In this brief guide, Kevin Moorcroft explains some features of different screw drive types.
Anyone regularly using simple slotted drive screws – with a traditional straight bladed screwdriver, will be aware of its shortcomings, and with a power driver then even more so. The old-style slotted drive will “cam-out” easily, is almost impossible to centre and if the tool or slot is even slightly worn, then is extremely different to install or remove. Consequently, with the need in industry for a faster, more reliable approach a variety of different styles are now available.
The first seems to have been the Phillips – a cross-head design centred in the screw head with a matching drive tool, either manual or powered. The sides of each blade are slightly tapered and rounded so that the tool will rise out of the cross-head rather than break off the head. However, this limits tightening torque which led to the Pozidrive screw with flat sides and a more sophisticated design so as to engage more firmly with the driver, enabling much higher torque values to be achieved.
Importantly from an efficient assembly point of view, the self-centring action of these drive styles greatly aids ease and speed of use, either manually or in automatic feed systems. A more recent development is the six-armed star Torx approach which is a precision parallel-sided design for higher torque and finds great popularity in the motor vehicle industry.
High performance, precision drive
Interestingly, one of the oldest head styles and still one of the highest performing and commonest is the hex drive – with internal socket formed in the head or with external hexagonal shape to accept a socket wrench tool. Internal hex drives, often referred to generically as Allen key drives, are a precision drive with high performance, available in an extensive size range, whereas the traditional external hex head is more common on lower cost bolts and setscrews.
Both offer stability in operation but often need some “help” in centring. This is commonly achieved by providing a spherical end to the drive for an internal hex or a chamfered lead-in on an external hex head.
Machine screws, bolts, self-tappers, self-drilling screws and woodscrews are all available with at least two or three of these drive styles and making the right choice can make or mar an installation or assembly process. In case of doubt, consulting with your supplier is always advised.
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