PA 66 kompound malzemeden imal edilmiş asansör kapı makaralarının tribolojik özelliklerinin incelenmesi
Analyses of the tribological characteristics of  elevator door rollers made of compound PA 66 material
Yazar: YUSUF EREN ERDOĞDU
Danışman: YRD. DOÇ. DR. İBRAHİM MEHMET PALABIYIK
İstanbul Teknik Üniversitesi / Fen Bilimleri Enstitüsü / Makine Mühendisliği Ana Bilim Dalı
Derece: Yüksek Lisans, 2014
Konu: Makine Mühendisliği = Mechanical Engineering ; Polimer Bilim ve Teknolojisi = Polymer Science and Technology

Özet:
Son yıllarda, polimerler endüstride hızla artan bir ivmeyle, özellikle demir, çelik gibi metallerin yerini almaya başlamıştır. Düşük maliyet, hafiflik, kolay işlenebilme gibi özelliklerinden dolayı tercih edilen polimerlere, katkı maddeleri yardımıyla istenilen mekanik özellikler ve estetik görünüm kazandırılabilmektedir. Günümüzde sabit veya hareketli işlevi olan birçok makine elemanı polimer malzemelerden imal edilebilmektedir. Özellikle hareketli makine elemanlarının imalatında kullanılan polimer malzemelerin tribolojik özelliklerinin uygunluğu çok önemlidir. Bundan dolayı polimer malzemelerin tribolojik davranışlarını gözlemleyebilmek amacıyla birçok test yöntemi uygulanmıştır ve çok sayıda test cihazı geliştirilmiştir. Tribolojik testler, tribotest cihazları veya tribometre adı verilen düzenekler aracılığıyla gerçekleştirilmektedir. "Tribometre" kavramı ilk olarak sürtünme ölçmek amacıyla tasarlanmış bir düzenek için 1774 yılında kullanılmıştır. Yeni ve daha verimli çalışan tasarımlar için farklı malzemelerin farklı koşullardaki tribolojik davranışlarının tribolojik testler aracılığıyla incelenmesinin bir zorunluluk haline gelmesiyle birlikte daha modern tribotest cihazları geliştirilmiştir. Bu cihazlarda yapılan tribolojik testlerden yararlanılarak, parçaların bakım aralıkları, ömürleri, güvenilirlikleri belirlenmektedir. Testlerden elde edilen veriler ışığında polimerlerin sahip oldukları özelliklere en uygun alanlarda kullanılması sağlanmaktadır. Malzemeden beklenen performansa ve işleve göre, o malzeme, istenen özellikleri bünyesinde barındıracak şekilde belli sınırlar içerisinde geliştirilebilmektedir.
Polimer malzeme kullanımının yaygınlaşmasıyla birlikte diğer sektörlerde olduğu gibi asansör sektöründe de sıklıkla polimerlerden imal edilmiş parçalar kullanılmaya başlanmıştır. Bu parçalardan biri de asansörlerde kapı panellerini taşıyan ve performansı asansör hızını, konforunu, güvenliğini önemli ölçüde etkileyen asansör kapı makarasıdır.
Yapılan çalışmada mühendislik alanında kullanılan ilk polimer olan poliamid 66'nın MoS2 katkılı halinden imal edilmiş asansör kapı makaralarının belirli normal yükler altındaki aşınma davranışları, özel olarak tasarlanmış çift disk deney tesisatıyla deneysel olarak incelenmiştir. Bu çalışma için hem kayma hem de yuvarlanma hareketinin birlikte oluşarak aşınmaya sebebiyet verdiği şartları simule eden özel bir tribotest cihazı tasarlanmıştır. Çift disk test cihazı tasarlanırken ve çalışma yürütülürken çeşitli parametreler göz önünde bulundurulmuştur. Bu parametreler, aynı zamanda aşınmayı da doğrudan etkileyen kayma hızı, yük, sıcaklık, yüzey pürüzlülüğü ve nemdir. Deney parametreleri, asansörlerin pratikteki kullanımlarına göre ve buna ilaveten daha zorlu şartlar da göz önünde bulundurularak geliştirilebileceği sınırlara göre tayin edilmiştir. Deneyler, ortam sıcaklığı 18-20 °C ve nem oranı yaklaşık %60 olan laboratuvar koşullarında yürütülmüştür.

