Fitted Shaft Flange Bolts Vital to Safety at Sea
by SKF U.K. Ltd.
The SKF Supergrip bolt provides an accurate fit for propeller shafts enabling them to handle the high turning torques of today's large diesel engine vessels (courtesy of SKF).
In recent years, there have been a number of accidents at sea caused by the failure of conventional propeller shaft flange bolts, where the conventional bolts have broken resulting in the loss of propulsion, and endangering the safety of different vessels. Indeed, figures from the American Bureau of Shipping (ABS) state that propulsion shaft failures in offshore supply vessels and tugs amounted to 66 percent of the total failures, according to a paper recently published by the University of Gdansk, Poland. Contributing factors, such as low speed operation and bad weather, can cause propeller loading to vary widely, meaning that shaft bending may occur because of ship hull deformation. Nevertheless, most of these failures have predominantly been due to the direct drive propulsion systems that have been built into most of the vessels that have been constructed in the last few years. This design means that there is no gearbox and instead a large cathedral diesel engine drives the propeller directly. This does, however, mean that high torque forces are transmitted directly from the engine through the propeller shaft, and thus the associated flange bolts, rather than via traditional gearbox mechanisms.
"This can lead to extremely high levels of torque in the shaft arrangement, with problems typically manifesting themselves when the direction of the engine running is rapidly changed; a situation that often occurs when the vessels are undergoing critical maneuvers and shaft speeds," says Phil Burge, communications manager at SKF. "As a result, torque loading is often distributed unevenly across the conventional flange bolts, so that a small number are subjected to most or all of the applied torque; in some cases, this can mean that the torque loading on individual bolts can more than double as a result of a change in direction of propeller rotation, causing bolts to fracture or sheer."
In an attempt to resolve this problem oversized conventional bolts are cooled down before installation. However, this configuration is particularly difficult to dismount and typically leads to changes in the mechanical and physical characteristics of the bolt materials. One method of preventing shaft failures is through the use of the expandable SKF Supergrip bolt, which uses dedicated tapered bolts and sleeves, and is mounted and dismounted using hydraulic tools and a proven oil injection method. "This technique of torque transmission is being increasingly specified by marine safety organizations, including Det Norske Veritas (DNV), the ABS and Lloyds, to minimize the stresses transmitted through the engine and propeller shaft couplings, thus ensuring correct shaft alignment and torque transmission," Burge says.
The latest generation of SKF Supergrip bolt, for example, is based on the SKF oil injection method developed in the early 1940s. The technology has been advanced over the years to develop different connection systems with new mechanisms to save time and money, while increasing safety across a wide range of applications. "These new SKF Supergrip bolts are easier to install and remove, as well as being more cost effective than conventional bolting solutions, as the torque transmitted is mainly by the shear strength in the bolt, meaning fewer bolts are needed," Burge adds.
The SKF Supergrip bolt is inserted into the hole by hand with a clearance gap and the sleeve is expanded in the hole to eliminate the clearance by pulling the tapered shank into the tapered bore of the sleeve. The bolt is then hydraulically tensioned against one nut while the other nut is tightened manually. A preload is exerted on the bolt by releasing the pressure on the tensioner. Sleeve expansion and tensioning of the bolt are then carefully controlled using a specially developed hydraulic tensioner. This installation technique ensures that the SKF Supergrip bolt provides an accurate fit for propeller shafts enabling them to handle the high turning torques of today's large diesel engine vessels.
This illustration shows how the SKF Supergrip works (courtesy of SKF).
The torque in a coupling is transmitted in two ways-by the shear strength of the expanded bolt in the hole and by friction between the flange faces created by preloading the bolt. Using these sophisticated devices means that loads between the bolts are more uniformly distributed so that the coupling halves are held together considerably more securely. Perhaps most critically, the absence of clearance between the bolt and hole eliminates coupling slippage so there is no risk of bolt seizure or failure, allowing safety to be enhanced significantly.
Removal of the SKF Supergrip bolt is as simple as the fitting operation. Firstly, the preload on the nuts is removed, then oil is injected between the bolt's tapered face and the sleeve's tapered face allowing the SKF oil injection method to force the bolt and sleeve apart. Once apart, the assembly can be removed by hand as the initial clearance seen at installation will once again be present.
"This simple but highly effective technology was developed using specially designed product design and CAD software for 3-D modeling and virtual prototyping. Systematic calculations were made on the SKF Supergrip bolt construction in a variety of scenarios, with particular attention being paid to the difference between having a gap or no gap and different sleeve lengths, in order to understand what was happening and how. Ultimately, the results demonstrated that all the theoretical assumptions were correct," Burge says.
FEM analysis was also carried out, which showed that the bolts can achieve a perfect fit that will handle higher torques than a bolt with a gap, mainly because bolts with a perfect fit will spread the load equally on all the bolts. Indeed, bolts that do not create a perfect fit will result in uneven load distribution, which naturally increases the risk of bolt failures or breakages.
One of the key commercial goals of all vessel operators is to maximize availability while minimizing operating costs. Inevitably, this means keeping vessels at sea for longer, with maximum safety, while also reducing maintenance and overhaul costs. "By using the latest generation of SKF Supergrip bolt, engineers can achieve a perfectly fitted solution to deal with the root causes of many of the accidents that occur at sea, eliminating hazards and improving uptime considerably," Burge says.
For more information:
SKF (U.K.) Limited
Sundon Park Road
Luton, Bedfordshire LU3 3BL
Phone: +(44) 1582 490049