Bearing Technical Information

We hope that the bearing technical information shown below will help you to gain a better understanding of bearings and their performance. If you require further bearing technical information or an explanation of any of the sections below, please contact us.

1. Bearing Material

For chrome steel and stainless steel material composition and details on the different grades of stainless steel used in our bearings, please see our MATERIAL TABLES

SAE52100 Chrome Steel (no prefix)
This is the standard steel for most ball bearings. It is harder than stainless steel and gives greater life ratings. It also has superior low noise qualities to standard 440 grade stainless steel. Chrome steel actually has a low chromium content and is not corrosion resistant so not suitable for corrosive environments or for dry (no lubricant) bearings as chrome bearings require a protective oil coating on the exterior surfaces which can contaminate the inside of the dry bearing. Chrome steel can tolerate continuous temperatures of up to 120C. Above this temperature, chrome steel undergoes greater dimensional change and the hardness is affected, reducing load capacity. It can withstand up to 150C intermittently but above this temperature, bearing life is significantly reduced.

AISI440 and KS440/ACD34/X65Cr13 Martensitic Stainless Steel (prefix "S")
More resistant to corrosion due to the greater chromium content and the addition of nickel, 440 grade stainless steel is the most commonly used for corrosion resistant ball bearings. The chromium reacts with oxygen in the air to form a chromium oxide layer, known as the passive film, on the surface of the steel. It is hardenable and gives a good combination of strength and corrosion resistance. It is magnetic unlike some 300 grades. The load capacity of 440 grade is approximately 20 percent less than chrome steel so life ratings will be slightly reduced. This grade exhibits good corrosion resistant when exposed to fresh water and some weaker chemicals but may corrode in seawater environments or in contact with many aggressive chemicals. The corrosion resistance also depends on the surface finish. Iron particles and other impurities left on the surface during maching can lead to premature localised corrosion while surface irregularities or poorly finished surfaces also increase the likelihood of corrosion. KS440/ACD34/X65Cr13 grade stainless steel with a lower carbon content is used by EZO Japan and has greater corrosion resistance and superior low noise qualities to the standard AISI440C grade. Corrosion resistance can be increased by passivation (see section below). The 400 grade stainless steel will also withstand higher temperatures than chrome steel, coping with up to 250C constant and up to 300C intermittent with reduced load capacity. Above 300C, bearing life can be considerably shortened.
A note on passivation....
Passivation is a process by which free iron particles and other impurities are removed from the surface of stainless steel by immersion in nitric or citric acid, thus regenerating the passive film. This reduces the likelihood of surface discolouration so making it a useful process in some corrosive environments. Passivation does not increase the resistance of stainless steel to pitting corrosion. This means that where a bearing has incidental contact with, say, salt spray, passivation may be beneficial but it will not offer long term protection in harsher applications.

AISI316 Austenitic Stainless Steel (prefix "S316")
Used for greater corrosion resistance or where bearings must be non-magnetic, bearings made from this material are semi-precision and fine for applications such as marine pulleys but not suitable for precision instrument use. The main problem here is that 316 grade stainless steel is non hardenable, therefore as a softer steel, it will only support low loads and low speeds. The dynamic load rating of a 316 grade bearing may only be 10% of the 440 grade equivalent whereas the maximum speed may be 5% or less of the 440 stainless steel version. 316 grade stainless steel exhibits good corrosion resistance in sea atmosphere and may perform well submerged in seawater. However, as the passive film on the surface of stainless steel relies on the presence of oxygen to regenerate itself, in a low oxygen underwater marine environment (e.g under washers or o-rings) the steel may be prone to pitting or crevice corrosion although 316 grade is still much more resistant to corrosion than 440 grade. Bearings made from 316 grade stainless steel can be used at high temperatures provided a suitable cage material is used. Due to the difficulty of using 316 grade for the cage, 304 grade stainless steel is normally used for metallic cages and nylon for non-metallic cages. Please remember that, as 316 grade bearings are far less popular, minimum quantities may apply and some smaller instrument bearings may not be available.

