Compression Limiters

Proper length specifications of both the Compression Limiter and the plastic component are crucial to the proper performance of the bolted joint. The recommended maximum length of the Compression Limiter is the minimum thickness of the plastic component. This assures that when the proper load is applied to the bolt two critical conditions will be met:

• The bolt will be in contact with the Compression Limiter, eliminating the possibility of creep.
• The plastic host will always have a small amount of compression applied.

The amount of compression on the plastic host will be at most the combined thickness and length tolerances of the two components and the amount of compressive deflection on the Compression Limiter. In reality, with good SPC and production controls, the actual compression will be much less.

SPIROL rates our Compression Limiters by matching the load required to compress the Limiter 2.5% of its nominal length to the clamp load of the nominal sized fastener. See Table.

Compression Limiters are rated by the load required to compress the Limiter to a defined, safe distance that meets the requirements of:

• Maintaining integrity of the Compression Limiter, preventing rupturing or excessive swelling.
• Maintaining the integrity of the plastic host by keeping any localized compressive strains within generally accepted, safe limits.
• Maintaining the load of the fastener by preventing stress relaxation, thereby assuring the continued integrity of the bolted connection.

For all engineered thermoplastic materials used in durable manufactured products, allowing 3%-5% maximum compression has been determined to be a safe, conservative limit. Most plastics are perfectly safe being compressed 5%-7%; many even further. Plastics have the characteristic that they will very quickly exhibit stress relaxation in the areas of high compression eliminating the potential of stress cracking and allowing the Compression Limiter to take the fastener load.

The following design guidelines should be considered when a Compression Limiter is used to ensure its effectiveness in the plastic assembly:

1. Bolt is tightened to 75% of its proof load.
2. Bolt head applies a clamp load on the plastic and bottoms out on the Limiter.
3. Host thickness is greater than or equal to the length of the Limiter, and able to absorb the clamp load.
4. The installed Limiter is aligned with the Insert (when used) to prevent a jack-out condition.
5. Plastic compression ~ 3% - 5%.
6. Limiter is compressed between bolt’s head and mating component.
7. Correct hole size ensures retention of the Limiter.
8. The mating component can withstand compression force generated by bolt.

How does a designer choose between machined or formed Compression Limiters? Formed steel is often the most economical option. However, for short Compression Limiters, particularly when the length to diameter ratio is less than 1, it makes most sense to use a Machined Limiter because it difficult to form a Limiter with a short length and maintain square ends.

Below are guidelines as to how to choose between Machined or Formed Compression Limiters: