Globoid dişli sistemleri için alternatif dişli malzemelerinin belirlenmesi ve incelenmesi
Investigation of alternate bronze materials about globoidal wormgear
Yazar: MUSTAFA SAĞLAM
Danışman: DOÇ. DR. OSMAN HAMDİ METE
Sakarya Üniversitesi / Fen Bilimleri Enstitüsü / Makine Mühendisliği Ana Bilim Dalı / Makine Tasarım ve İmalat Bilim Dalı
Derece: Yüksek Lisans, 2024
Konu: Makine Mühendisliği = Mechanical Engineering

Özet:
Globoid teknolojisi, dünya çapında kullanılan yüksek teknoloji sınıfındaki üç farklı dişli teknolojisi harmonic, cycloid, globoid arasında en gelişmiş ve en zor olanıdır. Diğer dişli sistemlerinin aksine globoidin giriş ve çıkış milleri birbirine tam 90 derece açıyla durur. Bu sayede globoid dişli teknolojisi, 90 derece açıyla çalışması gereken yüksek teknoloji uygulamalarında tek çözüm olmaktadır. Bu çalışma kapsamında savunma sanayinde oldukça fazla kullanılan globoid dişli-vida setinde dişli malzemesi için alternatif bir malzeme aranmaktadır. Alternatif malzeme aramamızdaki amaç malzemenin ekonomik yönden istenilen maliyetleri ve kırılma tokluğu olarak istenilen aralığı sağlayacak malzemeyi seçmektir. Her malzeme için spektrometre raporu alınacak ve alternatif malzemeler oluşturulacaktır. Bu kapsamda 3 farklı türde 6 adet malzeme belirlenmiştir. Bunlar bronz alaşımlarından alüminyum bronz, fosfor bronzu; çelik alaşımlarından 8620, 4140, 1050; Kestamid türünden ise kestlub olarak belirlenmiştir. Farklı türde malzemelerin denenmesindeki amaç ise globoid dişli setinden oluşturulan redüktörlerin küçük hacim, düşük ağırlık ve yerine göre düşük devirlerde kullanılmak istenmesidir.
Tez kapsamında belirlenen deney düzenekleri ve deney numune sayısı ise şu şekildedir: Her malzeme için minimum 3 adet olacak şekilde çekme, sertlik ve V çentik darbe deney numunesi, 6 malzemeden 1'er adet 2 inç dişli ve 1'er adet karşılık vidası olacak şekildedir. Numunelerin boyutları ve şekilleri ASTM E8 referans alınarak belirlenmiş ancak standartta deney cihazının yapısına göre değiştirilebileceği belirtilen noktalar, deneyi yapacak olan akademisyen tarafından belirlenmiştir. Mevcut olarak kullanılan bronz alaşımlarının küçük boyutlarında tedarik imkanı oldukça zor olduğu için sertlik testleri dişli üzerindeki fatura yüzeyi üzerinden yapılmıştır. Deneyin asıl amaçlarından biri ise dişliyi karşılık vidasından tahrik ederek kırmaya çalışmaktır. Bu sebeple üretilen setler kırılma testi için standart bir gövde içine alınarak incelenecek ve ardından grafikler ile karşılaştırmalar yapılacaktır. Grafiklerin karşılaştırmalarından çıkan sonuçlar dikkate alınarak kullanım koşullarına göre dişli ve vida için en uygun malzemeler belirlenecektir. Buna göre, her koşul için tek bir malzeme cinsi kullanılması yerine; kullanım koşullarına göre en uygun malzeme belirlenmiş olacaktır. Bu sayede maliyet ve tedarik süreleri de optimize edilmiş olacaktır.
Globoid dişli sistemleri üzerine faaliyet gösteren firma; AGMA 6135 A02 göre üretim yapmakta ve buna göre malzeme seçimlerini yapmaktadır. TSEK belgesi için çalışmalarını sürdürmekte ve buna yönelik olarak malzeme seçimleri için kendi standartlarını belirlemek istemektedir. Bu bağlamda malzeme seçimi kriterleri, tarafımca yapılmış olan deneylere göre belirlenecektir. Malzemelerin tedariği ve üretimi firma bünyesinde gerçekleştirilmiş, mevcut deneyler Sakarya Üniversitesi laboratuvarlarında yapılmıştır.