Plastic - acetal resin (prefix "AC")
Bearings made from acetal resin with balls made from 316 stainless steel or glass are more corrosion resistant. They will however, corrode in the prescence of certain chemicals for which made-to-order polyethylene or polypropylene bearings with glass balls may be a better choice. These are generally termed as "plastic" bearings and like 316 stainless steel bearings, are not suitable for anything other than low loads and low speeds and should not be used in temperatures of greater than 90C. These types are also low precision so not suitable for instrument use. The smaller bearings are not usually available in these synthetic materials.

Ceramics - Silicon Nitride (prefix "CB" or "CC")
Some types may be available with steel rings and ceramic balls (hybrid) or "all ceramic" bearings with ceramic rings and balls. These types may not be stock items and could be subject to minimum order quantities. There are many advantages to silicon nitride such as a lower friction coefficient, much greater hardness and temperature resistance. Silicon nitride has 40 percent of the density of bearing steel but is about twice as hard. The lower density means that the balls exert less force on the outer raceways reducing wear while the extra hardness means greater wear resistance. Some ceramic materials such as Zirconia or Alumina are heavier and not as suitable for hybrid bearings although they can be used in full ceramic bearings. Hybrid bearings are also capable of higher speeds (usually up to 30 percent) and can also operate better with limited lubrication as the lower friction material generates less heat. However, ceramic bearings can be significantly more expensive, particularly "all ceramic" bearings partly due to the material and partly due to very low production quantities. The cost may be prohibitive for some sizes or quantities.
WARNING: Ceramics are often overrated particularly hybrid bearings.It is often thought that they will provide incredibly high speeds which is not correct unless you use special retainers or no retainer and the bearing still needs to be high quality. Customers often expect very low frictional torque with low noise and vibration levels. This may be possible but the bearing rings must have very good roundness and a high quality raceway finish while the balls must also have very good roundness and surface finish. There are many cheap hybrid bearings on the market that do a worse job than a good quality bearing with steel balls. Good hybrid bearings often prove too expensive for an application. Most sizes are made to order.
For further information, see cerbec's comments at
http://www.cerbec.saint-gobain.com/HybridBearing/Hybrid.asp
Click on the following for a compatibility check for several different materials and chemicals:
http://www.coleparmer.com/techinfo/ChemComp.asp

2. Retainer (back to top)

Retainers keep the balls evenly spaced around the raceway preventing ball to ball contact and thus allowing higher speeds. They also help to retain grease around the balls and raceways. For greater accuracy and to prevent any additional friction, it is important that the retainer is not allowed too much radial movement. To achieve this, the retainer is guided by either the balls or one of the rings. See the sections below for information on how each cage type is guided.

Metal crown/ribbon  
This standard retainer is manufactured from carbon steel for chrome bearings and AISI304 or AISI430 grade stainless steel for stainless bearings. These were often made from brass which also offered a high temperature capability but this is much less common due to higher cost of brass and advances in steel technology.

For higher temperatures, stainless steel is usually recommended. The crown cage and ribbon cage perform the same function but the crown cage is used primarily on smaller miniature bearings and thin-section bearings where space is more limited.Steel cages are preferred for arduous   operating conditions and where high levels of  vibration are experienced.

• Good for low to medium speeds
• Can withstand higher temperatures according to the type of steel (see "Bearing Material" section)
• Crown type - inner ring guided
• Ribbon type - mainly ball guided

Nylon crown (TW)
This moulded synthetic retainer has better sliding characterisitics than the steel cage and produces fewer fluctuations in running torque. It can increase maximum speeds by up to 60 percent so is generally used in high speed applications and has good low noise properties. This retainer is not suitable for low temperature applications as it loses elasticity below about 35°C. In vacuum applications, it may become brittle.
• High speed and low noise
• Max temperature range approx -35 to +110°C
• Ball guided

Phenolic crown (TP)
This retainer is also used for high speed applications. Generally more expensive, it does have advantages over the synthetic type such as absorbency allowing it to be vacuum impregnated with oil for long life application.
• Good oil retention.
• Can operate well with marginal lubrication
• Max temperature approx 140°C
• Inner ring guided

Full complement (F/B)

A full complement (or full ball) bearing contains extra balls and has no retainer. It is used for its greater radial load capacity although axial load capacity is very small. These bearings can only be used at low speeds due to ball to ball friction. An exception is a hybrid full complement bearing (ceramic balls) which can be used for very high speeds. Improved steel and hardening techniques have increased the load capacities of bearings with cages and the full complement bearing is much less common now.
• Higher radial load capacity
• Low speed only (except with ceramic balls)
• Low axial load

