Civil Engineering Reference
In-Depth Information
11.3
Mechanical Characteristics
The mechanical function of the luminaire housing is threefold: it accommodates the
various component parts of the luminaire, such as the optical system and the various
components of the electrical system; it protects these against external influences; and
it provides the means of mounting the luminaire in the installation.
11.3.1
Material
Aluminium alloys, in which other elements have been added to the pure aluminium to
improve its mechanical and physical properties and its chemical protection, are used
in the manufacture of cast, extruded and sheet-metal luminaires. Cast aluminium
refers to the process in which molten aluminium alloy is poured (cast) in a mould.
Extrusion is the process in which softened aluminium alloy is pressed through the
openings of a die. Cast and extruded aluminium alloys are much used in housings for
road, tunnel and floodlight luminaires. They can be employed in humid and damp
atmospheres without having to add protective finishes.
Plastics may be used for complete luminaire housings, for transparent covers,
and for many smaller component parts. All-plastic housings can, of course, only
be employed for light sources that have a relatively low operating temperature. In
the case of LED luminaires it is important to take into account the fact that plastic
is not a good heat conductor. Plastic covers are of methacrylate or polycarbonate.
Methacrylate maintains its high light transmission properties over a long period,
but its impact resistance is relatively low. The impact resistance of polycarbonates
is very high and thus offers a high degree of protection against vandalism. It can
be chemically treated to protect it from yellowing under the influence of ultraviolet
radiation.
11.3.2
Strength
The luminaire and its mounting attachments should be of sturdy construction to
ensure a good, steady positioning of the luminaire and its contents. Any weakness
here could lead to changes in the luminaires light distribution and consequent changes
in the planned lighting quality of the installation as a whole. The most critical part of
a luminaire as far as its strength is concerned are the mounting brackets. The strength
required here is covered by a safety factor: the mounting bracket(s) must be able to
support at least five times the weight of the luminaire itself. With road-lighting and
outdoor floodlighting luminaires, the mounting brackets must also be strong enough
to withstand the highest conceivable wind loading for the location. Here a good
aerodynamic shape for the luminaire can be advantageous, as it also serves to reduce
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