The most popular way out of the misunderstanding o

  • Detail

Walk out of the misunderstanding of linear motion bearing design

there are many misunderstandings in the design of linear motion bearings, including that all steel shafts are the same, linear bearings cannot be used in harsh environments, the closer the contact between bearing race and rolling element, the stronger the load capacity, ignoring the correct selection of bearing lubricant, fast running linear bearings will shorten the motion cycle of bearings, and the rolling element will not wear Do not pay attention to surface polishing, the use of small bearings will reduce the height of the system, the shaft will be damaged when there is a groove in the shaft, and the greater the hardness of the material, the better the bearing performance, etc. In order to better help designers avoid these misunderstandings in the design of linear motion bearings, this paper will analyze the above misunderstandings one by one

all steel shafts are the same

the most commonly used material for the shaft of linear motion bearing is medium high carbon steel. The Engineer shall confirm with the supplier whether the carbon content, straightness, curvature, surface finish, hardness and hardened layer depth of the steel used are suitable for the application environment of the bearing. Impurities in the steel will lead to premature bearing failure, because it may produce large Hertzian contact stress on the bearing. Insufficient chemical composition and uniformity of the material will affect the machining performance of the shaft, especially the ability to reduce roughness or high points. Generally, its surface needs to contain flat areas with troughs, and it also needs enough hardness and hardening depth to support the Hertzian contact stress under high bearing load, so as to ensure that premature failure below the surface does not occur

linear bearings cannot be used in harsh environments

please pay special attention to the fact that when linear bearings are used in low temperature or deep-sea environments, the design structure, correct sealing and material selection are all crucial factors. One of the misunderstandings to be avoided is to ignore the impact of the thermal effect of dimensional changes caused by drastic changes in temperature (for example, -40~85 ℃) on the axial and radial assembly clearances of bearings. It may require special sealing and lubrication options with scrapers, and when selecting coatings and materials, the salt spray test may not ultimately measure the corrosion resistance of a particular application. For example, standard salt spray is different from complete immersion. In addition, there are many options to protect the surface, all of which have their own advantages. For rolling bearings, TDC (thin dense chromium plating) coating has better wear resistance than TDN (thin dense nickel plating) coating, which is not due to its friction characteristics, but its higher hardness. However, TDN, which is better than TDC in the standard salt spray test, performs poorly in terms of bearing contact stress

the closer the contact between the bearing race and the rolling element, the stronger the load capacity is

people usually mistakenly believe that the stronger the load capacity is, the better the bearing performance is. For rolling bearings, the ratio between the groove radius and the rolling element radius is the closeness factor. Because the material characteristics will have certain restrictions on the stress, the increase of geometric consistency (closeness) will expand the area of the contact surface, thus improving the load capacity of the bearing. However, due to certain limitations, when the tightness becomes great, the performance of the bearing begins to decline, and the load capacity also decreases. Part of the reason is the increase in the difference in tangential shear as a function of relative surface acceleration. At the same time, the reduction of tolerance and high closeness factor will also lead to high friction characteristics, which proves that too much closeness is not a good thing

Figure 1 there are many misunderstandings in the design of linear motion bearing products

do not pay attention to the correct selection of bearing lubricant

lithium base grease with medium thickness can be applied to most bearings. It is wrong to think that lubricant is not important, or that grease is better as bearing lubricant than lubricating oil. Correct selection of lubricant is an important link in the application of linear bearings, which may be the root cause of complete or insufficient bearing performance. The film layer between the rolling element and the bearing race is EHL (elastohydrodynamic lubrication layer). This lubricating layer has extremely high stress and can resist both physical (such as heat and shear) and chemical (such as pollutants and solvents) shocks (such as shocks that can lead to lubricant failure) at the same time. As the main protective layer of the bearing surface, it can prevent premature damage. Lubrication is an important factor to ensure bearing performance

there are many kinds of lubricants, from high viscosity grease with EP additive for high load to low viscosity lubricant with preservative for chemical protection; From synthetic lubricants for reducing viscosity damage or preventing evaporation loss, to neutral or organic lubricants for low friction properties or compatibility considerations. Some special lubricants are specially designed for silencing or resisting oxygen rich environments or extreme temperature conditions. Therefore, the key to choose the right lubricant is to understand the operating environment in detail

