ROLLING CONTACT BEARING: Types, Materials & Lubrication.

ROLLING CONTACT BEARING 

The purpose of a rolling contact bearing is to support a load while permitting relative motion between two elements of a machine.  

The term rolling contact bearing refers to the wide variety of bearings that use spherical balls or some other type of roller between the stationary and the moving elements.  

The main functions of the most common types of bearings are as follows: 

  1. Supporting a rotating shaft. 
  1. Resisting radial loads. 
  1. Resisting a combination of radial and axial (thrust) loads. 

Some bearings are designed to carry only thrust loads.  

The primary applications of rolling contact bearing involve supporting linear as well as linear motion. 

TYPES OF ROLLING CONTACT BEARING 

  1. Single-Row, Deep-Groove Ball Bearing  
Single-Row, Deep-Groove Ball Bearing  
Single-Row, Deep-Groove Ball Bearing  

When the term ball bearing is called out by people, they generally tend to image the single row, deep groove ball bearing. It is sometimes also called as Conrad bearings. 

For rotation of the shaft with the bearing, the inner race is pressed with a slight interference fit on the shaft. 

In the inner as well as outer races of the deep groove ball bearing there is a spherical rolling element.  

The spacing of the balls is maintained by retainers or “cages.”  

While designed primarily for radial load-carrying capacity, the deep groove allows a fairly sizable thrust load to be carried.  

With the help of the shoulder on the shaft, a thrust load would be applied on one of the sides of inner race. 

There would be a transmission of load from the side of the groove, through the ball, and then to the opposite side of the outer race. Finally, the load would be transferred to the housing. 

For the free rolling of the balls, the radius of the ball is slightly smaller than the radius of the groove. 

The contact between a ball and the race is theoretically at a point, but it is actually a small circular area because of the deformation of the elements.  

Because the load is carried on a small area, very high local contact stresses occur.  

To increase the capacity of a single-row bearing, a bearing with a greater number of balls, or larger balls operating in larger-diameter races, should be used.  

  1. Double-Row, Deep-Groove Ball Bearing  
Double-Row, Deep Groove Ball Bearing  
Double-Row, Deep Groove Ball Bearing  

To enhance the radial load carrying capacity of the deep groove type of bearing, additionally row of balls is introduced which shares the load. 

This leads to greater load bearing capacity in the same space which is available. 

There is an effect to the misalignment capability with an increase of width in the double row bearings. 

  1. Angular Contact Ball Bearing  
Angular Contact Ball Bearing  
Angular Contact Ball Bearing  

To allow greater thrust loads, one side of each race in an angular contact bearing is higher as compared to single row, deep groove ball bearing. 

  1. Cylindrical Roller Bearing  
Cylindrical Roller Bearing  
Cylindrical Roller Bearing  

Greater radial load capacity is achieved when the spherical balls are replaced with cylindrical rollers. 

The pattern of contact between a roller and its race is theoretically a line, and it becomes a rectangular shape as the members deform under load.  

The contact stress levels generated in the cylindrical roller bearings are less as compared to similar sized ball bearings which allow the smaller bearings to carry the given load. 

Thrust load capacity is poor because any thrust load would be applied to the side of the rollers, causing rubbing, not true rolling motion.  

It is recommended that no thrust load be applied.  

Roller contact bearing are often fairly wide, giving them only fair ability to accommodate angular misalignment.  

  1. Needle Bearing  
Needle Bearing  
Needle Bearing  

Needle bearings are actually roller bearings, but they have much smaller diameter rollers.  

To carry a given load, a smaller radial space is typically needed for needle bearings. 

This makes it easier to design them into many types of equipment and components such as pumps, universal joints, precision instruments, and household appliances.  

The cam follower is another example in which the antifriction operation of needle bearings can be built-in with little radial space required.  

Needle bearings have lesser thrust and misalignment capabilities as compared to other roller bearings. 

  1. Spherical Roller Bearing  

The spherical roller bearing is one form of self-aligning bearing, so called because there is actual relative rotation of the outer race relative to the rollers and the inner race when angular misalignments occur.  

This gives the excellent rating for misalignment capability while retaining virtually the same ratings on radial load capacity. 