Çalışmada kontrollü değiştirilebilen parametreler kayma hızı ve normal yük iken, ölçülebilen parametreler ise aşınma miktarı ve ara yüzey sıcaklığıdır. Sistemde normal yük, kompresörden gelen sıkıştırılmış havadan doğan basınç kuvvetinin pnömatik pistonlar aracılığıyla aktarılmasıyla elde edilmiştir. İstenilen yüke denk gelen basınç, regülatör üzerinden ayarlanabilmektedir. Kayma hızı ise, karşı yüzey olan krom kaplı çelik diskin tahrik aldığı redüktöre bağlı bir DC motor üzerinde frekans ayarlaması yapılmasına imkan veren bir invertör aracılığıyla kontrol edilebilmektedir. Bunun yanında, aşınma miktarı belli aralıklarla hassas terazide ölçülmüştür. Ara yüzey sıcaklığı da aynı aralıklarla kızıl ötesi termometre yardımıyla temas yüzeyleri üzerinden ölçülmüştür. Ayrıca sistemde, disk üzerine yerleştirilen bir mıknatıstan her bir dönüşte uyarı alan manyetik sensör ve bu sensörün bağlı olduğu sayaç yardımıyla toplam dönme sayısı görülmüş ve bununla da mesafe hesaplanmıştır. Optik mikroskop yardımıyla da yüzeydeki malzeme transferi görüntülenmeye çalışılmıştır.
Bu çalışmada MoS2 katkılı poliamid 66 malzemeden imal edilmiş numune makaralar 18-20°C ortam sıcaklığında test edilmiştir. Her testte aynı anda dört numune bulunmak üzere sırasıyla 4 MPa, 3 MPa, 2 MPa ve 1 MPa yüzey basınçlarında ve tek kayma hızında (1,25 m/sn) çalışılmıştır. Testler 1269,20 km boyunca sürdürülmüştür. Çalışma sonucunda yük miktarı arttıkça aşınma miktarının da buna bağlı olarak arttığı görülmüştür. Ara yüzey sıcaklığının başlangıçta kayma mesafesi arttıkça arttığı görülmüştür. Ara yüzey sıcaklığında, yüksek yükler altında fazla derecede artış gözlenirken, düşük yüklerde belirgin değişmeler gözlenmemiştir. Çalışma sonunda transfer film oluştuğuna dair bulgulara rastlanılmamıştır.

Abstract:
During recent years, in the field of industry, polymers have started to replace metals like iron and steel. Polymers, preferred because of properties such as low cost, lightness and formability, can easily be modified mechanically and aesthetically by the use of additives. Nowadays, many machine elements; either dynamic or static, can be manufactured from polymer materials. The compatibility of tribological properties of polymer materials, especially in polymers used in dynamic mechanical elements, is of great importance. For this reason, many testing procedures and equipment have been developed to observe the tribological behaviour of polymer materials. Tribological experiments are run using tribotest devices or setups called tribometers. The concept of "Tribometer" was first used for a test set-up which was built in order to measure friction in year 1774.In time, investigating tribological behaviours of various materials under various circumstances has become a must in order to produce new and more efficient designs; resulting in development of modern tribotest equipment. Making use of the outputs of tribological tests run with this equipment, maintenance intervals, life cycles and reliability of components can be calculated. In the light of the results evaluated from the tests, proper usage of polymers in accordance with their technical attributes is maintained. According to the performance and the function of the material, keeping the vital properties stable, that specific material can be developed to a certain extent. For instance, wear resistance of PA 66 material can be increased using additives like MoS2, PTFE and graphite. MoS2 has a high affinity for metals. Once attracted to the metal, it fills the metal's microscopic pores, making the metal surface slippery. This makes MoS2 the ideal lubricant for applications in which nylon wears against the metal. MoS2 usage also brings advantages during molding, such as reducing molding time and decreasing shrinkage. It is also known that, MoS2 which increases the pv value of polymers, does not always improve the tribological properties of polymers but sometimes may have an unfavourable affect on them. As the usage of polymer material has become widespread, the usage of components made of polymer in elevator production has started to increase, as well as the other areas. One of these components is the elevator door roller which carries the door leaves and which has great importance in determining the speed, comfort and safety of the elevator.
For this study, a special tribotest device has been designed, which can simulate the conditions in which both sliding and rolling motion cause wear together. Certain parameters have been considered while two disk test device was designed and the study was conducted.Those parameters are, sliding speed, load, temperature, surface roughness and moisture, which at the same time affect the magnitude of wear. Experiment parameters were determined considering the practical use of elevators and additional heavy conditions. Experiments have been run in the lab of the ambient

