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Title: A tribological investigation of windscreen wiper performance
Author: Dobre, Oana
ISNI:       0000 0004 8504 6330
Awarding Body: Imperial College London
Current Institution: Imperial College London
Date of Award: 2018
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This project focused on understanding windscreen-wiper/glass interactions with an aim to reduce friction and wear, improve wiping quality and prevent friction induced vibration, (also known as squeak). To achieve this, the contact between windscreen and wiper was simulated under laboratory conditions using a UMT2 Tribometer, which loaded a stationary rubber profile against a rotating glass disc. Then, a range of measurement and analytical techniques were used to characterize the effect of conditions on different aspects of wiper performance. Different surface conditions were reproduced by applying a range of common treatments to the glass, including hydrophobic and hydrophilic coatings. In addition to this, a new method of partially forming self-assembled monolayers was devised in order to produce test specimens with a controlled range of surface energies. It was shown that friction reduces with increasing surface energy, which is attributed to a smaller volume of water being entrained into the contact. Following this, a range of non-steady state friction behaviours were studied. These included combined friction and wear tests, where under severe conditions it was shown how friction performance is dominated by the formation and removal of burs, which prevents water from being entrained into the contact. In addition, drying tests were conducted to understand "tacky" behaviour (i.e., the peak in friction peak under partially lubricated conditions). This was shown to be controlled by decreasing the surface tension of the water, through the addition of detergent, and provided evidence to support the theory that water menisci are responsible for increasing surface area. Static friction behaviour was also investigated, and the effects of start-up velocity and stationary duration on friction were quantified and explained. The practical implications of these results are discussed in terms wiper design and material selection. To study friction induced vibration, (FIV), friction, sound and high speed video measurements were combined with finite element modelling of a rubber wiper/glass contact. In agreement with previous research, FIV only occurs when the friction versus speed curve has a negative gradient; a factor, which, in combination with the low stiffness of the materials, can lead to vibrational instabilities in the mixed regime. Results also showed that friction induced vibration is strongly affected by surface condition, and only occurs for a certain range of surface energies. This is explained by the fact that both high and low surface energies alter the gradient of the Stribeck curve thereby preventing FIV (i.e., low surface energies prevent sufficient liquid entering the contact and high surface energies attract water molecules to the surface of the glass producing a film that reduces friction). In order to study the source of squeak, simultaneous measurements were realised by a high speed camera, microphone, and laser Doppler vibrometer (LDV). This showed that although both the wiper and glass vibrate with the same frequency, it is the latter that transmits sound to the air. Results from the high-speed camera and microphone have shown that the frequency of the rubber vibration equals to the frequency of the emitted sound and the water vibration. This frequency is the same as the eigen frequencies determined from a finite element model of the wiper, which was developed. These observations led to the conclusion that friction induced noise occurs only when bending modes of the wiper are excited and this has important implications for the control of FIV since it shows that emitted sound can be eliminated by modifying the blade geometry during the design stage. Another important observation is that the frequency of squeak decreases with increasing volume of water present on the glass. This is attributed to the water effectively adding mass onto the vibrating system and hence reducing its natural frequency. Additionally, capillary waves have been for the first observed in the water surrounding the wiper contact. Based on the understanding gained, a number of recommendations are made regarding means of reducing windscreen wiper noise. Finally, in order to monitor the wiping performance of the rubber wiper, a fluorescence microscopy technique was developed to view the sliding contact. This has enabled the fluid film thickness within the blade/glass contact to be assessed and also manufacturing defects, such as notches and inclusions, to be identified as the cause of wipe quality issues, such as hazing and hairlines.
Supervisor: Reddyhoff, Thomas ; Dini, Daniele Sponsor: Bosch (Firm)
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral