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Design Notes for LubroGlide OilLess bearings and other general bearings. When designing bearings for machines, there is no established standard as to what type of bearing should be used where. Here LubroGlide OilLess (Graphite Embedded / Self Lubricating) bearings are compared to conventional bearings, sleeve bearings and roller bearings. Essential design considerations are outlined in an attempt to help you select an adequate bearing and recommend the use of LubroGlide OilLess (Graphite Embedded / Self Lubricating) bearings for a better machine design. OilLess Bearing, Sleeve Bearing and Rolling Element (Spherical/Cylinder/Taper) Bearing: Bearings are classified into three types: 1. General plain bearings (Gun metal, Bronze, white metal, etc.). used in fluid lubrication conditions 2. LubroGlide OilLess bearings (self-lubricating type also used in fluid lubrication conditions) display incomparable performance unattainable with a general plain bearing, especially in a boundary lubrication or dry condition 3. Roller element bearings.
1 and 2 above may be interchangeably used under fluid lubrication conditions. LubroGlide OilLess (Graphite Embedded/Self Lubricating) bearings, however, are advantageous to general plain bearings because the former can be used under a non-lubricated conditions, still exhibiting good resistance to seizure and wear. In general, General plain bearings, LubroGlide OilLess bearings and roller bearings have different features and are applied in different fields. However, in specific cases where all are applicable, LubroGlide OilLess (Graphite Embedded / Self Lubricating) bearings perform superior in the following environments 1. Oscillating motion with a small angle, intermittent operation and vibration load. 2. Locations where noise is impermissible. 3. Locations where foreign matters tend to enter. 4. Applications where operating temperatures are high or low. General plain bearings often seize in the following cases: A. When oil film is torn, due to: excess load, low speed motion, oscillating motion, reciprocation, intermittent operation or foreign matter invasion. B. When ambience temperature is too high or too low. C. When normal lubrication is interrupted due to inadequate oil supply system or some other cause. Situations indicated above, being very unfavorable to general plain bearings, are where LubroGlide OilLess (Graphite Embedded / Self Lubricating) bearings exhibit their superior characteristic features. A. General instructions for bearing design: a. Wall Thickness of Bearing:
b. Length and Thickness of Bearing and Chamfering
c. Oil Groove and Oil Holes
d. Role of Oiling Periodical Oiling is suggested for OilLess Bearing because it will reduce co-efficient of friction and the Bearing temperature, thus improving the Bearing life. Oiling also facilitates performance under high load conditions and secures safety. Other advantages of Oiling include: removal of wear particles, sealing effect against foreign matters, prevention of rust on the shaft material, etc. Lubricating Oil: In general, motor oil is used. However, grease is recommended for conditions where considerable vibration occurs which scatters oil, or a low speed operation under a high load, which disrupts a lubricating, oil film. Grease: Grease is superior to lubricating oil in sealing effect, water proof ness and adhesiveness but is inferior in its cooling effect. Grease is often applied when Bearings are frequently splashed with water or where there is considerable vibrating or textile machine.
B. Service life OilLess and General Bearings The service life of sleeve type bearing, which depends on the amount of wear, is greatly affected by friction conditions, regardless if it is dry, boundary lubrication or fluid film lubrication. Wear amount can be estimated by the following experimental formula. Theoretical estimation becomes almost useless when a rapid change in wear amount occurs due to lack of lubricant supply or invasion of foreign matter. W = K*P*V*T
W : wear amount (mm) If the specific wear amount K is known, the wear amount W for a given running time T is determined and hence, the life of a bearing is calculated from an allowable wear amount of the bearing. Note that the estimated life given by the above formula should be used only as a reference because other factors which also affect the bearing life have not been taken into consideration. These factors include the influence of velocity and load, different modes of operation, type of lubricant used, clearance, surface roughness of mating material, invasion of foreign materials, etc. Experimental values of specific wear amount K for radial-journal sleeve bearings & OilLess bearings
The invasion of foreign particles or the lack of lubrication will lead general oil lubricated bearings to a sudden rise in specific wear amount K, followed by seizure. In contrast LubroGlide OilLess (Graphite embedded) bearings are not sensitive to foreign particles and lack of lubrication and stand longer on such conditions C. Loading Condition PV Value
PV value (product of bearing pressure
P and velocity V. N/mm2 {kgf/cm2}*m/s {m/min} ) is an
important parameter in selecting an OILES bearing. The friction calorie Q