  1. Tapered Roller Bearing  
Tapered Roller Bearing  

Tapered roller bearings are designed to take substantial thrust loads along with high radial loads, resulting in excellent ratings on both.  

They are often used in wheel bearings for vehicles and mobile equipment and in heavy-duty machinery having inherently high thrust loads. 

  1. Thrust Bearings  
Thrust Bearings  
Thrust Bearings  

The bearings discussed so far in this article have been designed to carry radial loads or a combination of radial and thrust loads.  

Many machine design projects demand a bearing that resists only thrust loads, and several types of standards thrust bearings are commercially available.  

In thrust bearing following rolling elements are used: 

  1. Spherical balls. 
  1. Cylindrical rollers. 
  1. Tapered rollers. 

Thrust bearings have the capacity to take very less or no radial load. 

Most thrust bearings can take little or no radial load.  

Then the design and the selection of such bearings are dependent only on the magnitude of the thrust load and the design life.  

Manufacturer’s catalog includes dynamic as well as static load rating. 

MOUNTED BEARINGS  

Mounted bearing is mostly preferred over unmounted bearings for heavy and special machines which are manufactured in small quantities. 

There is a provision provided in the mounted bearing to directly attach the bearing unit with the frame of the machine using bolts. This feature is not provided in the unmounted bearings. 

The most common configuration for a mounted bearing: the pillow block.  

Steel, cast iron, or cast steel are used to produce the housing which consist of holes or slots for attachment during the assembly of machine. 

The bearings themselves can be of virtually any of the types discussed in the preceding sections; ball, tapered roller, or spherical roller is preferred.  

Based on the conditions of the use of such bearings the misalignment capability is an important application. 

This capability is provided either in the bearing construction itself or in the housing. 

Because the bearing itself is similar to tho.se already discussed, the selection process is also similar. 

Most catalogs provide extensive charts of data listing the load-carrying capacity at specified rated life values.  

Again, several bearing types and sizes are available.  

Take up unit means a bearing which is fitted in a housing, that allows the movement of the bearing where the shaft remains in its place as the bearing is mounted in a frame. 

Used on conveyors, chain drives, belt drives, and similar applications, the take-up unit permits adjustment of the center distance of the drive components at the time of installation and during operation to accommodate wear or stretch of parts of the assembly. 

BEARING MATERIALS  

Rolling contact bearing exerts a load on small areas. 

Contact stresses are normally highly, regardless of the type of bearing. 

Contact stresses of approximately 300 000 psi are not uncommon in commercially available bearings.  

The balls, rollers, and races are produced from a very hard, high strength steel or ceramic to sustain high stresses. 

The most widely used rolling contact bearing material is AISI 52100 steel, which has a very high carbon content, 0.95% to 1.10%, along with 1.30% to 1.60% chromium, 0.25% to 0.45% manganese, 0.20% to 0.35% silicon, and other alloying elements with low, but controlled, amounts.  

Impurities are carefully minimized to obtain a very clean steel.  

For sustaining high contact stresses, the material is through hardened in a range of 58-65 on the Rockwell C scale. 

Tool steels such M1 and M50 are also typically used as materials of the bearings. 

In order to make the core stronger, case hardening is used on steels such as AISI 3310, 4620, and 8620 which helps in achieving high surface hardness. 

Careful control of the case depth is required because critical stresses occur in subsurface zones. 

Some more lightly loaded bearings and those exposed to corrosive environments use AISI 440C stainless steel elements.  

Silicon nitride is a ceramic material and hence can be used to make rolling elements and other components. 

Although their cost is higher than that of steel, ceramics offer significant advantages. 

Their light weight, high strength, and high temperature capability make them desirable for aerospace, engine, military, and other demanding applications 

LUBRICATION  

The functions of lubrication in a rolling contact bearing unit or in a general bearing are as follows:  

1. The main intention of the lubrication is to produce a low friction film between the rolling elements and the races of the bearing and at certain specific points of contact with cages, retainers, guiding surfaces, and so on. 

2. To protect the bearing components from corrosion.  

3. Dissipation of heat from the bearing unit.  

4. To carry heat away from the bearing unit.  

5. Helps in removing the contaminants and moisture from the bearing.  

Rolling contact bearings are usually lubricated with either grease or oil.  

