CHAPTER 6 FRICTION AND WEAR ANALYSIS FOR BUSHING

Size: px

Start display at page:

Download "CHAPTER 6 FRICTION AND WEAR ANALYSIS FOR BUSHING"

Harvey Morris
5 years ago
Views:

1 CHAPTER 6 FRICTION AND WEAR ANALYSIS FOR BUSHING 6.1 TEST RIG SETUP FOR THE FRICTION AND WEAR ANALYSIS Knowing the frictional coefficient is important for the determination of wear loss and power loss conditions; an appropriate test rig is used to determine friction of floating bush bearings. The frictional coefficient of bearings in lubricated conditions has been examined in experiments. The force known as friction may be defined as the resistance encountered by one body moving over another. This broad definition concludes two important classes of relative motion: sliding and rolling. The ratio between this frictional force and the normal load is known as the coefficient of friction and is usually denoted by the symbol µ and mathematically it can be represented by, µ = F f /F n 191 P a g e The magnitude of the frictional force is conveniently described by the value of the coefficient of friction. The friction coefficient under relative motion and impeding motion is defined by static and kinetic friction coefficients. These two types of friction coefficients are conventionally defined as follows: µ = F f /F n and µ f = F k /Fn, where F f is the force just sufficient to prevent the relative motion between two bodies, F k is the forces needed to maintain relative motion between two bodies, and Fn is the force normal to the interface between the sliding bodies [39]. As described elsewhere [40], six categories can be used to characterize friction testing devices: 1. Gravitation-based devices 2. Direct linear force measurement devices 3. Torque measurement devices 4. Tension-wrap devices 5. Oscillation-decrement devices 6. Indirect indications Gravitation-based devices have been proposed for at least 500 years, and some of them are shown in the notebook sketches of Leonardo da Vinci. In some configurations, like

2 flat-on-flat testing or pin-on-disk testing, the friction force can be measured directly with mounting strain gauge on structure, a load cell or similar force sensor mounted in line with the contact. In other systems, such as swept circular contacts (disk brakes, drum brakes, rotation seals, etc.), friction coefficients are obtained from torque measurements and component dimensions. Tension-wrap devices use the differences in tension resulting between the ends of a sheet of material or a wire wrapped over a circular body. 6.2 PRINCIPLE & TEST RIG FOR BUSHING ANALYSIS Figure 6.1 Schematic diagrams for the Experiment test rig The basic principle used in the construction of test rig is shown in the figure 6.1 above. With the help of above principle we constructed test rig for finding out cp efficient of friction for different bushings. The above schematic diagram helps us to understand the frictional force offered by the system and its measurement. For measuring frictional force transducer known as the load cell was used. The new measuring system works on the following principle. The normal contact of a bearing with a journal is achieved using the cable brake principle, while the friction of the bearing is measured from the force difference at the two ends of the cable due to friction torque during running. 192 P a g e

3 Figure: 6.2 Zoom view for test ring working principle The measuring principle of friction in bearing can be illustrated in Figure 4.9 and Figure The journal of bearing is connected to the principle axis of the motor whose rotation can be easily modulated. The two upper and lower parts of the bearing are attached to the journal with a soft string (as the cable) wrapped around them. The upper end of the string is fixed to a cantilever leaf spring, while the lower end is tied to a standard weight. With the strain gauges stuck at the root of the leaf spring, the tensile force of the upper end of the string can be precisely measured. The advantage of this system lies in that the stretching of the string in a line provides only normal contact between the bearing and the journal. This avoids the journal suffering from deflection, which is of great importance for spindles. The upper end of the string is fixed to a cantilever leaf spring and the lower is tied and hook up to standard weight. The Load cell which mounted on the cantilever leaf spring is calibrated with different weights at the string end before friction tests. The test rig was operated nearly 25 to 30 minutes at no load to achieve balance conditions and take into account the effect of friction from the bracket, shafts, motor etc.the string forces at both the upper and lower ends are adjusted to be equal before measuring, which determines the initial normal load. This can be achieved by adjusting the pressure gauge 193 P a g e

