Rolling element bearings are standardised machine units. They transmit forces, moments and rotations, including the guidance of shafts and spindles. Transmission of forces and movements generate friction. Such frictional resistance proportionally increases torque and thus generating additional undesirable heat gain. Any reduction in frictional resistance minimises these criteria. This directly reduces the required input power torque of a machine, thereby enabling a more compact design, higher efficiency and a simple construction of machines.
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The design of bearings may, for example, provide for free linear movement of the moving part or for free rotation around a fixed axis; or it may prevent a motion by controlling the vectors of normal forces that bear on the moving parts. Many bearings also, facilitate the desired motion as much as possible, such as by minimizing friction.
Bearings consist of two rings of different diameter known as "ring" , between which they are positioned the main elements to which is entrusted the function of:
to support the rotary element
to reduce the friction of rotation
to preserve the rotation seat
to reduce the danger of intrusion of dust and debris in the work areas of the spheres
The movement, in roller bearings , also called a "rolling", it is facilitated by the interposition between the fixed part and the pin of rolling elements (balls or rollers). In this type of bearings the lubricant is very important and in many cases it is advisable to provide protections to prevent leakage; and also to prevent the entrance of contaminate substances, including moisture.
Bearings are composed of three different elements:
Outer ring : that is the larger circular element
Inner ring: that is the smaller circular element
Rolling(s) element(s) : in case of mechanical bearings, are various types of rollers or balls and their cages.
Bearings can be so constructed as to be:
Mono-element : bearings can not alter in their structure without that this being damage, as it is not designed to be divided in its various parts.
To take apart: a classic example is present in the hubs of the bicycles and motorcycles, where the outer ring is kept closed by the fork and the nut.
Break up : these bearings are divided into two or more parts.
There are two different types of bearings. The principal differences are :
sliding friction ( for example plain bearings)
rolling friction ( for example rolling bearings)
For plain bearings, sliding friction occurs when two surfaces, generally separated by a third medium ( for example air and/or lubricant) , move relative to each other.
The maximum forces transmitted by plain bearings are determined basically in the contacting area between the component elements.
Their main advantage of plain bearings lies in their very simple and thus inexpensive structure. They are frequently used in maintenance-free designs and require less space in the machine design.
Their main disadvantages are limited ability in transferring forces, less accuracy in guiding shafts and spindles plus a relatively large starting torque.
Rolling element bearings operate with the principle of rolling friction.
The applied load is transmitted by rolling elements, rotating around and between two bearing rings, normally guided by ring raceway grooves. Usually the rolling elements are positioned within the bearing by separators, also termed retainers or cages.
Rolling bearing also require lubricant separation of the metallic parts.
This along with good surfaces finish ensures smooth running surfaces and reduces friction from the transmitted forces, resulting in minimizing power loss.
In practice no pure rolling motion occurs within rolling bearings. The basic rolling motion of the rolling elements will include elements of sliding friction.
An excessive amount of sliding friction may cause serious damage on the bearing components and thus may cause premature failure.
Sliding friction is common, but it may cause problems in the event of high speeds, accelerations or the bearing runs under too lightly applied load. There is supporting evidence that rolling bearings require an effective load to perform efficiently.
Rolling element bearings, compared to plain bearings, have the following advantages :
less starting torque, energy conservation
greater availability of different types and sizes
many capable of taking combined radial and axial loads
greater load carrying capabilities within limited design space
maintenance free, sealed " for-life" design arrangements
less lubrication consumption
very precise guidance of shafts
suitable to operate with high temperature
available in special designs to meet particular applications
If balls and rollers are not arranged over the whole volume, are maintained equidistant between them of a structure called "cage", which can be simple or massive and made of steel, bronze, brass or plastic material. A cage fulfils several function as to separate the individual rolling elements ( in order to reduce friction to a minimum), to guide and position the rolling elements between the raceways, to retain the rolling elements.
When bearings are separable, the cage also is necessary to retain the rolling elements.
In some specific cases of bearings ( for example in needle roller bearings, in axial bearings and in cylindrical roller bearings) the cage also is necessary to guide the rolling bodies as it serves to keep aligned so that they can roll with minimum friction.
The cage can be centred on the rolling elements or on one of the two rings and is guided radially.
Under certain conditions a cage may be omitted from the assembled bearing type. This is termed full complement bearing. This enables a maximum load carrying capacity by utilising the bearing cross sectional area with the optimum number of rollers. This causes higher friction therefore lower speed capabilities.
For rolling bearings fitted with cages, however, minimal sliding friction occurs when the respective surfaces of rolling elements and cage pockets.
