sample. Toughness is measured in Joules (J). Minimum values of toughness are specified for different applications. A value of 40 J is regarded as reasonable for most service conditions.

    Steels with ferritic or martensitic structures show a sudden change from ductile (safe) to brittle (unsafe) fracture over a small temperature difference. Even the best of these steels show this behaviour at temperatures higher than -100 deg C and in many cases only just below zero.

    In contrast austenitic steels only show a gradual fall in the impact toughness value and are still well above 100 J at -196 deg C.

    Another factor in affecting the choice of steel at low temperature is the ability to resist transformation from austenite to martensite. 

  • Is stainless steel non-magnetic?

    It is commonly stated that “stainless steel is non-magnetic”. This is not strictly true and the real situation is rather more complicated. The degree of magnetic response or magnetic permeability is derived from the microstructure of the steel. A totally non-magnetic material has a relative magnetic permeability of 1. Austenitic structures are totally non-magnetic and so a 100% austenitic stainless steel would have a permeability of 1. In practice this is not achieved. There is always a small amount of ferrite and/or martensite in the steel and so permeability values are always above 1. Typical values for standard austenitic stainless steels can be in the order of 1.05 – 1.1. 

    It is possible for the magnetic permeability of austenitic steels to be changed during processing. For example, cold work and welding are liable to increase the amount of martensite and ferrite respectively in the steel. A familiar example is in a stainless steel sink where the flat drainer has little magnetic response whereas the pressed bowl has a higher response due to the formation of martensite particularly in the corners.

    In practical terms, austenitic stainless steels are used for “non-magnetic” applications, for example magnetic resonance imaging (MRI). In these cases, it is often necessary to agree a maximum magnetic permeability between customer and supplier. It can be as low as 1.004.

    Martensitic, ferritic, duplex and precipitation hardening steels are magnetic.

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