Torque Tension Control
Coating Lubricity or Torque Tension
As efforts are made to use a fastener at its ultimate capacity, to economise on fastener size, strength and numbers used, the lubricity of the fastener assumes a great importance.
The aim of any fastener engineer using a fastener in a normal tensile joint would be to be able to tighten similar fasteners to give a uniform known tension.
The factors that control this are:
- Accuracy of applying and monitoring the tightening torque.
- Underhead dimensions surface roughness and lubricity performance of coating.
- Conditions (and coating) of the surface which is being turned against the bearing surface i.e. the surface under the head of a bolt or nut when tightened
- The thread condition and coating lubricity.
- The mating thread condition and coating.
The main criteria are that the variables outlined above are acceptably controlled. The specifications for the manufacture and mechanical performance of fasteners will usually control the fastener variability satisfactorily; e.g.ISO 68, ISO 724, ISO 898, ISO 965, ISO 6157 and the coating lubricity can be controlled by process control and testing according to testrequirements such as BS 7371 Pt. 2 or ISO 16047.
Lubricity of Coatings
This can be expressed in two ways, the Coefficient of Friction µ and the Torque Coefficient (K factor). The figures are not the same and it is important that they are not confused with each other. The coefficient of friction takes into account many more parameters to give a more accurate result.
Torque = Torque (Nm) =T
Coefficient K = Induced load (kN) x Nominal diameter (mm) = F.d
It is seen that this uses the dimension d, so the validity of K-factor is limited to fasteners of normal diameter, so calculations using K can only accurately be used with the same dimension fastener.
The coefficient of friction µ is usually derived from the work of Kellermann and Klein1
T – P
0.577d2 + 0.5 Db
- T is the tightening torque (Nm)
- F is the clamp force (kN)
- P is the pitch of the thread (mm)
- d2 is basic pitch diameter of thread (mm)
- Db is effective diameter of bearing surface under nut or bolt head (mm)
This is µtot which is the sum of the effective coefficient of friction on the bearing surface (µb) (e.g. under the head of a bolt or the rubbing surface of a nut if the nut is turned) and in the thread (µth). These figures will vary as contact materials change.
Coefficient of friction between bearing surfaces
Where Tb is torque to overcome bearing surface friction (Nm).
Coefficient of friction between threads
Tth – P
Tth this torque required to overcome thread friction and induce tension F (kN).
It should be noted that in a typical joint the energy is divided approximately:
- Overcoming thread friction µth – 40%
- Overcoming bearing friction µb – 50%
- Producing clamp force F – 10%
Changes in friction have a large effect on clamping force.
To monitor coating lubrication performance, it is customary and usually considered only necessary to consider µtot. Sophisticated equipment is required to measure thread or bearing torque requirements separately which is useful to assess the effects of varying bearing surface or thread contact materials. Anochrome Group has purchased this type of equipment and has assessed the individual coefficients of friction for popular material combinations which are quoted in the table below. This can be used as a guide to compute tightening torques, but it should be considered that variations may occur. Changes to any of the parameters, or contamination of contacting surfaces, can significantly alter characteristics.
Types of Coating
Lubricated coatings fall into two categories:
1. Integral lubricated coatings – where the lubricant is part of the coating and is found distributed throughout the coating.
2. Surface lubrication – in this case, the lubricity is given by an impregnated surface coating or a wax or oil.
In some cases, to give the required performance, both types of coatings are required, type 2 on top of type 1.
The integral lubricated type is usually regarded as the better as it is easier to apply, does not require an extra coat, cannot be omitted,and its performance is more consistent in that on reuse, the coating will still be able to supply lubrication to the joint.
Most zinc flake coatings have a lubricated top coat others have integral lubrication properties such as Geomet®500 and Dorken®KL105 which has a lubricant throughout the coating.
Electroplated coatings and some other coatings are not normally supplied with an integral lubricant so surface lubricants are used.
Performance Enhancing Lubricants / Waxes
These coatings are often more economical than the resin bonded dry film lubricants (see below) and can be applied to a large number of standard finishes. Typical lubricants/waxes that can be applied are:
- Rustarest – Oil
- Torque N Tension 11 or 15
- A3 Wax
- Wax 47/60
In a number of cases, A3 wax has been used to give the lubricity performance of cadmium under high pressure applications. Also similar waxes have been used to ensure that self-drilling screws can give their desired performance.
