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This thesis sets out to develop a condition monitoring technique for self-lubricating composite liner bearings. Self-lubricating bearings have been widely employed within the Aerospace industry since the 1950s. The focus of this thesis is primarily on helicopter main rotor applications, where self-lubricating bearings are located within the pitch-control link rodends. Such bearings are designed to be subjected to reciprocating motions and, due to their conformal geometry, they are able to handle high magnitude loads. Through the use of coupon and full-bearing test benches, an understanding of the wear characteristics of the liner composite was achieved, leading to the development of a series of condition monitoring techniques. Temperature and Acoustic Emission (AE) were chosen as the two focal points for experimental data gathering. Within a helicopter’s main rotor pitch control system, it can be assumed that all pitch-control bearings function at near-identical operating conditions. This assumption gave rise to a comparative condition monitoring technique being developed. Methods such as cross-sample correlation and creation of control charts were employed with successful outcomes. Acoustic Emission monitoring was successfully utilised in order to identify tribological condition changes with respect to the sliding contact surface. Such changes occur, as the liner material is consumed and in time transitions from a PTFE rich to a glass fibre rich composition. The wear depth of such transition is always constant and its detection can therefore be employed as a condition monitoring technique. A self-developed Acoustic Emission analysis technique was also successfully created, where the reciprocating nature of the sliding contact is exploited. Outcomes from this technique are compared with conventional methods such as RMS.

This thesis sets out to develop a condition monitoring technique for self-lubricating composite liner bearings. Self-lubricating bearings have been widely employed within the Aerospace industry since the 1950s. The focus of this thesis is primarily on helicopter main rotor applications, where self-lubricating bearings are located within the pitch-control link rodends. Such bearings are designed to be subjected to reciprocating motions and, due to their conformal geometry, they are able to handle high magnitude loads. Through the use of coupon and full-bearing test benches, an understanding of the wear characteristics of the liner composite was achieved, leading to the development of a series of condition monitoring techniques. Temperature and Acoustic Emission (AE) were chosen as the two focal points for experimental data gathering. Within a helicopter’s main rotor pitch control system, it can be assumed that all pitch-control bearings function at near-identical operating conditions. This assumption gave rise to a comparative condition monitoring technique being developed. Methods such as cross-sample correlation and creation of control charts were employed with successful outcomes. Acoustic Emission monitoring was successfully utilised in order to identify tribological condition changes with respect to the sliding contact surface. Such changes occur, as the liner material is consumed and in time transitions from a PTFE rich to a glass fibre rich composition. The wear depth of such transition is always constant and its detection can therefore be employed as a condition monitoring technique. A self-developed Acoustic Emission analysis technique was also successfully created, where the reciprocating nature of the sliding contact is exploited. Outcomes from this technique are compared with conventional methods such as RMS.

Self-lubricating bearings are widely used within the aerospace industry and are commonly found, for example, in the pitch control assembly of a rotorcraft. Due to their high importance the components located within such systems are marked as critical parts and therefore the monitoring of their current health state is of great value. The bearing liner which provides the sliding surface between inner and outer races is a Polytetrafluoroethylene (PTFE) and glass fibre woven composite. Low friction coefficients are achieved via the deposition of PTFE third-body particles, which occupy the valley-features at the roughness scale on the non-conformal sliding surfaces. As these third-bodies form, the woven matrix composite surface layers containing PTFE are consumed up to a point where only the structural reinforcing glass fibres are present, resulting in a higher coefficient of friction and accelerating the wear process. A cylinder-on-flat oscillating wear test bench developed within Cardiff University allowed for the gathering of physical data including temperature and Acoustic Emission (AE) signals during an accelerated wear test of the liner material. A radial load of 2.5 kN and oscillation frequency of 5Hz were applied to replicate typical operating conditions within a pitch control system. Frequency analysis techniques were carried out on the AE data, successfully identifying the transition from healthy contact into the failure region.

