Description

 

Spring washers, sometimes called disc springs, are a subtype of washers. They lend their mechanical capabilities to the unique profile of the material: when subject to a load, 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, eliminate rattle, maintain assembly tension, compensate for expansion or contraction in materials after assembly, or to absorb intermittent shock loads and provide a controlled reaction under dynamic loads.

 

 

Image credit: Wikipedia; Apex Fasteners; Borrelly

 

 

Operation

 

Deflecting under a load, a spring washer is very much like a compression spring. As such, the deflection rate of a wave washer is conditional of the washer's dimensions and materials. Thicker, taller spring washers can sustain more load. When under stress, the raised irregularities of the disc spring flatten to a predetermined degree. Spring washers are either under static or dynamic load. In a static load application, the basic function of the washer is to retain load and the elastic load rating of the washer material may be exceeded. For a dynamic load, the washer functions as a flexing spring and the yield limit must not be exceeded or permanent disfiguration will occur.

 

Spring washers undergo elastic deformation which is reversible and is determined by the material of construction. If the washer is overstressed or near the end of its cycle life, it will experience plastic deformation where some of its deflection is irreversible; this is the first sign of washer fatigue.

 

Spring washers have a small footprint and are valued in small machines where a coil spring would be unsuitable. Spring washers can also deflect in a linear, progressive, or regressive means and these devices show a symmetrical deflection around the washer's axis. Spring washers can be stacked to create spring sets or series where deflection and load capacity can vary without materially affecting the forces involved. 

 

Production

 

Production methods of spring washers include stamping, machining, hot forged, rolled, or some combination of these. The following infographic explains common production protocol for spring washers according to material and application requirements.

 

Table credit: Schnorr Corp.

 

Some spring washers will undergo a slight, acceptable plastic deformation after its first compression and it will not return to its previous height, though the washer is mechanically sound. This is known as 'setting,' and a manufacturer may choose it preset the spring washer in a process called scragging. Other production options include shot peening and adding corrosion resistant coatings to enhance the spring washer's life.

 

Configurations

 

There are several types of spring washers, as well as multiple options for their implementation.

 

Types

 

There are six primary instances of spring washers. Other types may exist as a specialty option through manufacturers.

 

  • Belleville washers can support high loads with small deflections. The load and deflection capability is dependent on height/thickness ratio. These are common in thermal expansion applications. 

Image credit: Seastrom Washers

  • Crescent washer is meant for lighter loads and produces a small deflection. There is a uniform spring rate throughout the washer's deflection. This is used in flexible, load-cycling applications.

  • Dome washers have a very high load capacity with a small overall deflection. While similar in appearance to a crescent washer, a dome washer has ground curves to create a flatter load-bearing surface.

  • Finger washers are notable for their split construction with protruding flanges. They combine the flexibility of a crescent washer with the distributed load points of a wave washer. They are common for noise and vibration dampening instances.

Image credit: Sodemann Industrifjedre

  • Wave washers offer moderate load capacity and deflection, and are typically used as cushions or spacers. These have multiple waves within the washer. More information about this type of washer, as well as a specification filter, are available in GlobalSpec's Wave Washer Selection Guide.
  • Single wave washers are similar to crescent washers but have flattened load points on the washer. This is meant to minimize abrasion against sensitive surfaces.

 

Sets and Series

 

Spring washers, with the exception of finger and single wave washers, are stackable and different permutations exist to vary deflection rates and load capacities. The optimal spring arrangement is the one which uses the least number of washers, which is best achieved by large outside disc diameters. The following video interprets the stacking and series options for all washers--except wave washers--and how they affect washer performance.

 

Video credit: BelleFlex Tech

 

Wave washers, once stacked, produce wave springs which are not subject to series or sets. Instead, deflection and load ratings are based upon how the wave washers are positioned to compress. In-depth coverage of wave washer stacking is available in the Wave Washer Selection Guide.

 

wave spring info, selection

Image credit: Spira Footwear

 

Specifications

 

Washer Dimensions

 

Dimensions of a spring washer are critical to its successful implementation since they are valued for their small size. Like all springs, their dimensions also change depending upon the deflection.

