What are the benefits of fine threaded fasteners over coarse threaded fasteners?
The potential benefits of fine threads are:
1. Size for size a fine thread is stronger than a coarse thread . This is both in tension (because of the larger stress area) and shear (because of their larger minor diameter).
2. Fine threads have also less tendency to loosen since the thread incline is smaller and hence so is the off torque.
3. Because of the smaller pitch they allow finer adjustments in applications that need such a feature.
4. Fine threads can be more easily tapped into hard materials and thin walled tubes.
5. Fine threads require less torque to develop equivalent bolt preloads.
On the negative side:
1. Fine threads are more susceptible to galling than coarse threads.
2. They need longer thread engagements and are more prone to damage and thread fouling.
3. They are also less suitable for high speed assembly since they are more likely to seize when being tightened.
Normally a coarse thread is specified unless there is an over-riding reason to specify a fine thread, certainly for metric fasteners, fine threads are more difficult to obtain.
How do you select a fastener size for a particular application?
When selecting a suitable fastener for a particular application there are several factors that must be taken into account. Principally these are:
1. How many and what size/strength do the fasteners need to be? Other than rely upon past experience of a similar application an analysis must be completed to determine the size/number/strength requirements. We can assist you if required!
2. The bolt material to resist the environmental conditions prevailing. This could mean using a standard steel fastener with surface protection or may mean using a material more naturally corrosion resistant such as stainless steel.
The general underlying principle is to minimise the cost of the fastener whilst meeting the specification/life requirements of the application. Each situation must be considered on its merit and obviously some detailed work is necessary to arrive at a detailed recommendation