TechArchive

Original article date: July 2000

What’s the best compression limiter for your application, and what points should you bear in mind? Here are a few simple guidelines

Compression limiters protect the plastic components of an assembly from compressive loads generated by bolt tightening. The objective is to keep compressive stresses below the elastic limit of the plastic, preventing unwanted distortion and cracking. In practice, as the bolt is tightened, the plastic compresses and the stress in the plastic increases until the head of the bolt (or washer if one is used) comes into contact with the compression limiter. Thereafter, the compression limiter and plastic will compress at the same rate. The compression limiter will absorb additional clamping loads without further significant compression and increased stress in the plastic material.

There are two basic types of standard compression limiter: split seam and solid knurled. The split seam type is produced from carbon steel in heat-treated and non-heat-treated versions. In the Spirol Industries range, these are designated Series CL250 and Series CL200 respectively. The solid knurled type is produced from brass in symmetrical and headed versions, Series CL100 and CL110 respectively. Since these compression limiters are designed to meet specified proof loads, the dimensional specifications are different. The split seam design has a lead to aid insertion. The solid design has a pilot, allowing it to stand freely in the hole prior to complete installation.

A properly designed bolted joint must meet the following criteria:

• The head of the bolt (or washer) should always seat against the compression limiter under load. This will prevent deterioration of the bolted joint resulting from diminishing clamping load due to plastic creep

• The compression in the plastic component should not exceed its elastic limit, when compressed to seat the bolt against the compression limiter in the worst-case tolerance condition of the assembly

• To ensure that the compression limiter will not yield prior to the bolt under excessive clamping loads, the proof load of the compression limiter should be equal to or greater than the proof load of the bolt

• The clearance between the maximum bolt diameter and the minimum installed inside diameter of the compression limiter should be sufficient to compensate for normal misalignment.

The Inserts and Tubular Products Divisions of Spirol Industries have joined forces to produce a 12-page design guide to assist design engineers in the selection of compression limiters for their plastics components applications.

The recommended maximum length of the compression limiter is the minimum thickness of the plastic component. The compression of the limiter at the recommended tightening torque assures that there will always be some compression on the plastic component.

The recommended minimum length of the compression limiter is the maximum thickness of the plastic component, minus the maximum allowable compression of the plastic component, plus the compression of the compression limiter at the proof load.

A further determination needs to be made to verify that, at the recommended clamping load, the head of the bolt (or washer if one is used) will seat itself against the compression limiter at the maximum tolerance condition.

• Spirol

July 2000