Araç yolcu koltuğunun sonlu eleman modeliyle titreşim analizi
Küçük Resim Yok
Tarih
2023
Yazarlar
Dergi Başlığı
Dergi ISSN
Cilt Başlığı
Yayıncı
Bursa Teknik Üniversitesi
Erişim Hakkı
info:eu-repo/semantics/openAccess
Özet
Araçlarda konforu etkileyen önemli etkenlerden biri taşıt koltuklarındaki titreşimdir. Titreşime maruz kalan yolcu yorgunluk, konsantrasyon eksikliği, bel ağrısı gibi problemlerle karşılaşmaktadır. Bu nedenle, koltuktan yolcuya aktarılan titreşimlerin ortadan kaldırılması veya sınırlandırılması konforlu seyir için kritik öneme sahiptir. Bu noktada koltuğun doğal frekansları, sönüm oranları, mod şekilleri gibi dinamik özelliklerinin tasarım aşamasında belirlenmesi ve bunlara bağlı olan titreşim iletkenliklerinin incelenmesi gerekir. Bu çalışmada, şehir içi ulaşımda kullanılan bir taşıttaki ikili yolcu koltuğunun metal iskeleti ve buna ahşap sırtlıklar eklenmesi halinde modal analizi gerçekleştirilmiştir. Sonlu eleman yazılımı ortamında iskelete eklenen ahşap sırtlıkların doğal frekansları ve mod şekillerini nasıl değiştirdiği gözlenmiştir. Yapılan çalışmada ahşap sırtlıkların 2., 4. ve 5. modlarda koltuğun doğal frekanslarını az da olsa arttırdığı, diğerlerinde ise azalttığı gözlenmiş, sırtlıklar eklendikten sonra bazı mod şekillerinin başlangıca göre değiştiği görülmüştür. Ayrıca iskelet için çekiç testi yapılarak doğal frekansları, sönüm oranları ve mod şekilleri deneyle de ölçülmüştür. Deneysel çalışmada koltuğun bazı frekanslarının sayısal modeldekine yakın ölçüldüğü görülmüş, ilgili modların şekilleri karşılaştırılarak ve Modal Güvence Kriteri (Modal Assurance Criterion–MAC) indisleri hesaplanarak bunlar arasındaki benzerlikler gösterilmiştir. İkinci aşamada yolcu koltuğunun zeminden gelen titreşime karşılık ileri geri doğrultudaki titreşim iletkenliği sayısal olarak incelenmiştir. Hesaplamalar ANSYS Random Vibration ortamında gerçekleştirilmiş, koltuk ayaklarından uygulanan ivme Spektral Güç Yog?unlug?u (Power Spectral Density–PSD) değerlerine karşılık koltuk üst çerçevelerinin üzerinde belirli noktalardan çıkış ivme PSD değerleri hesaplanmış, bu ikisinin oranından iletkenlikler elde edilmiştir. Ahşap sırtlık eklenmesi halinde bazı doğal frekansların biraz arttığı bazılarının da azaldığı görülmüştür. Bu durum, eklenen parçanın hem kütle hem de katılık özelliklerinin koltuğun dinamik özelliklerini etkilediğini göstermektedir. Yapılan çalışmalarda ahşap katman eklendiğinde maksimum iletkenlik değerlerinin düştüğü fakat yüksek frekanslarda özellikle sol ayaktan ivme uygulandığında iletkenliklerin bir miktar arttığı görülmüştür. Ahşap parçanın yapının sönüm ve mod şekillerini etkilemesinden dolayı bu değişimlerin meydana geldiği değerlendirilmektedir. Çalışmanın üçüncü aşamasında profil, sırtlık tahtası kalınlığı ve bağlantı mesafelerindeki değişiklikler mevcut durum ile karşılaştırılarak iletkenliğe etkisi incelenmiştir. İlk durum için profil kalınlığı değişimi incelenmiştir. Kalınlıklar mevcut duruma göre arttıkça veya azaldıkça doğal frekanslar artmıştır. Buna rağmen sol koltuğun ilk doğal frekanstaki iletkenliği kalınlık azaldığında artmış, kalınlık arttığında ise azalmıştır. Fakat sağ koltukta her iki durum için de iletkenlikler artmıştır. İkinci durumda ayak bağlantı mesafelerindeki değişiklik incelenmiştir. Hem orta bağlantı hem de simetrik bağlantı durumunda doğal frekansların mevcut duruma göre arttığı görülmüştür. Ayrıca sol koltukta yine ilk moddaki iletkenlik değerleri hem orta bağlantıda hem de simetrik bağlantıda mevcut duruma göre artmıştır. Sağ koltukta ise iletkenlik her iki bağlantı durumunda da mevcuda göre azalmıştır. Son durumda sırtlık tahtası kalınlığındaki değişimin frekanslara ve iletkenliğe olan etkisine bakılmıştır. Sırtlık tahtasının kalınlığı azaldıkça doğal frekanslar artmış, kalınlık arttıkça ise frekanslar azalmıştır (6.mod hariç). Yine yapının ilk modu ele alındığında sol koltukta kalınlık arttıkça iletkenlik fark edilir şekilde düşmüş ve kalınlık azaldıkça iletkenlik artmıştır. Sağ koltukta benzer bir durum gözlenmiştir. İletkenlikteki değişim oranları hem kalınlık arttığında hem de azaldığında benzerdir.
