Comparison of Bearings
--- For the Bearing Choosing of High-speed Spindle Design
小猴卡通图片Xiaofan Xie
Dept. of Mechanical Engineering, University of Utah
ABSTRACT: Bearing is the most important part of my project – High Speed Spindle; meanwhile, it is difficult to design or to choo. This paper is a detailed review highlighting the differences of most types of bearings which are utilized in industry field or currently developing the days.
1．Basic Concepts
The concept behind a bearing is very simple: Things roll better than they slide. Bearings reduce friction by providing smooth metal balls or rollers, and a smooth inner and outer metal surface for the balls to roll against. The balls or rollers "bear" the load, allowing the device to spin smoothly.
There are many types of bearings available, each ud for different purpos and different conditions. Two major types of bearings are contact bearings and noncontact bearings. By considering the ball ma
terial, I parate the contact bearings to steel bearings and ceramic ball bearings. On the other hand, there are bearings that can run without any contact between the sliding surfaces in the bearing. The noncontact bearings are hydrostatic/hydrodynamic bearings, air bearings, and magnetic bearings.
Numbers of properties for a certain bearing should be considered during the process of design. The include speed limits, applied load, accuracy, stiffness, manufacturability, cost, etc. Most of them will be discusd between the different bearings as follow.
Becau my final project is design of high speed spindle, the bearing lection or design should under the high speed condition. Limited by the speed property of steel bearing, steel bearings will not be concerned in this paper.
2. Speed Limits
Becau the density of ball of the ceramic bearings is only 40% that of steel, substituting steel
乌鸦喝水图片
balls with ceramic ones greatly facilitates high-speed rotation. In a machine tool spindle under position preload, the centrifugal force on ceramic rolling elements is substantially lower, resulting in superior high-speed performance. [1]
Hydrostatic/hydrodynamic bearings have only viscous friction associated with a fluid film layer being sheared during the motion of the bearing. But they can experience hydrodynamic effects on high speed condition if the lands are too wide, and considerable heat can be generated as a result. [2]
Same as Hydrostatic/hydrodynamic bearings bearings, air bearings have only viscous friction associated with a air film layer being sheared during the motion of the bearing. When using high-speed spindle, the bearing gap should be large enough to ensure that the friction power is less than twice the pumping power.
Magnetic bearings do not limit the speed or acceleration of components they support. System of 100,000 rpm and higher have been built for applications ranging from special pumps to spindles for ultrahigh-speed machining.
Bearings Approximately maximum speed (DN*)
Steel bearings 1,000,000
Ceramic bearings 2,000,000
Hydrostatic/Hydrodynamic bearings 1,000,000
Air bearings4,400,000
Magnetic bearings4,500,000
Table 1. Speed Comparison
* Top speed alone is not enough to describe a bearing. A better measure is the product of bearing diameter in mm (D) and top speed in rpm (N), which is DN number.
3. Applied Load
Ceramic bearings
Hydrostatic/hydrodynamic bearings distribute the load over a large area, huge loads can be supported. For example, machine tools with multitude carriages often ud hydrostatic/hydrodynamic bearings, and offshore oil platform desks, which may weight 20,000 tons, are transferred from the fabrication yard to a barge using fire hos to supply water to hydraulic bearings on the desk’s feet.
Becau of the different film layer, approximately only one-fifth of load can be support by air bearings than hydraulic bearings.
Virtually any magnetic load can be supported by a suitable magnetic bearing, depending on how much one wishes to pay and how much room on has. Increasing the proportion of the load that is supported by permanent magnets decrea the current that must pass through the coils and the resultant heat generated.
Figure 1. General Comparison of Bearing Types (rpm vs. load) [3]
4. Accuracy
Ceramic ball have a smoother finish than steel, so the vibration and spindle deflection is reduced allowing higher speeds and better performance.
Overall accuracy of motion of a hydrostatic/hydrodynamic bearing depends on the accuracy of the components. Hydraulic linear motion bearings have been built with submicron/meter accuracy.
Same as hydrostatic/hydrodynamic bearings, the overall accuracy of motion of an air bearing depends on the accuracy of the components. An air bearing averages out local irregularities to make it perhaps the smoothest running of all bearings.
