Most Engineers Select Bearings Wrong. Here's Why

[00:00:00]With my decade of experience in machine design,  if I had to name one mechanical component that is most mysterious, it would be the bearings.  If I show the bearing to people who are not mechanical engineers, someone like my mom, my  father, for them the bearing is very simple.

[00:00:20]It's just a bearing. They will probably say it's  used in wheel of car or buses. So they rotates and move smoothly. But if I show this same bearing  to a mechanical engineers, they will immediately identify it by its actual technical name, a deep  groove ball bearing. They can also tell the name of many other bearings. Many engineers can even  tell the exact size of the bearing just by looking at the bearing number. And honestly, that level of  knowledge is enough for many mechanical engineers, especially maintenance engineers. But if I saw  this bearing to a mechanical design engineers, especially machine design engineers whose  job is designing industrial machines like me, our job is not just understand an existing bearing  or finding its size or replacement. Our main job is to selecting completely new bearing for a  completely new machine or new application and that required a deeper bearing knowledge like bearing  internal clearance, bearing precision grade, bearing speed limit, basic dynamic load rating,  the recommended mounting fits and tolerance, recommended lubrication and more importantly  estimating the required bearing size as per the specific loading condition. But what  makes the bearing mysterious is even the designers with years of experience still end  up with wrong bearing selection. For example, most engineers think we should always select a  radial bearing for radial load, thrust bearing for axial load and angular contact bearings or  taper roller bearings for the combined loads. But this is not how the bearings are selected in real  machines. If you start selecting the bearings only based on the direction of the load, you will often  end up with an unnecessarily complex and expensive bearing arrangement. Moreover, that's just for  single bearing application. But when we support a shaft by two bearings, the selection rules of  bearing change completely because now we are not longer selecting just a bearing. We are selecting  a bearing arrangement, one that can handle the load and also compensate the axial and angular  misalignments. Then comes to the bearing sizing.

[00:02:47]Most engineers use the L10 bearing life formula.  But the bearing rating life L10 only predict the fatigue failure. It cannot predict the permanent  deformation of the bearing under the peak load or during the slow moving speed which can fail a  bearing long before its rated life is reached.

[00:03:09]And this is why if you are the one who selecting  the bearings for new machine or new application, you cannot simply rely on textbook theories  or some few formulas. You need to understand all the hidden mistakes in bearing selection  and their sizing. And this is exactly what we are going to learn in this video. So let's just  start with the first mistake. Selecting the wrong type of bearing. Most beginner engineers think  that selecting the type of bearings is entirely depends on the direction of the load. If the load  is pure radial, choose a radial bearing. If it's pure axial, choose a thrust bearing. Or if it  is a combined load or load acting at an angle, choose an angular contact bearing or tapper roller  bearing. But this is not how we actually select the bearing for industrial machines. First thing  we need to understand is there are two different bearing arrangements. One is single bearing  arrangement used to support a single point load or movement and another is two bearing  arrangement used to support an entire shaft.

[00:04:18]Let's start with single bearing arrangement. For  example, supporting a gear, a timing pulley or a sprocket. In these application, the load is  concentrated at one location and the shaft itself is relatively short. So, a single bearing is  often enough. Now, before we selecting any type of bearing, we must have three mandatory inputs.  First, the shaft diameter and the available space around the bearing. Second, the magnitude and  the direction of the applied load. And third, the operating speed of the application.  Once we have selected the bearing type, then maybe we need additional inputs to choose the  exact bearing. But here is something that surprise many new engineers in industry. For almost  any new application, regardless of direction of load or amount of the load, we almost always  start with the same bearing, a deep groove ball bearing. Let me explain it with an example.  Let's say our shaft diameter is 20 mm and the applied load is pure radial. So our first choice  is a deep groove ball bearing and we will start with the light series. For 20 mm shaft, it would be  6004 bearing. Its outer diameter is 42 mm. Now, if there is enough space in housing for 42 mm  outer diameter, we will simply continue with this bearing. But suppose the housing is too compact.  In that case, we may start with a needle bearing or another suitable bearing depending on available  space. But in nearly 99% of machine design, this is rarely a problem because the design  process is usually work like this. First, we determine the shaft diameter based on the  required bending or torsional strength. Then we select the bearing as per the shaft diameter  and then finally we design the housing as per the selected bearing's outer diameter. So in  most of case we can start with simply a deep groove ball bearing. Now the next step of bearing  selection is to calculate the bearing rating life L10 of the selected deep groove ball bearing. If  the calculated life satisfying our requirement, we can simply continue with that selected bearing.  But suppose if the bearing life is not sufficient, then our first approach shouldn't be changing  the bearing type. Instead, we should try with a heavier series of deep groove ball bearing with the  same shaft diameter. For example, instead of 6004, we can try with 6204 or even 6304 which have  the same bearing shaft diameter 20 mm but has much higher dynamic load rating and therefore a longer  bearing life under the same loading condition.

