Couplings have been around since the dawn of the industrial age, and even before that, when simple machines like spinning machines and windmills needed to connect rotating shafts. Couplings have been essential to the power generation and shipping industries since the advent of the steam turbine in 1884. As turbines and cargo ships grew larger, the power and torque that needed to be transmitted increased. This placed greater demands on the shafts and couplings—and, in the case of bolted couplings, the bolts themselves.
Bolted couplings
“Couplings are ubiquitous in the power generation and shipping industries, and they haven’t really changed much,” says Martin Walsh, an engineer with more than three decades of experience working with large bolted couplings. “If you look at a coupling from 60 or 70 years ago, the design and concept are pretty much the same as they are today. But the engineering behind it is not. Bolts, in particular, have made couplings better and better, and they can transmit more torque with a smaller coupling size.”
One of the most important functions of a bolted coupling is to keep the shaft aligned. In offshore applications, shafts often run at low speeds, and misalignment can cause vibrations and put unnecessary loads on bearings. In power generation applications, speeds can reach up to 3,600 rpm, and even the slightest vibration or uneven loading can seriously affect the turbine’s ability to operate at full power. As a result, a lot of time and resources are invested in optimizing shaft alignment.
“Once you achieve alignment, you need to bolt it down to maintain alignment during operation,” says Walsh. “In the future, you may unbolt and disconnect the shaft. When you put it back together, it needs to fit perfectly, just as it did when you first put it together, so you don’t lose all the effort you put into aligning it.”
Bolted Connection Solutions
For many years, the most common bolted connection solution was the standard through bolt, which was relatively cheap and practical. The bolt is simply inserted into a threaded hole and tightened with a nut at the other end, creating a friction connection. However, the torque that can be transmitted through friction is very limited, and excessive torque can cause the parts to slip and misalign. The resulting slight movement and uneven loading can cause the bolts and threads to break. This requires reassembling the coupling and re-aligning the shaft.
In theory, a mating bolt inserted into a threaded hole can deliver greater torque capacity due to the torque increase caused by the direct shear force of the bolt cross section. In practice, however, the bolt diameter shrinks during the tightening process, so it is difficult for the bolt to achieve a true fit. This can cause a gap between the bolt and the threaded hole, causing problems such as slippage and bolt failure.
The need to establish and maintain shaft alignment even after the coupling is disassembled and reassembled has led to the increasing use of expansion sleeve bolts. Expansion sleeve bolts expand in the threaded hole, thus ensuring a true bolt fit and a much more even load distribution. This eliminates movement and slippage, and when the expansion sleeve bolt is reinstalled, it automatically realigns the shaft.
Expansion Bolts
“Expansion sleeve bolts are perhaps the biggest advance in precision in the past thirty years,” said Steve Brown, global product manager (expansion bolts) at Nord-Lock. “They have many advantages, such as easy installation, precise fit, easy disassembly and realignment, and they can be restored to concentricity and reused if the hole conditions are suitable.”
Engineering analysis is a key factor driving the advancement of bolted couplings. “Seventy or eighty years ago, engineers were too cautious and couplings and bolts were often over-designed and larger than they needed to be. This was the case in many industries because sophisticated computational and simulation capabilities were not available at the time,” says Walsh.
Today, many OEMs have access to computer modeling and simulation techniques to test the effects of temperature, different materials and operating conditions. Due to the complexity of rotating couplings, finite element methods are increasingly used to identify weak links and torque tolerances in specific installations. Shear tests are successfully used to demonstrate the physical limitations of different bolting solutions.
Walsh continues: “We hope to conduct further analysis to see how the new expansion sleeve bolt design performs compared to the old bolts in transmitting higher torque. Comprehensive finite element analysis is a significant advantage, demonstrating the potential to reduce the number of bolts and reduce the size of the coupling, especially in industries such as wind turbines, where space constraints mean people want to use as few bolted couplings as possible.”
As turbines and cargo ships grow in size and power, and need to transmit higher torques, designing smarter, smaller couplings will continue to be a top priority in the future.
C.A. Parsons and Turbinia
In 1884, British engineer Sir Charles Algernon Parsons invented the steam turbine.
Although the first prototype produced only 7.5 kilowatts of electricity, it showed great potential for power generation and driving ships. In 1893, the Parsons Marine Steam Turbine Company was established and began building the Turbinia test ship to prove the advantages of this new technology.
The new ship was equipped with three axial-flow turbines connected to three shafts, each driving three propellers. The Turbinia was built in 1894 and was the fastest ship in the world at the time, with a maximum speed of 34 knots (63 km/h), while the fastest battleship of the Royal Navy could only reach 27 knots.
In 1897, Turbinia unexpectedly participated in the naval review for Queen Victoria’s Diamond Jubilee, demonstrating her speed and power in front of the royal family and senior naval officers. In the following two years, Parsons’ turbine was adopted by the Royal Navy and soon equipped on passenger ships traveling across the Atlantic.
Sir Charles Algernon Parsons’ design enabled steam turbines to be rapidly scaled up to produce cheap and abundant electricity. In 1899, the first megawatt-class turbine was built in a power plant in Germany. During Parsons’ lifetime, all the major power plants in the world used his invention.
Tightly connected
The need to assemble components by bolting has been around for a long time. For a long time, stud bolts were the standard threaded fasteners. These bolts are inserted into the screw hole and tightened with nuts at both ends to form a friction connection. However, excessive torque can cause slippage and misalignment, causing damage to the bolt and screw hole.
The expansion sleeve bolt is a new invention that can overcome these problems. It expands in the screw hole, ensuring a true bolt fit and a much more even load distribution. It is also easy to install and remove, and is suitable for retrofitting.