Typically, achievable rotational accuracy is m µ50 and m µ1.0system has been built. Since magnetic bearings depends on a clo-loop rvo system to a achieve stability, the performance of the position nsor and rvo controller will directly affect the accuracy of the system.
5. Stiffness
Ceramic (silicon nitride) ball have a 50% higher modulus of elasticity (resistant to deformation) than steel, which means a 15 to20% increa in rigidity, improving stiffness.绿华岛
Hydrostatic/hydrodynamic bearings can easily be in the Newton per nanometer range and do not have loss of contact problems that sliding or rolling contact bearings that are preloaded against each other have.
Air bearings stiffness can easily be in the 100 N/μm range and do not have the problem of loss of contact that sliding or rolling contact bearings can experience.
The steady-state stiffness of magnetic bearings can be esntially infinite, depending on how the clo-loop control system is design. Magnetic bearing dynamic stiffness depends on the frequency of applied load and the bandwidth of the control system.
6. Damping Capability
Ceramic bearings have good damping capability becau of the nice smooth finish of the ceramic balls.
王者情侣昵称
The thin oil film in the bearing gap gives hydrostatic/hydrodynamic bearings excellent damping capa
bilities in both normal, via squeeze film damping, and tangential, via viscous shear, bearing directions.
The thin, low-viscosity air film in the bearing gap gives air bearings moderate to low damping capabilities in the normal and tangential bearing directions respectively.
A magnetic bearing’s damping capability is attained from the clod-loop control system. Additional magnetic bearings modules can be added at various points along a shaft and ud as vibration dampers. In this mode, the gap measurement signal is differentiated and ud as a velocity feedback signal.
凤梨怎么保存
7. Friction
Ceramic bearings are “anti-friction” bearings and the friction is reduced drastically, and meanwhile the micro-weld problem is solved.
Hydrostatic/hydrodynamic bearings, air bearings and magnetic bearings have absolutely zero friction becau of the “noncontact” characteristic.
8. Thermal performance
做一个淡淡的女子
Becau hardness and strength of silicon nitride do not deteriorate at high temperatures when compared to tho of bearing steel, silicon nitride holds a lot of promi as bearing material for high temperatures. [4]
Energy is input a hydrostatic/hydrodynamic bearing in the form of a flow at a pressure. The oil oozes out of the bearing and into a drip pan. In the pan its flow rate and pressure are esntially zero, so all the power that is reprented by the initial flow and pressure is expended in the viscous shear the fluid undergoes as it oozes out of the bearings. This power is dissipate as heat. The temperature ri of the oil depends on how much heat is conducted by the machine. One must be careful to consider this effect, which gives motivation for using as low pressure and flow rate as possible. In general, hydrostatic bearings are not ud where speeds greater than about 2 m/s are encountered becau viscous shear of the fluid in the bearing gap also generates too much heat.
运营是什么意思
The viscosity of air is very low, so air bearings are tolerant of small changes in bearing
clearance caud by viscous heating. But it is important to realized that air cools as it expands, and thus for a precision machine it is important to minimize flow and the resultant refrigeration effect.
Magnetic bearings can generate significant amounts of heat and therefore may require external cooli
ng devices, such as recalculating chilled water jackets. For system where the load does not vary greatly, a large percentage of the load can be supported by permanent magnets which minimize coil size and current required to levitate load.
特别想念一个人的诗句9. Size and Configuration
Ceramic bearings take small space as general steel bearings.
Hydrostatic/hydrodynamic bearings take up very little space themlves, but the plumbing requirements may be significant.
Same as hydrostatic/hydrodynamic bearings, the plumbing system of air bearings will take more significant space than the bearings themlves.
Magnetic bearings are typically 2-10 times than the rolling element bearings they can replace; however, in many applications, accommodating a magnetic bearing’s larger size is not too much a problem.
10. Weight
Ceramic ball is 60% lighter than a steel ball.
Becau of their simplicity, hydrostatic/hydrodynamic bearings have very high performance-to-weigh ratios, but only if one excludes the size and weight of the pump, oil collection and distribution system, and oil temperature control system.
Air bearings have moderate-to-high performance-to weight ratios.
Magnetic bearings are very heavy compared to the rolling element bearings they replace. In some applications, such as precision mechanical gyroscopes, the forces encountered by the bearing are so small that the weight of the required bearings is inconquential anyway.
11. Maintenance and Requirements