3. Closures (back to top)

Shields (ZZ)
Most sizes are available with metal shields. Shields are designed to prevent larger particles from entering the bearing and also to keep grease inside the bearing. They may be pressed into the bearing’s outer ring (non-removable) or retained by a circlip (removable). As the shields make no contact with the inner ring, they do not increase starting or running torque. Shields on stainless steel bearings are generally made from AISI 304 grade stainless steel.
• Prevent contamination by larger particles
• Reduce lubricant leakage
• No torque increase

Contact seals (2RS)
The standard bearing seal consists of nitrile rubber bonded to a metal washer. High temperature teflon seals (up to 250C) or Viton seals (up to 230C) are available on some sizes. The inner lip of the seal rubs against the bearing inner ring to provide an effective seal against smaller particles such as dust and moisture while preventing lubricant leakage. Contact seals produce much higher frictional torque levels than shields and reduce the maximum speed of a bearing.
• Good protection against contamination
• Greatly reduce lubricant leakage
• Reduce maximum speed by approx. 40%
• Greatly increase bearing torque
• Temp. range –30°C/+110°C (nitrile rubber) or up to 230C (Viton) and 250C (Teflon)

Non-contact seals (2RU)
These seals are also made of nitrile rubber bonded to a metal washer but do not rub against the bearing inner ring and therefore do not have the same effect on bearing torque and maximum speed as contact seals so can be used for low torque, high speed applications. They offer superior protection over metal shields but do not provide as effective a seal as the contact type.
• Good protection against contamination
• Reduced lubricant leakage
• No torque increase
• Do not affect maximum speed
• Temp. range –30°C/+110°C

4. Load Rating (back to top)

Dynamic load rating
The official explanation for this is... "The dynamic load rating is that constant stationary radial load which 90% of a group of identical chrome steel bearings, with only the inner ring rotating, can endure for one million revolutions before the first signs of fatigue develop". Yes, 1 million revolutions sounds a lot but is it really? If you take a bearing running at 5000 rpm with the max dynamic load applied to it, it will last for 1,000,000 revs divided by 5000 = 200 minutes or 3 hours and 20 minutes!! This should tell you that these figures are used in the calculation of life ratings but bearings should not be subjected to such loads in normal application unless you don't expect them to last very long. AISI440C/KS440 stainless steel bearings will achieve approximately 80% of the figure quoted. For life ratings, please contact SMB.

Static load rating
This rating represents the purely radial load which will cause a total permanent deformation of the balls or raceway equal to one ten-thousandth of the ball diameter. This may be tolerable for certain applications but not where any smoothness or accuracy is required. Static load ratings for stainless steel bearings are approximately 75% of the load ratings for chrome steel bearings.

The load capacity of a bearing may be limited by the lubricant. Certain lubricants are only suitable for light loads while others are designed for high load applications. Load ratings are higher for full complement bearings (see Retainer). The axial load capacity of a radial ball bearing can be increased by specifying loose radial play.

Axial load rating
Small and thin-section deep groove ball bearings should not be subjected to axial (thrust) loads greater than 25 percent of the bearing's static load rating. For larger bearings (e.g. 6001, 6201, 6301 upwards) the figure rises to nearer 50 percent. To exceed the recommended limits will have a detrimental effect on bearing life.

5. Radial Play (click here for radial play tables) (back to top)

                     Radial     
  
  Also referred to as radial play, it is the amount of play
  or looseness between the inner and outer ring or more
  specifically: average outer ring raceway diameter minus
  average inner ring raceway diameter minus (2 x ball diameter).
  Radial play should not be confused with tolerance grade and
  is entirely separate.

                Axial   


  Clearance measured along the bearing axis is known as axial
  play. Axial play is approximately 10 times the radial play value.
 

Radial play (or internal radial clearance) is an important consideration when choosing a bearing. The radial play in the bearing before it is fitted can be called the "initial" radial play. "Residual" or "operational" radial play is what is left when the bearing has been fitted.There should normally be a slight residual radial play in the bearing to minimize ball skidding and reduce axial play (end play). Correct selection of the initial radial play can avoid faster bearing wear and reduce unwanted play.

