FEATURE RACKS, CABINETS & ENCLOSURES


GUIDE RAILS – A SIMPLE COMPONENT WITHA DIVERSE RANGE OF POSSIBILITIES


Nicole Jeroschewski product manager atHEITEC EPS discusses guide rails, PCBs and enclosure design G


uide rails are used to hold in place printed circuit boards in 19”


subracks. Additionally, they enable the use of coding pins for coding plug-in units, which prevent boards being inserted in the wrong slots. There are many different configurations for guide rails. The most important aspect of interfacing a board with the front end of a system is the groove width and the fitment of ESD contacts in line with IEC 60297-3-101 and IEC 60297-3-103 standards. However there are several design possibilities. Even small variations in the material


used may contribute to a significant improvement in the sliding characteristics of a guide rail. The material used further influences the structure of a guide rail, its flammability and its temperature resistance. This shows that, even for an apparently simple component such as a guide rail for printed circuit boards, there can be numerous differences in the design from various manufacturers. Guiding and sliding properties and strength can be decidedly influenced by this – along with cost. Among other things, the groove width


of guide rails may vary greatly from different manufacturers, which has a significant affect on their guiding features. Other design features, such as contact springs on Heitec plastic guide rails for example, influence them greatly. Through these pressure springs a printed circuit board may be designed for a low-tolerance specification, independently of the material thickness which may vary from 1.6mm up to 2.7mm. Because the pressure springs on the Heitec guide rails hold the printed circuit board against the reference area on the right-hand side of the guide rail which is required by the IEC 60297-3- 101 standard, thus ensuring a perfect- fit contact of the plug connector on the printed circuit board with the backplane (see detail X Figure 1). The pressure springs have the


additional advantage of balancing out any tolerance and thus guide rails with different groove widths are not needed. With guide rails without these pressure springs for printed circuit boards, the related plug connectors are typically


26 OCTOBER 2017 | ELECTRONICS


not designed for a low-tolerance specification and it may result in the backplane not achieving an optimal contact, which may lead to damage. Anyone who has bent the pin of a backplane when plugging in a printed circuit board knows how irritating it is to have to dismantle the entire system in order to insert a new backplane. A further feature of Heitec plastic guide


rails is the curvature in the centre of the guide rail. It ensures a slight pre- tensioning of the printed circuit board guide as a result of which the printed circuit boards, which are also subject to enormous loads as defined in tests in accordance with EN 50155 and IEC 61587-1, are held perfectly in place and can be optimally guided. The advantage of these design details is evident (see Figure 2). Printed circuit boards may be guided at any time to a low-tolerance specification and in line with standards. In times of increasingly complex


technology the issue of cooling or the dissipation of heat is of great


Figure 1:


Typical construction of a system with a front system, guide rail, printed circuit board and backplane


Figure 2:


Comparison of design details of Heitec guide rails with those of typical versions on the market


significance. It is therefore important to pay attention to good cooling characteristics for each individual component of a 19” system – guide rails are no exception to this. Air flow is significantly improved by incorporating ventilation openings and a “slim” design as a result of which guide rails also contribute to an effective dissipation of the heat in the overall system. Depending on the area of application,


Figure 3:


Overview of Heitec guide rails


size and weight of a printed circuit board, various designs of guide rails are required which also differ in their mounting options. According to the requirements, there are guide rails which can be screwed into place and some which are fixed onto the horizontal rail by a hook – a combination of both is also not unusual. Typical lengths of guide rails are derived from the depth of the printed circuit boards, as determined in the standard IEC 60297-3-101. Heitec covers a broad range of applications with its extensive selection of guide rails (see Figure 3). Lighter boards may generally be designed using plastic guide rails – for simple standard applications for which no ESD protection and no coding is required, in the 2 HP variant without the option of fitting an ESD clip to the guide rails for complex applications in the 4 HP variant with chambers that, among other things, can accept a centring pin from the handle of the front system for the prompt discharge of static charges and with the option of inserting up to two ESD clips and an additional coding option. ESD clips are used in order to discharge electrostatic charges from the printed circuit board. For double ESD protection they are also used for discharging static from the front system. The use of aluminium guide rails is


recommended for particularly heavy printed circuit boards. This is also available in a three-part version with a plastic centre section for insulated guiding of a printed circuit board and better contact and guiding characteristics through the use of the contact spring.


Heitec www.heitec-eps.com T: +49 (0)9126 2934 142


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