4 scale at 00 reading. Adjust the pointer on torque arm to match with the Stationery pointer fitted on the frame. After this leaf spring end is fixed with load cell. The force difference will occur while the journal rotates, which reflects the total friction between the journal and the sleeve bearing. By analyzing the relation between normal load and friction force, the friction coefficient of the bearing can be obtained. Since the radius of the convex cylinder of the bearing wrapped with the string is much larger than the radius of the concave surface of the bearing against the journal, there is no possibility of slip taking place between the bearing and the string. For taking reading an electronic display with mother board is used. It mainly comprising of Micro controller, analogue to digital I.C., display, voltage regulated and operational amplifier IC Seven segment LED were utilized for display, Whiston Bridge circuit etc. were utilized in the construction of Mother board. The test rig in this research work has the following parameters. The leaf spring has a length of 600 mm, with a breadth of 205 mm and a thickness of 3 mm. The beam type rectangular Load cell was mounted on the structure. Load cell having two strain gauges on upper and two strain gauges on bottom of the load cell which are used to constitute a Bridge circuit, with one serving as measuring sensor and the other as a temperature-compensating sensor. The rotation of the motor can be modulated from 0 to 1500 rpm. Silk thread of a diameter of 3 mm that cannot elastically elongate is selected as the cable (string) in the experiment. Table No. 6.1 Technical specification for test rig for bushing Sr. No. Description Journal mm diameter, 148 mm length Bearing 38 mm outside diameter, 48.2 mm length with pressure gauge and arm Loading Consisting of loading bracket 1 No. having 3.5 Kgs. Weight and dead arrangement weight 5 kgs. And 2 kgs. And 1 kgs. 2 Nos. each Drive Motor ½ HP rpm. D.C. shunts motor with armature voltage control to speed variation. Range of speeds 100 to 1500 rpm. Lubricant Recommended oil SAE P a g e

5 6.3 ESTIMATION FOR COEFFICIENT OF FRICTION The working principle of the testing system consists of two key aspects. One is to convert tensile force of string into normal force acting on bearing using cable brake principle. The other is to separate the bearing into two halves so that the normal pressing force can be transferred to the interface of the bearing and the journal. The inner diameter of the bearing is designed a little larger than the diameter of the journal to assure proper contact. As shown in the zoom view in Figure 6.2 the tensile forces at two string ends are set to the value of the weight F1 before the journal rotates. When the journal turns at a constant speed, the tensile force of the upper string end will increase to F2 because of the action of the friction force, while the tensile force of the lower end remains the same value of F1 as the weight. From point A B C D A at the circumferential section of the string, the tensile force varies from F1 to F2. The increase of tensile force at upper string end from F1 to F2 due to rotating friction will result in further deflection of the cantilever beam, and hence we can measure force with the help of Load cell mounted at the end of cantilever. Little consideration will show us that due to the applied force through string, there will be slight rotation angle of the bearing assembly bracket, but the value of the rotation angle are quite small. For the given example test as shown below, the rotation angle is about to 3 0. Therefore, the contacts of the upper and lower bearing parts against the journal can be regarded as remaining on the vertical line through the bearing axis during testing. 6.4 EXPERIMENTAL SETUP FOR WEAR ANALYSIS FOR THE BUSHINGS The tests on the adhesion wear has been done on three different Bushing material specimens and its values are given in Table(6.6, 6.7 & 6.8).With the help of arrangement made in the wear testing machine it was possible to record reading on every 10 Minutes for the 60 Minutes. During the Test duration readings were recorded for the Wear and Coefficient of friction. Wear results are recorded after every 10 minutes for 60 minutes cycle mentioned in Table 6.6, 6.7 and P a g e

pressures and the surface velocities, the wear rate on the gudgeon pin

can be calculated using popular Archard law.

easily obtainable and it is the function of several variables including

6 Figure 6.3 General view of test rig for bushing With the knowledge of constant pressures and the surface velocities, the wear rate on the gudgeon pin (also called wristpin) and connecting rod small end bush bearing surfaces can be calculated using popular Archard law. Using the Archard law instantaneously, and averaging over the cycle gives the cycle average wear rate. The wear coefficient constant, which is an input to the problem, is not easily obtainable and it is the function of several variables including lubricating conditions, sliding velocity and lubricity of the used lubricant. In this experiment study, a single value for the wear coefficient is used all three materials to make it more identical and realistic. 196 P a g e

Also we can see the cantilever structure along with off white string attached to hook for applying load.