temperature of 18-20 °C and about 60% moisture. During load determination, eligibility of pv limits as well as the practical use of elevators were taken into consideration. pv limit is an important factor determining the operating conditions of polymers. One must be careful about the pv limit, since pressure and temperature increases with increasing wear, therefore causing the temperature rise above the limits between which the material can show normal sliding behaviour. Four different pv values were considered as 1,25 MPa.m/sec, 2,5 MPa.m/sec, 3,75 MPa.m/sec and 5 MPa.m/sec, since the tests were performed with constant velocity and different surface pressures.
In this study, the wear behaviour of the elevator door rollers made of a MoS2 additive version of Polyamid 66 is experimentally investigated under certain nominal loads utilizing the specifically designed two disk experimental set-up.
The samples, being more frequent in the beginning, have been controlled with certain intervals. In each control, first the interface temperature has been measured with infrared thermometer, followed by a weight check in the sample, using a precision balance. In each experiment, the disk surface has not been cleaned until the end, in order to avoid affecting the film formation. At the end of the experiment, this time not to affect surface roughness, the material transferred to the disk surface was cleaned with the help of cotton and acetone.
In the study, sliding speed and the nominal load are set as the control parameters while wear rate and the interface temperature are the measurable ones. In the set-up, nominal load is maintained by transferring pressure force coming through the pressurized air from the compressor, using pneumatic pistons. The pressure can be adjusted from the regulator so that the desired load is maintained. Sliding speed is regulated by the utilization of an invertor which enables frequency adjustment on a reducer coupled DC motor, from which the counter face ( chrome plated steel disc) is driven by. Additionally, wear rate has been periodically measured by accurate scale. The interface temperature has been measured at the same frequency as well, by taking data from the interacting surfaces with the help of an infrared thermometer. Furthermore, total number of revolutions and corresponding distance have been calculated with the data taken from the rotational speed sensor mounted on the device. Material removal have been viewed using an optical microscope.
In this study, sample rollers made of MoS2 added Polyamid 66 whose density is d=1,14 gr/cm3 were tested in dry conditions. 4 samples were analized in each test. The surface pressures have been altered as 1 MPa, 2 MPa, 3 MPa and 4 MPa, whereas the sliding speed was kept constant as 1,25 m/sec. In the study, 1 MPa surface pressure correspounds to 98,17 N normal load, 2 MPa surface pressure corresponds to 196,35 N normal load, 3 MPa surface pressure corresponds to 294,52 N normal load and 4 MPa surface pressure corresponds to 392,70 N normal load.The test kept running for 1269,20 km in total. During the study, 16 samples were tested.. During the test, first there had been polymer material transfer to metal surfaces and metal surfaces were covered with polymer. Then it has been observed that particles were come off from the samples. It's also been observed that, after surface coverage, there had been material transfer from metal surface to polymer sample surface.
At the end of the study, it has been observed that, the position of the samples on test device had no effect on wear progression under various loads. Checking the similarity between the samples, it has been concluded that the test device was suitable to test 4 samples in each run. Slight axial eccentricities had negligible effect

on the test results. Analyzing the test results, it's been concluded that wear quantity had a linear increase with the magnitude of nominal load in the tests run under 1,2 and 3 MPa surface pressures. During the 4 MPa test, the similar behaviour has been observed till 400 km sliding distance; whereas after 400 km sliding distance, the wear quantity has been observed to increase acceleratively. As a general result, it has been observed that increase in wear quantity was directly proportional to the increase in load magnitude.
It has also been observed that the interface temperature increase was directly proportional with the increase in sliding distance, at first. Under high magnitude load, the interface temperature has experienced high increase, whereas no significant increase has been observed under low magnitude loads. Having the test results analized, it has been observed that the interface temperatures had started to increase starting from ambient temperature,till the first measurement performed at 13,5 km distance value, afterwards followed by an increase with a decreasing acceleration to a certain point, and after 400 km distance, it's been seen that interface temperature value has remained almost constant with minor variations. When the co-relation is searched, it has been determined that the interface temperatures were higher under high pressures, lower and more stable under low pressure values. Considering this, one can conclude that increasing load magnitude results in an increase in surface temperatures, between the test intervals. When the interface temperatures are checked, it's been measured that the temperature had increased to 52,2 °C under 4 MPa surface pressure, to 45,5 °C under 3 MPa surface pressure, to 35,7 °C under 2 MPa surface pressure and to 28,7 °C maximum under 1 MPa surface pressure. These temperature ranges are considered as ineffective on the wear behaviour of PA 66 with MoS2 additive, therefore in this study, interface temperature is considered as a result of the applied loads, more than a factor affecting wear quantity. No evidence of transfer film formation has been found in this study.