Designers always agonise and are in the grey zone when deciding about the amount of bolt tension that they must specify for a particular assembly to perform true to its desired function. While the answers are not simple they are not too complicated either.
The amount of bolt tension needed for an assembly will be determined by the job the assembly is supposed to perform. An assembly where the bolt is used only to hold the parts in place may not need very high bolt tensions but where the job of the assembly and the joint face is to resist movement and transmit loads then higher the bolt tension better is the joint rigidity. In any case, the tension in the bolt cannot exceed the designated load carrying capacity of the bolt despite all preapplied forces and those encountered during operation.
While the above may be obvious to an engineer, ironically higher bolt tension also aids in loosening. Hence the choice of low, medium, high or very high bolt tension must be looked at holistically.
To understand this further we need to dissect the components of the tightening torque and their roles..
The three components of a tightening torque are
TTT = TH +TT+TBT
= F x Dh /2 x µ h +F x 0.58 x dp x µ T + F 0.16 x P
Where,
TTT is the Total Tightening Torque,
TH is the Torque required to overcome the rotational friction under the Head,
TT is the torque required to overcome the rotational friction in the Threads
TBT is the Torque required to create the Bolt Tension
F is the Final Bolt Tension,
Dh is the Mean Head Diameter,
dp is the Pitch Diameter,
P is the Thread Pitch and
µ h andµ T are co-efficient of Friction under the Head and the thread respectively.
Bolt Tension represents the energy that is stored in the bolt. This energy is independent of the friction. Like for every other energy, be it thermal or kinetic, this energy (Potential in nature) also wants to come down to a lower state but is prevented from doing so due the head friction torque and the thread friction torque in a torqued fastener. This is the reason a tightened fastener stays tight under static conditions.
While opening a bolt other torques oppose the opening but the Bolt Tension Torque aids in opening, because it always is wanting to become zero.(The energy wants to come down to a lower state.)
Thus Opening Torque TOT = TH+TT – TBT is always lesser than the tightening torque due to the component TBT. TBT = F 0.16 x P can also be called as ‘Inner Loosening Torque’ ILT or the ‘Intrinsic Loosening Torque’ which is resident in every torqued assembly. This ILT aids in loosening either when you deliberately open the bolt or in dynamic conditions. ILT being the function of Bolt Tension F is higher when the bolt tension is higher and vice a versa.
In dynamic conditions, particularly in transverse movement situations, when there is a transverse movement of the bolt with respect to the threaded hole, a ‘side sliding’ movement under the head, this ILT becomes active and leads to actual rotation of the bolt. Under such conditions the designer must decide if the final function of the bolt is to transmit load or just to retain a part to decide on the quantum of bolt tension. The following two situations will make it clearer.
- The bolts which are used to secure a crown gear onto a hub connected to the axle in a differential gear box:
Here the power from the propeller shaft must be transferred to the axle through the pinion mated to the crown gear. In turn the power is transmitted to the axle via the hub and the crown gear faces. The sliding frictional force between the hub and crown gear face is responsible for the power to be transmitted without any slippage. The sliding frictional force in turn is proportional to the Normal Force which is nothing but the Tension in the Bolts securing the Crown Gear to the Hub. In this application higher bolt tension results in better joint performance. There also is a severe transverse loading in this case and under no circumstance should side slipping be allowed and hence chemical locking of the bolts is a must.
2. The Bolt which is used to secure an idler gear in a typical IC Engine timing Gears:
Here the bolt is used to secure a bush on to the surface of the engine housing. There is no power transmission at the joint interface. In fact the bolt along with the bush and the idler gear is a floating cantilever. But this cantilever is also subjected to minute transverse movements owing to co-axiality errors of other driving or driven mating gears with respect their own mounting shafts. There is no requirement of High Bolt Tension though. Here the bolt used to fix the idler gear also must be chemically locked. Higher bolt tension will increase the inner loosening torque and lead to quicker loosening of the bolt.
