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This paper studies the lubrication characteristics of ball bearings for cryogenic turbopumps. First, the frictional coefficients between 440C and a Ag coating, 440C and solid PTFE (polytetrafluoroethylene), and 440C and a PTFE coating in LN2 (liquid nitrogen) are obtained using a ball-on-disk testing machine under a high sliding speed in the range of 0 to 8 m/s and a high contact stress in the range of 2.5 to 3.5 GPa. Dynamic and power loss models of high-speed and high-load ball bearings are established to study the key factors affecting the heat generation characteristics. In order to verify the correctness of these two theoretical models, a coupled fluid-thermal finite element model is built to evaluate the temperatures of the outer ring under different bearing speeds, which are then proved by experiments with ball bearings for cryogenic turbopumps. The results show that the power loss due to the spinning-sliding of the ball and the churning and drag of LN2 account for more than 80% of the total power loss; the spin-roll ratio of the ball on the inner raceway is a key indicator for this type of ball bearing, and the relatively small radial clearance and contact angle are suggested. Both of the proposed theoretical models have sufficient accuracy and can be used in the performance evaluation and optimization design of bearings.

In the present study, a flow visualization experiment in the clearance between a clear floating-ring and a rotating journal was carried out in order to clarify the effects of the recess geometry on the flow characteristics. Five types of the floating-rings with different recess geometries, i.e., the plane rhombus, the plane circle, the revise circular Young Leaf Mark, the dome and the revise dome geometries, were operated at a rotational speed of up to 40,000 rpm using liquid nitrogen as the working fluid to observe the interaction between source flow from the recess and rotating flow induced by the journal rotation in the clearance. The cavitation cloud inside the clearance was induced by viscous frictional heating and the pressure drop. The influence of the sweepback angle (θs) of the recess leading edge on the flow characteristics was investigated by comparing flow coefficient (Cf) and the cavitation cloud area ratio (Ac) obtained from the visual image. Based on the experimental results, it may be confirmed that the effects of the recess geometry on the flow characteristics of the cryogenic hybrid journal bearings are mainly caused by the influence of the θs value on the flow condition.

In the present study, a flow visualization experiment in the clearance between a clear floating-ring and a rotating journal was carried out in order to clarify the effect of the sweepback angle (θs) of the leading edge of the recess on the flow characteristics. Four types of the floating-rings with different recess geometries, i. e., the Young Leaf Mark, the square and the non-recessed geometries, were operated at a rotational speed of up to 45,000 rpm using liquid nitrogen as the working fluid to observe the interaction between source flow from the recess and rotating flow induced by the journal rotation in the clearance. The cavitation cloud inside the clearance was induced by viscous frictional heating and the pressure drop. The influence of the θs value on the flow characteristics was investigated by comparing flow coefficient (Cf) and the cavitation cloud area ratio (Ac) obtained from the visual image. Based on the experimental results, the dependence of the flow characteristics on the rotational speed was confirmed to be affected by the sweepback angle of the leading edge of the recess.

In the present study, two types of nine-recess hybrid journal bearings (HJBs) with different recess geometries, i.e., the Young Leaf Mark and the square, were operated at a rotational speed of up to 67,000 rpm using the cryogenic journal bearing test apparatus at JAXA to clarify influences of the journal rotating direction on the cryogenic HJB. Liquid nitrogen was supplied to the test bearings with a pressure of up to 4 MPa. The test bearings were 60 mm in diameter and 25 mm in width, with a 0.055 mm clearance and a recess area ratio of 0.186. Based on the experimental results the dependence of the rotor vibration on the rotational speed was confirmed to be affected by the journal rotating direction. The test HJBs with the positive sweepback angle of the recess leading edge showed the smallest rotor vibration in the high-rotational-speed operation where the hydrodynamic effect was predominant.

In the present study, a nine-recess hybrid journal bearing (HJB) having a Young Leaf Mark recess was operated at a rotational speed of up to 75,000 rpm using the cryogenic journal bearing test apparatus at JAXA, to clarify an onset speed of the rotor instability supported by the cryogenic HJBs. Liquid nitrogen was supplied to the test bearing with a pressure of up to 4 MPa. The test bearing is 60 mm in diameter, 25 mm in width, with a 0.055-mm clearance and a recess area ratio of 0.186. When the HJB was operated at a rotational speed of over 74,000 rpm in liquid nitrogen, hard frictional contact between the bearing surface and the journal surface occurred, which consequently caused considerable damage to the Cr2O3 coating film on the journal. The results confirmed that the HJB surface damage had relatively little effect on HJB performance. Therefore, cryogenic HJB tested can be considered to have sufficient robustness when the rotational speed is below 60,000 rpm.