BEARINGS DESIGN GUIDELINES

BEARING DESIGN GUIDELINES

The main factors considered while designing / considering the type of bearing to be employed in machined parts are as follows:

1.Space availability within the designed space

Dimensions of a bearing are predetermined by the machine’s design with the shaft diameter determining the bearing bore diameter. For the same bore diameter, different outside diameters and widths are possible.

However the design factors  determine the optimal size , basis shaft and casing

shafts with small diameter
(approx. d < 10 mm)

o   deep groove ball bearings

o   needle roller bearings

o   self-aligning ball bearings

o   thrust ball bearings

o   shafts with normal diameter
all bearing types

·       very limited radial space

o   needle roller bearings

o   deep groove ball bearings in the 618 or 619 series

o      toroidal roller bearings in the C49, C59 or C69 series
bearings without inner or outer ring and raceways machined directly on the shaft or in the housing

2. LOADING PATTERN

When selecting bearing type based on load criteria, you should bear in mind that:

·       Roller bearings accommodate heavier loads than same-sized ball bearings.

·       Full complement bearings accommodate heavier loads than the corresponding bearing with a cage.

Combined radial and axial loads
The direction of load is a primary factor in bearing type selection. Where the load on a bearing is a combination of radial and axial load, the ratio of the components determines the direction of the combined load

The suitability of a bearing for a certain direction of load corresponds to its contact angle ,the greater the contact angle, the higher the axial load carrying capacity of the bearing.. ISO defines bearings with contact angles ≤ 45° as radial bearings, and the others as thrust bearings, independent of their actual use.

To accommodate combined loads with a light axial component, bearings with a small contact angle can be used.

 Deep groove ball bearings are a common choice for light to moderate axial loads. With increasing axial load, a larger deep groove ball bearing (with higher axial load carrying capacity) can be used.

For even higher axial load, bearings with a larger contact angle may be required, like angular contact ball bearings or tapered roller bearings. These bearing types can be arranged in tandem to accommodate high axial loads.

When combined loads have a large alternating axial load component, suitable solutions include:

·       a pair of universally matchable angular contact ball bearings

·       matched sets of tapered roller bearings

·       double-row tapered roller bearings

Where a four­-point contact ball bearing is used to accommodate the axial component of a radial load, the bearing outer ring must be mounted radially free and should not be clamped axially. Otherwise, the bearing may be subjected to unintended radial load.

3. SPEED

The permissible operating temperature of rolling bearings imposes limits on the speed at which they can be operated. The operating temperature is determined, to a great extent, on the frictional heat generated in the bearing, except in machines where process heat is dominant.

Selecting bearing type on the basis of operating speed, the following should be considered

·       Ball bearings have a lower frictional moment than same-sized roller bearings.

·       Thrust bearings cannot accommodate speeds as high as same-sized radial bearings.

·       Single row bearing types typically generate low frictional heat and are therefore more suitable for high­-speed operation than double or multi-row bearings.

·       Bearings with rolling elements made of ceramics (hybrid bearings) accommodate higher speeds than their all-steel equivalents

MISALIGNMENT

Bearing types vary in their ability to compensate for misalignment between the shaft and housing:

·       Self-aligning bearings
Self-aligning bearings can compensate for misalignment within the bearing. Values for the permissible misalignment are listed in the relevant product section.

·       Alignment bearings
Alignment bearings can accommodate initial static misalignment because of their sphered outside surface. Values for the permissible misalignment are listed in the relevant product section.

Rigid bearings
Rigid bearings (deep groove ball bearings, angular contact ball bearings, cylindrical, needle and tapered roller bearings) accommodate misalignment within the limits of their internal clearance For rigid bearings, any misalignment may reduce service life.

Tolerance for  fits are to be considered while  designing for rigid cases and shaft fits - these are based on bearing size/type and are usually considered by the manufacturer/designer for that particular temperature rating , basis conditions of preload/ static loads and safety factors.

5.

TEMPERATURE

The permissible operating temperature of rolling bearings can be limited by:

·       the dimensional stability of the bearing rings and rolling elements – as given in the table generalized.

