Bearing Selection - How To Select A Bearing?


 

 

How to choose a bearing? - Explanation of The Bearings And Guidelines And Tips For Selecting Bearings

 

 

Background

Bearing Selection

By Solaris2006 [GFDL or CC-BY-SA-3.0], via Wikimedia Commons

Bearing is a mechanical component consists of two bodies when one is fixed and the other can move relatively to the other. Bearing are very common in every motion mechanism, especially in robotics and automation. The bearing allows motion between mechanical elements and lowers the friction between them. The motion can be either angular or linear.

 

The first bearings were invented in ancient times. Large stones were placed on round wood logs so they can roll on the logs. In this way the wooden logs were rotating between the stone and the ground. Even after thousands of years, the principle I the same – only the size, the accuracy and the technology have changed.

 

 

 

 

General Bearing Structure

Bearing internal structure consists of outer ring, inner ring, balls or other round shaped bodies, cage and protective plates.

 

Ball Bearing Internal Structure

By Silberwolf [CC-BY-2.5], via Wikimedia Commons

 

Suppose that one of the rings is static (Say the external) and the second ring is dynamic (say the internal). When the dynamic ring starts to rotate then the friction between it and the round balls cause it to slip one on the other. At the same time, the round balls roll on the static ring. If the dynamic ring rotates at a speed of 1000 rpm, then the round balls rolls at a speed of 500rpm.

 

 

Ball Bearing Animation

 

A plain bearing is also consists of internal and external rings but without the internal bearing bodies. This type of bearings consists of low friction between the two rings and the motion is relatively between the two rings.

The cage roll is to maintain an equal distance between the internal bearing balls.

The protective plates roll is to prevent dirt and dust from entering inside the bearing huts lowering the performance and life time of the bearing.

 

 

 

How To Select A Bearing? - Bearing Specifications

 

 

Radial Force

Force acting perpendicular to the rotation axis of the bearing

Bearing Radial Force

 

Axial Force

Force acting Parallel to the rotation axis of the bearing

Bearing Axial Force

 

Combined Forces

A Combination of radial and axial forces:

Bearing Combined (Radial and Axial) Force

 

Bearing Material

Material from which the bearing is made. There are many common materials such as: metal, plastic and glass.

 

 

Rotation Speed

Every bearing has a maximum allowable rotation speed. As with any design of a mechanical system, you should never design the bearing to rotate at 100% of the maximum allowable speed. Safety factor should always be considered.

 

 

Shielded Bearing

Bearing protective plates that cover the entire structure of the bearing. Typically, protective plates shall be fixed to the outer ring by “C” Shaped tightening ring. These plates provide good protection against dust and dirt, but not perfect insulation. The protective plates are pressed against the outer ring but there is always some very small gap in which dirt and dust can enter.

 

 

Sealed Bearing

Sealed bearing are protected bearing based on the same principle as shielding plates but in this type of bearing the plates replaced with rubber plates. The rubber plates create perfect insulation that prevent dust, dirt and even liquid to enter inside the bearing. However there is a downside – the rubber ring may touch the inner ring and reduce the rotation speed of the bearing.

 

 

Loading Direction

Loading direction is the direction of the stresses in the bearing. For example, the following picture: 

Bearing Load Direction

© SKF

 

The bearing in the image placed in a way to withstand stress that exerted from the left side. If the same force will be exerted from the right side, the bearing might fail. Therefore, it is very important to analyze what is the load direction.

When using bearing with shifted rings (as in the picture), various loading direction can be addressed by using several bearing arranged in three main methods:

 

Tandem Arrangement

Bearing Tandem Arrangement

© SKF

In this arrangement, the load lines are parallel and the bearing is distributing evenly the axial and radial forces. However, this arrangement allows axial forces in one direction only. If additional force is exerted on the opposite direction, or combined forces are expected, the a third bearing or different arrangements should be considered.

 

Back-to-Back Arrangement

Bearing Back-to-Back Arrangement

© SKF

In this arrangement, the load lines are acting towards the outside of the bearings. Axial forces acting on both directions are handled well but each direction is handled with one bearing. This arrangement is a rigid one and can handle well with torque exerted on the bearings.

