Coarse and Fine Thread trivia

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.

Just thought I'd throw this out for interest's sake.

1/2-13, preload (lb)= 15,450, bolt head torque 85 ft-lb, nut torque 95 ft-lb

1/2-20, preload = 17,400, bolt head torque 95, nut torque 105

The advantage is that you can torque the fine thread a little bit higher than the coarse thread without exceeding the yield point of the material, and then achieve a higher clamping force. Of course each individual situation will be different based on whether there are washers, coatings, lubricants and so on. In the case of using fine threads for joints with gaskets, it is easier to attain the needed clamping forces to satisfy the gasket seating and maintenance factors for proper sealing.

Wolfie