7 Figure 6.4 Photographic images for Experimental setup for Bushing The wear test rig photographic views are shown in the figure These four views give us complete set up details for the test rig which was fabricated and mofied at institute. As shown in above figure 6.4, load cell was mounted on frame structure along with rectangular section leaf spring. Also we can see the cantilever structure along with off white string attached to hook for applying load. We can also see in the figure seven segment red color display. We can also see the pressure gauge and lubricant oil supply pipe. As stated earlier, in this experiment we used common lubricating oil i.e. SAE 30 oil for all set. The same lubricant in all test setup would assist us to analyze the behavior of all bushings from tribological point of view 197 P a g e

Figure 6.5 Photographic images for load bracket used to apply force on bushing The above figure 6.

The leaf spring one end whose cross section is rectangular in shape, utilized to fix string on it.

The normal load is applied through hook by way of placing standard weights on it.

1), we can see that applying load on the bush bearing is the normal load and the force measured on leaf

8 Figure 6.5 Photographic images for load bracket used to apply force on bushing The above figure 6.5 described the load bracket and its mounting on the test rig. The leaf spring one end whose cross section is rectangular in shape, utilized to fix string on it. We can see that after the string is wound over load bracket strings other end is attached with hook. The normal load is applied through hook by way of placing standard weights on it. While referring schematic diagrams for the Experiment test rig (figure 6.1), we can see that applying load on the bush bearing is the normal load and the force measured on leaf spring with the help of load cell is frictional force. The load cell mounted on the frame structure in cantilever beam shape would help us to measure precisely force developed in the string end due to friction. 198 P a g e

Before the test and after the test the weight of the specimen was measured by a precise electronic weighing machine A&D Japan makes shown in the figure 6.

The readings are mentioned in the Table 6.2, 6.3 & 6.4.Generally wear rate is calculated per Kilometer distance traveled by the bearing.

9 Before the test and after the test the weight of the specimen was measured by a precise electronic weighing machine A&D Japan makes shown in the figure 6.7 with an accuracy of g (Model MC-1000). Using the mass loss technique wear rate can be calculated. The lubricant used in wet condition is SAE30.The readings are mentioned in the Table 6.2, 6.3 & 6.4.Generally wear rate is calculated per Kilometer distance traveled by the bearing. Here we have taken in all cases speed as the 900 rpm, therefore we have not opted for such calculation. Figure 6.6 Three different investigated Bushings Figure 6.7 Digital weighing machine MC (1100 grams x g.) 199 P a g e

10 6.5 EXPERIMENTALINVESTIGATION FOR POWER LOSS AND Table 6.2 Readings for and power loss first test set Load Applied: 20 N Lubricant: SAE30 OIL Speed: 900 rpm. Sr. No. Time Duration (Minutes) Brass Loss Brass (Watt) Gunmetal Loss Gun Metal (Watt) Cast Nylon Loss Cast Nylon (Watt) Rema rks Average P a g e FIGURE 6.8 Relationship b/w Loss and Time at 20 N &900 rpm

11 TABLE 6.3 Readings for and power loss second test set Load Applied: 30 N Lubricant: SAE30 OIL Speed: 900 rpm Sr. No. Time Duration (Minutes) Brass Loss Brass (Watt) Gunmetal Loss Gun Metal (Watt) Cast Nylon Loss Cast Nylon (Watt) Remarks Average FIGURE 6.9 Relationship b/w Loss and Time at 30 N &900 rpm 201 P a g e

12 TABLE 6.4 Readings for and power loss third test set Third Test Set : Load Applied : 40 N Lubricant : SAE30 OIL Speed: 900 rpm Sr. No. Time Duration (Minutes) Brass Loss Brass Gunmetal Loss Gun Metal Cast Nylon Loss Cast Remar ks Average FIGURE 6.10 Relationship b/w Loss and Time at 40 N & 900 rpm 202 P a g e

13 6.6 GRAPHICAL REPRESENTATION FOR FOR BUSHING Figure: 6.11 Representation of the Vs. Time (A, B & C) 203 P a g e