[00:07:17]But even that doesn't satisfy the required  life then we can consider increasing the shaft diameter. If the design allow it or if it's not  possible only then we move to cylindrical roller bearings which provide a much higher life than  a ball bearing under pure radial load. But let's say if the applied load is combined load where  both radial and axial loads are acting together.

[00:07:44]Even in this case, our first choice should be  still a deep groove ball bearing, not an angular contact ball bearing. Because a deep groove ball  bearing can also accommodate axial load up to a certain limit, typically up to about half of  their basic static load rating. For example, a 6004 bearing can safely carry around 250 kilograms  of axial force which is more than enough for many industrial application. So if the actual  load is relatively small, let's say 5 to 10 kg, there is a no reason to move to an angular  contact bearing. We should move to an angular contact bearing only when the thrust load becomes  too high. And for very heavy combined loads, we move to taper roller bearings. Now you might  be wondering that why I'm trying so hard to stay with a deep groove ball bearing because first it's  economical and readily available and second it is also the simplest bearing to install. There is no  need to worry about preload where angular contact bearings and taper roller bearings always required  proper preloading which add an another level of cost and engineering complexity. So as a designer  we always try to make a deep groove ball bearing as our default choice and only move to a specialized  bearing when the applications actually demands it. Even for the pure axial load, I will still  first check whether a deep group ball bearing can handle it. And only when axial load exceed its  capability, I move to thrust bearing or another suitable bearing. But here's the another common  misconception. Many engineers believes a thrust bearing is always the best choice for the axial  load. But that's not true. Thrust ball bearing and thrust roller bearings are designed for only  centrally applied axial load. If the load is offset, they don't perform efficiently and also  they cannot take any radial load. For example, in any rotary indexing table, the operations are  usually happen near the outer edge of the table, not at the center. So in that situation a thrust  bearing doesn't work. Instead we typically use a pair of angular contact bearings. I usually prefer  to a standard fixed bearing block with angular contact bearing in back to back arrangement. And  in case of heavier load or large table a cross roller ring bearing should be used. Now before we  move to the next mistake of the bearing selection, I would like to thanks the sponsor of the  today video BKZ industry. If you are involved in machine design, industrial maintenance or  equipment manufacturing then you already know selecting the right bearing is only the part  of the job. But choosing a reliable bearing manufacturer is equally important. BKZ industry is  a professional bearing manufacturer with over 15 years of manufacturing experience and an ISO  certified production factory supplying bearings for wide range of industrial applications. Their  product range includes standard bearings such as deep groove ball bearing, taper roller bearings,  spherical roller bearings along with many other commonly used industrial bearing types. They  are also manufactured a specialized bearing solutions including solid lubricant bearings,  self-lubricating bearings and customized bearings solutions. So whether you are designing a  new machine or replacing the bearing during maintenance or looking for a customized bearing  solution, BKZ bearings offer a comprehensive range of product to meet your different industrial  requirements. If you do like to know more about BKZ industry, browse their bearing catalog or  request a quotation for your product, simply click on the link in video description or the pin  comment below. Thanks again to BKZ bearings for sponsoring the today video. Now let's see how we  select the two bearing arrangements to support as.

