Calculation of Tension to Torque is very important in case of an assembly. The conditions experienced by a fastener isn’t easy to identify, so the tension and torque relationship should be considered carefully.
Generally, direct measurement of Tension in a bolted joint is not easy. To find out the appropriate amount of torque required to tighten a nut in a bolted joint and then attempt to relate this Torque to Tension is a common practice in the industry.
A twisting force is required to tighten a bolt in a bolted joint; this twisting force is known as Torque. In other words, Torque is the measurement of the required twisting force of a nut bolt joint. But for proper installation, measurement of Torque alone is not sufficient.
Torque and Tension acting on a bolt, Image Credit: smartbolts.com
When a wrench or tensioner is used to tighten a bolt, a load is applied to the bolt. Due to which, the bolt stretches out and tries to return back to its original length creating a compression pulling the bolt head and the nut towards each other, clamping the joint together. This stretch of a bolt that results the clamping force to the fastener assembly is known as Tension.
Torque and Tension acting on a bolted joint, Image Credit: securitylocknut.com
It means a bolt can behave like a spring, and after it is fully tightened, it grabs all the parts of an assembly and gives a secure and tight joint.
Fastener Loads during Tightening, Image Credit:WhitePapers
WhereFM: bolt tension or preload before any common settling. FK: clamp load in the clamped pieces MA: applied torque or input torque MG: thread torque or the reaction torque to the input torque.
Joint Diagram, Image Credit:WhitePapers
From the above diagram, we can see when a bolt is tightened, the bolt gets stretched like a spring, causing tension while the assembly is compressed due to clamp load.
To get a secure joint appropriate amount of clamp force must be decided. Higher amount of clamping force is responsible for a bent joint or a damaged bolt and lower amount may result a unsecured and loose joint.
So it is difficult for a technician to make a decision for the accurate Tension required for tightening a bolt that will result a precise amount of clamping force.
Bolt Torque charts are also available to guide a field operator to find tension and torque. Since the torque-tension relationship is affected by many variables like surface texture, lube oil, human error etc., the only preferable procedure to determine the correct Torque is experimentation under actual assembly conditions.
The Equation to find Tension to Torque
The torque value used for a specific nut/bolt assembly is always a matter of concern.
Whenever a bolt is tightened in a bolted joint, Torque is the required twisting force to move a nut along the threads of the bolt. The formula to calculate torque and tension relationship in an assembly is
T = (KDF)/12
Where, T=torque measurement in ft-lbs
D=bolt diameter in inches
F=desire clamp load or tension in lbs.
K=nut factor or torque coefficient, ranging from 0.03 to 0.35 (Dimensionless)
This is an industry-accepted formula applied for the estimation of torque value that will give a certain tension or clamp load.
Here, the value of the nut factor varies according to the condition of the fasteners.
K values used in different conditions are:
Plain, non plated bolts, k=0.20
Rusty bolts, K=0.30+
The readymade torque charts are available for the operators to find tension to torque where the above mentioned formula is used to prepare these charts. Using these charts for different types of fasteners one can easily determine the required torque and tension to get a secure joint.
But in reality the conditions are quite different, so these estimates should be used as a guide only to get an approximate idea about the required loads during tightening.
Torque and Tension relationship
The Torque applied to a fastener decides the Tension created because the bolt can stretch out just like a spring due to the application of Torque.
Though Torque and Tension are related to each other linearly, but it is not predetermined that same amount of Torque applied gives same amount of Tension always.
Certain factors like human errors, debris, dust, lubrication oil, rust and material category may produce different results against a specific amount of Torque applied to an assembly.
While tightening a fastener, the twisting force or Torque applied is mainly absorbed in three main areas: first, under head friction which may absorb 50% or more of the total Torque applied, second thread friction absorbs around 40% of the applied Torque.
The remaining 10% of the Torque produces a clamping force that holds the whole assembly together. It implies an increase in any one of the friction components of 5% may reduce the Tension by half.
Energy transfer takes place during the tightening of a threaded fastener. To get a proper command over tightening process, a good knowledge of Torque and turn in the development of Tension is required.During fastening of a screw different processes undergo like turning of the screw head, development of tension from turning moment and finally a clamping force which holds all the parts of the assembly together.
In the above figure, the area under the torque-angle curve gives a clear view of the energy utilized for tightening the fastener.
The torque and angle curve is a representation of total energy necessary for tightening the fastener. The whole area under the curve is the total energy required and the top most portion is the elastic clamping energy that holds all the components together.