QUALITY CONTROL IN THE TIGHTENING OF BOLTS

A survey was completed in the United States of automobile service managers which indicated that 23% of all service problems were traced to loose fasteners, with even 12% of new cars being found to have fasteners loose. Presented below is information and techniques intended to assist an Engineer in ensuring that threaded fasteners are tightened so that the desired preload is achieved.

Introduction

The drive for improved quality throughout manufacturing industry has had an impact on the assessment of the accurate measurement of assembly line torques. It is no longer sufficient just to run a nut down a bolt until it stops and hope that it is tight enough.

The critical weakness in many products is the region of joints which exist in the design. Bolted joints in particular can be a source of concern for the Engineer. A single bolt, inaccurately or incorrectly tightened, can lead to the failure of the complete product. Too high a tightening torque and the Engineer sustains the risk of a bolt shank or thread stripping failure. Too low a specified torque and the bolt tension can be inadequate to meet functional requirements. Failure to meet the tightening specification can have unfortunate consequences for the reliability of the product. Such failures could occur either during production assembly or during subsequent maintenance on the product after it had entered service. Either is obviously undesirable.

The most prevalent controlled method of tightening threaded fasteners is by tightening so that a specified torque is achieved. This method is generally known as torque control. The major problem related to this method is that the clamp force generated as the result of an applied torque is dependent upon the design of the fastener and the prevailing frictional conditions. Despite these problems, it is still the most popular way of ensuring that an assembled bolt complies with an engineering specification.

Distribution of the Tightening Torque within the Fastener

A major problem with using the torque control method to indirectly control the clamp force in the fastener is that only a small proportion of the torque is actually used to extend the bolt. The majority is absorbed by friction between the nut face and the joint and in the threads. Some fasteners use a nylon insert or have a distorted thread so that a torque is required to run the nut down an unclamped bolt thread. Interestingly, for the same stress in the bolt, the required tightening torque between the prevailing and non-prevailing bolt types varies by less than 4%. The prevailing torque type nut however results in significantly less preload (16%). An example of the distribution of torque within the fastener for a M10 Property Class 10.9 bolt is shown in the piechart. (The chart is from the BOLTCALC program.)

Torque Auditing

Checking that a torque controlled tightened bolt complies to a specified torque is known as torque auditing. Most major manufacturing companies have in-house quality audits on their products which include the checking that threaded fasteners have been tightened so that they conform to the engineering specification.

There are two approaches used to audit installation torque. The first is dynamically, which, by the use of in-line transducers attached to the tightening tool, the installation torque is measured directly. The second approach is by an operator or inspector measuring the torque after the installation has been completed.

Dynamic Torque Auditing

The dynamic method gives results which are independent of operator reading accuracy. Generally powered torque tools are used in conjunction with a computer based data collection system. Because this method allows the automatic storage and retrieval of tightening data it can be an important tool in statistical process control. The disadvantage of the method is that it is generally expensive in terms of capital cost and the technical support skills needed.

Torque Auditing after Assembly

 

There are three basic methods for the checking of torques applied to bolts after their installation; namely, taking the reading on a torque gauge when:

1. The socket begins to move away from the tightened position in the tightening direction. This method is frequently referred to as the "crack-on" method.

2. The socket begins to move away from the tightened position in the un-tightening direction. This method is frequently referred to as the "crack-off" method.

3. The fastener is re-tightened up to a marked position. With the "marked fastener" method the socket approaches a marked position in the tightening direction. Clear marks are first scribed on the socket and onto the joint surface which will remain stationary when the nut is rotated. (Avoid scribing on washers since these can turn with the nut.) The nut is backed off by about 30 degrees, followed by re-tightening so that the scribed lines coincide.

The torque in all three methods should be applied in a slow and deliberate manner in order that dynamic effects on the gauge reading are minimised. It must always be ensured that the non-rotating member, usually the bolt, is held secure when checking torques. The torque reading should be checked as soon after the tightening operation as possible and before any subsequent process such as painting, heating etc. The torque readings are dependent upon the coefficients of friction present under the nut face and in the threads. If the fasteners are left too long, or subjected to different environmental conditions before checking, friction and consequently the torque values, can vary. Variation can also be caused by embedding (plastic deformation) of the threads and nut face/joint surface which does occur. This embedding results in bolt tension reduction and affects the tightening torque. The torque values can vary by as much as 20% if the bolts are left standing for two days.

"Crack-On" and "Crack-Off" Methods of Torque Auditing

With the "crack-on" and "crack-off" methods there are two ways in which the breakaway torque can be checked. The first is manually by having the operator "feel" the point of breakaway. The second is by use of electronics and a strain-gauged torque wrench and recording the breakaway torque automatically. The automatic method removes the operator variability which does occur.

Marked Fastener Method of Torque Auditing

The most consistent and accurate method for the checking of applied torques after the installation has been completed is by the "marked fastener" method. From tests carried out under laboratory conditions, the maximum torque checking accuracy possible, if the above guidelines are followed, is approximately ± 5% of the value actually applied to the nut.

The specification of the tightening torque is of crucial importance in determining the reliability of the joint. Different thread and joint surface finishes all have an effect on tightening torque. Special fasteners, such as those having a prevailing torque, flanged heads, or reduced shanks, also require special consideration when determining the tightening torque. Prevailing torque fasteners use a nylon insert or have a distorted thread so that a torque is required to run the nut down an unclamped bolt thread. For free spinning nuts, the prevailing torque is zero.

Threads Locked by Adhesive

It has been shown by extensive test work that locking the male to female thread by means of adhesive gives the fastened assembly excellent resistance to vibration loosening. The adhesive may be applied in liquid form at the assembly stage, or the threads may have been coated previously with an anaerobic adhesive which cures when the parts have been assembled.

Most thread locking adhesives tend to display a thread friction coefficient which is higher than what is normally present in the threads, also, a slight prevailing torque characteristic will be present. By the correct specification of tightening torque both these effects can be accounted for accurately. Full cure of the adhesive is normally achieved within 24 hours but is dependent upon the finish applied to the fastener, the bond gap and ambient temperature. With the liquid applied adhesive an activator can be used to improve curing time.

Measuring the assembly torque, dynamically, at the assembly stage does not present any problem when using adhesive to lock the threads together. Problems are present however when the assembly torque is required to be checked after assembly. The use of any of the three methods for torque auditing of threads which have been bonded by adhesive does present problems. If the cured bond is broken to check the torque, the vibration resistance of the fastener assembly may be impaired. Secondly, the hardened adhesive in the threads increases the thread's friction characteristics so that on re-tightening less torque goes into achieving preload and more in overcoming friction. Due to these reasons any of the three methods for post assembly torque auditing are unsuitable for threads which have been bonded together by adhesive.

A common method of checking that the specified assembly has been achieved is to re-tighten the joint up to the specified torque whilst checking that the male relative to the female member does not move. The effect of the adhesive is to augment the assembly torque so that the breakloose torque is some 10% to 30% above the value of the assembly torque. Hence if the torque specification was correctly achieved, no rotation of the male relative to the female member should occur at the assembly torque value.

To assist the Engineer in overcoming the problems associated with the use of threaded fasteners and bolted joints, Bolt Science has developed a number of computer programs. These programs are designed to be easy to use so that an engineer without detailed knowledge in this field can solve problems related to this subject.