The Experts - the Relation Between Torque, Preload and Friction

In this video, Damien Thomas, Technical Manager at Nord-Lock Group, explains how torque, preload and friction interact.
"The break down of torque shows that only 10% of the input torque is useful. All the rest is dissipated by friction. By applying a low friction lubricant, the effect of friction will be greatly reduced," Damien Thomas says.
The Experts is a series of videos in which Nord-Lock Group shares knowledge about bolted connections and how to solve bolting challenges.
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[@albertojosedanielssoncaram6826]

Muy bien DAMIEN ......fisica pura....TORQUE, fricción, lubricantes, tensión, tests, disipación de fuerzas.........

[@ninemilliondollars]

For installing wheels on a car, the lug nuts are tightened using a torque wrench until the indicated reading is between the manufacture's specifications for minimum and maximum values. However, they do not specify using a lubricant. Nor do they consider the rust that occurs over the years. Would this mean don't use a lubricant; always tighten dry? Seems the manufactures already took this into consideration and you can only tighten dry to the specs. Do you have any comments you can offer?

[@NordLockGroup]

The friction coefficient used in the calculation of the appropriate torque is the most essential parameter. Two strategies can be implemented to control the torque/tensile relationship while making it achievable and controllable easily on an industrial scale: 1) for large volume applications with very standardized items - both on first fit installation and in after market retrofits - one fix range of torque is specified by the manufacturer and consequently an appropriate friction coefficient must be achieved; or 2) for low volume applications, well optimized joints or non-standard designs and components, the friction is defined by the type of materials, coatings or lubricants usable and consequently a suitable torque is determined by the designer.
In case 1) all parts are standardized so fasteners come with a pre-set state that always have the same friction coefficient and therefore match the right preload at the specified torque value. Hence, that one torque value fits all joints and fasteners can be assembled right out of the box. This approach implies specifying a single combination of material and coating for the fasteners - usually integrating a lubricant in the top coat or in a dry film, hence lubrication is « built-in » - so operators never have to apply any extra lubricant during the tightening operation. It is typical on high volume assembly lines in industries where standardization is key - for example, railway rolling stock, automotive cars or trucks.
In case 2) joints are managed one by one and for each the designer calculates the appropriate torque based on the type of materials, coatings, finish or lubricant available. Hence each bolted joint can have its unique torque value or lubricant recommendation. It is typical in power generation equipment for example.
Provided that truck industry falls into the category of high volume and very standardized joint designs, wheel/hub joint is managed as per case 1) described above. Wheel nuts and studs come with a pre-defined friction value as per manufacturer’s specs to avoid requiring the application of an extra lubricant during the tightening operation, exactly as you described. It can be based on typical friction values for uncoated/coarse finish parts if they are black, or with a lubrication top coat or dry film if they are coated. By controlling the friction of the fasteners, manufacturers of trucks can define a single range of torque for all their trucks without the need of an extra lubricant during the mounting.

[@mdriponhowlader3314]

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[@meeraprasannaayyagari5704]

By coating the nut face/bolt head face, aren't we going to loose preload sooner? It's the friction between then that can prevent loosening of the nut once tightened, right?

[@NordLockGroup]

It’s definitely admitted from a mechanical stand point that the friction keeps the bolt and the nut locked after assembly - otherwise the nut would naturally spin backwards to release the accumulated preload. Hence, most of locking fasteners rely on increasing the friction to lock the fasteners. Vice-versa, lubricants reduce the friction hence the resistance against self-loosening of fasteners. Nevertheless, the characteristics of lubricants may change between assembly and in service conditions: ageing make most lubricants dry out quickly and therefore not affect the locked state of the fasteners, while others remain fluid and keep friction low.
At last, most of joints are designed and built with sufficient safety margins to prevent the eventuality of loosening and therefore lubrication has no effect on them during service, but in case of loosening-sensitive joints it’s true that lubrication may help fasteners unscrewing.