Torx Design

Torx head screws are technically superior to the other designs. (They also look cool).

Torx screw head/drivers were developed by Camcar Textron in the late 1960’s. Torx heads are characterised by a 6-point star-shaped pattern. ISO (International Organisation for Standardisation) specification ISO 10664:2005 – specifies the shape and basic dimensions of the hexalobular internal driving feature for bolts and screws, including the gauging method.

Torx screw are found in engineering, electronics and applications requiring precision and high torqueing abilities. Sometimes related to applications requiring “tamper resistance”, since they are not as widely used as more common types.

Design Considerations

Torx head screws resist cam-out better than other types like Phillips head or slotted head screws.

Phillips head design causes the driver to cam-out, preventing overtightening. Cam-out, while seemingly possitive, risks damage to the driver, screw head, workplace and application. Torx heads are desing to prevent cam-out.

One can understand during the widespread automation of the early 20th Century, cam-out can be beneficial to speed up construction. With the development of torque-limiting devices and automatic tools that precisely measure and stop at the desired torque, this benefit has diminished.

The Torx design facilitates the application of higher torque with a similar sized tool without damaging the head, tool and application compared with other designs.

In a hex head design, if the radial force is too great for the material, the corners will be rounded off and the part/tool may fail. In the Torx design, the internal sidewalls are straight and internal facets angles are much smaller than in the hex design and this is greatly minimised.

Hence for a given torque, the potential for damage is much lower. This allows the head of the fastener to be smaller for the same required torque. This is an advantage where space is limited.

Practically, the Torx design facilitates the application of higher torque on the same fastner before the occurrence of cam-out. Simply put, the Torx head is more precise, avoids cam-out, requires a smaller driver, reduces damage to the tool, fastener and application.

The Downside

There is always a price to pay for good design. But we think this is worth it.

Those unfamiliar with the Torx design have a disproportionate expectation of the necessary torque required to secure the fastener. While not an issue with automotive sized fasteners that most of us are familiar with. But with small precision horological sized screws this can be an issue.

In practice this means those unfamiliar with the required torque, will apply far too much torque to Torx heads. Worst case, the head will be sheared off and/or the thread is damaged – this happened with our case backs from an unnamed Swiss Assembly house. So it can happen to the experts, (we will not use this house ever again).

As a side note, our Compressor Case has been designed to move. That means it gets more water resistant the higher the pressure. This means that all our case/bracelet screws, retaining rings, etc. do NOT need to have the f@#* tightened out of them.