Abstract:
Globoid technology is the most advanced and difficult of the three different high-tech gear technologies used worldwide: harmonic, cycloid and globoid. Unlike other gear systems, the input and output shafts of the globoid stop at exactly 90 degrees to each other. This makes globoid gear technology the only solution for high-tech applications that require a 90 degree angle operation.
Within the scope of this study, an alternative material is sought for the gear material in the globoid gear-screw set, which is widely used in the defense industry. The purpose of searching for an alternative material is to select the material that will provide the desired range in terms of economic costs and fracture toughness.
A spectroscopy report will be obtained for each material, and alternative materials will be developed. Within this scope, six materials of three different types have been identified. These include aluminum bronze and phosphor bronze from bronze alloys; 8620, 4140, and 1050 from steel alloys; and a type of nylon called 'kestmide' specified as 'kestmide-lub'. The purpose of testing different types of materials is to use reducers made from globoid gear sets in small volumes, low weight, and, depending on the application, at low speeds.
The experimental setups and the number of samples determined within the scope of the thesis are as follows: For each material, there will be a minimum of 3 tensile, hardness, and V-notch impact test samples. Additionally, for each of the 6 materials, there will be 1 gear with a 2-inch diameter and 1 corresponding screw. The dimensions and shapes of the samples are determined based on ASTM E8 standards, with the flexibility to be adjusted according to the structure of the testing apparatus, as specified by the academician conducting the experiment.
Tensile testing machines work by applying a standard speed to pull a material and measure its stress-strain curve. This curve illustrates how the material behaves during tension. Tensile tests on metals or metal materials are primarily based on DIN EN ISO 6892-1 and ASTM E8 standards. Both standards specify the sample shapes and tests. The purpose of these standards is to define and introduce the test procedure in a manner that ensures comparable and accurate characteristic values will be determined, even when different test systems are used. As a result of this experiment, force (F) and elongation (Δl) curves are obtained. However, the more accepted curve is the stress-strain curve. Therefore, the applied force is converted to stress values by dividing by the initial cross-sectional area of the sample (σ=F/A0), and the force values are transformed into stress values to obtain the stress-strain graph.
As a result of the tensile test, strength values such as the elastic modul, yield strength, and ultimate tensile strength, as well as ductility values such as elongation at fracture, reduction in area, and toughness, can be determined. These characteristics depend on the type of material, its chemical composition, and its metallurgical structure. Hardness is defined as the resistance of materials to plastic deformation. Hardness tests are crucial mechanical tests that allow for quick and non-destructive control of both materials and manufactured parts. Commonly used hardness measurement methods in technology are based on measuring the size of the permanent indentation obtained on the sample. Hardness measurement involves measuring the resistance exhibited by the material when a standard indenter is pressed into it. In Brinell hardness measurements conducted on a manual machine, appropriate scale charts are mounted on the main indicator to ensure accurate representation of the materials hardness values. Since there is no suitable chart for plastic material, Shore D is used for measuring the hardness of kestlub material. During the experiment, when measuring phosphor bronze, since the indicated value on the scale exceeded 180, the hardness value of phosphor bronze was determined using Rockwell for a more accurate measurement.
Engineers desire to understand the behavior of materials regarding which temperatures they become ductile or brittle, how much energy they can absorb during fracture, or how much fracture energy they might possess.
The fracture energy value determined through notch impact testing, like the results of tensile tests, is not used as a numerical value in engineering design calculations. Instead, the fracture energy value is solely used to evaluate the fracture behavior of the material and to gain insight into its ductility or brittleness.
In fact, the V-notch test symbolizes the true dynamic behavior of materials. Considering gear mechanisms, they are often subjected to sudden force changes and loads. For globoid gears, our expectation from alternative materials is that they exhibit ductile behavior against these sudden shocks. Materials with low load-bearing potential are preferred for applications where manual rotation is performed.
One of the main purposes of the experiment is to attempt to break the gear by driving it from the corresponding screw.