generated in unit area of bearing for unit time is obtained from the energy loss Q =µ*P*V Where, P: pressure N/mm2 {kgf/cm2}, V: velocity m/s {m/min} and µ: coefficient of friction Assuming that the co-efficient of friction is a constant, the friction calorie Q, a criteria in selecting of bearings, is proportional to PV value. During continuous rotation of a shaft, the friction heat generated at the bearings and the heat lost by radiation maintains the bearing temperature at a constant balance. After an extended period of operation, however, the co-efficient of friction may increase due to changes in the sliding surface conditions. The existence of foreign particles, deteriorated lubricants, wear particles, material fatigue and other factors will influence the co-efficient of friction. As a result, when the beating temperature rises, eventual damage to sliding surfaces or seizure is considered to occur. The lower the bearing temperature stays, or in other words, the smaller the PV value is, the lighter the load conditions of the bearing is considered to be. Safety design of bearings means that bearings are to be designed to lower PV value which leads to a longer service life. Furthermore, under certain conditions, the PV value can exceed the maximum. Maximum Allowable PV Value The maximum allowable PV value is defined as the maximum product of the pressure on a projected unit area of the bearing and the velocity allowable in the design of a bearing. The PV value is calculated from the formula below:
P = load N{kgf} / projected area of bearing mm2 {cm2} In the case of a radial journal bearing, the projected area is obtained by multiplying the inner diameter of the bearing by the bearing length. In the case of a rotational operation, the following should apply. Bearing Load In general, the bearing pressure is obtained by dividing the maximum load imposed on the bearing by the pressure supporting area of the bearing. The pressure supporting area is defined as the projected loading area which contacts with the shaft, projected in the direction of the load in cases of a cylindrical and spherical bearings. Velocity The main cause of generated heat is the work done at the friction surface of the bearing. It is known from experience that the rise in temperature at the friction surface is affected more by the velocity than by the pressure. With the same PV value, the larger the V value is, the higher the bearing temperature will be. When used in a high velocity operation, it is recommended that the bearings should be designed and used in such a manner that the co-efficient of friction be reduced by positive supply of oil to enhance both cooling and lubricating effectiveness, in order to take advantage of their wear resistance and anti-seizure. Direction of Motion and PV Value In the case of continuous, one directional, rotational motion of a radial journal, proper fluid film lubrication is easily accomplished for trouble-free operation. In the case of a thrust bearing, however, careful consideration must be given in designing the lubrication method and oil grooves so to insure proper lubrication of the sliding surface. More over, when intermittent operations, oscillating motion or reciprocating motion is encountered. The maximum allowable PV values, indicated in the catalog, are the values applicable to a rotational operation. For a reciprocating motion or rotational and oscillating motion of a thrust bearing, about half of the maximum allowable PV value should be employed as indicated in the catalog. Even a lesser value be applied for an oscillating motion. Operation Intervals Operations may be either continuous or intermittent. Intermittent operations can be advantageous for general type bearings because of intervals which allow generated friction heat to cool down. This enables a PV value to remain relatively high. The disadvantage of intermittent operations is that frequent operational interruptions tend to cause inadequate lubrication, resulting in increasing wear amount. Moreover, scoring or seizure may occur when restarting. The heavy load imposed in an intermittent operation is liable to cause boundary lubrication condition. LubroGlide OilLess Graphite embedded bearings, in particular, has a high load carrying capacity and displays excellent performance in intermittent operations with high load because of the tough film of solid lubricants covers the sliding surface. Oscillating Motion and OilLess Bearing The oscillating motion is considered to be one of the most severe conditions to bearings because it passes through points of zero velocity in each cycle of motion. Oil film is liable to be disrupted, fatigue and wear of material be accelerated and wear particles tend to remain longer. The ball bearings which are designed mainly for rotational motion have a very small contact area causing, extreme high contact stress to develop at their pressure supporting areas. They are, thus, unsuitable for oscillating motion because of material fatigue. The sleeve bearings which have large contact area are generally considered better for this application. Fretting Wear and OilLess Bearing In some cases, like that of the weaving machine application, slight oscillating and reciprocating motion are being manufactured, yet without close study, it appears only as a slight vibration. The type of wear developed in this situation is fretting corrosion or fretting wear. The cause of fretting corrosion is considered to be the fine wear particles generated by friction. Wear is further accelerated when these particles oxidize and create a highly abrasive