Fastener Retention Overview

There are three methods that can be employed to determine how much tension is exerted on a fastener; using a torque wrench, measuring the amount of stretch, and turning the fastener a pre-determined amount (torque angle). Of these methods, use of a stretch gauge is the most accurate.

It is important to note that in order for a fastener to function properly it must be “stretched” a specific amount. The material’s ability to “rebound” like a spring is what provides the clamping force. You should know that different materials react differently to these conditions, and ARP engineers have designed each fastener to operate within specific ranges.

On the other hand, if a fastener is over torqued and becomes stretched too much – you have exceeded the yield strength and it’s ruined. If the fastener is longer than manufactured – even if it is only .001,˝ it is in a partially failed condition. Therefore, ARP has engineered its fasteners with the ductility to stretch a given amount and rebound for proper clamping.

Heat, primarily in aluminum, is another problem area. Because the thermal expansion rate of aluminum is far greater than that of steel it is possible to stretch a fastener beyond yield as the aluminum expands under heat. An effective way of counteracting material expansion is through producing a more flexible bolt.