 

  • Outer diameter (OD) is the straight-line width across the washer to its furthest edges.
  • Inner diameter (ID) is the width across the bored hole of the washer.
  • Diameter across slots (DAS) is the measurement of the washer minus the sloped flanges; this is only applicable in belleville washers and may be the same as the inner diameter.
  • Diameter of center rotation (DCR) is distance between the center points of the washer flanges. It is essentially the pitch of the washer.
  • Cone height (CH) is the measurement of the washer between its lowest and highest points while at rest. This should never be more than 40% of the plate thickness
  • Plate thickness (T) is the width of the material used for the washer's construction.
  • Washer envelope is the space allocated for a washer's deflection, as the washer's diameters enlarge when under load.

 

The accompanying graph illustrates how cone height, represented by h, and plate thickness, represented by t, affect a washer's load carrying capacity.

 

Graph credit: Century Spring

 

Spring Washer Material

 

Spring washer operation is dependent on the elastic deformation of it's material, which should have good tensile strength and elastic limits. A high fatigue limit is not a product of the material, but can be enhanced by the use of quality metals with a smooth, carbon-free surface. Young's Modulus is a measurement used to predict a material's load and deflection capability.

 

  • Steel is the most common material for spring washers due to its ease of process and its high tensile strength. Chrome vandium alloy is a higher quality steel with a good through-hardening capability; it is used to make very thick spring washers. Specialty washers offering corrosion resistance include steel-nickel alloys. High-temperature applications incorporate chrome-molybdenum into the material structure to create washers that can operate over 400°C.
  • Copper alloys offer non-magnetic yet electrically conductive washers, along with some corrosion resistance. These alloys have lower strength values than steel.

  • Nickel-chrome and nickel-chrome-cobalt alloys have very high tensile strengths with very low elastic ratios. These washers will have more set if overloaded, but they have extreme longevity and corrosion resistance. They can also operate in a large range of temperatures, from absolute zero to over 700°C. These expensive alloys are difficult to manufacture. 

Spring Rate and Force

 

Spring rate is the amount of load (lb./N) needed to move the spring a predetermined distance (in/mm). Spring force is the measurement of the resisting pressure resultant of a compressed spring washer. Manufacturers will usually specify this in their inventory. Both of these specifications are altered with the use of a spring stack, and spring force is directly dependent of how the load is distributed on the washer.

 

The spring rate and force can be calculated easily with this spreadsheet calculator courtesy of Circlips Australia.

 

Spring Washer Stress and Fatigue

 

The type of stress placed on the spring washer depends on the type of load, be it static or dynamic.

 

Static Load

 

When a washer carries an unchanging load, the spring washer has a permissible stress point. While it should not exceed the tensile strength of the material, it may exceed the yield strength if the consumer accepts the possibility of setting the washer.

 

Dynamic Load

 

Dynamic loads feature varying loads on the spring washer. Dynamic loads are subject to permissible stress as well as critical stress. Permissible stress values of tensile strength and yield strength should not be exceeded, as their operational responsibility is higher than in a static load. Critical stress is the value of stress placed on the concave underside of the washer, which is the first place mechanical distress appears in the form of cracks.

 

Steel-based spring washers can have their fatigue estimated. In Circlips Australia's spring washer calculator, it provides two values for stress labeled as 'Point II' and 'Point III', based on the cycle of deflection. Using the maximum percentage of the washer's deflection, determine the greater value between Point II and Point III. Now determine the value of the same point, but substitute with the washer's minimum deflection. Extrapolating this value on the appropriate graph and comparing it to the cycle line segments will provide an estimate of fatigue.  

 

For thicknesses <1.25 mm: 
For thicknesses 1.25--6.0 mm:

For thicknesses 6.0--16 mm:
Graphs credit: Schnorr Corp.
  
Spring washers of specialty construction will have different fatigue points and the manufacturer should be consulted regarding these washers' fatigue ratings.
 

Friction and Lubrication

 

Friction in spring washers has four origins: (1) The elastic deformation of the material in each deflection; (2) the edges of the washer that act as load-bearing surfaces; (3) the contact of the washer with its guide; and (4) abrasion between washers in parallel stacking. The friction in washer stacks is considerably higher than in a similarly dimensioned coil spring. The level of friction is reliant on the geometry of the washers, the materials used, how the washers are stacked, and the amount of deflection.