A number of things can alter the radial play during the fitting process. A tight shaft fit where the shaft is slightly larger than the bearing inner ring (often called an interference fit or a press fit) will stretch the inner ring so making it bigger. This reduces radial play by up to 80% of the interference fit. The same thing happens if the outer ring is a tight fit in the housing. This can squash or compress the outer ring also reducing radial play. A difference between the shaft and housing temperatures can also be a problem. If a bearing inner ring gets hotter than the outer ring, it will expand more and reduce radial play. This can be calculated as follows:
Chrome Steel:  0.0000125 x (inner ring temp - outer ring temp °C) x outer ring raceway diameter in mm.
440 Stainless Steel:  0.0000103 x (inner ring temp - outer ring temp °C) x outer ring raceway diameter in mm.
The outer ring raceway diameter can be roughly calculated as: 0.2 x (d + 4D) where d is the bore in mm and D is the outer diameter in mm.

There can also be problems where, for example, the shaft is made of different material to the bearing and housing and expands more due to a different expansion coefficient. In such a case, a bearing with a looser radial play may be needed.

In most cases a standard radial play is suitable and preferable as these bearings are usually more readily available and may be cheaper but there are certain conditions where a non-standard clearance is recommended as long as other conditions such as temperature or interference fit are not present. A tight radial play is better for greater rigidity and running accuracy if the load is purely radial. This may be worth considering for very low noise, low vibration applications which is why many of our small electric motor bearings are MC3 radial play. However, in other applications, a tight radial play may be highly undesirable. If there is a high axial load, a loose radial play is preferable as it increases the bearing's axial load capacity. Also, a loose radial play will better accommodate misalignment between the shaft and housing and cope better with heavy loads or shock loads.

Finally, radial play has nothing to do with precison grade or tolerance. It is often believed that a loose bearing means a low precision bearing and that, when there is too much play, a higher precision grade will solve the problem. In this case, the answer is often to use a bearing with a tighter radial play or use a tighter shaft/housing fit or introduce an axial preload to the bearing (see below). Using a higher precision grade will make no difference to the "looseness" of the bearing. You can have a P4 (Abec7) grade bearing with a loose radial play just as you can have a P0 (Abec1) bearing with a tight radial play.

  Preload
In many low noise, low vibration or high speed applications, zero radial play is desirable. This gives greater rigidity, reduces noise and vibration, gives greater ball alignment and running accuracy and can eliminate ball skidding under high acceleration. This is achieved by applying a preload to
the bearing. A preload is an axial load deliberately applied via the inner or outer ring to offset the outer ring against the inner ring and reduce the radial play to zero. Preload is usually applied by the use of wave or spring washers, springs or by clamping. If the bearings are glued on to the shaft or housing, it may be possible to use weights to keep the bearing preloaded while the adhesive cures. The amount of preload should be small. Excessive preload can cause the bearing to be too tight leading to very high frictional torque and rapid failure.

For the actual clearances used in the radial play groups, please see our RADIAL PLAY TABLES.

6. Maximum Speed (back to top)
A number of factors affect speed limitation such as temperature, load, vibration, radial play, retainer, lubricant, ball material and closures. The speeds quoted in our catalogue pages are only approximate and valid for bearings used on a horizontal shaft with a metal cage, standard tolerance grade and radial play, medium loading, rotating inner ring and suitable lubricant (see below). Vertical shaft applications will necessitate a reduction of approximately 20 percent. Temperature excesses and heavy loadings will also require slower speeds. Bearings fitted with contact seals cannot achieve the same speeds due to increased friction between seal lip and bearing inner ring. The choice of lubricant may also have a significant effect on the speed rating. The maximum rpm at which a lubricant can effectively operate varies from type to type. The following adjustment factors are approximate and are based on bearings with a metal crown or ribbon cage. The maximum speed of a bearing can be increased by the use of a delrin or phenolic cage provided a suitable lubricant is used. The use of ceramic balls will increase bearing speed by up to 30 percent.

