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Cat. no.e1254f 2011 d-2_172-175E第5部_P170-187改.qxd 11.2.24 9:20 PM ページ 172 6. LUBRICATION
Purposes of Lubrication
Table 6.1 Comparison of Lubricating Methods The main purposes of lubrication are to reduce friction and The lubricating methods in zones A and B are summarized in wear inside the bearing that may cause premature failure. The If packed grease deteriorates, seizure may occur.
Limitation of temperature rise is possible.
May allow penetration of dust or cutting fluid.
effects of lubrication may be briefly explained as follows: (1) Reduction of Friction and Wear
Since new oil is always fed, no fear of oil deterioration.
Dust and cutting fluid cannot easily enter.
Oil supply quantity varies depending on the oil viscosity and Direct metallic contact between the bearing rings, rolling temperature, so control of a small flow rate is difficult.
elements, and cage, which are the basic parts of a bearing, is It is difficult to confirm that oil is actually fed.
Since the oil flow rate is high, dust and cutting fluid cannot prevented by an oil film which reduces the friction and wear in enter and seizure hardly ever occurs.
Since oil leaks, it is difficult to use for vertical spindles.
Because of cooling by oil, the bearing temperature can be (2) Extension of Fatigue Life
Since oil quantity control is possible, the optimum quantity of The rolling fatigue life of bearings depends greatly upon the oil is fed and heat generation is low.
Confirmation of whether oil is actually fed to bearing is difficult.
Besides little heat-generation, there is a cooling effect of the air, viscosity and film thickness between the rolling contact surfaces. A heavy film thickness prolongs the fatigue life, but it Since new oil is always fed, no fear of oil deterioration.
Dust, cutting fluid cannot easily enter.
is shortened if the viscosity of the oil is too low so the film Environmental pollution mist is slight.
where f0 : Coefficient determined by bearing and lubricating Grease Lubrication
speed spindle bearings, or spindle motor bearings.
(3) Dissipation of Heat
(1) Recommended Greases
The following equation shows the mean life of wide range Circulating lubrication may be used to carry away frictional Lithium base greases with mineral oil as the base oil have good heat or heat transferred from the outside to prevent the sticking properties and excellent characteristics for rolling log t = 6.12 – 1.4 n/N
bearing from overheating and oil from deteriorating.
max – (0.018 – 0.006 n/Nmax) T
bearings. These are usually usable over a temperature range of (4) Others
Fig. 6.1 Oil Quantity and Temperature Rise Adequate lubrication also helps to prevent foreign material As grease for high speed machine tool spindles that require from entering the bearings and guards against corrosion or low temperature rise and long life, a consistency No.2 grease with a synthetic base oil (diester, diester + mineral oil, etc.) is (3) Quantity of Grease for High Speed Spindle Bearings
Table 6.2 lists the brand names and properties of greases To operate bearings at high speed with grease lubrication, the For machine tool spindles in which high accuracy is important, widely used in machine tools main spindles and ball screw recommended quantity to be packed is 10% to 20% of internal it is necessary to prevent excessive temperature rise of the space. If too much grease is packed, during running in, spindle to reduce thermal deformation.
(2) Grease Life
abnormal heat generation occurs and this may cause the Bearing heat generation is divided into a load term determined Grease life depends greatly upon operating temperature; grease to deteriorate. To avoid such a risk, it is necessary to by the bearing type and load, and a speed term determined by • Oil mist lubrication • Jet lubrication therefore, it is necessary to keep the temperature of the run in spindles for a sufficient time. Based on their experience, bearing (including atmospheric temperature) cooler, in order to NSK determines the packing quantity which allows easy Lubrication
Generally, the speed term is greater, but if a lubricating running in and will provide sufficient lubrication. For the method resulting in a small speed term is selected, the High performance wide range grease is often used for high amount, please refer to the tables on Page 175.
influence of the load term cannot be disregarded. Therefore, it Table 6.2 Grease Brand Names and Properties is important to select a low heat generating bearing (load term) and lubricating method (speed term).
Regarding heat generation, both the lubrication method and quantity of lubricant have important effects. Lubrication using a small amount of grease is common since this method is economical, maintenance free, and there is little heat generation. At high speeds, to maintain a constant low temperature, the oil-air lubrication method, which requires a minimum quantity of oil, was developed.
The relation between oil quantity and heat generation (frictional loss) and temperature rise is already known as shown in Fig. 6.1. Therefore, for machine tool spindles, to avoid excessive temperature rise, adoption of a lubricating method aiming at either zone A or B is necessary.
E第5部_P170-187改.qxd 11.2.24 9:20 PM ページ 174 6. LUBRICATION
The Recommended Grease Quantities for High-speed Spindle Bearings
(1) Oil Mist Lubrication and Oil-Air Lubrication (Minimal Oil Quantity Lubrication)
Angular contact ball bearing : 15% of internal space Cylindrical roller bearing : 10% of internal space Spray oiling is a method of spraying oil by turning it into a mist using compressed air. It is also called oil mist lubrication.
Oil-air lubrication is a method of feeding oil continuously by injecting oil into a compressed air stream by means of a mixing valve that intermittently discharges the minimum quantity of oil using a constant-quantity piston.
Fig. 6.2 shows the recommended oil quantity for the lubrication methods described above, each quantity is for one bearing.
In case of oil mist lubrication, it’s necessary to adjust the oil quantity to accommodate for the effects of the branches in path tubing, and leakage from the gaps around the spacers.
Please ask NSK, as the oil quantity should be increased, in cases where the dmn value is higher than 1 800 000.
For the position of the spray nozzle, please refer to Page 192.
(2) Jet Lubrication
Fig. 6.2 Recommended Oil Quantity for Each Bore Size of Bearing Jet lubrication is mainly used for high speed to several nozzles, jets of lubricating oil under a constant pressure pass through the bearings. At high speed, the air surrounding the bearing rotates together with the bearing and forms an air wall. The speed of the jet from each nozzle must be faster by 20% than the circumferential speed of the inner ring outside surface. Since the jet lubrication uses a large quantity of oil, there is much agitation resistance, so it is necessary to dissipate the heat effectively using a large oil discharge outlet and forced discharge.
For machine tool spindle bearings, this method is used in some applications as a means for stable operation at ultra high speeds (see Fig. 6.3).
For the position of the spray nozzle, please refer to Lubrication
Fig. 6.3 Spindle Structure with Jet Lubrication The grease quantity of “xxTAC20(29)X(D)” should be same as the double row cylindrical roller bearing’s, which is assembled with this bearing together.
Use the grease listed on Page 173, and multiply 0.93 (density) to the quantity above, for the weight of the grease.
For the recomended grease quantity for angular contact thrust ball bearing for ball screw support, please refer to Page 110-113.
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