Written by Tara B. Meinck | Application Engineer
SPIROL International Corporation, U.S.A.
Plastic compressed by a bolt’s bearing surface area (highlighted in red).
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 be unretained by the bolt, resulting in a poor joint.
There are several methods to ensure sufficient bearing surface under a bolt’s head, including the use of a flanged bolt, washer, or a headed compression limiter.
One important question to ask before assembly: exactly how much plastic should be compressed? Ideally, the length of a compression limiter is equal to or slightly less than the host thickness. The amount of material compressed under a bolt’s head will vary depending on the application’s loading and plastic properties.
However, this area of compression must be large enough to withstand forces that might pull the assembly apart, yet small enough to allow sufficient plastic compression so that the limiter contacts both the bolt and the mating component.
Assembly considerations
Several factors including the speed and assembly method must be considered when determining the ideal and most cost-effective solution for a specific application.
In one example, various fastener combinations were manually assembled to determine the approximate differences in efficiency.
These are the results:
The assembly with a flanged bolt was the quickest, followed by that with a headed compression limiter — which is important to orient properly. As expected, the addition of a third component (a washer) significantly slowed the assembly process, requiring twice the assembly time.
If an assembly is automated, it’s imperative to ensure the design is as efficient as possible. The addition of a third component, such as a washer, might be undesirable in this case because of feeding and alignment challenges.
Other possible factors affecting efficiency include the number of components used and the ease of orientation. Bolts, headed compression limiters, and some washers require orientation. Due to their relatively low-head to outer-diameter ratio and short length, headed compression limiters and washers are more difficult to mechanically orient than bolts.
Conversely, symmetrical compression limiters require no orientation. An assembly with a flanged bolt requires one component’s orientation and those with a headed compression limiter or washer require two components to be oriented.
The use of a headed compression limiter or flanged bolt in serviceable assemblies may be ideal because the washers for such applications could not be omitted accidentally during re-assembly. These are also preferable in applications where there are multiple assembly locations or poor quality-control.
Cost considerations
Typically, fasteners are the least expensive components in an assembly. The following table shows the representative costs for each component combination discussed based on an annual usage of one-million assemblies that incorporate an M6 joint.
The relative cost differences between the bolts and compression limiters vary depending on the component supplier and the characteristics of the bolts. Of these three potential combinations, the method with a washer, bolt, and non-headed compression limiter provided the lowest component cost for controlling a bearing surface.
However, as noted, the cost of the fastening components is generally the least significant compared to the overall cost of an assembly.
This table shows the estimated overall cost analysis of each configuration, assuming it’s $50/hour for labor to assemble one-million components.
What’s missing in this analysis are the administrative costs for ordering, handling, and maintaining the inventory of components. The addition of a third component may also increase these expenses.
If the assembly process is automated, the technology required to feed and orient a washer will also increase the cost. Regardless, a flanged bolt or washer can replace a headed compression limiter in most applications to augment efficiency and lower the overall cost of the assembly.
Always consult an application engineer who specializes in fastening and joining to ensure a properly configured joint is used for each application.
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