Under normal ambient temperatures (approximately 70°F) and relatively slow speeds (under 500 rpm), grease is satisfactory.  

At higher speeds or higher ambient temperatures, oil lubrication applied in a continuous flow is required, possibly with external cooling of the oil.  

Oils used in bearing lubrication are usually clean, stable mineral oils.  

Under lighter loads and lower speeds, light oil is used.  

Heavier loads and/or higher speeds require heavier oils up to SAE 30.  

A recommended upper limit for lubricant temperature is 160°F.  

Many factors are considered while choosing a correct oil or grease, so it’s important to have a discussion with the bearing manufacturer regarding the application. 

In general, a kinematic viscosity of 13 to 21 centistokes should be maintained at the operating temperature of the lubricant in the bearing.  

Manufacturer’s recommendations should be sought.  

In some critical applications such as bearings in jet engines and very-high-speed devices, lubricating oil is pumped under pressure to an enclosed housing for the bearing where the oil is directed at the rolling elements themselves.  

A controlled return path is also provided.  

The temperature of the oil in the sump is monitored and controlled with heat exchangers or refrigeration to maintain oil viscosity within acceptable limits.  

Heat removal and reliable lubrication is provided by such systems. 

Greases used in bearings are mixtures of lubricating oils and thickening agents, usually soaps such as lithium or barium.  

The soaps act as carriers for the oil which is drawn out at the point of need within the bearing. Additives to resist corrosion or oxidation of the oil itself are sometimes added.  

Classifications of greases specify the operating temperatures to which the greases will be exposed, as defined by the American Bearing Manufacturers’ Association (ABMA). 

PRELOADING  

Some bearings are made with internal clearances that must be taken up in a particular direction to ensure satisfactory operation.  

In such cases, preloading must be provided, usually in the axial direction.  

On horizontal shafts, springs are typically used, with axial adjustment of the spring deflection sometimes provided to adjust the amount of preload.  

When space is limited, the use of Belleville washers is desirable because they provide high forces with small deflections.  

Shims can be used to adjust the actual deflection and preload obtained.  

On vertical shafts the weight of the shaft assembly itself may be sufficient to provide the required preload.  

BEARING STIFFNESS  

Usually, the radial stiffness is most important because the dynamic behaviour of the rotating shaft system is affected.  

The critical speed and the mode of vibration are both functions of the bearing stiffness.  

The critical speed of the shaft assembly gets reduced when a softer bearing (lower stiffness) is used. 

Stiffness is measured in the units used for springs, such as pounds per inch or newtons per millimetre.  

Of course, the stiffness values are quite high, with values of 5,00,000 to 10,00,000 lb/in reasonable. The manufacturer should be consulted when such information is needed, because it is rarely included in standard catalog. 

SEALING 

Special shields and seals are mostly used for the bearing to operate in dirty or moist environments. 

Seals are generally provided on either or both sides of the rolling elements. 

Shields are normally attached to the stationary race which is made of metal, and tend to remain clear of the rotating race. 

Seals are made of elastomeric materials and do contact the rotating race.  

Although such bearings are likely to give many years of satisfactory service, extreme conditions can produce a degradation of the lubricating properties of the grease.  

The presence of seals also increases the friction in a bearing. Seals tend to have the provision of placing them outside the bearing and specifically in the housing or at the shaft/housing interface. 

On high-speed shafts, a labyrinth seal, consisting of a non-contacting ring around the shaft with a few thousandths of an inch radial clearance, is frequently used.  

Grooves, sometimes in the form of a thread, are machined in the ring; the relative motion of the shaft with respect to the ring creates the sealing action. 

General Recommendations for Achieving Long Life for Bearings:  

1. Choose a bearing with an adequate rated life using the procedures outlined in this article.  

2. Always check whether the bearing has a fine surface finish as it should not be damaged by rough handling, corrosion, vibration, improper installation practices, or electric current flow exposure. 

3. Ensure that operating loads are within the design values.  

4. Clean lubricant of appropriate viscosity should be supplied to the bearing according to the recommendation of the manufacturer. 

Provide external cooling for the lubricant if necessary.  

For an existing system, this is the factor over which you have the most control without a significant redesign of the system itself.  

5. If there is a scope of redesign, then the system should be designed at a low speed. 

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