 

Face-to-Face Arrangement

Bearing Face-to-Face Arrangement

© SKF

In this arrangement, the load lines are acting towards the inside of the bearings. Axial forces acting on both directions are handled well but each direction is handled with one bearing. This arrangement is a less rigid then the last one and deals less with torque exerted on the bearings.

 

 

 

Basic Rating Life

Bearing is one of the few mechanical components that have an actual formula to calculate the life time. The formula calculates the bearing life time according to standard ISO 281:1990:

 

Formula to calculate the bearing life

When:

– Bearing life (in million hours)

– Characteristic of the bearing dynamic load (kN)

– dynamic load (radial + axial) on the bearing (kN)

– Lifetime Coefficient. P=3 for ball bearing and p=10/3 for sleeve bearing.

 

If the rotational speed is known and constant, then the following formula should be used:

 

Formula to calculate the bearing lifeWhen:

– Rotational Speed of the bearing (rpm)

– Life time (in million hours)

 

There are three thumb rules about the life time of bearings. Higher bearing life time achived with:

  1. Lower bearing rotational speeds
  2. Lower bearing loads
  3. Larger bearing Diameter.

 

If you cannot determine the exact life time a bearing should have, the following table can be used to help assess the bearing life time based on the application of the bearing. To save cost of a very reliable bearing (too reliable), it is recommended to follow this table and select the appropriate life time (reliability = money).

 

L10h Type of Application
300 - 3000 Machines used in domestic, agricultural equipment, instruments, technical equipment for medical use
3000 - 8000

Machinery intended for short use: Manual appliances, freight elevators in garages and workshops, construction machinery

8000 - 12000 Machines designed to work reliably during a short actions: lifts, cranes
10000 - 25000 Machines designed to work eight hours a day but not regularly: transmissions for different uses, industrial electric motors
20000 - 30000 Machines designed to work eight hours a day and regularly: metalworking machines, woodworking machines, industrial machinery engineering, cranes, fans, conveyor rails, printing equipment, separators and centrifuges
40000 - 50000 Machines for continuous 24 hours a day: Turn in machining equipment, medium-sized electric machines, compressors, pumps, machinery textile industry
60000 - 100000 Water industry machinery, cable manufacturing machines, equipment, vehicles and marine propulsion
>100000

Large electric machines, power plants, pumps Mines, Mine Fans

 

 

Static Bearing Load

Static load characteristic is a characteristic of the bearing and indicated by the symbol    and measured in units of force (N). It is used for calculations related to bearings rotating at very low speeds (n <10rpm), and perform very slow periodic movements.

 

 

Dynamic Bearing Load

Dynamic load characteristic is a characteristic of the bearing and indicated by the symbol   and measured in units of force (N). It is used for calculations related to the rotating bearings and loads exerted on them.

 

 

Equivalent Static Bearing Load

 

Bearing Equivalent Static Load

© SKF

 

Equivalent static load is a characteristic that summarizes the forces acting on the bearing at rest or at very low rotation speed. This is one of the most important characteristic when selecting bearings. Equivalent static load is calculated using the following formula:

 

Formula to calculate the bearing equivalent static load

When:

– Equivalent Static Bearing Load (kN)

– Radial force exerted on the bearing (kN)

– Axial force exerted on the bearing (kN)

– Radial bearing load coefficient (each bearing has its own coefficient)

– Axial bearing load coefficient (each bearing has its own coefficient)

 

 

Static Safety Factor

Static Safety Factor is one of the characteristic that affect the decision whether to choose one bearing or the other. The static safety factor is calculated using the following formula:

 

Formula to calculate the bearing static safety factor – Static Safety Factor

– Static Bearing Load Coefficient

– Equivalent Static Bearing Load

Checking against the following table for the case where the bearing is supposed to work: 

 

Type of Motion Smooth movement without vibration Standard Shocks

Ball Bearing (noise irrelevant)

0.5 0.5 >1.5

SleeveBearing (noise irrelevant)

1 1 >2.5

Ball Bearing (Standard)

1 1 >1.5
SleeveBearing (Standard) 1.5 1.5 >3
Ball Bearing (Quite Motion) 2 2 >2
Sleeve Bearing (Quite Motion) 3 3.5 >4
Ball Bearing (Non-rotating) 0.4 0.5 >1
Sleeve Bearing (Non-rotating) 0.8 >2

 

 

* A Basic Table describing two bearings. Consult bearing manufacturers regarding these figures.

