22.03.2022 | Category : spectrum
The most common screw head drives
It's the profile that counts.
To make a secure bolted connection, it is not only the size of the tool (profile size) that matters – it's just as important to know the most common fastener profiles, their advantages and disadvantages. The following article presents the most important types of fastener heads:
- Slotted-head screw
- Cross-slotted screw head (Phillips recess, Phillips or PH fastener)
- Pozidriv screw head (PZ)
- Internal hex drive (Allen)
- External hex
- Torx® or star screw head (TX)
The flat-slotted screw head is the most "traditional" kind. Right up to the 1930s, it was used more widely than any other type. The fact that many other profiles have superseded it today is mainly due to two major problems of this type of drive:
- Since the slot is quite wide, it is hard to guarantee that the screwdriver is really located in the centre. Insufficient centring, however, means there is a growing danger that the screw will wobble when it is screwed in.
- The risk of the tool slipping is high. In addition, there is only one position to apply the tool, i.e. once you have slipped, you have to pay more attention to reapplying the screwdriver, which costs time and nerves.
The cross-slotted screw
In 1933, J.P. Thompson invented the cross-slotted screw, which effectively eliminated the disadvantages mentioned above. He sold his patent to the Phillips Screws company, which is why the cross-slotted screw system is still called Phillips recess, Phillips or just PH. Common sizes are PH0, PH1, PH2, PH3.
- The cross-slot drive is self-centring, which stops the screw wobbling. But that is not all: the design makes it easier to apply the screwdriver. Thanks to the improved positive locking characteristics, the danger of slipping off sideways is significantly lower than with the slotted screw.
- Due to these positive locking properties, the crosshead screw can also be tightened or loosened by machines.
- The profile tapers to a point: the sides of the screwdriver taper and are not parallel to each other. This design may compensate for misalignments between the fastener head and screwdriver, but when the fastener is tightened or loosened, an axial force is generated at the same time, which can force the tool out of the profile ("cam-out effect").
- The Phillips recess is easily confused with the Pozidriv system (see below). However, the tools are not compatible.
The cross-head drive has been improved over time. Today, there are several different types that can be tightened or loosened safely and easily with power tools and to high torques.
One prominent variant is the Pozidriv system ("PZ"), which is used especially in woodworking ("Spax"). In contrast to the classic cross-head drive, the flanks of the Pozidriv profile do not taper. Instead, this design is provided with additional, much smaller, tapering notches in the centre of the drive, which give it its star-like appearance. Common sizes are PZ0; PZ1; PZ2; PZ3; PZ4.
- Due to the parallel flanks, no significant axial force is generated and the tool sits more securely in the screw head.
- Power transmission is better than with the Phillips drive.
- There is also less wear on the fastener head and screwdriver or bit.
- The Pozidriv is often confused with a normal Phillips – but the tools are not compatible. If the wrong tightening tool is used, the fastener head will be damaged.
Internal hex drive (Allen)
The internal hex drive is better known under the name "Allen". The tool used is a simple hexagonal key. The width across flats is the distance between opposite sides in millimetres (or inches).
- The tool can be centred easily and safely. The fastener can be tightened straight and without wobbling.
- The fastener is suitable even for hard-to-reach places, as the tool is also available as an angled version.
- A powerful notch stress is generated in the corners of the inner hexagon under load, which can cause damage to the tool or fastener.
- The drive is not suitable for large torques because the hexagon socket must be smaller than the nominal diameter of the screw due to the design.
- The drive is unsuitable for outdoor use, as dirt and water collect in the hexagon socket and lead to corrosion.
With the external hexagon, the entire fastener head serves as the drive. The width across flats is the distance between the opposite sides in millimetres (or inches).
- In relation to the screw thread, the usable diameter of the fastener head is greater than with the internal hex drive. This means larger torques can also be applied.
- Hexagonal bolts can be tightened from above with a shell tool or from the side with the aid of a ring spanner or open-ended spanner.
- As a rule, tools for external hexagon fasteners are relatively large and heavy, which can cause problems when working in confined spaces. However, there are correspondingly manufactured light-weight, more manageable and thin-walled spanners available.
- As with the internal hex drive, large notch forces occur at the corners of the flats and this can quickly lead to slight deformations on the drive. As a result, the use of spanners and sockets with extremely low tolerances is recommended. They offer an ideal fit, excellent positive-locking characteristics and perfect force transmission.
The Torx drive eliminates all the disadvantages of the classic Phillips drive. Like the hexagonal drive, it is also available as an inner and outer profile. The Torx drive offers a much better grip than most other drives, there is no axial force or cam-out effect, slipping off is almost impossible.
- The Torx drive offers much better force transmission than most drive types and its parallel profile flanks prevent axial forces from pushing the bit out of the fastener head when tightening.
- Because the force is transmitted over a larger surface and centrally, this drive system involves extremely low wear and can also be used by assembly robots.
- The absence of a cam-out effect protects the fastener head and very high torques can be applied.
- The tightening action can be effected quickly and safely.
There are now even further developed versions of the Torx drive. "Torx Plus", for example, has lobes that are more square to allow higher torques and minimise wear. In this way, drive action is even more positive. In addition, tamper-proof variants of Torx and Torx Plus can be found on the market: "Torx TR" and "Torx Plus TR". "TR" stands for "tamper resistant". A fastener with a TR drive can only be tightened or loosened with a suitable bit.