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zajišťování šroubových spojů

Washers reduce costs in propeller assembly

During the worst shipping crisis in history, Mecklenburger Metallguss reinvented the ship propeller to help the world’s largest container ships sail through the storm.

It’s late in the evening and heavily loaded lorries escorted by police cars, blue lights flashing, move through the sleepy town of Waren, north-eastern Germany. Yet the residents of the quaint little houses aren’t worried; they know it’s just another ship propeller about to leave town. Waren, with some 20,000 inhabitants, is home to Mecklenburger Metallguss GmbH (MMG) – a world leader in the design and production of propellers for large container ships. One of its crea-tions, a 131-ton, huge six-blade propeller for Maersk, holds the world record for the largest ship propeller.

It’s a tricky transport challenge. Hamburg, Germany’s largest sea-port, is more than 200 kilometres away and stopping on the autobahn is time-consuming for the convoy of long and heavy vehicles. Some years ago, a whole A-road, along with two railway tracks, had to be moved as the company grew and propeller transports kept jamming the traffic. So, what is an XXL ship supplier like MMG doing this far from the sea?

“We started manufacturing propellers 70 years ago, as the area was a Soviet occupation zone and the East German ship building industry had to be rebuilt after the war,” explains Jörn Klüss, Head of Design and Construction at MMG and a Waren native. “Back then, ship propellers were a lot smaller. Today, the know-how in the region is what makes us stay.”

CARGO CAPACITY has grown 1,200 percent over the past 40 years. Fifteen years ago, ships moved 5,000 TEU (twenty foot equivalent unit, a standard 6.1-metre shipping container). Today, ultra-large container ships load 22,000 TEU.

However, container demand collapsed during the financial crisis in 2009, and orders for new propellers stopped. “What saved us was our  ‘Retrofit’ programme,” says Klüss, “a new generation of propellers that optimises efficiency in old ships.”

UNTIL THEN, propellers were built for ships operating at the highest possible speed. After the crisis, ships started slow steaming with engines running below capacity to cut fuel consumption. Ships that used to cross the oceans at 25 knots (46.3 km/h) slowed down to 18 knots (33.3 km/h) or less.  “The slower an engine runs, the larger you can go on the propeller,” explains Klüss. “By analysing individual operating profiles, we adapt the number of blades and the diameter to determine the most efficient individual propeller.”

The company also analysed the propeller cap – the part of the propeller behind the blades that protects the steel components of the propeller shaft from seawater corrosion. It created a new energy saving cap (MMG-escap) with a new fin design that straightens the hub vortex, reducing the required torque and preventing wear on the rudder. These innovations made it possible to increase efficiency by up to 10 percent, sav-ing roughly EUR 200,000 for an Asia-Europe voyage.

MANY SHIPPING COMPANIES showed interest in upgrading old propellers with the MMG-escap, which alone increases propulsion system efficiency by up to 3 percent. Conventionally, propeller caps are fitted to the propeller with bolts secured by a chemical locking adhesive. But this requires the ship to dock for at least three days, incurring docking fees of about USD 15,000 per day, plus the cost of removing the ship from its sailing schedule. “We started thinking about attaching the new cap underwater, using divers,” says Klüss. “But that excludes the use of adhesives, which need oxygen to harden. That is how we found out about wedge-locking washers and the inventor of this technology – Nord-Lock.”

HOWEVER, the strictly regulated shipping industry relies on classification societies to ensure safety at sea and define technical standards for ship construction and operation. A ship cannot operate without classification, as it won’t get insurance or freight orders. The Nord-Lock washers hadn’t been tested and approved for use with the special copper alloy used in the propeller cap, so a certification that the washers efficiently secured the propeller cap was urgently needed. Every ship building country has its own classification organisation, and MMG works with all of them. In this case, they contacted DNV GL, one of the largest.

“If the shipping company and classification society agree, you may implement an application and perform a subsequent verification,” explains Klüss. “The Nord-Lock washers already had multiple certifications and DNV GL was motivated to test for another one due to our innovative steel-copper alloy combination.”

THE FIRST UNDERWATER ASSEMBLY of a propeller cap with Nord-Lock washers was carried out in 2014 on a large European container ship. Three divers only needed 1.5 days to do it during regular port time and without docking fees – a success. MMG, DNV GL and Nord-Lock met in September 2016. Less than a year later, the washers had been tested and approved.
“The shipping industry is very conservative,” says Klüss. “But we’ve convinced them of the diver solution’s merits, and Nord-Lock is now our new standard for all bolted joint applications.”