Cages of steel or brass are usually centred on the rolling elements.
The massive cages, centred on one of the two rings allow higher rotational speed.
These are necessary when there are movements that overlap those of simple rotation, especially when prevailing heavy acceleration conditions.
The cages of rolling bearings are mechanically stressed by frictional forces, deformation and inertia. For these reasons, the mounting of the cages and the choice of its material are of fundamental importance for the performance of that element.
Polyamide cages : single and double row deep groove ball bearings, self aligning ball bearings, angular contact ball bearings, cylindrical roller bearings are equipped with polyamide cages 6.6, thermally stabilizing reinforced with glass fibres. The capacities of such cages to work with poor lubrication allow continuing operations without the risk of engage or future damage. These cages are unsuitable for working at temperatures below of -40°C, as they lose elasticity and are unsuitable for high operating temperatures, that depending on the type of bearing, that are included between 80°C and 120 °C - if bearings are used for a short time the temperature ranges changes between the 140 ° C to 210 ° C.
Steel cages: these cages have a relatively high resistance and weigh little. To reduce friction and wear can be hardened and subjected to surface treatments. The massive steel cages are used in large bearings. To improve the fluidity and wear resistance some massive steel cages are carbonitriding. The cages in issue endure at temperatures up to 300 ° C and out of them have no effect on either the lubricants based on mineral or synthetic oil, or the organic solvents used to clean them : there is however the risk of corrosion in the presence of water.
Brass cages : these types of cages are used for medium and small bearings. The brass cages can not exceed 300 ° C and on them does not affect the lubricants ( including synthetic oils and greases) but they can be cleaned with very normal organic solvents. We do not recommend the use of alkaline agents. Pay attention to the use of ammonia because it favors the arise of stress corrosion cracking.
In bearings lubricated with grease, part of this adheres to the cage and form a reserve ( of lubricant ) , ensure a good lubrication of the bearings work surfaces themselves.
Bearings without cages
Under certain circumstances a bearing may be used without cages. In such cases the bearings are full complement.
The shielding of the bearing may vary depending on the need, the application and lubrication of the bearing. They are present in the following versions :
Close , where the rolling bearing parts are protected from external agents.
They can be of two types:
Shield : in these ball bearings or roller bearings are protected within the two rings by a pair of thin metal flanges. It is effective against dust.
Tin : in these ball bearings or roller bearings are protected within the two rings by a pair of thin rubber flanges, watertight. In this case the lubricant is necessary, sufficient for the whole life of the bearing, is inserted by the manufacturer during assembly. This type of protection is effective against dust and fluids;
Open, where the rolling parts are exposed to the external environment, and therefore it is not recommended to use in harsh environments (dust or corrosive fluids).
For internal clearance of the bearings it means the total possible displacement of one ring respect to each other in radial or in axial direction. The initial internal clearance is greater than what it has in operation because the rings will expand or contract following the couplings.
This play is of great importance in order to obtain a zero operation or a light preload. The internal clearance of the bearings defined as "normal" was chosen so that, when the same bearings are mounted with the recommended match it obtained an appropriate operating clearance.
The bearings to operate must have a specific play, in order to going up to operating temperature their thermal expansion does not lead to a tightening of the bearing, compromising the fluidity that is the duration and strength of the bearing.
The play in question is of two types :
Radial , play that defines the radial displacement of the inner ring of the bearing characterized by the empty space it has between the balls / rollers and the two rings
Axial , play that defines the axial displacement (along the axis ) of the inner ring of the bearing.
The preload can be expressed as a force or as the displacement which gives rise to the same force.
Depending on the applications it is useful that a set of bearings gave, in operations, a positive or negative play ( it is useful note, that in the great majority of cases, the play during operations must be positive). There are some cases in which it is suitable to have a negative game, that is a preload with the aim to increase the stiffness of the whole or the precision of rotation ( for example shaping spindle, conic pinions of cars, small electric motors or mechanisms with oscillating movements. The application of a preload is advisable when the bearings must operate without load or at high speed in the presence of very low loads. In these cases, the preload serves to avoid damage due to rubbing.
It's good to keep in mind that for optimal operation the bearing must always be subject to a certain minimum load.
Depending on the bearings the preload may be axial or radial. The optimum preload to the working temperature depends on the load applied to the bearing.
Cylindrical roller bearings (because of their constructive form ) can be preloaded radially only
Thrust ball bearings can be only preloaded axially only
Single row angular contact ball bearings and tapered roller bearings ( They are mounted in opposition with a second bearing of the same type) they can be preloaded axially only, and in this way the application of axial preload also support a radial preload.