7. Shaft/Housing Fit (back to top)
Bearing rings under a rotating load may need to be firmly located by an interference fit or other means such as a nut or adhesive. This prevents them from creeping in a circumferential direction which gives rise to increased wear. A bearing ring is subjected to a rotating load when the load is applied to all points of that ring during operation. For example:


Inner ring rotating load: e.g. a bearing in a vacuum cleaner motor belt driving the roller brush. The shaft and bearing inner ring are rotating. The load is in a constant direction in relation the the bearing so as the inner ring turns, all parts of that ring are subjected to the load. The outer ring does not rotate so the load acts on only one point of the outer ring.


Another possibility is a static inner ring and rotating outer ring but this time, the load rotates with the outer ring. As above, the load acts on only one point of the outer ring while all parts of the inner ring are subjected to the load. Both of these applications require an interference shaft fit and a clearance housing fit.


Outer ring rotating load: e.g. a bearing in a pulley. The shaft and inner ring are fixed while the outer ring and housing (the pulley) do rotate. The load is in a constant direction in relation to the bearing so as the outer ring turns, all parts of that ring are subjected to the load. The inner ring does not rotate so the load acts on only one point of the inner ring. This application requires a clearance shaft fit and an interference housing fit.

This example involves a static outer ring and rotating inner ring, the load rotating with the inner ring. As above, the load acts on only one point of the inner ring while all parts of the outer ring are subjected to the load. Both of these applications require a clearance shaft fit and an interference housing fit.

This means that usually only one ring is subjected to an interference fit. There may be instances where a fluctuating load direction will require interference fits for both shaft and housing. This may also be true where there is excessive vibration associated with the application. Make sure that interference fits do not reduce the radial play of the bearing to an unacceptable level or early failure will occur. Excessive interference fits can also cause high stress which may fracture rings.

The material of the shaft and housing should be taken into consideration. An aluminium housing will expand more than a steel housing so an interference fit in an aluminium or plastic housing will require a greater interference than a steel housing. Greater interference fits are required in thin walled housings and also on hollow shafts.

The standards of roundness and surface finish which apply to the bearing should also apply to shaft and housing. This is very important for electric motor and other quiet-running applications. Miniature and thin-section bearings are particularly susceptible to distortion which leads to higher noise and vibration levels. Care should be taken where shaft and housing materials have a different expansion coefficient to the bearing steel (12.5 x 10-6 per °C for chrome steel bearings and 10.3 x 10-6 per °C for 440 stainless steel). This may lead to an increase or reduction in radial play.

Interference fits can affect rotational accuracy by distorting bearing rings. If rotational accuracy is important, a combination of close bearing tolerances and close shaft/housing tolerances should be used to obtain the correct fit with the minimum interference. It should also be noted that an interference fit can reduce radial play by up to 80% of the size of the interference fit. If further advice on shaft and housing fits is required, please contact us.

8. Tolerance (click here for tolerance tables) (back to top)
Tolerances control the dimensional accuracy of the bearing. We use ISO tolerances and these are given in microns or thousandths of a millmetre. AFBMA tolerances in ten-thousandths of an inch are also often used with bearings. The equivalents are as follows: P0 = Abec1; P6 = Abec3; P5 = Abec5; P4 = Abec7. Tolerances have no effect on radial play although it is sometimes mistakenly thought that improving the tolerances will produce a bearing with less play. Assuming that the shaft and housing are manufactured to the same tolerances as the bearing, higher bearing tolerances will produce better mating between shaft/housing and bearing, lower noise and vibration due to improved roundness and lower starting and running torque (also subject to radial play and lubricant). For exact tolerance limits, please view our TOLERANCE TABLES. For an explanation of how tolerances are measured, see TOLERANCE TIPS.

9. Frictional Torque (back to top)
This affects the free-running of the bearing. Spin a bearing containing stiff grease with your finger and not much happens - relatively high frictional torque. Try a bearing with no lubrication and it will spin freely - low frictional torque. The effort required to rotate a bearing depends partly on the accuracy of the bearing components and raceway finish but much more so on the load and speed applied to the bearing, the lubrication and the closures. The greater the load, the greater the deformation of the bearing components leading to increased resistance. The higher the speed, the greater the lubricant drag. Instrument oils will often produce lower torque levels but the difference between these and many low torque greases is actually quite small, particularly if a low grease fill is used. A standard low torque grease such as Multemp SRL grease may give an increase of only 20 percent over a Aeroshell 12 oil. This can drop to under 5 percent for very low torque greases if a low (e.g 10 to 20 percent) fill is used. Initial torque levels for a greased bearing are briefly higher as the grease takes a short time to "run in" or be distributed inside the bearing. Contact seals will greatly increase the torque figures as will high viscosity lubricants.The effort required to rotate a bearing from rest (starting torque) is slightly greater than the effort required to keep it rotating (running torque).

