One of the biggest challenges when working with bolted joints is achieving
a precise, accurate preload.
Inadequate preload is a common cause for bolted joint failure, which is why
it’s so important to use the appropriate bolt tightening methods. In this blog
post, we’ll look at some of the most common bolt tightening methods and
their advantages and drawbacks.
1. Torque control
The most common method of managing preload,
with calibrated torque wrenches allowing operators to hit the desired
torque value
Once a bolt is tightened, the shank experiences both direct and torsional
stress, which needs to be taken into account when calculating total stress
and its relation to yield. The prevailing industry wisdom is total stress should
be 90 percent of yield.
A major drawback of using this method: small changes in frictional conditions can lead to more significant
shifts in bolt preload, as most of the torque in play is used to counter friction. Friction stabilizers can diminish
this effect, and using flanged nuts and bolts can help improve accuracy.
2. Angle control
Also known as the “turn of nut” method, angle control was first introduced in the years after World War II for
applications requiring a certain tightening angle. It’s a method that’s been used with power wrenches, where
the bolt is tightened to a preset angle past the elastic range, leading to a small difference in preload due in part
to the yield stress tolerance.
One disadvantage of this method is determining the precise angle, which should be done through
experimentation. Note that fasteners can only withstand a few reapplications before they wear out.
3. Yield control
This technique can lead to highly accurate preload by keeping friction and scatter to a minimum. This method
takes its inspiration from the “sense of feel” an operator would have when using a wrench, giving them a
precise sense of the fastener’s yield point.
This method replicates that feel. As the bolt is tightened, the torque wrench senses a change in slope and halts
tightening as the yield point is reached. One downside to using this method is the cost of the tools, as a wrench
with the control circuitry needed is more costly than conventional wrenches.
4. Bolt stretch
This method is used to tighten bolts that have diameters greater than 20
mm, as these bolts need an extremely high tightening torque.
This method employs a hydraulic ram placed over the nut and a threaded puller which attaches to the ram. The
threaded section of the bolt should extend past the nut for this method to succeed.
5. Heat tightening
This is perhaps the least commonly used of the bolt tightening methods on our list, and typically only used for
extremely large bolts.
Heat tightening uses the thermal expansion characteristics of a bolt, heating the bolt with a direct flame,
sheathed heating coil or carbon resistance elements to make it expand. From there the nut is marked using the
angle of turn approach, and the system gets time to cool down.
As the bolt tries to contract, the clamped material constricts it to create a preload. The drawback here is the
length of the bolt, which becomes difficult to measure, as the system needs to return to ambient temperature
for each measurement.
6. Tension indicating
This bolt tightening method involves measuring bolt tension through things such as
load indicating bolts and washers, along with techniques that calculate the length change of fasteners.