Self-lubricating bearings are widely used within the aerospace industry and are commonly found in the pitch control assembly of rotorcraft and, as such, are critical parts due to their importance within the system. The dry lubricant present within these bearings acts as a consumable material which occupies the troughs of the inner race therefore forming a smooth sliding surface between inner and outer race via the deposition of third body particles. For this research the liner material is in the form of a woven composite, consisting of Polytetrafluoroethylene (PTFE) as the dry-lubricant and glass fibres as the reinforcing material, cured in a resin matrix. A cylinder-on-flat oscillating wear test bench developed within Cardiff University allowed for the gathering of physical data including temperature and Acoustic Emission (AE) signals during an accelerated wear test of the liner material. A radial load of 2.5 kN and oscillation frequency of 5Hz were applied to replicate typical operating conditions within a pitch control system. Frequency analysis techniques were carried out on the AE data, successfully identifying the transition from healthy contact into the failure region.

The paper describes a novel method for testing the wear rates of liner materials for self-lubricating bearings. The apparatus differs from similar machines in that it is designed for reciprocating contact, to simulate motions encountered in helicopter main rotor pitch link bearings, and is designed to test up to 4 samples at the same time, reducing the effect of variability in environmental conditions. This paper describes key aspects of the test rig along with initial results. Self-lubricating bearings have been in widespread use since the mid-1950s, predominantly in the aerospace industry where they have the advantage of being low maintenance components. They usually consist of a spherical bearing with the inner and outer elements separated by a composite glass fibre / PTFE resin-bonded textile liner. Previous work found that the wear of typical composite liners has three distinct phases – initial wear in, steady-state wear phase, final wear-out, and that humidity and environmental temperature can have a strong influence. Typical bearing-scale tests are long duration, and so a method of accounting for environmental factors and allowing the rapid screening of materials and test conditions is described here as an aid to technology development.

Self-lubricating bearings are widely used within the aerospace industry and are commonly found, for example, in the pitch control assembly of a rotorcraft. Due to their high importance the components located within such systems are marked as critical parts and therefore the monitoring of their current health state is of great value. The bearing liner which provides the sliding surface between inner and outer races is a Polytetrafluoroethylene (PTFE) and glass fibre woven composite. Low friction coefficients are achieved via the deposition of PTFE third-body particles, which occupy the valley-features at the roughness scale on the non-conformal sliding surfaces. As these third-bodies form, the woven matrix composite surface layers containing PTFE are consumed up to a point where only the structural reinforcing glass fibres are present, resulting in a higher coefficient of friction and accelerating the wear process. A cylinder-on-flat oscillating wear test bench developed within Cardiff University allowed for the gathering of physical data including temperature and Acoustic Emission (AE) signals during an accelerated wear test of the liner material. A radial load of 2.5 kN and oscillation frequency of 5Hz were applied to replicate typical operating conditions within a pitch control system. Frequency analysis techniques were carried out on the AE data, successfully identifying the transition from healthy contact into the failure region.

Self-lubricating bearings are widely used within the aerospace industry and are commonly found in the pitch control assembly of rotorcraft and, as such, are critical parts due to their importance within the system. The dry lubricant present within these bearings acts as a consumable material which occupies the troughs of the inner race therefore forming a smooth sliding surface between inner and outer race via the deposition of third body particles. For this research the liner material is in the form of a woven composite, consisting of Polytetrafluoroethylene (PTFE) as the dry-lubricant and glass fibres as the reinforcing material, cured in a resin matrix. A cylinder-on-flat oscillating wear test bench developed within Cardiff University allowed for the gathering of physical data including temperature and Acoustic Emission (AE) signals during an accelerated wear test of the liner material. A radial load of 2.5 kN and oscillation frequency of 5Hz were applied to replicate typical operating conditions within a pitch control system. Frequency analysis techniques were carried out on the AE data, successfully identifying the transition from healthy contact into the failure region.