One of the significant factors affecting comfort in vehicles is vibration in seats. Passengers exposed to vibration face problems such as fatigue, lack of concentration, and low back pain. Therefore, eliminating or limiting the vibrations transmitted from the seat to the passenger is critical for comfortable travel. At this point, it is necessary to determine the dynamic properties such as natural frequencies, damping ratios, and mode shapes during the design phase, and accordingly to examine the transmissibility of the seat. In this study, modal analysis was carried out when the metal frame of a double passenger seat in a vehicle used in intercity transportation and wooden backrests are added to it. It was observed how the wooden backrests added to the backrest frame change the natural frequencies and mode shapes in the finite element software. It was noticed that wooden backrests slightly increased the natural frequencies of the seat in the 2nd, 4th and 5th modes, but decreased in the others, and it was seen some mode shapes changed compared to the beginning after the backrests were added. Additionally, the natural frequencies, damping ratios, and mode shapes of the frame structure were measured by the hammer test. It was seen from the experimental study that some natural frequencies of the seat were measured close to those belonging to the finite element model, and the similarities between them were drawn graph by comparing the relevant mode shapes and their Modal Assurance Criterion (MAC) indices. In the second stage, the back-and-forth vibration transmissibility of a double passenger seat against the vibration applied from the floor was investigated numerically using the ANSYS Random Vibration module. The Power Spectral Density (PSD) of the output acceleration from certain points on the upper seat frames versus the PSD input applied to the seat legs were calculated, and the transmissibility was obtained from the ratio of these two PSDs. It has been observed that some natural frequencies are slightly increased, and some are decreased if a wooden backrest is added. This shows that both the mass and stiffness properties of the added part affect the dynamic properties of the seat. In the studies, it was observed that the maximum transmissibility values decreased when the wooden layer was added. However, the transmissibility slightly increased when acceleration was applied especially, from the left foot at high frequencies. It is considered that these changes occur because the wooden part affects the damping and mode shapes of the structure. In the third stage of the study, the changes in profile, wood backrest thickness, and connection distances were compared according to the current situation, and the effect of transmissibility was examined. For the first situation, the profile thickness variation was examined. Natural frequencies increased as the thicknesses increased or decreased compared to the current situation. However, the transmissibility of the left seat at the first natural frequency increased when the thickness decreased and it decreased when the thickness increased. But in the right seat, transmissibility increased for both cases. In the second case, the change in connection distances was examined. It was observed that the natural frequencies increased compared to the current situation in both mid-connection and symmetrical-connection cases. In addition, in the left seat, the transmissibility values in the first mode increased compared to the current situation in both the middle connection and the symmetrical connection. In the right seat, the conductivity decreased in both connection conditions compared to the existing one. In the last case, the effect of the change in the thickness of the backrest on the frequencies and transmissibility was examined. As the thickness of the backrest wood decreases, the natural frequencies increase, and as the thickness increases, the frequencies decrease (except for the 6th mode). Again, considering the first mode of the structure, transmissibility decreased noticeably as the thickness increased in the left seat and it increased as the thickness decreased. A similar situation was observed in the right seat. The rates of change in transmissibility are similar both as thickness increases and decreases.
