What Is an Inch Screw?
An inch screw is a type of screw with dimensions standardized in inches.
While metric screws, standardized in meters, are commonly used in Japan, inch screws are prevalent in the United States, the United Kingdom, and Canada.
Uses of Inch Screws
Inch screws are utilized in various applications, including:
- Civil engineering and construction: electrical, plumbing, air conditioning, and piping systems.
- PC components: hard disks, chassis cases.
- Precision equipment: cameras.
- Music equipment: drums, speakers.
They are also used in aircraft manufacturing and imported furniture.
Principles of Inch Screws
The functioning principle of an inch screw is similar to that of a standard screw. It uses its inclined plane to lift heavy objects, generating a significant fastening or axial force. The screw stays tight due to the frictional force on its slope, which exceeds the gravitational force acting on it, thus maintaining the fastening force without loosening.
Types of Inch Screws
Inch screws are primarily classified into two categories: unified threads and wit screws.
1. Unified Threads
Unified threads are standards defined by the American National Standards Institute (ANSI). They come in two types: unified coarse threads and unified fine threads. The fine threads have a smaller pitch than the coarse threads and are less prone to loosening but require more turning during fastening. Metric screws also come in similar coarse and fine types.
2. Wit Screw
Wit screws are considered the world’s first standardized inch screws. Differing from metric and unified screws, which have a 60° thread angle, Witt screws feature a 55° thread angle.
Other Information on Inch Screws
1. Handling Precautions
Handling inch screws requires the same precautions as metric screws. However, extra care is necessary to avoid misuse, such as mistakenly using a metric screw in an inch screw hole. It is challenging to visually distinguish between metric and inch screws; therefore, measuring the outside diameter and pitch with calipers is recommended. If there is difficulty when tightening a screw into an unknown threaded hole, it is advised to measure the screw rather than forcing it.
2. Notation of Inch Screws
Inch screws are typically denoted by the following conventions: Coarse-grained screws are abbreviated as UNC, and fine-grained screws as UNF. The notation includes the screw nominal (thickness), number of threads, standard, and length. An example notation is 1/4-25UNCx5/8. Wit screws are indicated by adding “W” to their notation.