[00:12:16]So in two bearing arrangement the job of bearing  is not just provide a smooth guided motion but they also have to compensate the misalignment  between the two end of the shaft. And there are mainly two types of misalignments that we deals  with. One is axial misalignment and another is angular misalignment. Let's first understand the  axial misalignment. And there are two main reasons why it happens. The first is manufacturing  tolerance. For example, in applications like ball screw shaft, the distance between the two  bearings can be easily have a tolerance about 0.5 to 1 mm. That is a significant amount of  axial misalignment for any rigid bearing. The second reason is thermal expansion. For example,  the drying cylinder in paper machine or the large propeller shaft in centrifugal pump. These  driving shaft experience a significant change in temperature which changes the shaft length.  And this is where most engineers make mistake.

[00:13:26]They simply choose rigid bearing such as deep  groove ball bearing to locate the both end of the shaft. But the axial misalignment generates  a large force inside the bearing. As a result, bearing fail very quickly. So in these application  to compensate the axial misalignment. We use locating and non-locating bearing arrangement.  In this arrangement, the bearing at the one end locate the shaft both axially and radially. This is  usually, the drive end of the shaft and the bearing at the other end support the shaft only radially and  allow the shaft to move freely in axial direction.

[00:14:08]This allows the shaft to accommodate manufacturing  tolerance and thermal expansion. For the locating bearings, we can use a deep groove ball bearing  when axial load is relatively small or a double row angular contact bearing, a spherical roller  bearing, a matched pair of single row angular contact bearing in back to back or face to face  arrangement or match taper roller bearings or even the combination of two different bearings  are used such as cylindrical roller bearings for the radial load and a four point contact ball  bearing for axial load. Now for non-locating bearing there are two possible options. If  the axial misalignment is fixed such as in ball screw because of the manufacturing tolerance  we can simply allow the non-locating bearings to float inside the housing using a loose housing fit.  For this we commonly use deep groove ball bearing.

[00:15:12]But if the axial displacement continuously  change because of the thermal expansion, the bearing itself should accommodate the axial  movement internally. In that case, we use bearings such as cylindrical roller bearing with the flange  only on one ring. Needle roller bearings or CARB toroidal roller bearing. This bearing allows  actual displacement within the bearing itself.

[00:15:40]So there are many combination of locating and  non-locating bearing arrangement but these are the simply most commonly used. You can download  it from the video description. Now there are also the applications where the shaft is relatively  small and the thermal expansion is also not an concern but we need high stiffness and positional  accuracy. For example, in drilling a spindle in these applications, we use an adjusted bearing  arrangement. Here the shaft is located actually from the both sides usually using angular contact  ball bearing or taper roller bearing. Because of this, the bearing must be properly adjusted during  the assembly. And there are also the applications where high stiffness isn't required and a small  amount of axial movement is perfectly acceptable.

[00:16:36]In that case, we use float bearing arrangement.  Both bearings are allowed to float. Actually, the most common combination is use deep groove ball bearing or self-aligning ball bearing with a clear gap in housing. Now, so far we have  assumed that bearings are mounted on accurately machined surface and so only shaft have the axial misalignment. But that's not always true. Sometime we mount the bearing on welded structure or the  modular aluminum frame such as in belt conveyors.

[00:17:13]In these applications, the bearing mounting  surface may not perfectly align and creates a fixed angular misalignment. For such situation,  we use alignment bearings such as ball bearings unit also known as Y bearings. These bearings can  automatically adjust their angular position to compensate the angular misalignment. But sometimes  the shaft itself bent while the machine is running especially under heavy radial load. This creates  dynamic angular misalignment. For that we need self-aligning bearing such as self-aligning ball  bearing as spherical roller bearings or CARB toroidal bearing. These bearings can continuously  compensate for the shaft deflection while the machine is operating. Now after selecting the  right type of bearing the next step of the bearing selection process is to choose the correct bearing  size. But in case of bearing there are no direct formula to calculate the required bearing size.  Instead, we first select a bearing number based on the shaft diameter and then we check the bearing  life using L10 rating life formula. The L10 rating predicts how many revolutions the bearing is  expected to complete before the rolling contact fatigue failure under the given applied load. Then  based on the operating speed, we convert those revolutions into operating life in hour. If the  operating hours meet the application requirement, that means our selected bearing size is correct.  And if the life is not sufficient then we move to the larger bearing or a roller bearing as we  discussed earlier. Let's understand it with an example. Let's say we are designing an automatic  industrial machine and one of the driving shaft diameter is 20 mm and the applied load is pure  radial 100 kg and the operating speed is 1,000 RPM. Now since there is a pure radial load, let's  start with a deep groove ball bearing of a light 600 series for 20 mm diameter 6004. Its basic dynamic  load rating C is 9.95 Kilo Newton. Since the applied load is pure radial, the equivalent load  P is simply equal to the applied radial load.