One of the significant factors affecting comfort in vehicles is vibration in seats. Passengers exposed to vibration face problems such as fatigue, lack of concentration, and low back pain. Therefore, eliminating or limiting the vibrations transmitted from the seat to the passenger is critical for comfortable travel. At this point, it is necessary to determine the dynamic properties such as natural frequencies, damping ratios, and mode shapes during the design phase, and accordingly to examine the transmissibility of the seat. In this study, modal analysis was carried out when the metal frame of a double passenger seat in a vehicle used in intercity transportation and wooden backrests are added to it. It was observed how the wooden backrests added to the backrest frame change the natural frequencies and mode shapes in the finite element software. It was noticed that wooden backrests slightly increased the natural frequencies of the seat in the 2nd, 4th and 5th modes, but decreased in the others, and it was seen some mode shapes changed compared to the beginning after the backrests were added. Additionally, the natural frequencies, damping ratios, and mode shapes of the frame structure were measured by the hammer test. It was seen from the experimental study that some natural frequencies of the seat were measured close to those belonging to the finite element model, and the similarities between them were drawn graph by comparing the relevant mode shapes and their Modal Assurance Criterion (MAC) indices. In the second stage, the back-and-forth vibration transmissibility of a double passenger seat against the vibration applied from the floor was investigated numerically using the ANSYS Random Vibration module. The Power Spectral Density (PSD) of the output acceleration from certain points on the upper seat frames versus the PSD input applied to the seat legs were calculated, and the transmissibility was obtained from the ratio of these two PSDs. It has been observed that some natural frequencies are slightly increased, and some are decreased if a wooden backrest is added. This shows that both the mass and stiffness properties of the added part affect the dynamic properties of the seat. In the studies, it was observed that the maximum transmissibility values decreased when the wooden layer was added. However, the transmissibility slightly increased when acceleration was applied especially, from the left foot at high frequencies. It is considered that these changes occur because the wooden part affects the damping and mode shapes of the structure. In the third stage of the study, the changes in profile, wood backrest thickness, and connection distances were compared according to the current situation, and the effect of transmissibility was examined. For the first situation, the profile thickness variation was examined. Natural frequencies increased as the thicknesses increased or decreased compared to the current situation. However, the transmissibility of the left seat at the first natural frequency increased when the thickness decreased and it decreased when the thickness increased. But in the right seat, transmissibility increased for both cases. In the second case, the change in connection distances was examined. It was observed that the natural frequencies increased compared to the current situation in both mid-connection and symmetrical-connection cases. In addition, in the left seat, the transmissibility values in the first mode increased compared to the current situation in both the middle connection and the symmetrical connection. In the right seat, the conductivity decreased in both connection conditions compared to the existing one. In the last case, the effect of the change in the thickness of the backrest on the frequencies and transmissibility was examined. As the thickness of the backrest wood decreases, the natural frequencies increase, and as the thickness increases, the frequencies decrease (except for the 6th mode). Again, considering the first mode of the structure, transmissibility decreased noticeably as the thickness increased in the left seat and it increased as the thickness decreased. A similar situation was observed in the right seat. The rates of change in transmissibility are similar both as thickness increases and decreases.
Açıklama
Lisansüstü Eğitim Enstitüsü, Makine Mühendisliği Ana Bilim Dalı
Anahtar Kelimeler
Makine Mühendisliği, Mechanical Engineering