Approximate figures for frictional torque for can be calculated using a simple formula. This is only valid if the bearing has low torque lubrication (and the grease fill is not high), is open, shielded or has non-contact seals and is subjected to low speed and low load. For radial ball bearings, the axial load should be less than 20 percent of the radial load while the load should be purely axial for thrust bearings. Contact us if you need more accurate figures taking into account the speed and the lubricant viscosity. The measurements are in Newton millimetres (Nmm). This is a compound unit of torque corresponding to the torque from a force of one newton (approx 0.1 Kgf) applied over a distance arm of one millimetre.

Frictional torque (measured in Nmm or Newton millimetres)
Radial ball bearings:   0.5 x 0.0015 x radial load in Newtons* x bearing bore (mm)
Axial ball bearings:    0.5 x 0.0013 x axial load in Newtons* x bearing bore (mm)
*10 Newtons = 1 Kgf

10. Noise Rating (back to top)
Bearing rings and balls are not perfectly round and the balls and raceways, even after extensive fine grinding and polishing, are not perfectly smooth. There are machining imperfections in the form of rough or uneven surfaces. For example, if a bearing inner ring is rotating and the outer ring fixed, these imperfections will cause the outer ring to move radially in relation to the inner ring. The amount and speed of this movement contributes to the amount of bearing vibration and noise. Poor cage design can also increase bearing noise.

The smoothness or quietness of a bearing can be checked by accelerometers which measure vibration at the outer ring of a bearing, usually with the inner ring rotating at 1800 rpm. To understand how bearing vibration is measured, it is important to understand how vibration works. When measuring bearing vibration, we need to take into account both displacement and frequency as these two factors together tell us far more.

Firstly, a vibrating object moves or oscillates. This amount of movement is called displacement. When (for example) a bearing outer ring vibrates, the outer surface will move upwards to the upper limit, then down to the lower limit and then back to the start point. The measurement between upper and lower limit is called peak to peak displacement. The whole oscillation movement from start point through upper and lower limits and back to start point is called a cycle. This vibration cycle will repeat as long as the bearing is rotating. We can also measure the number of these cycles in a given time. This gives us the frequency. Frequency is most commonly expressed as cycles per second (CPS) or Hertz (Hz) which is the same thing.

Vibration is potentially damaging to a bearing and the equipment it is used in, increasing the rate of fatigue and therefore, shortening the life of the bearing. Displacement measurements do not tell us enough. Vibration in a bearing or a machine will usually occur at many different frequencies and they all contribute to fatigue so we need to take all of these frequencies of vibration into account in our measurements of vibration. We can achieve this by measuring vibration velocity.
Vibration velocity is displacement x frequency. If a bearing component is moving a particular distance (displacement) at a particular rate (frequency) it must be moving at a certain speed. Vibration velocity gives us a much better indication of how severe the vibration is. The higher the vibration velocity measurement, the noisier the bearing and the faster the bearing will fail as a result of fatigue. Vibration velocity is measured on a BVT machine (Bearing Vibration Tester) in microns per second or an Anderon Meter in Anderons. One Anderon equals 7.5 microns per second. The vibration velocity readings are separated into three frequency bands:
Low band (50 to 300 Hz); Medium band (300 to 1800 Hz); High Band (1800 to 10000 Hz)
These vibration velocity measurements are usually classified into V grades (e.g. V1, V2, V3 or V4). Although vibration velocity measures the fatigue potential, this is not the only cause of failure. Vibration force can cause deformation to balls and rings. Vibratory force can be very damaging at high frequencies where velocity readings may be quite low. For this reason we also measure vibration acceleration.
Vibration acceleration is an indication of vibratory force (force = mass x acceleration) and since force is damaging at higher frequencies, vibration acceleration is a useful measurement where a bearing will experience vibration frequencies above 2000 Hz. Vibration acceleration is measured in G (1G being the acceleration produced by the Earth's gravity or 9.81 m/s²) but you will often see these measurements converted to decibels (dB). These decibel measurements are usually classified into Z grades (Z1, Z2, Z3 or Z4). Bearings can be classified according to both vibration velocity AND vibration acceleration levels (ZV1, ZV2, ZV3 or ZV4).