If the calculated static safety factor    is lower than the one in the table, a “stronger” bearing should be selected that fits the load requirements and the desired safety factors.

 

 

Equivalent Dynamic Bearing Load

 

Bearing Equivalent Static Load

© SKF

 

Equivalent Dynamic load is a characteristic that summarizes the forces acting on the bearing at rest or at very low rotation speed. This is one of the most important characteristic when selecting bearings. Equivalent static load is calculated using the following formula:

 

Formula to calculate the bearing equivalent dynamic load

When:

Equivalent dynamic load (kN)

– Radial force on the bearing (kN)

– Axial force on the bearing (kN)

– Radial bearing load coefficient (each bearing has its own coefficient)

– Axial bearing load coefficient (each bearing has its own coefficient)

 

 

Friction Moment

Friction Moment is describing the resistance of the bearing to motion. When a torque equation of motion mechanism is calculated, these characteristic has to be taken into account. There are bearing with extremely low friction moment and some with higher friction moment.

The friction moment is calculated using the following formula:

 

Formula to calculate the bearing friction moment– Friction moment of the bearing (Nmm)

 - Coefficient of friction of the bearing (see table)

– Equivalent dynamic load (N)

– Inner diameter of the bearing (mm).

 

Friction Coefficient Type of Bearing
0.0015 Deep groove ball bearings
0.0020 Angular contact ball bearings single row
0.0024 Angular contact ball bearings double row
0.0024 Angular contact ball bearings four-point contact ball bearings
0.0010 Self-aligning ball bearings
0.0011 Cylindrical roller bearings with cage, when Fa≈0
0.0020 Cylindrical roller bearings full complement, when Fa≈0
0.0022 Needle roller bearings
0.0018 Tapered roller bearings
0.0018 Spherical roller bearings
0.0016 CARB toroidal roller bearings
0.0013 Thrust ball bearings
0.0050 Cylindrical roller thrust bearings
0.0050 Needle roller thrust bearings
0.0018 Spherical roller thrust bearings

 

 

A video demonstrating a production process of bearings: 

 

 

 


How To Select A Bearing? - Bearing Types 

 

Ball Bearing

 

Ball Bearing Internal Structure

By Silberwolf [CC-BY-2.5], via Wikimedia Commons

Rolling bearing based on spherical bearing bodies. Designed for higher speeds and smaller loads due to small contact area between the balls and the rings. Can also carry axial load in addition to radial loads. Cheap bearings relative to other types of bearings. 

 

Angular Contact Ball Bearing

 

Angular Contact Ball Bearing

© SKF

Bearing is based on the spherical bearing bodies. The difference between this type of bearing and the previous one is that the internal slide rails are asymmetric. These bearing support much better combined loads. It is important to diagnose the loading direction. Positioning the bearing in the wrong loading direction can cause malfunction in the robot or the moving mechanical mechanism.

 

 

 

 

 

Self Aligning Ball Roller Bearing

 

Self Aligning Ball Roller Bearing Animation

This bearing is very similar to the rolling ball bearing except that the slide rails are spherical instead of straight ones. This means that when a mismatch is created between the tracks of the inner ring and the outer ring tracks, the rolling balls can align itself in the center of the sliding tracks automatically. 

 

 

 

 

 

Ball Thrust Bearing

 

Ball Thrust Bearing

Thrust bearing based on spherical bearing bodies. Unlike the standard ball bearing, these bearing designed to withstand much higher axial forces and not intended to deal with radial force.