 
Lubrication is recommended for disc spring configurations, but care should be taken if the washer has received a protective coating. Oil is the most common lubricant and should be supplied generously. Grease is an option if relubrication can be taxing or irregular. Slip paints provide a permanent and clean friction solution, and add corrosion resistance.
 

Spring Washer Guides (Mounting)

 

Spring washers need a guide, either over a shaft or in a sleeve, to prevent lateral movement. Since both inner and outer diameters increase under deflection, a certain amount of clearance between the washer and guide is required. The table below is relevant for both sleeve and shaft mounted spring washers.

 

 

Spring Washer Hardening

 

Isothermal annealing is sometimes used to help strengthen a steel spring washer. Shot peening can be used for washers carrying dynamic loads to enhance fatigue life by creating compressed dimples in the surface of the washer. This will counteract some internal tensile strength and it will set more than non-shot peened washers. For this reason, surface bonding by means of shot peening is not meant for static loads.

 

Spring Washer Environment

 

Temperature Resistance

 

As temperature increases, the elasticity of the material and the load capacity of the washer decrease. The graph below indicates the performance of the washer under increasing temperature based on the washer's chemistry.

 

Graph credit: Schnorr Corp.

 

Corrosion Resistance

 

Non-steel washer springs are typically produced to prevent corrosion. For steel washers, an abridged variety of corrosion prevention methods are listed below.

 

  • Phosphating incorporates a zinc-phosphate layer and corrosion protective oil. This is sufficient for most applications but can be abrasive.
  • Browning produces an oxidized surface which is coated with oil. While not as good as phosphating, this is used when phosphating's abrasive finish is unacceptable.
  • Metallic coatings like zinc, cadmium, and nickel are all common. Zinc acts as a sacrificial anode that is attacked by corrosive materials before the washer's material. Cadmium provides high-quality protection, but is environmentally and biologically toxic. Nickel can be used as a coating and is resistant to many corrosives but since it acts as a cathode, the base metal can be attacked. For this reason, a nickel finish must always be a dense, non-porous coating.
  • Electroplating can use almost any metal as a surface coating, but hydrogen embrittlement is a major drawback of this method, which leads to a shortened fatigue life.
  • Peen plating uses a peening body along with a metallic powder that will be embedded into the washer in the shot peen process.
  • Metal spray is reserved for large spring washers where regular plating methods are challenging. This results in a thick coating, but is not as adhesive to the washer as other metallic coatings.
  • Chemical plating uses a nickel-phosphor alloy precipitated onto the surface with a chemical process. This has outstanding corrosion and abrasion resistance.

  • Dacromet coating involves zinc and aluminum flakes in a chromatic compound which is baked onto the washer.

Applications

 

Spring washers can be used as a type of lock washer to eliminate rattle or vibration that might unscrew a bolt. These are common in instances where a coil spring would be impractical or too large. Disc springs are common on pipe flanges to combat flange leakage from high temperatures and varying pressure. These small devices are also found in many types of valves and pumps, braking systems, types of couplings, machine presses, aerial cable cars, boiler suspension systems, and overload protection components in electrical transformers. Not until the 1930s did the disc spring become prominent, when formulas to calculate disc springs were developed pre-World War II in the United States. As such, the spring washer was used in anti-tank mines and artillery guns.

 

    

Piezoelectric pump; M4 AT mine. Image credit: Math Works

 

Standards

 

ADS AGS162 - Single spring washers

 

ADS AGS163 - Double spring washers

 

Resources

 

Century Spring - Disc Springs

 

RoyMech - Disc Spring Design

 

Valley Spring - Disc Springs, Belleville Washers UK

 

Circlips - Disc Spring Selection

 

WCL Company - Spring Washers: Descriptions/Designs

 

Schnorr Corp. - Handbook for Disc Springs

 

Bauer Springs - Disc Springs

 

Seastrom Washers - Spring Washers

 

Mubea Disc Springs - Belleville Washers

 

Key Bellevilles, Inc. - Disc Spring Applications

 

 


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