surface. Effective measures to prevent fretting corrosion are to reduce the co-efficient of friction and prevent wear particles from oxidizing. Plating shaft surfaces and applying grease may be effective. The NASA reported, however, that providing a solid lubricant film on the sliding surface is the most effective prevention for this type of wear. LubroGlide OilLess Graphite Embedded/solid lubricant bearings have proven to be effective for such applications. Since preventive effectiveness may differ, depending on actual service conditions, it is recommended that preliminary tests be performed to insure that the most effective bearing is selected. Impact Load and OilLess Bearing LubroGlide OilLess Graphite Embedded/solid lubricant bearings resistance to impact load is far superior to that of general ball bearings. Since impact load is a severe condition for bearings and can vary according to the environment, it is important to perform preliminary test in order to find the suitable bearing for practical use. Rotating Bearing When a bearing is rotated under a high pressure such as a sheave metal or a wheel bearing, fatigue of bearing material cannot be neglected. For operations under such conditions, it will be safer to limit the PV value to within one third of the maximum allowable PV value. D. Environmental Conditions and OilLess bearings Bearing Temperature and OilLess Bearing The life of a bearing is greatly influenced by environmental temperature and friction heat that is generated from oscillating and reciprocating motion. For a high temperature application, the PV value of the bearing should be limited to a small value. The heat resistance of plastic bearings is generally inferior to that of metallic bearings. When oil-containing bearings are used at a high temperature, oiling with high temperature grease or high viscosity oil will prevent lubricant in the bearings from leaking out or changes in lubricant viscosity. For low temperature applications, refrigerating machine oil or synthetic lubricant oil is recommended. In general, the bearing design can be made easier and safer for high or low temperature application if solid lubricant type bearings OILLESS LB#500, OILLESS LB#2000, OILLESS LB#1000 AND OILLESS LB#300 are used. Foreign Matter Invasion in OilLess Bearing If dusts or other foreign particles settle on the sliding surfaces, oil film or solid lubricant film will be disrupted. The direct contact of the mating shaft with the bearing material will eventually cause seizure or abrasive wear. Foreign materials, a formidable enemy against bearings, are prevented by applying seals or enclosing grease. Positive oiling may wash off the foreign particles once entered. Installing seals accompanied by oiling (including lubrication of seals) will lead to better results. E. Conditions of Mating Materials The performance of OILES bearings can be greatly affected by surface roughness, the use of plated or non-plated surfaces and mating materials which form the friction between two solids. Surface Roughness In case of solid lubricant Bearings, like OilLess LB#500, OilLess LB#2000, OilLess LB#1000 and OilLess LB#300 their Bearing lives are determined by whether or not a lubricating film of solid lubricants can be formed on sliding surfaces of either mating materials or the Bearings. Surface roughness of mating materials within 3 ~ 12 micron meter is sufficient for proper performance of the Bearings. A mirror finish, as in the cases of oil lubricating Bearings and plastic Bearings, is not necessary. Surface Treatment The three main reasons for treating mating surfaces are: 1. To improve corrosion resistance. 2. To improve surface hardness. 3. To improve surface smoothness for better lubricity. When plating is employed to attain purpose 1 above, hard chrome plating is highly recommended, because it prevents mating materials from rusting and abrasive wear from developing. hard chrome plating also contributes considerably to the lubricity improvement .For slight oscillating motions under high loads, nitriding treatment on shaft materials is effective. When used under highly corrosive environments such as in seawater, double or triple layered chromium plating is necessary. F. Handling instructions for OilLess bearings Installation Instruction for OilLess Bearings In most cases, OilLess Bearings are used with a press fit in the same manner as general sleeve Bearings. Fit the Bearings into housings using mandrels or press. In case of a relatively large interference, provide both the ID of the housing and the OD of the Bearing with chamfers, and fit the Bearing into the housing with mandrel for easy installation. Break-in Instruction for OilLess Bearings At the on start of the operations, contact surfaces of shafts and Bearing are smooth, however, microscopic irregularities are inevitable to develop after continued use. A deviation from true center alignment may also exist. Thus the initial contact between sliding surfaces could be local. Do not immediately start a regular loaded operation. It may result in damaging the bearing surfaces, leading to a shorter service life. Instead, gradually break-in operations so to smooth out the microscopic irregularities, and allow the entire pressure support area to slowly come in contact without causing damage. This is called a “break-in for familiarity”. |
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P and V, using the following formula.