fast running linear bearings will shorten the motion cycle of bearings

sliding bearings, radial bearings and oil film bearings in a narrow sense are extremely sensitive to the factor of speed in their motion cycle, but it is interesting that the motion cycle of rolling linear bearings only changes due to the influence of speed and acceleration on the system load. As long as the motion system is used as a factor affecting the load, the linear bearing will not be affected by PV (pressure velocity), will not be sensitive to heat removal factors, and will not be affected by whether to support the macro film factor (according to Bernoulli equation, the liquid boundary layer is the factor that dominates this kind of tension force should not be greater than 2% of the corresponding force value of the nominal tensile strength of the experimental wire, and the speed is the main variable). When the speed and acceleration exceed a certain standard, that is, the rolling element no longer rolls or begins to affect EHL (more than v=3~5m/s and a= 10g), at this time, the L10 motion cycle of the rolling linear bearing can be estimated by the equation in Lundberg and palmgren bearing failure theory: motion cycle = (rated load/equivalent load) 3 × Rated movement, where speed and acceleration are not factors

the rolling contact will not wear

the rolling contact will not reach the wear degree of the sliding bearing, which is greatly related to the low friction coefficient, but only microscopic wear. In bearings with significantly close geometry, there is a difference in the relative tangential surface velocity between the rolling element and the bearing race. The surface will obviously not run at the same speed. This difference will lead to shear of lubricant, damage the viscosity of lubricating film, and endanger EHL (which can lead to wear and premature damage). Depending on the load and environmental conditions, a higher frequency of relubrication cycles may be required

ignore the importance of surface polishing

and the plastic masterbatch process is simple. Not all so-called surface polishing are the same, and sometimes the appearance is deceptive. Surface polishing can be called RA, RRMS, RPK, etc. Some surface finishes are based on average readings, while others are peak to trough or peak to peak readings. For the load performance of bearings, surface polishing is very important. Usually, designers need to pay attention to the high points. For the same surface polishing value, the flat area with valley value is better than the flat area with peak to valley value. Too rough surface (long thorns or high points) usually means that polishing is more difficult, which is not conducive to the performance of the bearing. If the surface is too fine, polishing will affect the performance of the lubricant to form an appropriate film on the bearing surface, which is also not conducive to the performance of the bearing. Therefore, in most cases, our company Jinan new era Gold Testing Instrument Co., Ltd. sells countless impact testing machines nationwide. In bearing applications, the surface polishing should not be less than 2 RA, which is the key to determine the advantage of surface polishing

using small bearings will reduce the height of the system

many designers will fall into such a misunderstanding, that is, strive to find more compact and compact bearings. This misunderstanding ignores other parameters or components that actually determine the overall height of the system. These elements are usually drive systems, brackets, motors, or gearboxes. These components that will limit the size are very important to the system, and they need to be considered early in the design of the system. Ignoring this often leads designers to place additional washers or risers under small-sized linear bearings to build enough height to match the driver, motor or support. Through the overall design and the positive force sensor, amplifier and data processing system to complete the measurement and select the system components, we can avoid the waste of time and money, as well as the loss of misuse of system components

when the shaft is grooved, it will be damaged

on the contrary, it is not a bad thing for the shaft to be grooved. Sometimes the linear bearing will run under high load, and the shaft will have grooves after several runs. This phenomenon is called "stable state". When the bearing operates under high load, the Hertzian contact stress will be very large, large enough to melt the high carbon bearing steel with extremely high hardness. Because this is only the pressure effect, the molten material will not move or convert, resulting in the separation of the lower layer, and eventually leading to material damage or wear. The actual contact stress keeps stable at the position lower than the equivalent stress due to the increase of the contact area caused by the groove. The load will not cause further melting of the material

if this happens, the shaft begins to have grooves, and then remains stable. Therefore, the engineer should not rotate or replace the shaft at this time, because the bearing will bear another "stable state" cycle next, and if the ball is not hard enough, it will exceed the limit value

the greater the material hardness, the better the bearing performance.

higher material hardness represents a higher yield stress limit value, but this is not necessarily a good thing. High hardness also means increased brittleness, which means sacrificing the toughness of the material. It may break before bending, but sometimes bending is necessary

this is also a trade-off. Proper hardness of close contact bearing elements will optimize the performance of the bearing. Standard linear bearings basically have three main load bearing components: inner race, rolling element and outer race. If the ball is much harder than the inner race, it will wear the inner race due to high contact stress. If the ball is soft, it will wear itself, resulting in flat spots on the ball. These problems also exist in the outer race. Generally, for linear bearings, the outer race runs more cycles than the inner race, but according to the geometry of the inner race (such as tightness), the outer race is better to have a slightly higher or lower hardness than the inner race. In any case, the rolling element with slightly lower hardness (depending on the material) will be more ideal, which can optimize the overall performance of the bearing. (end)

Copyright © 2011 JIN SHI