FACTS:
CUSTOMER:  Mecklenburger Metallguss GmbH.
END CUSTOMERS: Shipping companies across the globe, ship yards mainly in Asia.
LOCATION: Waren (Müritz) in Mecklenburg-Vorpommern, Germany.
APPLICATION: Securing a ship propeller cap with Nord-Lock washers instead of adhesives.
NORD-LOCK GROUP SOLUTION: SMO washers for stainless steel bolts.

BENEFITS GAINED:
■ Excellent locking reliability.
■ Reduction of assembly errors thanks to ease of use.
■ Possibility to fix the propeller cap to an existing propeller under water.

“There’s no stock, no waste, and it can be done locally”

26 června 2018
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Text: Richard Orange

fotografie: Simon Van Boxtel

Since its launch in 2013, Dutch 3D printing start-up 3D Hubs has produced over a million parts, making it the world leader in distributed manufacturing. Chief Marketing Officer Filemon Schöffer explains the concept.

What is the thinking behind 3D Hubs?
“The current value chain in which goods are manufactured produces a lot of waste. Many products have been produced in very high amounts for economies of scale, but roughly one third never get sold. What distributed manufacturing can bring is ‘on-demand manufacturing’, so goods are only produced the moment they’re sold. There’s no stock, no waste, and it can be done locally.

“3D printing is a new manufacturing technology, and in that it’s doing really well. The scepticism is based on the consumer market, where few people see anything happening. So that would be my message: it’s in the manufacturing sector.”

So, what benefits does 3D printing bring to manufacturing?
“For injection moulding, the most common process used for manufacturing in China, you need to build a mould first, and that means that there are a lot of up-front costs. Without moulds, 3D printing is very price competitive for small-batch production. It’s also on-demand, so if you upload a file now, we can start producing instantly. And because it’s additive, you can do highly complex geometries.”

Are there any specific industries or applications that benefit?
“3D printing is completely conquering the prototyping market. Then industries that need small-batch production of highly complex geometries, such as prosthetics, hearing aids and dental implants. There are already a lot of commercial aircraft with 3D printed parts in them. Replacement parts are also a very large business, which really leverages the on-demand aspect of 3D printing.”

What are the benefits for the fastener industry?
That’s an interesting case actually. As fasteners, almost by definition, are standardised parts, they are typically not suitable for 3D printing – simply not price competitive. However, in a wider context, from a value chain perspective – 3D printing can offer a lot in terms of replacement parts and ‘ondemand’ turn-around. Especially in the technical service branch, we see 3D printing used
a lot for these benefits, even for standardised parts. This is where I would see fasteners of any kind benefit as well.

FACTS: FILEMON SCHÖFFER
TITLE: Chief Marketing Officer, 3D Hubs, Amsterdam, The Netherlands.
AGE: 32.
BACKGROUND: I’m an industrial design engineer and physicist, so I know a lot about manufacturing, but I’ve always worked in ads and creative campaigning.
LIVES: Near the 3D Hubs office among the start-ups, galleries and hip bars of Amsterdam’s trendy Westerpark district.
PASSION: AFC Ajax.
INTERESTING FACT: Filemon’s ancestor Peter worked with Gutenberg in the 15th century. “3D printing has a lot of potential to localise manufacturing of lots of things, it distributes both skill and know-how, and I think that’s comparable to what the printing press did.”

► Video: Watch how Superbolt uses 3D printing in their research and development

Safe and effective wind turbine maintenance

NEARLY TWO YEARS AGO, US company All Energy Management (AEM) began developing retrofits and training com-panies that service a fleet of 1,000 wind turbines in the UK, the US, Canada and Italy.

When embarking on repair work, it was found that the pins attaching the turbine blades to the rotor were wearing prematurely, along with the rotor holes. Line boring and welding when up on the turbine tower was not possible due to weight and space constraints. The only solution was to replace the rotor and pins, which took roughly 10 working days and cost USD 15,000.

Subsequently, AEM began discussions with Expander about developing a solution that would increase speed, improve efficiency and maintenance safety, and ultimately reduce costs. AEM developed a system to bore the holes out before installing the pins to ensure a reliable connection. Sets of three pivot pins and three different oversized sleeve options were supplied by Expander, which fitted perfectly into the holes depending on the degree of wear.

