An examination of how oval compression limiters can be applied to absorb accumulated variation
In multi-fastener plastic assemblies, maintaining precise alignment between components is often more complex than it initially appears. Even when each individual component meets its design tolerance, the combined effect – known as tolerance stack-up – can create misalignment large enough to compromise fastener fit, structural integrity, or long-term joint performance
SPIROL’s recent white paper, ‘Absorbing Centreline and Stack-Up Tolerances with Oval Compression Limiters’, provides a detailed examination of how oval compression limiters can be strategically applied to absorb accumulated variation while ensuring consistent, repeatable alignment across multi-point assemblies.
UNDERSTANDING CENTRELINE TOLERANCE
Compression limiters are used in plastic components to prevent over-compression during fastening and to create stable load paths at bolted joints. When two or more fastening points exist within an assembly, the positional accuracy of each limiter relative to the others – its centreline tolerance – becomes critical. As the white paper notes, “centreline tolerance defines the maximum allowable misalignment between two or more components in an assembly.” If the limiters drift beyond this allowable window, fasteners may no longer align properly with the mating component, forcing rework, assembly scrap, or design changes.
Maintaining tight centreline tolerances is especially difficult in plastic parts, where moulding variation, shrinkage, and warpage can shift hole locations. When these variations accumulate across multiple features, the result is a classic tolerance stack-up scenario.
TOLERANCE STACK-UP
Tolerance stack-up refers to the accumulation of individual dimensional variations from each component in a system. Even when each part is manufactured within specification, their combined variation can exceed allowable positional limits.
SPIROL’s paper raises a key design question: Will all fastening points still align at both the minimum and maximum stack-up conditions? In many assemblies, the answer is no – particularly when using rigid, round compression limiters that offer little accommodation for lateral deviation.
MANAGING Y-AXIS DEVIATION
A design engineer managing a multi-point assembly needed to ensure that no movement occurred along the y-axis. To do this, they used one round compression limiter as a datum, installed first to lock the assembly’s reference position. However, the remaining three fastening points experienced positional variation that exceeded allowable centreline limits.
Round limiters, while excellent for precise alignment, are unforgiving when misalignment occurs. As the white paper explains, “round compression limiters provide precise alignment but do not accommodate stack-up tolerances.” By replacing the remaining three round limiters with SPIROL oval compression limiters, the designer enabled controlled positional compliance where needed while maintaining an accurate datum at the primary fastening point.
THE ENGINEERING BEHAND OVAL COMPRESSION LIMITERS
SPIROL’s CL400 and CL460 oval compression limiters incorporate an elongated geometry that provides 2.25mm of additional clearance along one axis. This built-in compliance enables the assembly to absorb centreline variation without forcing the mating components out of alignment. Oval limiters are manufactured using a rolled-seam forming process that produces minimal scrap and supports high-volume manufacturing. According to the white paper, this results in “significantly lower costs compared to machined, cold headed, or powdered metal parts that can have costly unused material or slower production output.”
In addition to better tolerance absorption, oval limiters maintain the structural support required to protect plastic components during bolt tightening – ensuring joint integrity is not compromised.
STRATEGIC USE IN COMBINATION
One of the paper’s most important insights is that oval compression limiters should not replace round limiters universally. Rather, the strongest designs use both strategically.
Round limiters help maintain overall alignment by restricting excessive float along both axes. Oval limiters provide targeted compensation where misalignment is expected or unavoidable. SPIROL highlights that if too many oval limiters are used, “the entire plastic component could sit too far to one side of the mating component.” The engineered balance between the two shapes ensures structural stability while providing compliance only where needed.
MANAGING X-Y MISALIGNMENT IN A FOUR-POINT MOUNT
A second case study illustrates this strategy (Figures 3 & 4). In a four-point rectangular mounting pattern, misalignment can occur along both the x and y axes. The designer placed one oval limiter diagonally opposite the datum to absorb accumulated variation at the final fastening point.
During assembly, three round compression limiters are fastened first, fixing the component’s global position. Then, any mismatch between the components is absorbed at the fourth location by the oval limiter. This ensures a secure assembly without forcing components into alignment or introducing residual stresses.
ENSURING SUCCESSFUL DESIGN INTEGRATION
The white paper concludes by emphasising that the wide variety of compression limiter geometries makes specialist support essential. SPIROL recommends consulting its Application Engineering team early in the design process to confirm the best limiter configuration for a specific assembly. With nearly 80 years of fastening and joining experience, SPIROL provides design guidance, tolerance analysis, and recommendations tailored to the specific materials, assembly loads, and manufacturing constraints of each application.
Oval compression limiters offer engineers an effective solution for managing centreline tolerances and stack-up variation in multi-point plastic assemblies. By combining selective compliance with structural robustness, they enable more forgiving assembly processes, reduce the risk of misalignment, and improve overall product manufacturability. When used strategically with round limiters, oval limiters provide a cost-effective, high-performance method for absorbing accumulated tolerances without sacrificing the precision required in today’s engineered products.
Contributed by Peter Grant, sales application specialist at SPIROL Canada. www.spirol.co.uk