Due to the high cost of setting up the system for breaking under dynamic conditions and for accurate measurement, at this stage, the reducer was subjected to loading under static conditions. Reducers with a 1/40 transmission ratio form self-locking, so no additional locking mechanism was required. In the initial setups created for this test, a simple force lever and a structure capable of hanging weights at the end of the lever arm were constructed. However, it was observed that the gears used in the experiment were too resilient, making it difficult to reach the necessary weight limits for breakage. Lever arms of 500 and 1000mm did not provide a solution for this issue. Subsequently, continuously extending the lever arm exacerbated the weight problem.
The flexibility of the long lever arm slightly retained the load on the gear, thus preventing accurate measurement.
Globoid gears, each made from different materials, have been manufactured with corresponding globoid worm and bodies. The recesses in the reducer body's interior have been crafted to meet the required conventional and geometric tolerances. Synthetic Mobil SHC 634 has been used for lubrication in each mechanism. Data obtained from the experimental setup are calculated using the necessary equations to determine input-output mechanical powers and calculate breaking torques. As part of the experimental studies, the dimensions of the reducer for fracture testing were determined by the manufacturer. Examinations were conducted on a standard globoid reducer with a center distance of 2 inches (50.8mm) and a gear ratio of 1/40. Specific globoid worm, gear covers, worm caps, bodies, and output shafts were manufactured for each gear sample. Since the aim of the experiment was to measure and evaluate the torque that the gear could withstand, no special sample was produced for this test. Turning and milling processes were carried out according to the gear's technical drawing within the standard product.
In the setup where a press is used, the standard globoid reducer is attached to the press base, and the preferred load cell for accurate measurement is connected to the moving part of the press with necessary fasteners. A 500mm force lever is placed on the reducer output, and the force transmission between the force lever and the load cell is established using any steel alloy. With a movement of 3mm per second applied to the press, over time, it pushes the force lever downward, exerting pressure on the gear profiles and causing them to break under a certain weight. For certain gear alloys, it was necessary to give the force lever a specific angle. This is because when the force lever is brought to the zero position, it is very close to the base, and without appropriate weight measurement, the lever contacts the press base.
In the experiment, materials with six different properties exhibited various behaviors under applied force. To elaborate, in the first scenario, the gear profiles were fractured to obtain necessary data; in the second scenario, the gear profiles began to deform; and in the final scenario, due to the product's unique wedge system failing under the load, resulting in the wedge being cut, causing the force arm to become disengaged. As data from the points where the wedges were cut was deemed usable and indicative of structural failure, it was included in the evaluation results.
In conclusion, to evaluate the initial objective of this thesis, which is to achieve the desired fracture torque, it is necessary to assess other experimental methods. Among the six different materials tested in the experiment, one of them is the currently used gear material. Therefore, comparison has been facilitated. For the fracture test conducted under static conditions, the most helpful complementary experimental method is the stress-strain graphs obtained from tensile tests.
Our expectation from infinite gear mechanisms is that they can withstand the required torque values. The Kestlub material is deemed unsuitable for use due to its low load-bearing torque values. However, it can serve as a solution in applications where manual rotations are preferred. In the provided graphs, materials with low stresses experienced sudden fractures during the regular weight loading conducted during the fracture test.
Hardness was defined as the resistance of materials to plastic deformation. In the case of reducers breaking under high torques, we prefer the gear to be the material susceptible to damage. In infinite gear mechanisms, gears operate with a mating worm. Therefore, we expect the hardness of the worm, made from modified 4140, to be lower than that of the gear. Compared to the material produced with a maximum hardness of 40 Rockwell (375 HB), all the materials selected at this stage can be considered suitable.
The V-notch test symbolizes the true dynamic behavior of materials. Considering gear mechanisms, they typically experience sudden loads from the motor at the initial moment. For alternative materials in globoid gears, our expectation is that they exhibit ductile behavior against these sudden shocks. The experiment revealed the maximum load that materials in their normalized state could withstand. It was concluded that the Kestlub alloy is unsuitable for motorized applications. In the scope of this thesis, an optimal setup was devised to obtain accurate data in the fracture test, which will ultimately determine the main outcome. Experimental results were obtained to the extent allowed by the apparatus. The primary objective was to subject the gear to high torques in order to break it. However, due to the specific wedge system used to secure the gear set, torque values at the point where the wedges were cut under certain weights were taken into consideration. For addressing this issue, particularly encountered in steel materials, increasing the number of wedges used in the output shaft or modifying the wedge widths would provide a solution.