Spacer and Standoff Information
Spacer
Information
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 are not intended to be
used as precision bearings or shafts or as highly precise jacking or
adjusting devices.
Materials:
Brass: Is the most common material for high quality spacers. It
provides strength, corrosion resistance and electrical conductivity.
It is non-magnetic and will stand up well to most environmental
conditions, including heat and humidity. The weight of larger brass
spacers can be a disadvantage in certain situations.
Aluminum: Spacers provide a compromise between weight and
strength.Althoughtheycannotbeprovidedinsolderablenishes,
theycanbeplatedwithspecialcolorednishesbesidesthestandard
clearchromate.Anodizednishes haveinsulating characteristics
whenundamaged,andcanbeprovidedinablackmatte,non-reec-
tive surface. Aluminum is non-magnetic and can withstand severe
conditionswhenproperlynished.
Nylon: Is a general-purpose insulating material for spacers. Molded
threads are precise and will withstand torquing without stripping.
It is an excellent insulator and its surface lubricity allows wires to
beroutedagainstthespacerwithoutfearofchangtheinsulation.
Disadvantages of nylon are relatively low operating temperature,
cold-owunderhigh-compressiveloadingandatendencytoabsorb
up to 2% moisture in high humid ambients.
Phenolic: Are made from paper-base, resin impregnated, heat-cured
materials. High torque should not be applied to these spacers. They
are the insulating spacer of choice when higher temperatures are
encountered than can be withstood by nylon, or where greater
strengthwithoutcold-owisrequired.
Ceramic: SpacersareaGradeL-5Steatitematerial,glazedandred
at high temperature. They exhibit exceptional strength in compres-
sion and tension, but are quite weak in shear and torsion, as are
all glass-type materials. Ceramic spacers will withstand very high
operatingtemperaturesandveryhighvoltageswithoutash-over.
Due to the nature of the manufacturing processes, high tolerances
cannot be met in dimensions or threading, so these spacers should
not be considered as precise devices. Tightening torques on fas-
teners should be held to a minimum and no shear or torsional load
should be applied to spacers in use.
Alternate materials such as Stainless Steel are available in quantity
from the factory. Call for details.
Shape: Standard spacers are offered in round or hex external form.
Other forms can be supplied in quantity as specials from the factory.
Round spacers are generally used for minimum clearance require-
ments and as a general use shape. Hex spacers are used primarily
in threaded spacers where wrench use and tightening are required.
For this reason, hex spacers are only offered in threaded styles, while
round spacers are offered in clearance and threaded types.
Diameters: Outside diameters are graduated with regard to in-
ternal thread sizes for strength, minimum clearance requirements
and footprint area. In general, a minimum of two thread sizes are
providedforeachOD.Forspecicationsnotillustratedinthefol-
lowing pages, contact the factory or your sales representative for
more information.
Length: Standard spacers are generally offered in lengths of 1/8” to
1”. Extended length spacers are available up to six inches in gradu-
ated half and full inch increments in aluminum and ceramic only. For
metallic spacers, this is an issue of weight versus function. Brass and
steel spacers tend to be unacceptably heavy in the longer lengths
and serve no purpose which cannot be met by aluminum. Ceramic
spacers are offered in longer lengths, proportionally sized in OD for
high-voltage and high-compressive requirements.
Finishes:Variousnisheshavebeenselectedasstandardforeach
of the materials supplied. They represent the most practical and cost-
effectivenishforthemajorityofapplications.Alternativenishes
are available on special order in quantity from the factory, including
most MIL-spec requirements.
Threading: There are three possible ways to thread the inside of a
spacer: (1) Straight through with continuous thread throughout. (2)
Halfway from each end, meeting near the middle with no continuity
of the thread throughout. (3) Part way in from each end, with no
hole through the rest of the spacer. The method used on any style
of spacer depends on the length and its ratio to the diameter of the
tap. Excessively deep threading is costly and usually not necessary.
Threads are only made continuous where practical. Please refer to
the Thread Depth Table throughout the next section for standard
parameters. Special depths can be ordered in quantity from the
factory.
Mounting Footprint:Endnishtechniquesofoutsidechamfering
and countersinking reduce the total surface area in contact with the
mounting surface of the spacers, increasing pressure at this surface.
If the area is too small, fastening pressure will cause the spacer
to dig in to the surface, deforming and possibly cracking it. For that
reason, most thread sizes are offered in two different OD spacers to
allow a choice of pressure versus clearance requirements.
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HH Smith