Deep groove ball bearings can be preloaded axially only (they should have a radial internal clearance above the normal).
Angular contact ball bearings and cylindrical roller bearings they can withstand radial and axial forces acting simultaneously.
Underlying we can see the main effects of the preload :
long operating life : it offers operational reliability increases.
It has a positive influence on the load distribution in the bearings and therefore also on its duration .
Wear Compensation and and adjustments in operation: the wear and adjustments that occur in operation causing an increase of the play, that the preload is able to compensate.
Greater precision of shaft guide : the more precise guidance and the greater stiffness offered by the preloaded bearings mean that the engaging remains precise and constant over time.
The result that comes out is the silent operation and long lifetime of the couple.
Noise reduction : small is the play in operation better is the guidance of the rolling elements in the unload area and quieter is the rotation.
Increase of the stiffness : it is defined as the ratio ( relation) of the effort that acts on it and the elastic deformation that occurs within the bearing.
The friction in a bearing depends on the load that is applied and by multiple factors, for example depending on type and size of the bearing, speed of rotation, as well as the characteristics and the amount of lubricant. In a bearing the overall resistance to motion it is formed by rolling friction and from that of rubbing in rolling contact, in the contact areas between the themselves rolling elements and the cage and finally by the friction in the lubricant and by friction in bearings fitted with contact seals.
In bearings there is a limit of speed at which they can operate and it is represented by the working temperature and it may be accepted in relation to the lubricant used or to the material to which the components of the same bearings are made.
The speed depends on the heat generated by friction within the bearing and the amount of such heat that can be take away from the bearing itself.
The type and size of the bearing, its internal design, the load, the lubricating and cooling conditions and also the execution of cage, accuracy and internal clearance all have a role in determining the permissible speed.
In order to take advantage of the load capacity of a bearing, the respective rings or "washers" must be supported over their whole circumference and over the whole width of the track. The support must be solid and uniform, furthermore rings must be securely fixed to avoid that in the presence of wheel loads they revolves to their seats.
( Attention : rings that are not fixed in the correct way will damage bearings and parts that house them ).
When it has to choice the combination of bearings with their respective house is good to take into consideration the factors below-listed :
conditions of rotation: it refers to the movement of the ring's bearing respect to the load direction. There are three different conditions:
- "turning load " : it happen when the ring's bearing rotates and the load is fixed, so that all parts of the track are subject during a round to the load.
Normally, high loads do not rotate but oscillate are defined as such.
-"fixed charge" : valid in the case of a ring and of a fixed load, or in the case where load and ring rotate at the same speed. In this condition does not occur rotation of the ring on its seat, therefore after mentioned ring does not need to be forced in the same place.
-"indeterminate load direction." : It occurs when it has variable external loads, shock loads, and loads due to imbalance in high-speed machines , undergo changes of direction that can not be described accurately. In the presence of high loads it is appropriate that both rings will assembled with interference.
Load size : with the increase of load , the interference of the inner ring of a bearing in its seat it will be loosen , seen that the ring tends to grow larger. Under the effect of a turning load the itself ring can start rotating. Greater is the load, greater will be the required interference.
Bearing internal clearance : when the ring of a bearing is mounted with interference on the shaft or in the housing it is elastically deformed and causes a decrease of the internal clearance of the bearing. This factor depends on the type and size of the bearing.
Thermal conditions : in operation the bearings reach a temperature that is higher than that of the details of which they are mounted. This may causes a loosening of the couple of the inner ring on his housing while the outer ring expansion may prevent the axial movement into its housing.
Requirements relating to the accuracy of gear : to reduce weakness and vibrations should be avoided free couplings for those bearings that must have a high degree of driving precision.
Mould and material of the shaft and housing: the coupling in its seat must not lead to an irregular deformation of the affected ring. The different types of housings may not be suitable to accommodate outer rings mounted with forcing and adopted couplings must not involve greater interference. It's good to ensure an adequate support to the rings mounted in housings having thin walls or of light alloys, or of hollow shafts, with more forced make of those normally chosen.
Easy assembly and disassembled: bearings with a loose fit are easier to assemble and disassemble than those with forced coupling. In the case where applications require the use of forced couplings can be employed separable bearings or those with tapered bore and adapter or withdrawal sleeve. ( The majority of a rolling bearings are never removed from their houses, they stay in their machines or plants until the whole machine is scrapped. This is the replacement of bearings affects mainly larger rolling bearings, and bearings for important machinery where it is part of planned preventative maintenance schedules).