Fewer parts meant faster and simpler installation, while the Expander System also provided a perfect fit into the borehole, eliminating further movement causing wear. AEM has now been using the solution for over a year and is delighted with the results. “Instead of taking three days with four workers onsite to repair a turbine, it now takes us less than a day with only two workers required,” says Ian Sleger, Operations Manager. “The guys at Expander are really accommodating and the solution has freed us up to concentrate on other matters.”

The Making of Bolts

Bolts are one of the most basic components of engineering and construction, yet their production has become an advanced, high-tech process with multiple steps. Find out how raw steel is transformed into highly specified and exact metal implements.

First published in Bolted #1 2018.

BOLTS can come in a wide range of different sizes and shapes, but the basic production process generally remains the same. It starts by cold forging steel wire into the right shape, followed by heat treating to improve strength and surface treating to improve durability, before being packed for shipment. However, for more advanced bolt designs, the production process can expand by a number of additional steps.

As one of the leading suppliers of fasteners to the automotive industry, Swedish manufacturer Bulten is highly proficient in every step and facet of bolt production. “We do not produce catalogue parts – everything we produce is custom-designed, according to the customer’s specifications,” says Henrik Oscarson, Technical Manager at Bulten’s production plant in Hallstahammar, Sweden. “Depending on where the fastener will be used, there are a number of different options for producing exactly the right bolt.”

COLD FORGING STARTS with large steel wire rods, which are uncoiled and cut to length. The grade of steel is standardised across the industry, according to the requirements of ISO 898‑1. Using special tooling, the wire is then cold forged into the right shape. This is basically where the steel is moulded, while at room temperature, by forcing it through a series of dies at high pressure. The tooling itself can be quite complex, containing up to 200 different parts with tolerances of hundredths of a millimetre. Once perfected, cold forging ensures bolts can be produced quickly, in large volumes, and with high uniformity.

For more complex bolt designs, which cannot be contoured through cold forging alone, some additional turning or drilling may be needed. Turning involves spinning the bolt at high speed, while steel is cut away to achieve the desired shape and design. Drilling can be used to make holes through the bolt. If required, some bolts may also have washers attached at this stage of the process.

HEAT TREATMENT IS a standard process for all bolts, which involves exposing the bolt to extreme temperatures in order to harden the steel. Threading is usually applied before heat treatment, either by rolling or cutting, when the steel is softer. Rolling works much like cold forging, and involves running the bolt through a die to shape and mold the steel into threads. Cutting involves forming threads by cutting and removing steel.

Since heat treatment will change the properties of the steel to make it harder, it is easier and more cost-effective to apply threading beforehand. However, threading after heat treatment will mean better fatigue performance.

“The heat treatment can cause heat marks and minor damage to the bolt,” explains Henrik Oscarson. “For this reason, some customers demand threading after heat treatment, especially
for applications like engine and cylinder head bolts. It’s a more expensive process since you need to form hardened steel, but the threads will maintain their shape better.”

For long bolts, where the length is more than ten times the bolt’s diameter, the heat treatment can have the effect of making the steel revert to the round shape of the original steel wire. Therefore, a process of straightening often needs to be applied.

THE CHOICE OF surface treatment is determined by the bolt’s application and the requirements of the customer. Often, the main concern for fasteners is corrosion resistance, and therefore a zinc-plated coating applied through electrolytic treatment is a common solution. This is a process whereby the bolt is submerged in a liquid containing zinc, and an electric current is applied so that the zinc forms a coating over the bolt. However, electrolytic treatment does come with an increased risk of hydrogen embrittlement. Another option is zinc flakes, which offers even higher corrosion resistance, albeit at a higher price.

WHEN CORROSION RESISTANCE is not an issue – such as inside an engine or an application that is regularly exposed to oil – using phosphate is a more cost-effective option. Once the surface treatment has been applied, standard bolts are typically ready to be packaged. However, more advanced designs may require some additional assembly, such as brackets. Other bolts will also require some form of patching, either a locking patch or a liquid patch. A locking patch consists of a thick nylon layer over the threads, which helps improve grip. A liquid patch will help improve thread-forming torque.

ONCE THESE STEPS are complete, the bolt is finished. Now all that remains is some form of quality control to ensure uniformity and consistency, before the bolts can be packaged and shipped.

