3.0 RESULTS AND DISCUSSION 3.1 Table 3.2 Discussion
5
4.0 CONCLUSION
6
REFERENCES
6
Aim To compare the efficiency of different bearing materials through determining the frictional resistance to rotation
Abstract The main function of a bearing is to provide support and stability to a shaft as it is transferring energy or power from one end of the shaft to the other. There are mainly two types of bearing categories which are roller and journal bearings. Journal bearing are plain slider bearings where the bearing is held in place and sliding contact happens with the surface of the rotating shaft. Whereas roller bearings are bearing in which the journal rotates in peripheral contact with a number of rollers usually contained in a cage.
1. Introduction The friction produced in the bearing is unavoidable and will always be present as the product of the rotating shaft and a stationary surface on which it moves upon. It is imperative that the friction must be reduced as it is an unwanted by-product . Friction causes wastage of power in t he form of build-up of heat and wear. Thus, the choice of materials used are to be decided with further consideration. In less complex machinery, where the speed and the load are low, it is commonly found that the bearing rely on dry sliding contact against dissimilar materials. Such can also be said in oil-lubricated bearings. It also depends on the dry sliding contact as the shaft begins to move or returns to its original position. The experiment of the bearing friction alongst a selflubricating sintered bronze over a range of various materials provides an invaluable studying opportunity. To compare the results, a roller bearing is used to see the effect of changing to rolling contact.
2. Experiment Design 2.1Material s 1. Wall Bracket 2. Cord 3. 1N load hanger 4. 300mm diameter flywheel and shaft 5. Journal bearing in Cast Iron, Tufnol, Oil Impregnated Bronze and ball bearing
2.2 Methods Let W= weight of flywheel and shaft (N) w= weight of hanger plus load (N) x = offset from ) of resultant total weight r = radius of shaft journal R = radius of loading cord N = normal reaction of bearing at point of contact with journal (N) µ = coefficient of friction = tan x =
]
2.3 Procedure 1. Before the apparatus were used, the solid metals bearings were degreased using dry tissue paper to ensure that the dry friction coefficient can be measured 2. The flywheel was set up in turn with a pair of dry journal bearings . The cord was then winded up around the shaft and the load hanger was added 3. Weights were added to the hanger until the flywheel turned at a constant speed as the load descended after a gentle start was given. The test was repeated a few times with different low speeds to see if the change in weight was necessary. An average total weight required was recorded to each pair of dry bearings 4. The flywheel was set up using the metal journal bearings with thin machine oil as a lubricant. The test was repeated and the average results were recorded 5. The ball bearing was internally lubricated with grease and used. The results were recorded
3. Results and discussion 3.1 Table Bearing Material Cast Iron dry Tufnol dry Bronze dry Cast Iron oiled Bronze oiled Ball Race
w (N)
W+w (N)
x (mm)
Ɵ
Friction µ
20 21.3 18.3 18.3
247.59 248.89 245.89 245.89
1.292 1.369 1.191 1.191
8.743 9.268 8.054 8.054
0.154 0.163 0.142 0.142
16.3 0.5
243.89 228.09
1.069 0.035
7.225 0.236
0.127 0.004
Discussion 1. In the experiment , the use of different speeds didn’t not noticeably affect the coefficient friction. According to Coulombs’ third law of friction, t he sliding friction is independent is of its speed . However at speeds that are relatively much lower, the bearing surface and the shaft will interact more due to the prolonged period of interactions which would lead to a formation of static friction due to interlocking of surfaces[1] 2. As observed from the table above, the ball race bearing has the lowest coefficient friction of 0.004 however when comparing those that are under the journal bearing category, the bearing made of bronze and oiled showed to l owest coefficient friction compared to the others even so when in its dry state. In the other hand, the dry tufnol has the highest coefficient friction. Having a high coefficient friction reduces the efficiency. As energy is transferred through the shaft that is being held by the bearing, as both materials from both the shaft and bearing rub against one another, more heat is generated thus resulting an overall reduction in driving power. 3.
A good choice of a journal bearing from the list of materials above that would be the most suitable in integrating into a centrifugal waste pump would be the bronze mainly due to it having the lowest coefficient friction and also due to its resistance to corrosion from water[3]. This reduces the frequency of the maintenance that it has to undergo.
4. Frictional resistance of the roller bearing is significantly lower than that of the journal bearing due to the presence of the ball or rollers which has rolling contact which removes the sliding friction and substituted with a much lower rolling friction. However in the roller bearing, the initial resistance to motion only slightly more than their resistance to continuous motion.[2]
4.0 Conclusion
Based on the experiment, we can conclude that bronze bearing has the least coefficient friction and tufnol has the highest value. The coefficient friction can also be reduced significantly using lubricants which would improve efficiency.
Reference [1]http://www.mecheng.iisc.ernet.in/~bobji/funtri/assign/Friction%20Theories.htm [2]http://www.mecheng.iisc.ernet.in/~bobji/funtri/assign/Rolling.htm [3] http://www.engineeringtoolbox.com/metal-corrosion-resistance-d_491.html [4] School of Engineering Laboratory Handbook
