ISO standard metric threads, as presently designed, are not suitable for non pre-torqued applications. Screws and other threaded components have a relatively high incidence of failure. In most cases this failure is as a consequence of design or assembly faults. These faults often lead first to fatigue cracks and then to failure.
Design rules are available in most National Standards and in addition, manufacturers of screws tend to have good design rules in their handbooks and catalogues. For high tensile and fatigue loaded screw joints, pre-metric screw thread specifications are still valid.
The points below must be considered for thread selection:
* The limit of strength of the screw and load must match
* Torque should be based on the strength, surface treatment and lubrication of the screw
* The size of the contact surface of the screw head and nut
* The right size and hardness of the washer
* Hardness and surface structure under the washer or the screw head
* Tolerances of the angle between the screw and the contact surface
* Stiffness of the connected parts
* Quality of the screw
* Clamping length of the screw.
If the above requirements are met, screwed components usually resist fatigue loads. It is significantly more complicated when non pre-torqued threaded components are exposed to fatigue load. In particular, components containing M-threads, both coarse and fine pitch, tend to present significant problems.
A number of failures have occurred on threaded bars for chain mountings, hook shafts and similar equipment. These components are predominantly subjected to a pulsating load.
Even if the component is in low tension, the crack starts from the bottom of the thread and proceeds deeper and deeper into the material until the fracture occurs. The broken element in this case is a large part fatigue area and a small part final tensile area. In circumstances such as this, analysis of the failed component will show that the failure mechanism was predominantly attributable to fatigue. This indicates that the thread creates a very large tension concentration factor.
But what is the reason for the design of the bottom of an ISO M fine-pitched or coarse-pitched thread? Has the Standard been developed completely without regard to tension concentration factors? Or are there other strong reasons for having sharp edges instead of a well-rounded bottom section to the thread? These questions and the recorded failures point to a problem which can be reduced by changing the design of the bottom of the thread. Failures are common in both male and female threads. It is evident that ISO standard M-threads, as presently designed, are not suitable for non pre-torqued applications. A number of Swedish steel factories have, as a result of the experiences quoted above, concluded that they will not accept M-threads in hook shafts and similar equipment.
Thread to DIN 15403 is recommended for hook shafts with diameters of 50mm and greater. This is a round thread which gives much better fatigue resistance. For small dimensions, UNC and Whitworth threads used to be specified which at least had a defined bottom radius.