·       the cage – construction and material)

·       the seals

·       the lubricant – selection criterion basis design

CAGES

There are fundamental design differences between bearings which, together with the influence of bearing size, necessitate customized cage designs .

·       some bearing types need either split or snap-type cages, because they are assembled after the rings and rolling elements have been sub-assembled

·       other bearing types need roller-guided cages, to be self-containing

·       bearings of a certain combination of size and series need ring-guided cages, to limit contact stress between rolling elements and cage.

Specific functional demands, quantity of bearings being manufactured, material and manufacturing methods are chosen to provide the most reliable and cost-effective cage.

Cages are mechanically stressed during bearing operation by frictional, impact, centrifugal and inertial forces. They can also be chemically influenced by certain organic solvents or coolants, lubricants, and lubricant additives. Therefore, the material type used for a cage has a significant influence on the suitability of a rolling bearing for a particular application.

Steel cages:

Operating temperatures up to 300 °C (570 °F).

Types of steel cages:
Sheet steel cages

Stamped sheet steel cages are made of low carbon steel. These lightweight cages have relatively high strength and, for some bearing types, can be surface treated to further reduce friction and wear in critical conditions.
Machined steel cages

Machined steel cages are normally made of non-alloyed structural steel. To reduce friction and wear, some machined steel cages are surface treated.
They are not affected by the mineral or synthetic oil-based lubricants normally used for rolling bearings, or by the organic solvents used to clean bearings.

Brass Cages:

Operating temperatures up to 250 °C (480 °F).
Types of Brass cages

Sheet brass cages

Stamped sheet brass cages are used for some small and medium­-size bearings.

 Machined brass cages

Most brass cages are machined from cast or wrought brass. They are unaffected by most common bearing lubricants, including synthetic oils and greases, and can be cleaned using organic solvents.
In applications such as refrigeration compressors that use ammonia, machined brass or steel cages should be used.
Polymer cages

Polyamide 66

Polyamide 66 (PA66) is the most commonly used material for injection moulded cages. This material, with or without glass fibres, is characterized by a favourable combination of strength and elasticity. The mechanical properties, such as strength and elasticity, of polymer materials are temperature dependent and subject to ageing. The factors that most influence the ageing process are temperature, time and the medium (lubricant) to which the polymer is exposed Cage life decreases with increasing temperature and the aggressiveness of the lubricant.

Therefore, whether polyamide cages are suitable for a specific application depends on the operating conditions and life requirements. The permissible operating temperature  is defined as the temperature that provides a cage ageing life of at least 10 000 operating hours.

Some media are even more “aggressive” . A typical ex­ample is ammonia, used as a refrigerant in compressors. In those cases, cages made of glass fibre reinforced PA66 should not be used at operating temperatures above 70 °C (160 °F).

Polyamide loses its elasticity at low temperatures. Therefore, cages made of glass fibre reinforced PA66 should not be used in applications where the continuous operating temperature is below –40 °C (–40 °F).

Polyamide 46

Glass fibre reinforced polyamide 46 (PA46) is the standard cage material for some small and medium-­size CARB toroidal roller bearings. The permissible operating temperature is 15 °C (25 °F) higher than for glass fibre reinforced PA66.

Polyetheretherketone

Glass fibre reinforced polyetheretherketone (PEEK) is more suitable for demanding conditions regarding high speeds, chemical resistance or high temperatures than the PA66 and PA46. The exceptional properties of PEEK provide a superior combination of strength and flexibility, high operating temperature range, and high chemical and wear resistance. Because of these outstanding features, PEEK cages are commonly available for hybrid and/or super-­precision ball and cylindrical roller bearings. The material does not show signs of ageing by temperature or oil additives up to 200 °C (390 °F). However, the maximum temperature for high­-speed use is limited to 150 °C (300 °F) as this is the softening temperature of the polymer.

LUBRICATION

When a bearing has reached its normal speed and operating temperature, the lubrication condition of the bearing is:


k= viscosity ratio- which indicates lubrication condition.

V=Actual operational viscosity of oil

V1= Rated viscosity , a function of mean bearing diameter and rotational speed.

