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Lubricating pastes for common bolted joints

Lubricating pastes for common bolted joints

The bolted joint has its own specifics. Throughout the tightening process, the bolt connection is subjected to tension, torsion and possibly bending. As the preload increases, the pressure in the contact surfaces increases.

These basic assumptions should be used as a basis for selecting a lubricant for a common bolted joint.

There are many lubricating pastes that are claimed to have a low coefficient of friction. However, a low coefficient of friction in itself is no guarantee. It must also be taken into account how it was measured and according to what standard.

The coefficient of friction is not a single number. It is necessary to know its course and variance depending on the load during tightening of the bolted joint.

The test of the coefficient of friction of the bolted connection is carried out according to the standard ČSN EN ISO 16047, which specifies the conditions for testing the torque and clamping force of fastening components.

The friction value stated by the manufacturer based on another measurement method, such as the four-ball test* or Pin-on-Disc test**, is misleading for the tightening torque calculation.

These tests do not determine the friction in the thread or under the nut, including their dispersion. Neither the total friction nor the tightening factor, the so-called k-factor, can be derived from them.

The actual value of friction measured by a method other than EN ISO 16047 varies significantly.

* Four-ball test: a method of measuring the coefficient of friction for oils and lubricants. The lubricant's load resistance and general abrasion index are evaluated. It is divided into ASTM D 2596 weld test and ASTM D 2266 wear test.
** Pin-on-Disc : Measurement consists of pressing a firmly attached ball-shaped test body (PIN) with a predefined force into a test specimen (disc).

Customers often contact us with a request for a tightening torque calculation. They then want to tighten the fasteners themselves using their own lubricant.

In view of the above, we make control measurements of friction coefficients, including their dispersion, for lubricants that we have not historically measured. Thanks to our own laboratory, where we carry out the measurements in accordance with EN ISO 16047, we subsequently have accurate and realistic friction coefficients for individual lubricants over the entire range of permissible screw loads.

Based on the hundreds of measurements we have taken, we can say that in many cases the real friction coefficients differ from those commonly available in the material data sheets. The differences can be tens of percent and are often caused by different measurement methods.

When tightening the flange connection itself, the same lubricant that was considered in the calculation must always be used. Otherwise, the individual bolts may be over- or under-tightened and the seal may be under- or over-compressed.

Powertorque LF Booster is a solid lubricant in the form of a paste with a low and stable coefficient of friction designed for the normal assembly of bolted joints.

After mixing, the lubricant is applied with a brush to the threads, washers and nut faces or under the bolt head. It forms an effective lubricating film between the friction surfaces separating the screw thread and nut from each other. The coefficient of friction does not change with increasing pressure on the thread contact surfaces and under the nut head, see chart.

The bolted connections are cold tightened as standard. The temperature resistance of the lubricants rather gives information about their ability to ensure subsequent loosening of the bolted joint. Temperature resistance test of a tightened bolted joint with paste Powertorque LF Booster took place after it was heated to 600 °C and then cooled.

To give you an idea, here is a comparison of the friction curves of an unlubricated bolt, a lubricated bolt and a bolt tightened with paste Powertorque LF Booster.

Comparison of the coefficients of friction of LF Booster lubricating paste, high-temperature metal-based paste and lubricant-free bolted joint.

The graphs show that the coefficient of friction of Powertorque LF Booster nis constant over the entire course of the permissible bolt load, which is not the case with standard metal-based pastes. A coefficient of friction that varies over the bolt load is not desirable.

As friction worsens, metal-to-metal contact occurs at the contact surfaces of the threads. This phenomenon in the bolted joint subsequently causes seizure of the bolt and puts additional undesirable torsional stress on the bolt. The required preload in the bolt cannot be achieved during tightening.

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