SPACERS
AND STANDOFFS
DEFINITION:
Spacers
are mechanical devices used to physically or electrically separate board, chassis,
components, and other devices from each other.
They may also be used to locate, hinge, and guide parts in electrical
and mechanical assemblies. (They a re
NOT intended to be used as precision bearings or shafts or as high-precision
jacking or adjusting devices.
SHAPE:
Standard
spacers are offered in ROUND, SQUARE and HEX external forms. ROUND spacers are generally used for
minimum-clearance space requirements, to fit through holes in panels, and as a
general-use shape. HEX spacers are used
primarily in threaded spacers where wrench holding and tightening is
required. HEX spacers are also offered
in threaded styles and unthreaded styles.
ROUND spacers are offered in both clearance and threaded type.
DIAMETERS:
Spacers
are available in diameters ranging from 1/8" to 5/8". Outside diameters are graduated with regard
to internal thread sizes for strength, minimum clearance requirements. In general, a minimum of two thread sizes
are provided for each O.D.
LENGTH:
In
most materials, spacers are offered in.
For the metallic spacers, this is a function of weight vs.
function. Brass and Steel spacers tend
to be unacceptably heavy in the longer lengths, and serve no purpose which
cannot be met by Aluminum.
MATERIALS:
BRASS is the most common, general-purpose material for
high quality spacers. It provides
strength, corrosion resistance, electrical conductivity, and ease of
manufacture and-finishing. It is
non-magnetic and will stand up moderately well to extreme environmental
conditions, including heat and humidity.
ALUMINUM spacers provide a compromise between weight and
strength. They are used extensively in
commercial and the Aerospace Industry and are becoming more popular as size and
portability grow in importance.
Although they cannot be provided in solderable finishes, they offer a
large variety of special colored finishes.
Anodized finishes have insulating characteristics when undamaged, and
can also be provided in black matte, nonreflective surface. Aluminum is non-magnetic and can withstand
severe conditions when properly finished.
STAINLESS
STEEL exhibits all the properties of
Brass with the added feature of exceptional resistance to extreme environmental
conditions. It is the material of
choice for continued outdoor exposure or for use in equipment subject to
corrosive atmospheres, such as salt spray and chemical fumes. It is somewhat stronger than Brass or
Aluminum, but has a higher cost. It is
non-magnetic and non-solderable. Weight
factors for Stainless Steel are similar to those for Brass. Few finishes are available for the Stainless
spacers.
FINISHES:
Various
finishes have been selected as standard for each of the materials
supplied. They represent the most
practical and cost-effective finish for the majority of applications. Alternative finishes are available on
special order in quantity from the factory, including most MIL-Spec
requirements.
ZINC PLATING is provided as standard on Brass spacers. Zinc is bright, non-tarnishing (when
provided with a clear passivate conversion, as we do on all Zinc plating) and
it is solderable. Solderability is
often important for grounding purposes and continuity between printed circuit
boards. Zinc will flow at high ambient
temperatures (350 degrees Fahrenheit).
Zinc has largely replaced Cadmium for these applications since Cadmium
is considered an environmental pollutant.
IRRIDITE is a chemical conversion coating which is applied to
Aluminum to seal and protect the surface.
It produces an oxide coating which prevents further oxidation of the
surface, much as an Anodize does, but at much lower cost. Yellow irridite is the standard finish since
it provides visual proof of its presence.
It is difficult in a batch-dip process like Irriditing to have
uniformity of color piece-to-piece. For
that reason, some prefer Clear irridite which only shows the color of the bare
Aluminum below. Anodizing, either Clear
or in colors, is much harder and more uniform than irridite but at greater
cost.
NICKEL can be plated on Brass and provides a hard, bright,
non-oxidizing surface which will stand up to difficult ambient conditions. Nickel is not easily soldered, however, and
is only offered as a special plating on request. TIN plating (pure Tin, not a solder alloy) provides all of the
characteristics of Zinc with even greater solder ability. Since it is a pure elemental plating and
compatible with finishes applied to printed circuit traces, it is often
specified when the spacer is soldered to the PC trace to prevent contamination. Tinning or Hot-Tin-Dip are not Tin plating
at all, but an electroplate or immersion coating of solder. This plate has a tendency to destroy thread
fits and to darken and oxidize with age and exposure and is not recommended as
a finish on mechanical components.
THREADING:
There
are three possible ways to thread the inside of a spacer, (1) Straight through,
from one end to the other with the thread continuous throughout. (2) Halfway from each end, meeting near the
middle with no continuity of the thread throughout. (3) Only partway in from each end, with no hole through the rest
of the length of the spacer. The method
used on any style of spacer depends on the length and its ratio to the diameter
of the thread tap. Excessively deep
threading is costly and usually unnecessary in spacer applications. Threads are only made continuous where
practical on standard spacers.