[00:19:46]Let's first convert the load in kilo newton 0.981  kilo newton. Now substituting this value in L10 life equation the bearing rating life comes out  to be about 1043 million revolutions 1,000 RPM that correspond to about 17,000 operating hours.  Now for most of industrial machines running at 8 hour per day a typical target bearing life is  around 20,000 to 30,000 operating hours. So 17,000 operating life is below our target life.  Now to achieve the target bearing life instead of changing the bearing type or increasing the  shaft diameter we will first try with a heavier series of deep groove ball bearing 6200 series 6204  the shaft diameter is still 20 mm but the basic dynamic load rating increased to 13.5 kilo Newton the  calculated operating life becomes about 40,000 hours which is well above our target, and this is  how most engineers validate the selected bearing size. But here is a problem. Only few engineers  knows that the L10 rating live predict only rolling contact fatigue failure and it is valid  only when bearing is operating with adequate speed at proper lubrication where a full lubrication  film separate the rolling elements from the raceway. For example, let's take the same bearing  again. But now assume the operating speed is only 10 RPM. The calculated L10 operating life becomes  about 1.7 million hours. Even if we increase the applied load by four times, the calculated  L10 life is still much higher than recommended operating life. But this is a lie because in  reality if the bearing is running at very slow speed or subjected to very high peak loads the  lubricant cannot maintain a full film. This is called the boundary lubrication condition. In this  condition the rolling elements comes in contact directly with the bearing raceway. And because of  this metal to metal contact, the bearing can fail due to permanent dent or deformation long before the L10 life is ever reached.

[00:22:24]So in application where bearing operates under  boundary lubrication because of the slow speed, our bearing must be capable for carrying the load  directly without undergoing permanent deformation.

[00:22:39]For that we check bearing static safety  factor. The required safety factor depends on the application and loading condition and the level  of uncertainty. But as a general guideline, if we don't want any permanent deformation in  bearing, the static safety factor must be greater than 2. Let's calculate it for 6004 bearing. Its  dynamic static load rating Co 6.55 kilo N.

[00:23:10]So, the calculated static safety factor is only  1.66. This means this bearing is not suitable for this loading condition. So instead we can select  a cylindrical roller bearing NU 204. It's basic static load rating is 22 kN. Now the static  safety factor becomes 5.66 which is much more than sufficient and this ensure that even  if the bearing is operating at very low speed or under boundary lubrication it will not goes  under permanent deformation. More about boundary lubrication conditions in this video. In short  the bearing L10 rating life helps us to predict rolling contact fatigue failure while a static  safety factor helps us to prevent permanent deformation. So in case of slow moving speed or  high peak loading conditions, we should ensure the both. Now even after understanding all the  bearing selection and sizing calculation process, many engineers don't want to go through it. They  feel it's unnecessary because the general purpose ball bearings are not that expensive. They think  why spend engineering times and optimize the bearing size when I can simply choose a larger  bearing at almost the same cost. But this is actually a one of the biggest mistake because when  a bearing is oversized the applied load becomes very small compared to the bearing capacity.  As a result, the rolling elements do not get properly loaded and instead of pure rolling,  they start skidding. This increases friction, generate excessive heat and eventually it leads  to bearing seize. And that's why every bearing required a minimum load. As a general guideline,  the minimum load for the ball bearing should be about 1% of basic dynamic load rating. and  for roller bearing about 2%. The importance of maintaining this minimum load becomes even greater  in application with rapid acceleration prevent start and stop or when the operating speed exceed  the 50% of the bearing limiting speed. And this is it for this video. I hope this video will help  you to understand the bearing little more deeply.

[00:25:42]If you're new to this channel, I have created a  complete bearing playlist from basic to advanced topic. You can check this out on this channel.  Also, not forget to check out the BKZ bearing the links in description. And thank you so  much for watching this video. Tomorrows, I don't know if I'll reach it. Maybe I won't mind.