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.

 

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