Threads in pneumatics
We must first establish where the thread is located. Threads on a piston rod differ greatly from threads for air supply or a mounting thread for drives.
Threads for the air supply
Small diameters are designed as metric threads. Usually, this concerns the sizes M3, M5 and increasingly also the size M7. Sizes for metric threads are stated in millimetres [mm]. Larger threads are designed as Whitworth pipe threads. These pipe threads are then subdivided into two different designs (G-thread or R-thread). The size on Whitworth pipe threads in stated in inches ["]. Inch threads in pneumatics start with the size 1/8"
G-thread or R-thread?
G-threads have a cylindrical form in accordance with EN-ISO 228-1. R-threads have a conical form in accordance with EN 10226
In the case of a thread size 1/8", for example, the threads are specified as G1/8 or R1/8.
The following rules must be adhered to when using a combination of G and R-threads: Male G-threads (cylindrical) can only be screwed into female G-threads. Male R-threads (conical) can be screwed into female G or R-threads.
What are the dimensions of Whitworth pipe threads? 1 inch equals 25.4 mm. However, this conversion does not apply to pipe threads.
Overview of thread dimensions
| G 1/8"
| G 1/4"
| G 3/8"
| G 1/2"
| G 3/4"
| G 1"
| G 1 1/4"
|| 1 1/4
| G 1 1/2"
|| 1 1/2
| G 2"
Whitworth pipe threads and their features
What does the term Whitworth pipe thread actually mean and why is there a deviation in dimension between 1 inch (25.4 mm) and a G 1" thread (~33 mm)? The Whitworth pipe thread is named after the English engineer Sir Joseph Whitworth. When standardising pipe threads, a 1 inch pipe had an internal diameter of 25.4 mm. However, the walls of the pipe are relatively thick, resulting in an outside diameter of ~33 mm. Therefore a 1 inch pipe thread has an outside diameter of ~33 mm. Thanks to modern materials and production methods, pipes today can be manufactured with far thinner walls. However, because the standardised thread continues to apply as the reference size in pipe manufacture, today's pipes with a 1 inch thread (~33 mm) do not necessarily have an internal diameter of 25.4 mm, but can have a much larger internal diameter. This means that a pipe with a 1 inch diameter is not always 1 inch either in terms of its thread of its internal diameter.
Is there a limit for the thread length?
When using fittings from Festo, this question is irrelevant, as of course our fittings have been adapted to the required conditions. When using external fittings, the length of the fitting thread must be observed. The part of the fitting that can be screwed into a valve, for example, should not exceed a maximum length (Cmax). If you use Festo fittings, the length is guaranteed to be correct.
Overview Thread diameter to thread reach
| C max [mm]|
|G 1/8|| 6.5|
|G 1/4|| 8|
|G 3/8|| 9|
|G 1/2|| 10.5|
|G 3/4|| 12|
|G 1|| 13|
Threads on piston rods
The piston rods on cylinders are generally available in two designs, with either a female or a male thread.
Female threads have a metric standard thread on standard cylinders. Male threads, on the other hand, can also have a metric fine thread. Whether you select a standard or fine thread depends on the size of the thread and the size of the piston rod. Festo supplies for example the DSNU, DNC, ADN, … with a nut on the piston rod if a fine thread is used. This means that the customer does not have to procure a special nut. Of course, lost nuts can be replaced at any time by ordering them as spare parts.
Threads for mounting a drive
Depending on the type, our cylinders have a thread on the outside diameter or, on profile cylinders, they have female threads on the cylinder body. When mounting the cylinder, the following applies: when using a fine thread, certain cylinders such as the standard cylinder DSNU and the cartridge cylinder EGZ, are supplied with a retaining nut.
We don't always need threads …
Festo provides some valves and pneumatic cylinders with integrated push-in fittings.