Failure of a rotary tiller spur gear

Transcription

1 Engineering Failure Analysis 12 (2005) Failure of a rotary tiller spur gear Ibrahim Akinci *, Deniz Yilmaz, Murad Çanakci Department of Agricultural Machinery, Faculty of Agriculture, University of Akdeniz, Antalya, Turkey Received 8 April 2004; accepted 10 April 2004 Available online 4 January 2005 Abstract This paper analyzes the failure of a transmission gear in a rotary tiller. The failed gear was examined, the reasons for the failure were determined and preventive methods were explained. The failure types were abrasion and plastic deformation. The reasons for the failure were design errors and material faults. Selection of the gear material was wrong (the material was not hard enough) and the tooth profile was not correct. Ó 2004 Elsevier Ltd. All rights reserved. Keywords: Gear-tooth failures; Agricultural machinery failures; Wear; Abrasive wear; Plastic deformation 1. Introduction Failure studies have previously been carried out in the authorsõ institution to prevent damage in machinery parts and to develop the techniques of failure analysis [1 3]. In soil tillage implements, the rotary tiller is one of the main machines in minimum tillage farming. The machine is driven by tractor Power Take Off (PTO) shaft line. It is especially used in field, fruit and greenhouse agriculture. The rotary tiller is used to mix the topsoil layer and to prepare the seedbed preparation directly. In field works, it is operated under heavy loads and vibration. These effects can cause some problems such as breakdowns, failures etc. Most failures occurred in gear systems [4]. In this study, the failure types and failure reasons were explained and a sample failure that occurred in the transmission gear of a rotary tiller was examined. Its construction system, material features and working conditions were investigated and analyzed. Failure reasons were determined and preventive methods explained. * Corresponding author. address: iakinci@akdeniz.edu.tr (I. Akinci) /$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved. doi: /j.engfailanal

2 2. Failure types and reasons 2.1. Failure types Failure types are fracture, surface fatigue, abrasion and plastic deformation in gear mechanisms [5]. Fracture damage results from surface fatigue, high loads or abrasions. Surface fatigue results from tensile stress, compression stress and sliding stress under the gear surface. Abrasion damage is defined as loss of material at touching gear surfaces Failure reasons I. Akinci et al. / Engineering Failure Analysis 12 (2005) Failure reasons are faults of usage, heat treatment, design, manufacture and material [5]. Usage faults are caused by incautious operation and insufficient technical knowledge. These faults can cause important damage in machinery. Design faults include incorrect shape, dimensional errors, bearing faults, selection of wrong material and insufficient technical knowledge. In studies of gear design, usually a dimensional error has been made and or the wrong material has been selected. In research on gear failure types and reasons, 931 gears were investigated during 35 years [5]. Results show that the failure types were fractures of 571 gears, fatigue of 188 gears, abrasion of 122 gears and plastic deformation of 50 gears. The most encountered failure is fracture (61.2%) and the least is plastic deformation (5.3%) (Table 1). The failure reasons were determined as usage faults of 696 gears, heat treatment faults of 151 gears, design faults of 64 gears, manufacturing faults of 13 gears, material faults of 7 gears. Consequently, the most encountered failure is usage fault (74.7%) and the least one is material fault (0.8%) (Table 2). Table 1 Failure types Failure type Gear Number % Fracture Surface fatigue Abrasion Plastic deformation Total Table 2 Failure reasons Failure reason Gear Number % Usage faults Heat treatment faults Design faults Manufacturing faults Material faults Total

3 402 I. Akinci et al. / Engineering Failure Analysis 12 (2005) Failure analysis 3.1. Machine features The Rotary tiller is attached to a tractor threepoint linkage system and driven by the tractor PTO. Power is transferred from the PTO to the first gearbox. The drive direction changes by 90 at the horizontal shaft to the second gearbox. The Rotor shaft is used as power and movement transmission from the gearbox to the spade knives. The output shaft is horizontal and is driven by the second gearbox. A Clutch safety system is placed between the universal joint and the first gearbox to prevent overload. The Transmission system is shown in Fig. 1 and the technical features are given in Table 3. Fig. 1. Transmission system of rotary tiller. Table 3 Technical features of rotary tiller Technical feature Length (mm) 1250 Width (mm) 1930 Weight (kg) 450 PTO power (kw) 22 PTO revolution (min 1 ) 540 Gear in first gearbox (number) 2 Teeth in first gearbox (number) 10, 23 Gear in second gearbox (number) 3 Teeth in second gear box (number) 31, 43, 38 Transmission ratio of first gear box Transmission ratio of second gear box Transmission ratio PTO-rotor shaft Spade knives (number) 48 Spade knives (type) L Spade knives speed (m s 1 ) 4.74

4 3.2. Failure analysis of rotor shaft gear I. Akinci et al. / Engineering Failure Analysis 12 (2005) The failed component is the rotor shaft gear in the second gearbox. The failure types are abrasion damage and plastic deformation (see Fig. 2). The rotor shaft gear has 31 teeth, the tooth module is 6, the addendum circle diameter is 200 mm, the pitch circle diameter is 185 mm, the face width is 38 mm, the tooth thickness is 12 mm, and the tooth height is 15 mm (see Table 4). The gear material is SAE1020 steel. The flow stress out is 230 MPa, the rupture stress is MPa, the surface hardness is 69.2 R B, the carbon amount is 0.105% (wt) and the silicon amount is 0.024% (wt) (Table 5). Failure reasons were determined as design errors and material faults. The Gear material is SAE1020 steel which has a low rupture stress and hardness. This type of material should not be used in gear production. The gear material must be alloyed steel and the gear profile must be more rounded. For instance, SAE1050 steel should be used. This material consists of 0.45% C, 0.60% Mn, 0.10% Si, 0.040% P, 0.050% S. The surface hardness should be more than 40 R C. Fig. 2. Failured gear.

5 404 I. Akinci et al. / Engineering Failure Analysis 12 (2005) Table 4 Features of rotary gear Technical feature Teeth (number) 31 Teeth module 6 Addendum circle diameter (mm) 200 Pitch circle diameter (mm) 185 Face width (mm) 38 Tooth thickness (mm) 12 Tooth height (mm) 15 Table 5 Features of gear material Material feature Steel type SAE1020 Flow stress (MPa) 230 Rupture stress (MPa) Surface hardness (R B ) 69.2 Carbon (%) (wt) Silicon (%) (wt) Oil type SAE Results The results and suggestions are summarized as follows: 1. Failures types in the rotor shaft gear are abrasion damage and plastic deformation. 2. Failure reasons are design errors and material faults. 3. The gear material was SAE1020 steel. This type of material has insufficient hardness for gears. SAE1050 steel is preferred. 4. The gear profile is a trapezoidal shape. Tooth roots should be rounded more. Acknowledgement This study was partly supported by the Scientific Research Administration of Akdeniz University, Antalya, Turkey. References [1] Rende H. Failure and failure analysis of machine element. Lecture notes (unpublished). Department of Machinery Engineering, Akdeniz University, Turkey; [2] Yılmaz D, Akinci I, Canakci M. Failure and failure analysis on machine element, vol. 20. National agricultural mechanization congress proceedings, Turkey: Harran University; 2001 p [3] Eryürek B. Failure analysis. Istanbul, Turkey: Birsen Press; p [4] Akkurt M. Machine elements. II. Istanbul, Turkey: Birsen Press; p [5] Adanır H. Failure of saft and gears used in tractor. MSc Thesis, Yildiz Technic University, Turkey; p. 95.