BlindBolt Tightening Torques
Blindbolt UK | 19th October 2023
What is Torque?
Torque is a measure of force that can cause an object to rotate about an axis. In SI units torque is measured in newton meters, or Nm. If a fixing is tightened by a force of 90 N applied at the end of a spanner which is 0.3 m long, the torque is 90 × 0.3 = 27 Nm.
The effect of torque on a bolted fixing is to put a small tension, often called preload, into the bolt and compress the joined plates together.
Why Specify a Tightening Torque?
Tightening torques are specified to ensure that the fixing is not left in a loose condition. If the nut was left in a loose condition, repeated vibration might eventually cause the nut to come completely off the bolt.
The second reason for specifying a tightening torque is simply so that quality assurance inspections have a measure which can be readily assessed – it is not a matter of opinion or debate.
How Much Torque is Required?
This is a topic which is widely debated. In the UK, ordinary bolts (those carrying loads in shear and tension, used in clearance holes) do not need to be preloaded and therefore no tightening torque is specified. The fixings merely need to be “snug tight” or “spanner tight” for which no formal definition exists. The commonly accepted value in structural steelwork is “the force applied by an average steel erector with the usual size spanner”, which is a rather loose specification.
Within SCI publications P207 and P212, indicative values are given for certain bolt diameters. For example, the approximate torque equivalent to “spanner tight” is given as 110 Nm, which is simply the product of a force of 250 N applied at a lever arm of 440 mm, which was considered appropriate for a spanner used for M20 bolts.
As a comparison, the recommended torque for the wheel nuts of an ordinary car is usually between 110 and 120 Nm, and can be achieved with the wheel brace supplied with the car.
SCI Advisory Desk Note AD 302 recommends that generally, torque on ordinary bolts need not be checked. To verify that the nuts are not actually loose is sufficient.
Does the Preload Cause a Problem?
Applying a torque will introduce some preload into the bolt. The preload is very loosely related to the applied torque, but varies widely, depending on the friction between the threads of the nut and bolt. Damp conditions, contamination or lubrication all affect the relationship between torque and preload.
It might be thought that preload reduces the remaining tension capacity, by “using up” some of the resistance. What actually happens is that when external tension is applied to the joint, the compression between the clamped plates is relieved without adding to the force in the fixing. The fixing only experiences an increase in load once all the compression has been relieved, meaning that even a preloaded fixing can carry its full design tension value.
BlindBolt Torque Values
BlindBolts are used in clearance holes in the same way as ordinary non-preloaded bolts. This type of fixing is described as “Category A: Bearing type” in BS EN 1993-1-8 and is the most common form of fixing used in the UK.
Recommended tightening torques for BlindBolts are given on the BlindBolt website. As a comparison with the figure of 110 Nm noted for a standard bolt, the recommended tightening torque for a M20 BlindBolt is 65 Nm. The difference in values is not significant – as the previous discussion has clarified, all that is required is that the nut is not actually loose. The recommended torques provide the opportunity for the fixings to be checked after installation, for peace of mind rather than for any structural reason.
What Happens if the Recommended Torque Cannot be Achieved?
In some very unusual circumstances, there may be insufficient room to use a spanner long enough to reach the recommended torque. If the nut is not loose, the assembled BlindBolt fixing is satisfactory.
For standard bolts and BlindBolts used in clearance holes, tightening to a specified torque is not structurally necessary – the requirement is that the nut is not loose. For quality assurance reasons, if inspection is required, the recommended tightening torques provide a specification against which the fixing may be assessed.