INNOVATIVE ANCHORING SOLUTIONS
BOLT
Let’s look at three of the reasons: to prevent loosening, to prevent joint slip and to increase fatigue resistance.
3 REASONS
• Prevention of loosening – is critical if the bolt is going to perform its other purposes. There are invasive methods; wire-locking, locking washers; but these tend to impede the other purposes. Pre-loading works well because anything like embedment or vibration that leads to loosening, causes only small changes in the forces on the threads and interfaces. It is those forces that give rise to the friction that prevents rotation of the nut. A fully tightened HS bolt requires no further locking. However these loosening processes also lead to loss of preload so in general we need to be able to check and re-tighten the bolts.
• Preventing joint slip – in many renewable energy applications, friction grip joints reduce the manufacturing problems relating to preventing lateral failure of joints. With the heavy items in use and the need to make or remake joints in difficult environments at low cost, closely fitted means of control such as pins or interference fits are not practical. However friction co-efficient is very variable so sufficient preload to allow for this is important. If you only had the friction to worry about, all bolts would be taken as close to failure as the tightening method error would allow, including yield.
• Fatigue resistance – is a key feature in renewable devices such as wind turbines and wave machines. The high and low frequency variation in the environmental loading usually means that fatigue is a critical design driver. When bolts are loaded in tension, fatigue is important. You can soon spot the slack bolts on a wind turbine tower because they are the broken ones. A correctly preloaded bolt takes advantage of the stiffness of its surrounding structure to off-load the variations, to the point when fatigue is no longer a problem. However such connections will also see extreme loads which far exceed the “fatigue loads”. Preloading takes the bolt closer to its yield limit; excess preload takes it too close and will lead to yielding in near extreme events. Yielding takes the bolt slack, and a slack bolt is a… broken bolt!
When we come back to looking at practical bolt tightening methods, the issues show that we need the following from the “tightening” method:
• We don’t want torque, we want tension. • We need enough pre-tension to prevent fatigue and loosening
• It must be possible to limit the preload to prevent yielding
• We need an ability to check and re-tighten
Steve Gilkes Lloyds Register
www.lr.org
e = See enhanced entry online
TENSION: WHY?
IT IS USEFUL WHEN ASSESSING A BOLTING TECHNIQUE TO REMEMBER WHY WE PRE-LOAD BOLTS AND WHAT THE DANGERS ARE...
www.windenergynetwork.co.uk
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