Movement of a bearing axially free : when bearings are mounted it is essential that, in any operating condition, one of the two rings is free to move axially. This is ensured by the adoption of a clearance fit for the ring subject to a fixed load. When the outer ring is to be subject to the axial movement fixed load must take place in the housing bore.
Whilst rolling bearings, used in various applications, generally have radial location of their shaft and housing seats, they also require certain axial locating using the appropriate fits. Where heavy interference fits of shafts or housings provide clamping forces on the baring seats they do not guarantee axial location in all circumstances.
True the axial location of the bearings at their seats is best achieved by means of a closed-form arrangement usually by locking nuts and washers, housings cups, shaft shoulders, snap rings and etc..
The inner ring is fixed on the opposite side of the abutment using a lock nut with a lock washer, while the outer ring is fixed of the end cover, through the end cover; rarely with a thread nut. In application bearings must be protected to prevent penetration inside them of solid impurities and moisture and at the same time to retain the lubricant. The guards must have a minimum of friction and create less wear as possible, in order that the duration of the bearing performance is not compromised. The protections are of fundamental importance as they allow the correct operation of the bearing.
There are various types of protection:
non sliding contact : they haven't friction and don't undergo wear. They are not easily damaged by solid impurities and are particularly suitable for high speeds and high temperatures. These protections are used in applications in which the presence of impurities is not so high and there is no risk that water, vapour or other substances come into contact with the bearing. To increase the effectiveness of protective devices of this type, often , are used rotating disks that are mounted on the shaft.
sliding protections : they offer a reliable seal, especially if it reduces wear to a minimum. In addition, they are subject to mechanical damage. These protections are used for applications in which it can not be expected a type of effective external protections or because there is not enough space. These protections are used in all those applications in which it has a moderate presence of impurities, moisture may be present, splash and also when it requires a long operating life without maintenance. In these protections there are radial lips protection that are used in oil lubricated bearings. These protections are are ready for mounting and have a and have a metal reinforcement or they are provided with a sheet-metal box. The lip is normally of synthetic rubber and it is pressed against the sliding surface by a spring
external protection : this type of protection is the most simple,inasmuch it is constituted by the small light that is formed at the end of the shaft from the support. It is suitable for machines that works dry and dust-free ambient. There are different types of lubrication:
grease lubrication : the grease can be used for most of the applications as lubrication of rolling bearings operating under normal working conditions. The main advantage of the grease ( as lubricant) is that to be able to be more easily retained within the bearing system, especially when the shaft is inclined or vertical, also it helps to protect the system itself from impurities, moisture or water. The grease must only fill the bearing,while the the free space must be filled only partially. Lubricating greases are mineral or synthetic oils thickened.
PAY ATTENTION : when bearings have to operate at very low speeds and must be protected against corrosion it is advisable to fill the housing completely with grease.
The protection with this lubrication can be increased by obtaining one or more parallel and concentric grooves in the supporting hole in the end of the shaft. The grease that comes out of the light fills the grooves and prevent the entry of foreign substances. Protections in single or double labyrinth are used with this type of lubrication and their efficacy can be increased by introducing a grease based on calcium or lithium
oil lubrication : this type of lubrication is used only when high speeds or high operating temperatures prevent the use of the grease, when it has to take away the heat generated by friction or when adjacent parts are already lubricated with oil.
The oil lubrication presents several methods, which may be:
- lubrication in oil bath (???) that is the easier method, since the oil which is caught by the rotating parts of the bearing is distributed inside that and later flows again in bath ( ??? ).
When the bearings doesn't not rotate, the oil level, must be such as to reach the centre of the rolling body (It represents the lowest point of a bearing).
- lubrication with oil circulation is used when operation at high speeds entails a increase of the working temperature and an accelerated ageing of the oil. This lubrication carries out with the aid of a pump
- lubrication at oil jet it is used for very high speed operation where it is necessary that in the bearing goes in a sufficient amount of oil but not so excessive. In this type of lubrication an high pressure jet is directed towards a side of the bearing
- lubrication oil/air thanks to this method small doses of oil runs on each bearings. The oil is sent to the ducts at very specific pauses. The compressed air is necessary to cool the bearings and prevents the impurities to penetrate.
The lubrication in ball bearings or in roller bearings is very important since in absence of lubricant the bearing will damaged in a short time :
facilitates the sliding of the balls reducing the friction and the consumption of the moving parts
It protects the bearing from corrosion by external agents and from impurities
dissipates heat that is created due to the slip and friction.
Lubrication can be either grease or oil. Usually it is used the grease, but in some specific cases it prefers the lubrication with oil, especially within oil-lubricated mechanisms, where it is almost impossible to separate the bearing lubrication from that of other parties.