Preload springs, spacers, and washers are meant to maintain tension in an assembly where some slack may be present. Their capabilities can eliminate rattle, compensate for expansion or contraction of the assembly materials, or absorb intermittent shock loads. These products are made of elastic deformable materials, most commonly convoluted ductile, high-strength metal alloys which come in machined, welded, and open-ring varieties.
All of the preload devices covered in this selection guide lend their mechanical properties to the elastic deformation defined by their materials and dimensions. In preload washers, the raised irregularities of the spring washer flatten to a predetermined degree, while providing proportional resistance. Wave springs, better nominated as wave washers, are subject to axial stress that deflects similarly as spring washers. Preload spacers are unique products that offer axial deflection of up to .07 inches before plastic deformation occurs. As such, their capabilities are reduced to applications with fine tolerances.
Preload springs most often take the form of wave springs, as they can provide the same return force as a compression spring but with a fraction of the footprint. Wave springs come in a variety of examples, which are explained more in-depth in GlobalSpec's Wave Washer Selection Guide and Specification Filter.
Image credit: Smalley
|Preload spacers are adjustable load rings that fit around the shaft of a loose assembly. These spacers eliminate the need for selective shimming or ground spacers. They are available in a large variety of diameters--with load capacities of up to 250,000 lbs.--but also have precise deflections.||
Image credit: Temper Corp.
|Preload washers are commonly available as spring washers, which work similarly to preload springs. Many varieties of spring washers exist, with most of them being suitable for use in dynamic or ill-fitting assemblies. Washers offer the unique capability of being stacked however, which allows the user to change the both the deflection and resistance according to the application. For an unabridged account of spring washers, visit GlobalSpec's Spring Washer Selection Guide and the accompanying Specification Filter.||
Image credit: Wikimedia
Individual specifications for preload washers and preload springs can be found on their individual pages. There are several collective specifications that are relevant to all three products, however.
Inner diameter is the measurement directly across the cavity from inner edge to inner edge in springs, washers, and spacers. This is important when supplying preload springs, spacers, and washers that will fit over a shaft.
Outer diameter is the measurement directly across the cavity from outer edge to outer edge in springs, washers, and spacers. This is especially important when computing a washer's limits
Thickness or working height is the thickness of the product material, or the working height (installed height) of preload spring, spacer, or washer.
This is the maximum or optimal degree of stress the spring, spacer, or washer can attain before breaking or permanently deforming.
Spring rate is the measurement of the load needed to deflect a preload mechanism a predetermined distance.
Preload springs, spacers and washers are made of metallic elements such as aluminum, copper, nickel, and titanium. Sintered bronze, a copper-based alloy, is porous and can be impregnated with oil, graphite, or polytetrafluorethylene (PTFE). Nickel-based alloys contain nickel and one or more other metals, usually copper. Steel, a commercial iron that contains carbon in any amount up to 1.7%, is used in many preload springs, spacers, and washers.
Spring steel contains high amounts of carbon and provides high tensile properties. Suppliers recommend selecting spring steel based on both the application and the type of spring. Common spring steel types range from plain carbon grades to chromium, chromium-vanadium, nickel-chromium-molybdenum, silicon-manganese and silicon-manganese-chromium-molybdenum types. Hardened steel cannot absorb much shock or impact without breaking. Stainless steel is chemical and corrosion resistant and can have relatively high pressure ratings.
The versatility of preload hardware means that it may be employed in environments with corrosive or oxidizing materials. By providing a protective finish, the life span of the spring, spacer, or washer will be extended considerably.
Anodizing is a finishing process for aluminum alloys that consists of a protective oxide coating. It is resistant to corrosion and abrasion, and is typically transparent. This coating is very durable.
Black oxide coating is a special black rust that does not change the workpiece dimensionally. This coating is more aesthetic than functional.
Phosphating is a chemical or electrochemical treatment of the metal surface. It is corrosion resistance, and is suitable carrier for paints.
Galvanization is the process of coating a metal in molten zinc that metallurgically bonds to the substrate. This shields the base metal as well as reacting with corroding agents before the base metal.
Nickel plating provides corrosion and wear resistance, as well as an appealing appearance.
Tin plating covers the workpiece in a ductile layer of tin, which is also corrosion resistant and provides sacrificial anode for non-ferrous metals.
Zinc oxide coatings do not affect the surface integrity of the workpiece and acts as a barrier to further oxidation.
Zinc chromating provides a durable, yellow zinc coating to the material. It has becoming increasingly rare because of its toxicity.
The need to take up slack or play in assemblies is a universal manufacturing requirement. Some specific household and domestic examples where preload springs, spacers, and washers may be found include, but are not limited to: plumbing valves and pumps, pipe flanges, protection components in circuit breakers, lawn tractors and lawn mowers, sprinkler systems, boat motors, vehicle transmissions and braking systems, sewing machines, air compressors, blenders and mixers, and many other simple machines and systems.
Related Products & Services
Spring washers, sometimes called disc springs, lend their mechanical capabilities to the unique profile of the material: the irregularities of the washer compress with a proportionate resistance to return to their predeflected shape. Spring washers are employed in applications where assemblies need a part to take up play, maintain assembly tension, compensate for expansion or contraction in materials, or to absorb intermittent shock loads and provide a controlled reaction under dynamic loads.
Washers are disks of metal or non-metallic material placed beneath a nut, an axle bearing, or a joint, to relieve friction, prevent leakage, isolate, prevent loosening, or distribute pressure.
Wave washers or wave springs are wavy metal washers designed to provide a compensating spring force and sustain a load or absorb shock.