Loose Fasteners: Problems and Solutions
Example 1Problem: The base plate of a jaw crusher was fastened to the foundation beam with hex bolts (size M 36, property class 8.8). After the equipment had been in service for a short time, the bolts began to unwind, even though they were tightened correctly.
The clamping range of the bolted joint—the distance from underneath the heat to the first engaged thread—was only 90 millimeters, or only 2.5 Arial the nominal diameter of the bolt. Because this was a very rigid joint, the bolts’ elasticity could no longer compensate for the vibrations of the machine. The vibration then caused minute slips under the bolt head, which triggered rotational loosening of the fasteners.
Solution: Because there was enough space available, spacers with a length of 90 millimeters were applied. This, the total clamping range was extended to 180 millimeters, or about 5 Arial the nominal bolt diameter. This produced a very flexible joint, and rotational loosening was prevented.
Example 2Problem: An appliance manufacturer fastened a motor to a base plate with pan head machine screws (M 5 x 12, property class 4.8). Due to vibrations, these screws quickly unwound and became loose. The clamping range was only 0.8 Arial the nominal diameter of the bolt. However, the product design did not allow the clamping range to 5 Arial the bolt diameter.
Solution: There are two possible solutions. One is to use a thread-locking adhesive. Since the base plate is made of aluminum, a more economical solution is to use thread-forming screws (M 5 x 12). These screws not only prevent loosening, but they eliminate the expense of drilling and tapping holes. Thread-forming screws can be driven directly into molded holes. The firm, play-free thread fit protects the joint against vibration.
Example 3Problem: The guides of a conveyor were fastened to a thermoplastic component. The bolted joints consisted of ordinary pan head machine screws (M 4 x 12) and threaded inserts pressed into the plastic. Due to the constant vibrations, the screws were consistently becoming loose and had to be frequently checked and retightened.
Solution: Applying a thread-locking adhesive will prevent the problem. However, using thread-forming screws for plastics, instead of machine screws and inserts, is a more economical solution. This eliminates the cost of the inserts and the process of installing them. In addition, directly threaded joints are more resistant to vibration.