How to Ensure Bolted Joint Integrity When Using a Compression Limiter in a Plastic Assembly
Compression limiters are used to protect plastic components in bolted joints and maintain a threaded fastener’s clamp load by eliminating plastic creep. To function properly, the bearing surface beneath the bolt’s head must extend over the compression limiter to contact the plastic component. If this bearing surface is too small, the host component may not be retained by the bolt, resulting in a poor joint.
There are several methods to ensure sufficient bearing surface under the bolt’s head. These include pairing the compression limiter with a flanged bolt or a washer, or using a headed compression limiter.
The individual component cost, ease or complexity of assembly, and overall cost of each configuration influence which method is best suited for each application.
Ideally, the length of the compression limiter should be equal to or slightly less than the thickness of the host material. The amount of material compressed under the bolt’s head varies depending on the application’s loading and plastic properties. This area of compression must be large enough to withstand forces attempting to pull the assembly apart, yet small enough to allow sufficient plastic compression so that the compression limiter contacts both the bolt and the mating component.
Several factors, including speed and assembly method, must be considered when determining the most cost-effective solution for a specific application. To help engineers make that decision, we ran a series of tests to assess the speed and cost of assembly with compression limiters.
Various fastener combinations were manually assembled to determine approximate differences in efficiency. Assembly with a flanged bolt was the fastest, followed by that with a headed compression limiter, which must be oriented. As expected, the addition of a third component (the washer) significantly slowed the assembly process, requiring twice the assembly time.
When an assembly is automated, it is imperative to ensure the design is as efficient as possible. The addition of a third component, such as a washer, may not be desirable when automating due to feeding and alignment challenges.
Other common factors affecting efficiency include the number of components and ease of orientation. All bolts, headed compression limiters, and some washers require orientation. Due to their short length and relatively low head-to-outer-diameter ratio, headed compression limiters and washers are more difficult to mechanically orient than bolts. Conversely, symmetrical compression limiters do not need to be oriented. An assembly with a flanged bolt only requires one component’s orientation, while assembly with a headed compression limiter or washer requires two components to be oriented.
Use of a headed compression limiter or flanged bolt in serviceable assemblies may be preferable, because there would be no washer that could be accidentally omitted during reassembly. Headed compression limiters are also preferable in applications where there are multiple assembly locations or poor quality control.
Generally, fasteners are the least expensive components in an assembly. For example, an M6 flanged bolt costs approximately $83 per 1,000 pieces, based on an annual usage of 1 million assemblies.
Relative cost differences between bolts and compression limiters vary depending on the supplier and bolt characteristics. Of these three potential combinations, the method with a washer, bolt, and non-headed compression limiter provides the lowest component cost for controlling bearing surface.
However, the cost of the fastening components is often the least significant compared to the overall cost of the assembly. For example, assuming a $50 per hour labor rate, a washer, bolt and symmetrical compression limiter combination costs approximately $67,000 per million assemblies, compared with a total cost of assembly of $101,444 per million assemblies.
Not captured in this analysis are the administrative costs associated with ordering, handling and maintaining an inventory of components. The addition of a third component may increase these costs. Additionally, if the assembly process is automated, the technology required to feed and orient a washer will also increase overall cost. Regardless, a flanged bolt or washer can replace a headed compression limiter in most applications to increase assembly efficiency and lower the overall cost of the assembly.
Ultimately, the best method to ensure adequate bearing surface on the plastic in a bolted assembly depends on the application’s requirements and limitations. A washer may be preferred in lower volume or non-serviced
applications. In high-volume, automated or serviceable applications, a non-headed compression limiter with a flanged bolt is the easiest to assemble and provides the lowest total cost. Both configurations with a washer or flanged bolt will provide a lower cost solution than using a headed compression limiter.