THE PRODUCTION PROCESS

1. WIRE
Uncoiled, straightened and cut to length.

2. COLD FORGING
Moulding the steel into the right shape at room temperature.

3. BOLT HEAD
Progressively formed by forcing the steel into various dies at high pressure.

4. THREADING
Threads are formed by rolling or cutting.

5. HEAT TREATMENT
The bolt is exposed to extreme heat to harden steel.

6. SURFACE TREATMENT
Depends on the application. Zinc-plating is common to increase corrosion resistance.

7. PACKING/STOCKING
After quality control to ensure uniformity and consistency, the bolts are packaged.

 

Read more: Top ten tips for secure bolting

SpaceX competition winners chose wedge-locking washers

First published in Bolted #1 2018.

IN 2015, ELON MUSK, the billionaire behind the futuristic transport technology companies Tesla and SpaceX, launched the Hyperloop Pod Competition. It challenges university students to design the best transport pods for the Hyperloop– Musk’s dream where people will travel inside a pod that levitates on its tracks and races at almost supersonic speeds through a giant tunnel
network, which connects the major cities of the world.

During the 2017 competition, the WARR Hyperloop team from the Technical University of Munich was the one that finally raised the laser-sintered titanium trophy. During the competition, they broke a world-speed record for hyperloop pod travel, using Nord-Lock wedge-locking washers to secure each bolt of their pod.

THE 30-STRONG WARR Hyperloop team was divided into several sub-teams to manage areas ranging from CAD design and structure to procurement, finance and marketing. Sub-team leader for CAD design and structure, Florian Janke, says he was inspired by Musk’s vision for a superfast futuristic transport system, and especially the idea that people could one day travel from Munich to Berlin in just 30 minutes.

He says that, “When Musk launched his ‘SpaceX competitions’, I just had to be part of it. We did well in all the stages of the Hyperloop Pod Competition. In the last one, which focused on maximum speed, we achieved 324 km/h (210 mph).”

The WARR Hyperloop team’s lightweight pod smashed the previous 310 km/h (192 mph) record speed set by California-based Hyperloop One, whose pod reached this speed in a 500-metre tube. “There is obviously lots of acceleration and vibration when testing at such high speeds in a relatively short tube – 1.2 km (0.8 miles),” Janke explains. “It was essential that we had secure bolts, so we used Nord-Lock wedge-locking washers, which held the bolts firmly in place. They were perfect.”

The WARR team has registered for the next, third Hyperloop competition, and has already passed the first selection round. While some team members are active in the new, 2018 team, albeit in new roles and positions, most of the them are carrying on with their studies. A few are travelling from trade fair to trade fair showing the 2017 winning pod.

AS THE TEAM worked very closely with a lot of manufacturers in order to get financial backing and various parts, some team members have since had interviews with these companies, and are now considering working there.

Užitečné rady odborníka na šroubové spoje

Bolted got a unique opportunity to meet ­Japan’s foremost expert in bolting, Doctor ­Tomotsugu Sakai. His book Bolted Joint Engineering – Fundamentals and Applications continues to receive an enormous amount of support as the definitive work on bolt fastening.

Poprvé publikováno v magazínu Bolted č. 1 – 2017.

Jak byste definoval ideální spojení, o kterém (mimo jiné) hovoříte ve své knize?
„V ideálním případě by spojování součástí mělo být založeno na použití všeobecně dostupných, standardizovaných spojovacích prvků. Tato varianta je vždy lepší než spoléhání se na speciálně zkonstruované součásti. Co je však ještě důležitější – ideální spojení by mělo být provedeno tak, aby byla vyloučena jakákoli možnost selhání. Dojde-li k jedinému selhání, selhává celý koncept výrobku. Je třeba dbát na každý aspekt. Podle mého mínění je „hodnocení bez jakéhokoli opomenutí“ zcela zásadním.

Je použití maziv při provádění spojů výhodou?
„Ano, za předpokladu, že spojované prvky vůči sobě navzájem neprokluzují, je snížení koeficientu tření vždy výhodou. Pokud se spojované prvky nacházejí v „prostředí vedoucím k povolování“, pak k povolení dojde snáze, je-li koeficient tření nízký. Tato situace však nemusí vést k povolení pokaždé.

V „prostředí vedoucím k povolování“ se prvky nacházejí, pokud jsou opakovaně vystaveny prokluzu vůči sobě navzájem silou přesahující určitou prahovou hodnotu.

Jak způsobují vnější síly prokluz ve směru střihu, v osovém směru a torzním směru?

„Pokud působí síla ve směru střihu, dojde k prokluzu. Pokud působí síla v osovém směru, dojde k oddělení spojovaných prvků. Za těchto podmínek lze říci, že čím menší je koeficient tření, tím pravděpodobnější bude povolení.

