| | | The friction in a spherical plain bearing or rod end is primarily dependent on the sliding contact surface combination, the load and the sliding velocity. Because there are so many influencing factors which are not mutually independent it is not possible to quote exact values for the coefficient of friction. Under laboratory conditions, however, it is possible to record typical developments of the coefficient of friction for different sliding contact surface combinations. The friction during the running-in phase is higher than the value recorded during the subsequent test period. For example, for regularly relubricated steel-on-steel spherical plain bearings when a suitable highly viscous lubricant is used (e.g. the SKF grease LGHB 2). Guideline values for the coefficient of friction u will be found in Table |7 . They have been determined in laboratory trials. The coefficient of friction for the maintenance-free sliding contact surface combinations steel/PTFE fabric and steel/sinter bronze composite decrease with increasing specific load. At a constant specific load, friction will | | |
| | | be reduced to the given minimum value as soon as the transfer of PTFE from the sliding layer to the opposing steel surface has been completed. The friction torque for a spherical plain bearing or rod end can be calculated using M = 0,5 x 10"6mP dm where M = friction torque, Nm U = coefficient of friction (- Table F ) P = equivalent dynamic bearing load, kN dm = mean bearing diameter, mm, for radial spherical plain bearings dm=dkfor radial spherical plain bearings dm= 0,9 dk for angular contact spherical plain bearings dm= 0,7 dkfor spherical plain thrust bearings As operation progresses and as a result of negative influences (contamination, inadequate lubrication), even under very light loads, the maximum values of the coefficient of friction Table 1 | | quoted in the table may be approached or even exceeded if conditions are particularly unfavourable. In applications where friction is especially important, therefore, it is recommended that the maximum values of the coefficient of friction be used when determining the required power rating, for security reasons. For all bearings, which are operated under conditions of mixed or dry friction, there may be small differences between adhesive and sliding friction. Operation which is entirely free of stick-slip cannot be achieved. Experience has shown that stick-slip effects occur when the surrounding construction is "soft". In most applications, however, the effects are negligible. | | |
| | | Steel-on-steel 0,08 0,20 Steel-on-bronze 0,10 0,25 Steel/sinter bronze composite 0,05 0,25 Steel/PTFE fabric 0,03 0,15 Steel/PTFE composite 0,05 0,20 | | |