Fine vs Course Threads
Wednesday, 1 November 2023
Fine vs Course threads
In the last 40 years there has been a noticeable shift from Fine threads to Course threads, and rightly so, as Fine threads cannot be said to be technically superior.
In the last 40 years there has been a noticeable shift from Fine threads to Course threads, and rightly so, as Fine threads cannot be said to be technically superior.
Fine threads are still popular in the Automotive Industry, where the original manufacturer has greater control over assembly than existing in other areas.
Fine threads do offer:
However, in practice, most joints are dynamically loaded rather than statically loaded, so Fatigue strength, not Static Strength is the criterion that is critical.
Course threads have better Fatigue resistance because stress concentration at the thread root decreases as thread pitch increases. The argument that Fine pitches offer better resistance to vibration has been obviated by the development of mechanical and chemical locked systems.
Further advantages of Course threads are:
The table that follows summarises the pluses and minuses of Course and Fine threads.
The table also highlights the differences.
Thread stripping is insidiously dangerous in that it is not a sudden failure, and in fact is progressive. The first stressed thread will strip, and as the load transfers to subsequent threads, they will also strip. The progressive stripping may take hours to reach total failure, and until this happens there is no visual warning that the joint is coming apart. The operator has often left the scene well before the joint fails.
The key to preventing thread stripping are:
Fine threads do offer:
- Better accuracy of adjustment.
- Better resistance to vibration induced unwinding, due to the smaller helix angle.
- Higher static strength due to a larger stress area.
However, in practice, most joints are dynamically loaded rather than statically loaded, so Fatigue strength, not Static Strength is the criterion that is critical.
Course threads have better Fatigue resistance because stress concentration at the thread root decreases as thread pitch increases. The argument that Fine pitches offer better resistance to vibration has been obviated by the development of mechanical and chemical locked systems.
Further advantages of Course threads are:
- Less danger of thread stripping
- Thicker coatings, such as hot dip galvanised can be applied to reduce corrosion
- Less prone to damage, and generally quicker and easier to assemble
The table that follows summarises the pluses and minuses of Course and Fine threads.
The table also highlights the differences.
| Functional Properties | Screw Thread | |
| Course | Fine | |
| Static Strength | - | + |
| Dynamic Strength | + | - |
| Locking – unaided | - | + |
| Locking – aided | + | + |
| Resistance to damage | + | - |
| Coating thicknesses | + | - |
| Thread stripping | + | - |
| Ease of assembly | + | - |
| Cost | + | - |
Thread stripping is insidiously dangerous in that it is not a sudden failure, and in fact is progressive. The first stressed thread will strip, and as the load transfers to subsequent threads, they will also strip. The progressive stripping may take hours to reach total failure, and until this happens there is no visual warning that the joint is coming apart. The operator has often left the scene well before the joint fails.
The key to preventing thread stripping are:
- To provide maximum thread engagement; To ensure full thread engagement, a bolt thread should, after tightening, always protrude beyond the nut face by two full threads.
- Tighten bolts within their designed parameters.
- Wherever possible use matched strength bolts and nuts. Be aware that a nut, because of its geometry is inherently stronger than a bolt made of the same material. Matched nuts are not as hard as the bolts they are used with.
- Specify a Course thread.