Loosening fasteners

Loosening fasteners

A survey was completed in the United States of automobile service managers which indicated that 23% of all service problems were traced to loose fasteners, with even 12% of new cars being found to have fasteners loose. Presented below is information and techniques intended to assist an Engineer in ensuring that threaded fasteners are tightened so that the desired preload is achieved

The drive for improved quality throughout manufacturing industry has had an impact on the assessment of the accurate measurement of assembly line torques. It is no longer sufficient just to run a nut down a bolt until it stops and hope that it is tight enough.

The critical weakness in many products is the region of joints which exist in the design. Bolted joints in particular can be a source of concern for the Engineer. A single bolt, inaccurately or incorrectly tightened, can lead to the failure of the complete product. 

Too high a tightening torque and the Engineer sustains the risk of a bolt shank or thread stripping failure. Too low a specified torque and the bolt tension can be inadequate to meet functional requirements. Failure to meet the tightening specification can have unfortunate consequences for the reliability of the product. Such failures could occur either during production assembly or during subsequent maintenance on the product after it had entered service. Either is obviously undesirable.

The most prevalent controlled method of tightening threaded fasteners is by tightening so that a specified torque is achieved. This method is generally known as torque control. The major problem related to this method is that the clamp force generated as the result of an applied torque is dependent upon the design of the fastener and the prevailing frictional conditions. Despite these problems, it is still the most popular way of ensuring that an assembled bolt complies with an engineering specification.