Když jsem psal knihu Technologie šroubových spojů – základní principy a použití, vycházel jsem z obecného názoru na jev prokluzu spojovaných součástí vysvětlujícího prokluz na styčné ploše – tzv. „makro-prokluz“. Tento druh prokluzu je identifikovatelný pouhým okem. K jeho potvrzení stačí 0,1 mm posuvu. Kolem roku 1988 však bylo zjištěno, že před tímto „makro-prokluzem“ u spojovaných součástí dochází k tzv. „mikro-prokluzu“. V tomto případě je posuv tak nepatrný, že není možné odhalit ho pouhým okem. Tento jev však postupně snižuje osovou sílu. Mikro-prokluz byl poprvé popsán v žurnálu Japonské společnosti pro přesné technologie.

„Pokud jsou spojované prvky ve vzájemném styku, nedokáží běžné experimentální metody změřit míru prokluzu určité části styčné plochy nebo jiné části. Veškeré tyto hodnoty je však možné spočítat pomocí metody konečných prvků FEM. Tato metoda se začala v odvětví spojovacích prvků prosazovat někdy kolem roku 2000 a v dnešní době ji využívá většina výzkumů závitových spojovacích prvků. Článek doktora Satoshi Izumiho z roku 2006 informoval o tom, že postupné rotační povolování nastává spíše v důsledku mikro-prokluzu (nepatrného, okem neviditelného posuvu) než v důsledku makro-prokluzu (zřetelného, okem viditelného posuvu). Po přečtení tohoto článku jsem byl v šoku. Uvádí se v něm, že v případě opakovaného výskytu mikro-prokluzu dochází k nepatrnému rotačnímu povolování o velikosti 1 stupně na 1000 opakování (neboli 1/1000 stupně při každém mikro-prokluzu).  Jedna tisícina rotačního stupně je rozměrem, který je skutečně okem nepostřehnutelný. Díky metodě konečných prvků však může být tento jev skvěle prostudován. Její pomocí bylo prokázáno, že mikro-prokluz skutečně způsobuje rotační povolování.  Měl jsem pocit, že mám opravdu problém! [smích] Tento článek otřásl mou představou o kritické velikosti prokluzu.
Byl jsem si vědom toho, že mikro-prokluz způsobuje třecí opotřebení. Nemyslel jsem si však, že může vést i k rotačnímu povolení. V té době jsem neměl k dispozici žádný způsob, jak toto tvrzení otestovat. Jednalo se však o zkušenost, která člověku otvírá oči.“

Fakta: Mikro-prokluz
Prokluz, který je okem neviditelný. Postupně snižuje svěrnou sílu a časem může vést k viditelnému rotačnímu povolení (makro-prokluzu). Svěrnou sílu mohou taktéž snižovat jevy, jako jsou sesedání a relaxace materiálu. Společnost Nord-Lock Group uvedla na trh své podložky série X, které řeší obě formy prokluzu. Svým pružinovým efektem působí proti všem druhům ztráty svěrné síly. Jejich technologie závěrného klínu současně brání samovolnému povolování šroubového spoje.

Fakta: Dr. Tomotsugu Sakai

  • 1941 – narozen ve městě Okazaki v Japonsku
  • 1979 – po pracovní zkušenosti ve firmě Toyota Motor Corporation získal doktorský titul v oboru technologie. Zabývá se zejména zkouškami pevnosti a odolnosti a výzkumem a vývojem různých automobilových součástí.
  • 2001 – přechází do společnosti Toyota Techno Service Corp, kde se zabývá zejména vzděláváním a technický poradenstvím v oblasti šroubových spojů.
  • 2007 – odchází do důchodu a zakládá poradenskou kancelář pro šroubové spoje Sakai Consulting Office, kde působí dodnes.

Washers triple-protect nuclear transports

23 února 2018
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Text: Ulrich Schamari

fotografie: ILLUSTRATIONS: Daher Nuclear Technologies

First published in Bolted #2 2017.

THE CHALLENGE

Daher Nuclear Technologies GmbH, located in Hanau close to Frankfurt am Main, Germany, develops containers for transportation of radioactive substances. For obvious reasons, these containers must be extremely safe.

Designing a new container for uranium hexafluoride transports, the company had to consider the very stringent international and national requirements, including the recommendations of the International Atomic Energy Agency (IAEA) for transport by road, rail and sea. A container that fulfils these requirements must, for example, be resistant to the mechanical and thermal loads that can occur in case of an accident.