 

By Silberwolf [CC-BY-2.5], via Wikimedia Commons

 

 

 

 

 

Cylindrical Bearing

 

Cylindrical Bearing

Rolling bearing based on cylindrical bearing bodies. Designed for lower speeds applications due to increased friction but can deal with high loads and forces. Bearing performance lowered drastically if the sliding tracks are not aligned with each other.

 

By Silberwolf [CC-BY-2.5], via Wikimedia Commons

 

 

 

 

 

Cylindrical Thrust Bearing

 

Cylindrical Thrust Bearing

Thrust bearing based on cylindrical bearing bodies. Unlike cylindrical rolling bearing, destined to bear high axial loads and no radial loads. Bearings are usually cheaper but tend to erode more quickly because there are differences in the rotation of each point on the rollers.

 

By Silberwolf [CC-BY-2.5], via Wikimedia Commons

 

 

 

 

 

Needle Bearing

 

Needle Bearing

Rolling bearing based on needles as the body bearing. The needles are actually long cylinders with very small diameter. Due to the small diameter of the rolling bodies (the needles), the inner ring and the outer ring are very close one to the other. This contributes to a design when one has to choose an internal diameter bearing closely as possible to the axis. Bearing is characterized by quick degradation and low reliability.

 

 

 

 

 

Tapered Bearing

 

Tapered Bearing

Rolling bearing based on conical bearings bodies. The cones roll on the inner rail that is shaped as conical rails too.  Most of the bearing can deal with axial and radial forces but this type of bearing can deal much better with combined and inclined loads.  Usually, they are more expansive due to the complication of the manufacturing process.

 

By Silberwolf [CC-BY-2.5], via Wikimedia Commons

 

 

 

 

Tapered Thrust Bearing

 

Tapered Thrust Bearing

Thrust bearing based on conical bearings bodies. Bearing that, unlike standard tapered bearing, can deal with much higher axial loads and almost no radial loads. These bearings are usually expensive due to the complexity in manufacturing.

 

By Silberwolf [CC-BY-2.5], via Wikimedia Commons

 

 

 

 

 

 

Spherical Roller Bearing

 

Spherical Roller Bearing

Rolling bearing based on barrel-shaped bearing body (thick center and thinner at the edges). This bearing can deal much better with non-concentric bearing rings. Bearings are expensive due to their manufacturing complexity.

 

By Silberwolf [CC-BY-2.5], via Wikimedia Commons

 

 

 

 

 

Fluid Dynamic Bearing

 

Fluid Dynamic Bearing

These bearings consist of two sealed rings and no internal bearing elements. Pressurized liquid (usually, oil, water or air) is put inside and between the two rings. The advantages of these bearings are low cost and much lower friction mechanical bearings. However, these bearings are very sensitive to temperature changes and can fail in an instant when subjected to sudden high loads (shocks).

 

 

 

 

 

 

Spherical Bearing

 

Spherical Bearing

By Androstachys [CC-BY-SA-3.0 or GFDL], via Wikimedia Commons

Spherical bearing allows a slight angular movement of the drive shaft relatively to the bearing rings. The inner ring supports the drive shaft and can turn relatively to the outer ring.

 

 

 

 

 

Magnetic Bearing

 

Magnetic Bearing

© NASA

Magnetic bearing is bearing with no internal bearing elements. Bearing supports the rotating axis inside by electromagnetic force that cause “hovering” of the two rings in a constant distance. In this type of bearing there is no mechanical contact between the two rings. This bearing has almost zero degradation over time and characterized with extremely low friction. Another advantage is that this bearing has no speed limit. These bearings can turn around at any speed. Downside is that these magnets require constant electrical input voltage (for example, a machine that moves the axis and suddenly there is an electrical failure of the bearing may be damaged severely due to the collapse of the bearing).