In some instances, e.g. to dampen noise when inserting certain types of thread locking screw, the wax can be applied only to a portion of the thread of a fastener.
The oils applied, though they have a disadvantage of being slightly wet, have the attraction that their mobility can enhance the corrosion resistance, particularly when components are damaged in assembly. This is not usually the case with waxes because as they are dry films, they can sometimes suffer damage, although Anochrome Group does not use any standard materials that will detract from the corrosion resistance of the original coating.
Dry Film Lubricants
The Engineer’s demand for lower closely controlled friction coefficients, with no extruded material, has produced a requirement to eliminate molybdenum bearing greases and oils. The dry films used to replace these having the advantage of being tenaciously bonded to the surface giving improved reproducible performance over wide variations of temperature and under very adverse conditions. A major use of these materials is on stainless steel, to stop galling.
Dry film lubricants usually consist of a resin, used to bond a dry lubricant on to the surface of a component. The lubricants used are, in the main,P.T.F.E., Molybdenum Disulphide or Graphite, either individually or combined, to impart any of the following attributes.
- Low coefficients of friction.
- Good performance under high surface pressure conditions.
- High temperature, low temperature performance.
A number of these lubricants supplied as standard by the Anochrome Group, which include Torque Tip 28®(TT28), Molykote 321®, Molydag709®, Molydal 1870®and other lubricants and blends according to the requirement of the part.
The typical applications of these products are on:
- Turbo charger bolts
- Carburettor parts
- Lock parts
- Seat belt bolts and parts
- Brake and clutch pivots
- Bolts with special locking features
- High temperature nuts
- Clutch locking rings
- Machine slides
- Gearbox input shafts
- Stainless steel threads
If necessary, these coatings can be applied to local areas of components.
In most cases these coatings will enhance corrosion resistance if applied as a top coat and also have corrosion resistance properties in their own right.
These coatings can be applied in bulk, on parts up to 0.5 kg. wt.or 150 mm long. Longer parts can be spray coated or selectively coated on the thread or underhead feature.
Re-use of Fasteners
Interest is being shown in the torque tension performance of fasteners when they are re-used and our equipment has been used to investigate the change in performance during 10 re-usecycles. In these cases, the integral lubricants show a vast improvement over the “surface” type of torque tension control (See Torque Tension Lubrication graph PDF download above).
If required, please contact us for further information on the torque tension performance of coatings.
Torque Tension Performance Data
Data for guidance is given on the following graphs, but it should be noted that the actual component that is coated, its dimensions and the coating dimensions of mating parts can have a significant effect on the actual tightening performance.
Most figures have been obtained from testing parts that conform to the ISO specification for coarse series of bolts and in many cases M10 flange fasteners have been used.
Typical Torque Tension Specification Requirements
Integral Lubricant. Typical – Zinc Flake Re-use
Surface Lubricant. Typical -Zinc Electroplate + lube Re-use
Typical Performance Data for Coatings with Standard Lubrication
NOTE: The Clamping Forces specified are equal to 75% of the proof load of the property classes of bolts as given in ISO 898/1.The
Applied Torque figures are test requirements and are not recommended for use as assembly data.Thread locking and sealing features generally perform to this table. Please contact us for confirmation.This table complies with BS7371 Pt 2. Ford WZ100. Testing to Ford WZ101 is also available.Testing to International Standards such as ISO 16047 is recommended in the absence of customer standards.
Information presented in this data sheet is considered reliable, but conditions and methods of use, which are beyond our control, may modify results. Before these product are used, the user should confirm their suitability. We cannot accept liability for any loss, injury or damage which may result from its use. We do not warranty the accuracy or completeness of any such information whether orally or in writing. We reserve the right at anytime and without notice to update or improve products and processes and our information concerning the same.
Dorken® and Delta Protekt® are a registered trademarks of Eward Dorken AG
Geomet® is a registerd trademark of Metal coatings international Inc
gleitmo® is a registered trademark of Fuchs Lubritech GmbH
Geoblack ®is a registered trade mark of Dacral
Molykote® is a registered trademark of the Dow Corning Corperation
Molydag® is a registered trade mark of the Henkel corperation
Molydal ®is a registered trade mark of Molydal S A
If you would like further information on torque tension from an industry expert or to find out how Anochrome can assist you, please email us via the Contact page, or alternatively you can call us on 01902 397333.