A low noise/vibration rating is achieved by paying particular attention to the surface finish of the raceways and balls, the roundness of the rings and balls and correct cage design. We have three ratings for low noise bearings: EMQ (ZV2), EMQ2 (ZV3) and the quietest, EMQ3 (ZV4). These ratings are independent of precision grade, for example, a P6 bearing may be offered with any of the three noise ratings. To help reduce noise levels even further, low noise greases are available and the choice is now greater due to improved lubricant manufacuring techniques. These greases are more finely filtered and contain fewer, smaller solid particles. These particles generate noise when they pass between the balls and raceway.

External factors such as surrounding vibration can affect bearing noise. Another problem, particularly with smaller and thin-section bearings, as mentioned in "Shaft/Housing Fits" (section 7) is ring distortion caused by poor shaft or housing roundness. Dirt or dust contamination will also increase noise and vibration levels. Poor fitting practice or incorrect handling is sometimes to blame, causing shock loads which, in turn, create scratches or dents in the raceway.

11. Lubricants (click here for lubricant tables) (back to top)
Correct lubrication is critical to bearing performance, reducing friction, dissipating heat and inhibiting corrosion on balls and raceways. The lubricant will affect maximum running speed and temperature, torque level, noise level and, ultimately, bearing life. Lubricants are available for a whole range of applications. Silicon lubricants have wide temperature ranges and change viscosity less with temperature. They also have good water-resistance but are unsuitable for high loads and speeds. Perfluorinated lubricants withstand temperatures of up to 300°C and are resistant to most chemicals. while certain mineral or synthetic based lubricants are designed for high speed use. Low viscosity oils and greases are used where low lubricant resistance is required but higher viscosity lubricants may be specified for high load applications. Although greases are usually thought to be stiffer than oils, many modern low torque greases can even produce similar torque figures to some of the instrument oils.
• Oils – maintain their consistency well over a wide temperature range and are easy to apply. For very low torque applications, a light instrument oil should be specified. Higher running speeds are possible with oil but the obvious drawback with oil is the fact that it tends not to stay in place. For anything other than very low speeds, continuous lubrication through oil mist, oil jet or oil bath is normally necessary. An exception to this is the use of a retainer (cage) that can be impregnated with oil such as the phenolic retainer. Perfluorinated oils can offer improved performance at slightly higher speeds as they don't migrate (run out) as easily.
• Greases – are simply oils mixed with a thickener to so that they stay inside the bearing. Greases are generally more suitable for heavy loads and have the obvious advantage of giving constant lubrication over a long period without maintenance. Finely filtered greases are used for low noise applications. Lithium based greases are multipurpose, often low torque and high speed and widely used. Polyurea thickened greases have very good water resistance and a wide temperature range while aluminium complex gives excellent resistance to water washout. Many food applications will require an edible food grade grease. PTFE thickened greases can withstand very high temperatures. Surprisingly, too much grease can be bad for a bearing. A high fill will mean greater rolling resistance (higher torque) which may not be suitable for many applications but worse still is the risk of heat build-up. The free space inside a bearing is important in allowing the heat to radiate away from contact area between balls and raceway. As a result, too much grease can lead to premature failure. The standard fill is 25% – 35% of the internal space but this may be varied if required. A smaller percentage may be specified for low torque, low load applications while a much higher fill may be advisable for a high load application provided the speed is low.
• Dry Lubricants – are used primarily in vacuum applications or where standard lubricants are unsuitable. We use molybdenum disulfide for its hardwearing and low friction properties and the fact that it is insoluble in water and dilute acids. It is also effective within a wide temperature range of -180 to +300C. By burnishing the balls and raceways of a bearing, friction is reduced allowing higher speeds than with dry bearings.
We stock many different lubricants but for more information on our standard oils and greases, please see our LUBRICANT TABLES.