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11/30/2018

Auditing of Fasteners to Prevent Failures

Synopsis


Details

Fastening errors can never be eliminated if proper technique and tools are not used. Audit discussion must begin with an understanding that a process is under control, until such time as variances in expected result are found. That does not imply that there should be a lack of auditing; in fact, the contrary is true.


The critical question must be asked: If proper joint design and proper torque application still do not guarantee that effective tension has been achieved, how can we know if the installed fastener meets the specification intended by the designer?


The answer is in Auditing, a process used to validate, inform and confirm. Depending on how the word is used as a noun or a verb, the term audit has similar definition and meaning.


(n) the inspection or examination of a building or other facility to evaluate or improve its appropriateness, safety, efficiency, or the like: An energy audit can suggest ways to reduce home fuel bills.


(v) to make an audit of (a building or other facility) to evaluate or improve its safety, efficiency, or the like.


Even the most sophisticated fastening systems require torque calibration to ensure their accuracy or repeatability. Therefore, auditing is a necessary interrogatory to gain information about how a job was performed even when the process is being controlled perfectly. Clearly auditing is a preventative quality measure to help eliminate possible failure. Without an audit process we leave open the possibility of errors, failures or other additional costs.


Because of the variation in joints and the difficulty to create exact tension, it must be expected that Torque Auditing also have its pros and cons.


Traditionally there are there three main audit methods. These include first movement in the fastening direction, first movement in loosening direction (breakaway) and determining the rotation angle by marking the fastener; loosening it and determining the torque required to move the fastener back to the original position. While these methods can achieve the intended purpose, they have drawbacks.


First Movement
1. First movement testing in the fastening direction is performed with a hand torque device.  It is necessary to load the fastener and observe the movement. The problem is that it is difficult to perform as first movement is difficult to "feel" with a measuring tool and it is extremely difficult to observe movement. First movement can be done by hand with a dial or digital tool, it can be done with an outboard transducer with angle encoder, but this requires fixturing of the transducer that is often challenging. This method provides an estimate of the current torque but gives no indication of how much movement is required to achieve the desired torque if the current torque is less than the desired torque.  Further, if done incorrectly, it can radically increase the joint load.


Breakaway — Loosening Direction
2. Breakaway is a loosening test of the fastener and as a rule is always less than the actual torque. Breakaway provides data on what it will take to "break loose" a fastener under load. Loosening the fastener is not practical where a system is being audited "live" and loosening the fastener could cause leakage. Further, if done incorrectly, it can cause the joint to be incorrectly tightened, creating more problems than answers. If locking compounds or other joint security devices is used, this is not an effective method of testing, as the audit results will be inconsistent.


Mark, Loosen and Retighten
3. This is a test where the bolt and application are marked with chalk or similar, then the bolt is loosened and retightened to the original marking. This is often not practical and again it gives no indication of how much torque is required to achieve the desired torque if the current torque is less than the desired torque.

The goal of auditing is to improve process results. To do this it is necessary to gather information for evaluation and analysis. An audit that fails to provide relevant information to improve the fastening process is worthless. Sometimes achieving proper torque requires practical experimentation. In order to experiment data is required. Depending on the audit strategy or information desired different techniques should be considered. Torque testers, torque sensors, dial wrenches and digital torque wrenches can be used for the auditing process. quickly and independently.