14 Table 6.5: Summary of the experimental investigation for the bushings Phase Time Duration (Minutes) Average Brass Loss Brass (Watt) Average Gunmetal Loss Gun Metal (Watt) Average Cast Nylon Loss Cast Nylon (Watt) I Remarks II III RESULT DISCUSSION FOR BUSHING S C.O.F AND POWER LOSS The tests for the Co-efficient of frictions were done on three different material specimens and its average values are given in Table (6.5).With the help of arrangement made in the wear equipment it was possible to record reading on every 10 Minutes up to the 60 Minutes. Readings were recorded for the Wear and Coefficient of friction. From the figure 6.11 and above Table 6.2, 6.3 and 6.3 it is quite evident that the co-efficient of friction is very low for Cast Nylon compared to Brass and Gunmetal and therefore we have less power loss due to friction of the Cast Nylon. In the first phase keeping 900 rpm and load 20 N, the of Brass is approximately ten times more compared to Cast Nylon. Also the of Gun metal is nearly eight times more than Cast Nylon. Remaining two Tables reading 6.3 and 6.4 revealed the same facts i.e. the COF. of Cast Nylon is quite low with respect to Brass and Gunmetal. The graphical representation between loss and Time are shown in the figure 6.8, 6.9 and It is quite clear that in all three figures, the power loss due to friction for the Cast Nylon is lowest and highest for the brass. The summary for all three setup readings are mentioned above in the Table 6.5. In the first setup, Brass is five times and Gunmetal is three times higher compared to Cast Nylon. In the second setup Brass is four times and Gunmetal is 2.9 times higher compared to Cast Nylon. In the last set up Brass is 4.75 times higher and Gunmetal is 3.5 times higher compared to Cast Nylon. 204 P a g e

15 TABLE 6.6 Wear loss readings for all three bushings after every 10 minutes up to 60 minutes first set (Wear results were recorded after every 10 minutes for 60 minutes cycle) Sr.No Time (Min utes) Reduction in weight after test (mg)x10-1 Reduction in weight after test (mg)x10-1 Reduction in weight after test (mg)x10-1 Mater ial Time Min. BRASS GUN METAL CAST NYLON Figure 6.12 Relationship b/w Wear Loss and Time at 20 N & 900 rpm 205 P a g e

16 TABLE 6.7 Wear loss readings for all three bushings after every 10 minutes up to 60 minutes second set Sr.No Time Min. BRASS mg x 10-1 GUN METAL mg x 10-1 CAST NYLON mg x P a g e Figure 6.13 Relationship b/w Wear Loss and Time at 30 N & 900 rpm

17 TABLE 6.8 Sr.No Wear loss readings for all three bushings after every 10 minutes up to 60 minutes third set Time Min. BRASS mg x 10-1 GUN METAL mg x 10-1 CAST NYLON mg x Figure 6.14 Relationship b/w Wear Loss and Time at 40 N & 900 rpm 207 P a g e

18 6.8 RESULT ANALYSIS FOR WEAR WITH TIME The entire wear test for the three bushings was divided in three setups. The readings for wear were recorded in the Table 6.6, 6.7 and 6.8. The graphical representation between wear and time are represented in the figure 6.12, 6.13 and Referring to the readings recorded in the Table and 6.8, its graphical representations, it is quite clear that in the beginning of the test wear was pretty high but as the time goes on and establishing the lubricant film between the journal and bearing, the wear rate is reduced. After nearly 60 minutes in the all cases the wear rate reaches to nearly negligible. It is visible that the lowest wear rate was found in the case of Cast Nylon bushing while highest was found in the case of Brass. It is known fact that the brass is bit hard material compared to the Gunmetal and therefore wear rate of Brass is higher even compared to the Gunmetal. 6.9 CHAPTER SUMMARY With reference to readings recorded in Table 6.2, 6.3 and 6.4 and it s graphical representation between power loss versus time shown in the figures 6.8, 6.9 and 6.10, it is quite clear that out of three bushings the Cast Nylon bushing has got less coefficient of friction in all three different loading conditions. Also referring to the wear test readings recorded in the Table 6.6, 6.7 and 6.8 and its graphical representation between wear versus time shown in the figure 6.12, 6.13 and 6.14, we found least wear for the Cast Nylon in all three loading conditions. Therefore, if we opt for the Cast Nylon bearing in place of conventional bearing made either from Brass or Gunmetal, Cast Nylon would have less power loss and less temperature induced during the operation. In this investigation for all types of set up, speed and lubricant oil used were the same to have more realistic result with respect to I.C. engine. 208 P a g e

DEVELOPMENT OF DROP WEIGHT IMPACT TEST MACHINE

CHAPTER-8 DEVELOPMENT OF DROP WEIGHT IMPACT TEST MACHINE 8.1 Introduction The behavior of materials is different when they are subjected to dynamic loading [9]. The testing of materials under dynamic conditions