Tower Crane Bolt Loosening Problems Causes, Findings, Prevention and Solutions
(Last Updated On: May 10, 2023)
As a professional company providing bolt solutions, TorcStark often needs to study the connection problems of various types of bolts in various industries. Today, TorcStark will chat with industry elites and readers about the bolt connection of tower cranes.
How is The Tower Crane Installed?
If you want to talk about the bolt connection problem of the tower crane, you should start with its installation process.
Its simple steps are as follows
1. Fixed tower crane foundation 2. Install the standard section of the tower crane 3. Install the climbing bracket 4. Install the rotary 5. Install the slewing tower body 6. Install the cab 7. Install the balance arm 8. Install A frame assembly 9. Hoisting the jib 10. Wear wire rope and hook 11. Add standard section and equipment debugging.
In these 11 steps, each step requires bolts to be connected and fixed. So every little detail can cause a bolt to loosen.
Cause Analysis of Bolt Loosening in Tower Crane
1. In the actual installation, if you do not use a professional torque wrench (hydraulic wrench or electric wrench with torque control). Especially when the workers are working at high altitudes and the operating conditions are not good, it is challenging to apply the actual pre-tightening torque to the bolts. The tightening force of the bolts of the tower crane does not meet the specified pre-tightening force requirements.
2. For the tower lifting operation of the self-elevating tower crane, 4 or 8 connecting bolts must be loosened before each tower crane is lifted. After the jacking is completed and the standard section of the tower body is introduced, the introduced legal section, the lower standard section, and the upper rotary lower support are connected with bolts and are screwed to the bolt connection of the lower standard section. And the tightening operation of the bolt connection with the upper rotary lower support is often not tightened immediately because there is more than one standard joint for jacking and the jacking will continue. In this way, it is possible that after the jacking is finally completed, the connecting bolts of this layer are not tightened.
3. The tower crane frequently lifts and lowers heavy objects during standard work. The boom starts and brakes repeatedly, causing the tower body to bear the positive and negative bending moments and torques that change frequently, resulting in repeated changes in the stress on the connecting bolts of the standard section. This is the root cause of loose bolts.
4. When the tower crane rotates and the direction of the boom changes, the pressure and tension of the contact surface of the standard joint of the tower body change, and the force in the bolt change greatly, especially when the boom is turned to the opposite direction to the jacking and adding joints. The connection contact surface of the standard section on the original boom side changes from tension to compression, and the tension on the bolts is greatly reduced. Especially when the original preload is too much less than the accurate value, there may be no preload. This is another major cause of loose bolts.
5. When the tower ceiling is raised and added, the connection contact surface of the standard section on the boom side is under tension. The standard joint connection contact surface on the balance arm side is under pressure, and under the same tightening torque, the pre-tightening force in the bolts on both sides is equal at this time.
6. The construction unit is very slack in the management of the tower crane. The bolts that were originally equipped with double nuts (a thick stressed nut and a thin anti-loosening nut) were only installed with one nut during installation. The original anti-loosening measures of the bolt connection are gone, and it works under the action of alternating and frequent loads, so it is easy to loosen. There are also operators who are negligent in checking and tightening the bolts. Once the bolts are loose, they are not found to be tightened in time, which will deteriorate the stress of other bolts and cause a large number of bolts to loosen.
7. When the tower crane is working, the vertical force, bending moment, and torque are generated by the dynamic load. Therefore, the requirements for the bolt connection pair must be tightened strictly according to the specified torque, and must not be too tight or too loose. Too tight will cause the bolt to work at or close to the yield strength for a long time, which will cause the bolt to fatigue quickly and cause cracks and fractures. If it is too loose, the relative movement between the bolt and the connecting piece will cause the head of the bolt to bear repeated impact loads and be damaged.
Discovery and Prevention of Bolt Connection Problems in Tower Cranes
1. Use high-strength bolts in tower crane tower sections, slewing supports, lifting arms and balance arms of some models, and fixed feet.
2. The newly installed tower crane should check all the bolts (especially the connecting bolts of the standard section, the upper and lower supports, the connecting bolts of the crane, and the balance arm) when it is running for a week, and it should be checked again after one month.
3. When connecting high-strength bolts, use a torque wrench or a special wrench (wrench with torque control) to tighten according to the assembly technical requirements, and the exposed part of the screw thread should be 23 threads.
4. For the standard joints connected by bolts, the connecting bolts of the standard joints should be able to penetrate smoothly without hammering. After the bolts are tightened according to the regulations, the contact area of the end surface of the main limb should not be less than 70% of the contact surface.
5. The bolts for the slewing bearing should be replaced as soon as they are disassembled and tightened according to the requirements in the manufacturer’s instruction manual.
6. Regularly check and tighten the standard section bolts. According to experience, the standard section bolts should be checked at least once a week. If any looseness is found, use a professional torque wrench to tighten it in time.
7. Use professional tools and solutions to fasten the standard section bolts. Rotate the boom, and tighten the bolts on the compression side of the tower in turn. In daily use, when the tower crane is unloaded, move the trolley to the root of the arm, and check and tighten the bolts on the side of the balance arm from bottom to top. After this side is finished, rotate the boom 180 degrees, and check and fasten the bolts on the other side from top to bottom until it is all completed. After each jacking and adding joint, check and tighten the newly installed standard joint bolts according to the above method.
Solutions for Loose Bolts of Tower Cranes
After the bolts of the tower crane are loose, the bolts should be tightened in time according to the regulations. When tightening, a standard torque should be applied to the bolts to ensure that the bolts will not be cracked or broken.
1. When installing foundation bolts and column bolts, use a torque wrench with torque control to tighten them to ensure that the pre-tightening force of each bolt remains consistent.
2. Carry out detailed inspection, de-rusting, and lubrication of the connecting bolts of the tower body. Abnormal bolts such as cracks, wear, severe corrosion, and overall or local plastic deformation must be replaced.
3. When bolts, nuts, and washers are used together, bolts above grade 8.8 are never allowed to use spring washers to prevent loosening and flat washers must be used.
4. After the ordinary bolts are tightened, 3 threads are exposed.
5. High-strength bolts use elastic washers.
In general, the main reason for the loosening of the bolts of the tower crane is not to add the standard pre-tightening force and to neglect and check the bolt fastening in the later stage. When installing a tower crane, be sure to use a professional torque wrench to apply the specified torque to the bolts, and then the construction personnel should often check the tightening of the bolts. Anti-loosening measures should be applied when tightening bolts. Now, if you have any questions about tower crane bolted connections, welcome to contact us. TorcStark will provide you with bolting solutions for free.