By Julie Miller
Thumb screws offer a hand-adjustment feature, which allows for simple action to fasten, tighten, loosen, or remove the screws. The main benefit is that no tools are needed, so assembly teams and end-user consumers can adjust the screws with thumb and forefinger alone.
Thumb screws are used almost exclusively for the purpose of simplicity. However, they are also ideal for applications in which space is too restricted to allow enough clearance for the use of tools to fasten the screw.
Designers might choose thumbscrews for household, electronic and other consumer products, not just for the ease of fastening but also to avoid the risk of over-tightening. They are particularly beneficial for assemblies that require repeated removal and reinstallation.
When used for removal-and-reinstallation applications, such as safety covers or access panels, thumb screws can reduce the amount of time required for maintenance or inspection. Their hand-adjustment design also reduces the incidence of broken or stripped fasteners that might occur with frequent removal and reinstallation.
Thumb screws are not recommended for structural applications or high-vibration environments, such as automotive, railway or marine vehicles, where cyclical loading might cause them to become loose over time.
A screw drive that features either a tall head, a head with knurled sides, a wing, or a flat-sided vertical head can be categorized as a thumbscrew. Many oversized heads are designed with a spade shape and finished with a diamond-patterned for easy grip, and rounded heads often feature fine knurled ridges.
A shoulder pattern thumbscrew allows the head of the screw to sit flush against the mating surface. Several products include additional hex or screwdriver slots for tool-adjustment to ensure sufficient torque.
Threads can be coarse or fine. Coarse threads perform well with brittle materials or thicker coatings and platings.
Fully adjustable, thumbscrews behave like bolts in most applications and can remain partially engaged with a threaded hole in the substrate when making adjustments. The screws can be coupled with washers as well.
Fastener length and thread size varies considerably, and many thumbscrews are also available in metric sizes. The length is measured from under the head to the tip of the screw. Materials include brass, nylon, steel, stainless steel, aluminum and plastic. For consumer products, designers might choose a particular material based on its aesthetic appeal.