Tension-Indicating Bolts

If you are regular readers of ArchNews, you are probably aware we utilize ultrasonic pulse-echo technology for measurement of in-joint bolt tension. While a very powerful tool for joint development and validation, ultrasonics is not an i deal solution for all applications. The primary barrier is that the need for bolt preparation, specialized equipment and trained personnel tends to focus use to develop or troubleshoot joints rather than to tighten bolts in production or in the field. Several inventions have been commercialized to fill this need for off-the-shelf bolt tension/clamp load measurement. These products generally fall into two categories. The first utilizes an element contacting the bearing surface that indicates compressive loads, such as DTI washers that incorporate ridges that flatten with clamp load. Some add colored silicone in the channels under those ridges that squirts out as the washer flattens to make indication easier. The other technique is to directly indicate changes in bolt length. This is generally accomplished by inserting small diameter pin into a hole drilled through the head and into the bolt shank. As the bolt elongates during tightening the pin moves relative to the head and that movement causes an indicator located in the head to display the tension. While there are several display methods, the one most recently introduced uses a light-absorbing fluid to cause color change at a pre-determined tension. This bolt, called the DTI SmartBolt® was developed and is offered by Stress Indicators, Inc. of Bethesda, MD (www.smartbolts.com).

 

Figure 1 – Tested Tension-Indicating Bolt

We were curious as to how well this design would fill the needs for off-the-shelf bolt tension measurement so we purchased the bolt shown in Figure 1 from a large national industrial supplier. Bolts of 1/2, 5/8 and 3/4 nominal diameter are offered at an individual cost of $19.38 to $22.71. All are modified from Grade 8 hex head cap screws. The test bolt is 1/2-13 x 2 with a rated proof load of 14,500 lb, 85% of the 17,028 lb minimum load specified for a standard bolt. The instructions/specifications included with the bolt list a Design Tension of 11,000 lb selected to be 75% of the reduced proof load. As the bolt is tensioned the color in the indicating window darkens, until reaching the Design Tension at which the window appears black and no longer changes color.

To test the accuracy and repeatability of this system, we mounted the bolt into a 100 kN (22,727 lb) bolt tension load cell. The bolt was installed and tightened as suggested in the accompanying instructions. The head was turned with a box wrench to provide the best opportunity for judging indicator color. The inability to use a socket to both turn and read the head can be a limitation in some applications.

 

Figure 2 – Test bolt in the load cell used as the “control” tension. The nut is captured by tooling on the opposite side.

A technician tightened the bolt until he saw no incremental color change and stopped tightening (Figure 3). He then recorded the load cell tension. The bolt was loosened and the process repeated until ten tests were conducted. Because the total elongation required to generate a given tension in any bolt is influenced by the grip length (the length of the shank not engaged with mating threads) we performed the test at two different grip lengths. The first length was set to the common recommendation of having two threads visible beyond the end of the nut. The second test was conducted at a shorter grip length which left five threads exposed. The test results are summarized in Figure 4.

 

Figure 3 – Tension indicator before tightening (left) and tightening to the Design Tension (right).

Figure 4 – Test Results
Grip Length A – 2 threads exposed Grip Length B – 5 threads exposed
Test # Tension, lb Test # Tension, lb
1 8917 1 10278
2 9142 2 10065
3 9248 3 10503
4 10089 4 10207
5 9284 5 10077
6 9107 6 10538
7 9319 7 10645
8 9923 8 10727
9 9296 9 10633
10 8973 10 10106
N 10 N 10
MEAN 9330 MEAN 10378
STD DEV 383 STD DEV 258
X-3 STD DEV 8180 X-3 STD DEV 9603
X+3 STD DEV 10479 X+3 STD DEV 11153
MIN 8917 MIN 10065
MAX 10089 MAX 10727

The mean difference between the load cell reading and Design Tension of 11,000 lb was 15.2% and 5.6% for grip length A and B, respectively. The accuracy stated by the manufacturer is 10%. The repeatability of the two grip lengths expressed as 3 Std Dev / Mean, is 12.3% and 7.4%.

So, should these be considered good results? In our opinion, a bolt that could provide 10% accuracy in real world applications without the requirement for training or equipment has applicability. The breath of that applicability would obviously be dependant on cost-to-benefit analysis and use factors like robustness, temperature sensitivity, etc. These factors and bolt-to-bolt repeatability were not assessed in this limited test. As the tension reading of this bolt is grip length sensitive and no specific grip length is required by the manufacturer, it doesn’t appear that the Design Tension could be achieved within 10% accuracy in all cases. However, if the test results for grip length B were typical when tightening to a grip length at the limit of a specified range of 1/4” (so that complete coverage is possible by offering standard 1/4” length increments) it would seem that the bolt would be a good solution for certain applications. Products that utilize joint development testing, highly capable assembly equipment and repeatable components can achieve that level of bolt tension accuracy with standard fasteners. However the cost structure to design and assemble products in this manner is often justifiable only for medium-to-high volume and highly engineered products typified by the auto industry. In the more common instances where bolt tension is determined by a torque wrench and an installation torque established by a reference book torque table, the likelihood of getting within 10% of desired tension is somewhere between the proverbial slim and none.

Whenever the ability to achieve desired bolt tension at assembly is critical, maintaining that tension during use will also be important. While testing and advanced assembly processes, ultrasonics, strain gauged bolts and DTI washers can improve tension control at assembly, they can’t provide a simple visual indication of bolt tension after time or load application as the SmartBolt® is capable of. The resolution with which tension loss can be detected is hampered by the non-linear rate of color change, i.e. tension change is easier to detect at lower tension levels than near the Design Tension. Stress Indicators also offers a 2nd generation bolt with an indication transition range that is claimed to occur over a narrower tension range and is more distinctive as it changes color from yellow to green. This bolt was not available from the supplier carrying the tested bolt.

In summary, the DTI SmartBolt® performs the function it claims, though a design grip length and sensitivity to deviation from that length will need to be provided before the accuracy with which it performs that function can be evaluated. There are two applications for which the bolt seems most uniquely suited. First is when tension-critical bolts need to be replaced in the field and corrosion or other conditions change torque-tension characteristics or the advanced assembly processes used during production are no longer available. The second is where joint relaxation or loosening is a problem and visual access to the head is possible.

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