“Williams” Washers Cannot Support Bending Stresses

  1. Abstract

    “Williams” washers (WW) are excellent when used as spacers or when subjected to purely compressive stresses. When they are used under the head of a bolt or nut, the stresses are never purely compression. When WW experience a tensile stress (imagine a washer bending while used in a slotted hole) they tend to fracture perpendicular to the tensile load: this type of part is expected to fail when loaded in bending.

  2. Discussion

    "Williams” washers are typically made from a free machining steel round bar cut into disks and machined to geometry. One common free machining steel used to make these types of washers is AISI 12L14. The MnS stringers in these types of steels aid machinability yet sacrifice mechanical properties, i.e. strength is not isotropic. These MnS stringers run along the wire/rod drawing direction parallel to the axis (Fig 1). Many machine shops will offer a reduced price for goods that are fabricated from free machining steels as opposed to low carbon steels because tool life improves. Note, WW are commonly case hardened to improve their wear resistance.

    Inclusions in 12L14 steel
    Figure 1: Inclusions in 12L14 steel run parallel to the rolling direction.

    When free machining steels are stressed in tension or compression parallel to the stringers, the steel acts more or less like any other low carbon steel. However, when free machining steels have a tensile stress applied in the transverse direction to the stringers, the strength of the steel is greatly reduced as compared to other low carbon steels lacking these impurities.

    Nuts and bolts rarely apply a purely compressive stress to a washer therefore most washers experience bending stresses of some magnitude when used with fasteners. This is because mating holes can be oversized (Fig 2), slotted, off angle, and/or rough as well as the bearing surface of the fastener typically being smaller than the washer bearing surface. The surface of the washer in contact with the fastener (e.g. bolt head or nut) experiences compressive stresses and the surface in contact with the mating joint member experiences tensile stresses. These bending tensile stresses (Fig 3) act on the WW in the direction perpendicular to the stringers.

    Oversized Hole
    Figure 2: This hole size is exaggerated to highlight how bending loads could be applied to a washer.
    Figure 3: Forces acting on the washer would create a bending load. This would put the washer face in contact with the hole in tension; therefore, cracks tend to initiate from this face of the washer.

  3. Conclusions

    Most commonly when WW fracture it was because they were used in bending. Fracture is not because they were unexpectedly brittle or had “too many” inclusions. These types of washers were never designed nor intended for bending stresses.

  4. Solution

    Washer selection for use in applications such as oversized holes or slotted holes, that induce a tensile force on a washer, requires careful consideration. A low strength steel will have excellent toughness, ductility, and resistance to cracking under bending loads, however, using such a material for a washer may permit the fastener to embed causing plastic deformation of the washer while under the clamp load required to keep the joint tight. Conversely a very strong steel will have excellent compressive properties yet would be brittle which would increase the risk of a cracking. A balance of steel strength to resist deformation must be identified in conjunction with washer geometry as bending stress will reduce as washer thickness increases. Specifics of washer design go beyond the scope of this article, however as a general rule, a washers hardness should be greater than or equal to the fasteners hardness. Contact your fastener consultant for the appropriate washer steel chemistry, mechanical properties, and geometry for your application.

Tyler Olson
Fastener Enthusiast
Polycrew Contributor
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