 

 

 

 

 

Plain Bearing

 

Plain Bearing

Plain bearing is the simplest type of bearing without any internal bearing elements and based on sliding motion. This bearing is built from one ring (sleeve), where the drive shaft is turning inside it. Sliding bearing is very common and most affordable. It is a good choice where high radial loads may be subjected but has a very high degradation rate. This type of bearing is not built for high speed applications due to the high friction

 

By Silberwolf [CC-BY-SA-2.5], via Wikimedia Commons

 

 

 

 

 

How To Select A Bearing - So How To Select A Bearing?

 

 

Loads

 

Is there an axial load on the bearing? Is there radial loads? Or maybe a combination of both?

it is important to define the size and the direction of the loads.

Now begins the stage of trial and error. first choose a bearing from the catalog that corresponds to the expected loads.

Using the Characteristics of the bearing (,,,,,) Calculate the equivalent dynamic load and the equivalent static load.

First calculate the equivalent static load using the formula:

 

Formula to calculate the bearing equivalent static load

 

Compare the equivalent static load to the static safety factor. Choose a safety factor from the top table and use it in the following formula:

 

Formula to calculate the bearing equivalent static load

 

Now check the bearing manufactures tables and look for a bearing with a static load factor    fits to the calculated one. After a bearing has been selected, check the equivalent dynamic load of the bearing. To do so use the following equation:

 

or

Formula to calculate the bearing equivalent Dynamic load

 This is the equivalent dynamic load calculated according to the formula:

 

Formula to calculate the bearing equivalent Dynamic load

 

 Coefficient depends on the bearing type. m=3 for ball bearings, m=3.3 for sleeve bearings.

 This figure describes the bearing life in units of millions of turns.

 This figure describes the bearing life in units of millions of hours

– Rotation speed of the bearing

Now estimate how long the bearing should work. If you don’t know exactly how long the bearing should work, you can use the following table: 

:

 

Type of application
300 - 3000 Machines used in domestic, agricultural equipment, instruments, technical equipment for medical use
3000 - 8000 Machinery intended for short use: Manual appliances, freight elevators in garages and workshops, construction machinery
8000 - 12000 Machines designed to work reliably during a short actions: lifts, cranes
10000 - 25000 Machines designed to work eight hours a day but not regularly: transmissions for different uses, industrial electric motors
20000 - 30000 Machines designed to work eight hours a day and regularly: metalworking machines, woodworking machines, industrial machinery engineering, cranes, fans, conveyor rails, printing equipment, separators and centrifuges
40000 - 50000 Machines for continuous 24 hours a day: Turn in machining equipment, medium-sized electric machines, compressors, pumps, machinery textile industry
60000 - 100000 Water industry machinery, cable manufacturing machines, equipment, vehicles and marine propulsion
<100000 Large electric machines, power plants, pumps Mines, Mine Fans

 

 Put all the gathered data in the formula to get C. now check the bearing tables for the most suitable equivalent static load  and the equivalent dynamic load .

 

If one of the calculated figures is smaller than the one in the table, a “stronger” bearing has to be selected and the dynamic load has to be recalculated (X and Y are variables and depended on the selected bearing).

If the two calculated figures are higher than the one appearing in the table the one of the two following choices can be made:

  • Stay with the selected bearing with the knowledge that it has higher requirements than needed.
  • Choose a "weaker" bearing to lower cost and the safetey factor as well.

 

 

What is the bearing’s expected angular velocity?

 

Using bearings manufacturer’s catalogs, you can select the appropriate bearing by the indicated speed in the technical datasheets.

 

What are the geometric dimensions of the bearing? 

The critical part is the drive shaft. Therefore, the bearing selection process should be aimed towards matching the inner diameter of the bearing to the outer diameter of the drive shaft.

 

 

What are the environmental conditions of the bearing?

 

If dust or dirt is expected in the work environment, protected and sealed bearing should be considered.

If large environment temperature variations are expected, then avoid using hydrostatic bearings.

If a sensitive instruments for EMC or electrostatic positioned near the bearings, then avoid using magnetic bearings or find a way to protect the sensitive instruments.

 

 

 

 


 

Written by Eran Cenciper (Robot-and-Machines-Design webmaster)