The actual operating viscosity, ν, of the lubricant can be determined from the ISO viscosity grade of the oil, or the grease base oil, and the operating temperature of the bearing  

You can determine the rated viscosity, ν₁, from using the bearing mean diameter, d_m = 0,5 (d + D) [mm], and the rotational speed of the bearing, n [r/min]. 

Orange shading area shows lower operating temperatures- at higher temperatures , depending upon bearing desin the lubricant needs special attention. 

 The higher the κ value, the better the lubrication condition of the bearing and its expected rated life. This must be judged against the possible friction increase because of the higher oil viscosity. Therefore, most bearing applications are designed for a lubrication condition ranging from κ 1 to 4 .

·       κ = 4 indicates a regimen for which the rolling contact load is carried by the lubricant film – i.e. full film lubrication.

·       κ > 4 (i.e. better than full film lubrication) will not further increase the rating of the bearing. However, κ > 4 may be useful in applications where the bearing temperature rise is small and additional lubrication condition reliability is desirable. This would apply, for example, to bearing applications with frequent start-stop running conditions or occasional temperature variations.

·       κ < 0,1 indicates a regimen for which the rolling element load is carried by the contact of the asperities between rolling element and raceway – i.e. boundary lubrication. The use of fatigue life rating for lubrication conditions below 0,1 is not appropriate as it is beyond the applicability limits of the life rating model. Where κ < 0,1 , select the bearing size on the basis of static loading criteria by means of the static safety factor

·       For lubrication conditions with 0,1 < κ < 1, take into account the following:

·       If the κ value is low because of very low speed, base the bearing size selection on the static safety factor s₀ 

  

If the the static load that the bearing can accommodate, taking into account the possible effects of permanent deformation:

·       The bearing is not rotating and is subjected to continuous high load or intermittent peak loads.

·       The bearing makes slow oscillating movements under load.

·       The bearing rotates and, in addition to the normal fatigue life dimensioning operating loads, has to sustain temporary high peak loads.

·       The bearing rotates under load at low speed (n < 10 r/min) and is required to have only a limited life. In such a case, the rating life equations, for a given equivalent load P, would give such a low requisite basic dynamic load rating C, that a bearing selected on a fatigue life basis would be seriously overloaded in service.

In such conditions, the resulting deformation can include flattened areas on the rolling elements or indentations in the raceways. The indentations may be irregularly spaced around the raceway, or evenly spaced at positions corresponding to the spacing of the rolling elements. A stationary or slowly oscillating bearing supporting a load great enough to cause permanent deformation will generate high levels of vibration and friction when subjected to continuous rotation. It is also possible that the internal clearance will increase or the character of the housing and shaft fits may be affected.

K  value is low because of low viscosity, counteract this by selecting a higher viscosity oil or by improving the cooling. Under these lubrication conditions, it is not appropriate to calculate the basic rating life L₁₀ only, because it does not take into account the detrimental effects of inadequate lubrication of the bearing. Instead, to estimate the rolling contact fatigue life of the bearing, use the SKF rating life method.

Where κ < 1, EP/AW additives are recommended → Extreme pressure (EP) and anti-wear (AW) additives (below).

The speed factor nd_m is used to characterize the speed condition of the bearing.

·       If the nd_m of the bearing is lower than 10 000, the application is operating under low-speed conditions , regimen requires high oil viscosity to ensure that the rolling element load is carried by the lubricant film.

·       Operating conditions leading to nd_m > 500 000 for d_m values up to 200 mm, and  > 400 000 for larger d_m values, are typical of bearings operating at high speeds . At very high speeds, the rated viscosity drops to very low values. Lubrication conditions and κ values are generally high.

EP/AW additives in the lubricant are used to improve the lubrication condition of the bearing in situations where small κ values are in use, e.g. when κ = 0,5. Furthermore, EP/AW additives are also used to prevent smearing between lightly loaded rollers and raceway, for example, when especially heavy rollers enter a loaded zone at a reduced speed.

For operating temperatures lower than 80 °C (175 °F), EP/AW additives in the lubricant may extend bearing service life when κ is lower than 1 and the factor for the contamination level, η_c, is higher than 0,2 and the resulting a_SKF factor is lower than 3. Under those conditions, a value of κ_EP=1 can be applied, in place of the actual κ value, in the calculation of a_SKF for a maximum advantage of up to a_SKF = 3.