These mechanical accident loads are defined by a sequence of tests that include a 120-centimetre fall, followed by a 9-metre fall, followed by a fall from 1 metre onto a spike. The container must remain sealed, so that the subsequent thermal test, a fire, doesn’t jeopardise the container’s safety.

THE SOLUTION

Daher set out to design the container locks so that the locking bolts would, under no condition, come loose or be lost during the loading of the container onto a lorry or during transport. The company’s intensive search for the optimal solution led to Nord-Lock wedge-locking washers of type NL16-254SMO. These safety washers are an important component in Daher’s triple-protected locking system: the lock is secured with a bolt, which in turn is locked in position by another bolt. The wedge-locking washers from Nord-Lock are located under the second of these bolts. Each container has six locks and each lock is equipped with a Nord-Lock washer pair.

THE RESULT

Thanks to the use of Nord-Lock wedge-locking technology, the locking systems on the Daher transport container for the nuclear industry can no longer be worn by vibrations or stress, but remain tightly and securely locked. Daher was also pleased to find how cost-effective the use of the Nord-Lock product is, and how easy the maintenance is. If needed, the wedge-locking washers can be replaced at any time to ensure that the transport containers remain in top condition. The containers have a service life of more than 30 years – something that the Nord-Lock washers contribute to.

Odborné poradenství: Zlepšení odolnosti vůči únavě materiálu

Poprvé publikováno v magazínu Bolted, číslo 2 – 2015.

A: Odolnost šroubového spoje vůči únavě materiálu je ve srovnání s jeho odolností vůči statickému zatížení velmi malá. Chtějí-li konstruktéři zvýšit únavovou odolnost, mají dva základní prostředky, jak toho dosáhnout: zvýšení kapacity závitu a snížení střídavého zatížení závitu.

Pro zvýšení kapacity závitu je vhodné používat závity válcované, nikoli řezané. Pro zvýšení kapacity šroubového spoje je lepší použít několik menších spojovacích prvků než jeden větší.

Tuto kapacitu můžete zvýšit také použitím vylepšených spojovacích prvků, jako jsou například vícešroubové předepínací prvky Superbolt nebo matice Flexnut, které zlepšují rozložení zátěže v závitu a šroubovému spoji dodávají elasticitu.

Nejlepším způsobem zlepšení únavové odolnosti je snížení střídavé zátěže vyvíjené na závit. Toho je možné dosáhnout ve třech následujících oblastech: návrh spojovací sestavy, utažení spojovací sestavy a zajištění spojovací sestavy.

Fáze návrhu spojovací sestavy nabízí příležitost zlepšení rozložení zátěže šroubových spojů a snížení míry externího zatížení vyvíjeného na jednotlivé spoje. Je třeba mít na paměti následující principy:

1. Použijte nejvyšší možné předpětí
2. Minimalizujte možnost excentrické zátěže šroubu
3. Použijte největší možné styčné plochy
4. Použijte největší možné svěrné délky
5. Ve většině případů je vhodné použit předpětí vyšší, než je pracovní zatížení

Další možnosti zlepšení únavové charakteristiky ve fázi projektování spoje zahrnují: použití svorníků nebo šroubů se zúženým dříkem, použití elastických podložek, které vyvažují nepříznivé účinky relaxace materiálu, tečení materiálu a prodloužení v důsledku působení rozdílných teplot.

Co se týká utahování sestavy, je nejdůležitějším faktorem snižujícím střídavé zatížení závitu dosažení správné míry předpětí. Doporučujeme používat kalibrované nástroje s vysokou mírou přesnosti. Dále doporučujeme používat vhodný lubrikační prostředek, který umožní dosažení přesné míry předpětí a sníží riziko zadření závitu. Utahování je třeba provádět vhodným postupem a dodržovat při něm předepsanou sekvenci. Tak zamezíte nerovnoměrnému zatížení jednotlivých šroubů a zajistíte celistvost a jednotnost šroubového spoje.

V oblasti zajištění spojovací sestavy je vhodné používat prostředky zajišťující proti ztrátě předpětí. Sestavu je třeba zajistit také vůči působení faktorů okolního prostředí, jako je například koroze, která může vést ke vzniku únavových trhlin. Toho je možné dosáhnout výběrem vhodného materiálu a povrchové úpravy součástí a spojovacích prvků.

 

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