Specially designed lubricants influence the component's operational smoothness and noise levels considerably, and should also contribute additional features like neutral odour and compatibility with plastics. They also ensure lifetime lubrication, even at low temperatures. Klüber Lubrication has developed a range of versatile damping greases for car interiors to meet all these requirements. The greases stand out in particular for their capacity to reduce vibrations and prevent unpleasant noise while ensuring comfortable, smooth component operation satisfying the highest requirements.
The combination of base oil viscosity, additives and thickeners of these speciality lubricants was developed to ensure optimum damping properties. These are achieved through special manufacturing and homogenisation procedures as well as the purest ingredients. Damping properties can also be influenced positively by using the right lubricant application method and dosage in component manufacturing processes.
Damping greases in various shear viscosities and consistency grades
By selecting from a wide choice of shear viscosities and consistency grades from fluid to hard damping greases, designers can accommodate tolerances and achieve defined actuating forces.
The new damping lubricants made by Klüber Lubrication are available in shear viscosities ranging from L for smooth actuating torques (2 000 to 4 000 mPa s) to ES for very high actuating torques (above 20 000 mPa s) (see fig. 1). The shear viscosity of the grease is determined in a rotating cone viscosimeter at constant speed and temperature (see fig. 2).
Shear viscosity grade M offers good noise damping and excellent feel of seat rails and backrest recliners at acceptable actuating torques. Static friction has to be overcome for horizontal movements of the rail and manual backrest adjustment. When conventional lubricants are used, unpleasant breakaway torque is an issue in the transition phase between static friction and sliding friction. The grade M Klüber damping greases help to minimise this breakaway torque, reduce actuating torque and damp unpleasant noise. For gearshifts, window lifting systems and other lubrication points the ES grades can be used. They support oscillating movements and reduce stick-slip.
When developing a damping grease, the consistency grade is also considered. Consistency is influenced by the ratio of oil, thickeners and additives. This consistency is categorised by the National Lubricating Grease Institute (NLGI) in lubricants according to DIN 51818 giving consistency grades from 000 (fluid) to 6 (hard) (see fig. 3) and as unworked or worked penetration. The worked penetration of a lubricant is determined by means of a penetrometer where the consistency grade is determined according to the penetration depth of the cone (see fig. 4). Soft lubricants, i.e. NLGI class 1, allow smoother seat rail adjustment or gear shifting than comparable NLGI 3, i.e. hard lubricants, with the same kinematic viscosity and shear viscosity.
It makes sense to develop lubricants in close cooperation with the manufacturer of the lubricated component, as can be seen in the following example. A seat supplier faced the problem of "juddering" of opposing friction materials at low loads when adjusting the backrest manually through a small angle. With a greater adjustment angle and higher load, slipping occurred. To reduce the juddering, the friction coefficient between the opposing materials needed to be reduced. However, a certain friction coefficient was required to prevent backrest slipping. A component test rig was used to develop a damping grease by testing the the backrest adjustment mechanism with different lubricant samples for meeting these requirements. The speciality lubricant contains a combination of fully synthetic base oils and thickeners and is available in shear viscosities from M to S and NLGI class 2.
Lubricants as an integral design element
A damping lubricant for gear shifts was also developed to comply with many requirements. These included a good feel when actuating the gear shift, operability between minus 40 and plus 120 degrees Celsius, no silicone raw materials due to the lubrication point being close to electronic sub-assemblies and good compatibility with the plastics and elastomer components.
Various damping greases were tested on an oscillation-friction wear tribometer at an early stage to find the appropriate lubricant for these particular requirements. The damping lubricant Klübersynth LIP 84-42 stands out for its optimum combination of oil and thickeners, its shear viscosities between L and M and NLGI classes between 1 and 2. It is also suitable for use down to minus 60 degrees Celsius.
The damping greases described above were optimised to meet the particular requirements of the components right from the start and regarded as an integral design element of the component rather than the usual situation in which the lubricant is selected in a last-minute decision to solve an existing problem. The customer was able to save costs for component testing already in the pre-selection stage as several lubricant samples were tested on the oscillation-friction wear tribometer before to provide test data. The new range of damping greases is already used in practice for many automotive components, e.g. in seat rails, backrest recliners, gear shifts, window lifters, locking systems and sun roofs.
Design and operating conditions are key to successful development
In recommending the optimum lubricant, the specific requirements and operating conditions must be considered besides shear viscosity and worked penetration as described above. These include component materials, the specific temperature range, media influence, relative movements of the lubricated parts, rotating speed and loads on the friction bodies.
Klüber Lubrication has more than 100 test rigs to test lubricants, both standard and customised ones for individual component tests, for the ever-increasing requirements of the automotive industry. Klüber has approvals from almost all automotive manufacturers and their suppliers worldwide.
Author: Global Action Team Automotive Industry, Klüber Lubrication Munich