Some modern EP/AW additives containing sulphur-­phosphorus, which are most commonly used today, can reduce bearing life,testing of chemical reactivity of EP/AW for operating temperatures above 80 °C (175 °F).

STIFFNESS  The stiffness of a rolling bearing is characterized by the magnitude of the elastic deformation in the bearing under load and depends not only on bearing type, but also on bearing size and operating clearance.

When selecting bearing type on the basis of stiffness requirements you should consider, for bearings with the same size, that:

·       stiffness is higher for roller than ball bearings

·       stiffness is higher for full complement bearings than for the corresponding bearing with a cage

·       stiffness is higher for hybrid bearings than for the corresponding all-steel bearing

·       stiffness can be enhanced by applying a preload

CONSIDERATIONS FOR PRELOAD

Depending on the bearing type, preload may be either radial or axial. Super-precision cylindrical roller bearings, for example, can only be preloaded radially because of their design, while angular contact ball bearings or tapered roller bearings can only be preloaded axially.

Single tapered roller bearings or angular contact ball bearings are generally mounted together with a second bearing of the same type and size in a back-to-back (load lines diverge,  or face-to-face (load lines converge, arrangement. The same is true for single row angular contact ball bearings.

The distance L between the pressure centres is longer when the bearings are arranged back­-to­-back  compared to bearings that are arranged face­-to-face . The back­-to­-back arrangement can accommodate larger tilting moments.

If the shaft temperature in operation is higher than the housing temperature, the preload, which was adjusted at ambient temperature during mounting, will change. Since thermal growth of a shaft makes it larger both in the axial and in the radial direction, the back-to-back arrangements are less sensitive to thermal effects than the face-to-face arrangements.

When adjusting preload in a bearing system, it is important that the established preload value is attained with the least possible variation. To reduce variation when mounting tapered roller bearings, the shaft should be turned several times to ensure that the rollers are in correct contact with the guide flange of the inner ring. PRELOADING WITH SPRINGS

By preloading bearings it is possible to reduce the noise in, for example, small electric motors or similar applications. In this example, the bearing arrangement comprises a preloaded single row deep groove ball bearing at each end of the shaft . The simplest method of applying preload is to use a wave spring. The spring acts on the outer ring of one of the two bearings. This outer ring must be able to be axially displaced.

The preload force remains practically constant, even when there is axial displacement of the bearing as a result of thermal elongation.

The requisite preload force can be estimated using 

F = k d

where

F	preload force [kN]
k	a factor, described below
d	bearing bore diameter [mm]

For small electric motors, values of between 0,005 and 0,01 are used for the factor k. If preload is used primarily to protect the bearing from the damage caused by external vibrations when stationary, then greater preload is required and k = 0,02 should be used.

Spring loading is also a common method of applying preload to angular contact ball bearings in high-speed grinding spindles. The method is not suitable for bearing applications where a high degree of stiffness is required, where the direction of axial load changes, or where undefined peak loads can occur.

MOUNTING / ASSEMBLY/DISSEMBLY

When selecting bearing type, you should consider the mounting and dismounting requirements:

·       Is it required or beneficial to mount the inner and outer ring independently?
→ Select a separable bearing.

·       Is it required or beneficial to mount the bearing on a tapered seat or with a tapered sleeve?
→ Select a bearing with a tapered bore.
→ Consider using ConCentra ball or roller bearing units. 

Separable bearings
Separable bearings are easier to mount and dismount, particularly if interference fits are required for both rings.

Tapered bore

Bearings with a tapered bore can be mounted on a tapered shaft seat or mounted on a cylindrical shaft seat using an adapter or withdrawal sleeve .

SEALING

There are two reasons for sealing bearings or bearing arrangements:

·       keeping the lubricant in the bearing, and avoiding pollution of adjacent components

·       protecting the bearing from contamination, and prolonging bearing service life

Capped bearings (sealed bearings or bearings with shields) can provide cost­-effective and space-­saving solutions for many applications.

APPLICATION
Bearings are designed basis their application - usually customized to ensure specific applicability in case of precision bearings as per industry requirements.

All the above factors discussed is a brief introduction into the aspects considered in a design of bearing.