BS ISO 22423:2019 pdf free download – Foil bearings一Performance testing of foil thrust bearings – Testing of static load capacity, bearing torque, friction coefficient and lifetime.
7 Test methods
7.1 Principle
The take-off speed and the parameter values necessary to estimate the load-carrying capacity of the foil thrust bearing should be evaluated after sufficient preheating has taken place. The values of the parameters associated with the testing and estimation shall be presented in the test report (see Annex B).
7.2 Start-stop test cycle and evaluation of the take-off speed
The driving shaft is rotated using a driver and the foil thrust bearing begins to operate. Once the driving shaft begins rotating, the bearing specimen is moved close to the runner by moving the loading plate toward the driver. For practical purposes, the threshold (or recommended) value of a thrust pressure of a foil thrust bearing is 0,5 bar, which is the same as 0,05 N/mm2. The reference load, Fr, is calculated
as the product of the recommended value of a thrust pressure and the area of the top foil surface, Ai, as shown in Formula (1):
It is appropriate to determine the initial load, F, and load increment, as from 70 % to 90 % and 1 %, respectively, of the reference load, Fr, calculated using Formula (1) above. The bearing torque should be measured as the rotation speed is gradually increased. Figure 2 shows a typical example of variation in bearing torque, which is measured along with the rotational speed of the driving shaft. Typically, as the rotational speed increases, the bearing torque increases suddenly at a certain rotational speed and then decreases to a steady-state level with a fairly constant torque value. As the bearing torque decreases to its steady-state value, the rotational speed is determined as the take-off speed of the foil thrust bearing and should be recorded in the test report (see Annex 13). As the rotational speed decreases to zero, the bearing torque suddenly increases again and then decreases.
The following processes comprise a single start- stop test cycle.
a) Apply the initial load, Fw, on the rotating system and gradually increase the rotational speed of the motor.
b) After the driving shaft reaches the take-off speed, the state should continue for 10 s and then the power to the motor should be shut off to maintain a stopped state for 5 s.
c) The rotational speed, the accumulated number of start-stop cycles, the bearing torque, the
temperature inside the bearing and the temperature of the top foil surface should be observed
during the stop-start test.
7.3 Calculation of bearing torque and load
The friction force, F, may be measured using a load cell linked to the torque rod installed on the loading shaft shown in Figure 1. The bearing torque, M, generated by the rotation of the driving shaft is calculated as the product of the friction force, F and the distance, r, between the two centres of the loading shaft and load cell, as shown in Formula (2).