Installing and maintaning of bearings

The most important are the cleanness and competence. Alt parts that should be installed must be carefully prepared and the appropriate procedures and tools should be used. This catalogue will provide general guidelines, while more detailed instructions (for Y bearings) are given in the text preceding appropriate tables. There is one rule that must be applied: mounting and dismounting forces should not be transferred over roller units and raceway.

 

Preparations for mounting

Mounting of a bearing should be done in a dry space without dust. Working space must not be close to machines that produce dust or sward. All parts and tools should be prepared prior to installation process, and the procedure itself should be made clear. All bearing arrangement components (housings, shafts) should be thoroughly cleaned; any burrs that are left should be removed, as well as send particles on casted elements etc. Check the accuracy of measures and forms of all parts that shall be installed and that have any contact with the bearing. Bearing should be unpacked immediately before the installation. FKL factory made anticorrosive protection should not be removed, and by no means should you wash sealed bearings filled with grease. Only wipe the hole and sheet surfaces.

 

Mounting of bearing with cylindrical bore  

With non-separable bearings a ring with tighter fit is installed first. The seating surfaces on the shaft (or housing) should be lightly oiled before mounting. If they do not fit tight and the bearing is smaller, it should be driven into position by applying light hammer blows. This mounting dolly has got appropriate measures, and it is best when it has a cap (26a). That way the bearing (ring) shall not become angled. If a non-separable bearing is simultaneously installed into the shaft and the housing, a mounting ring (26.b), (26.c) should be placed between the bearing and the dolly. This board will evenly push both rings into the mounting space. In the case of separable bearings, rings are installed one after the other. This should be done carefully so that tracks are not damaged. A secondary cartridge is used (26.d) for that purpose. If s size is the same as the track measurements, but in d10 tolerance (track measurements for separable roller and needle roller bearings are given in bearing tables). Large bearings have to be heated prior to installation, because the installation forces increase with the diameter increase. Inner ring and housing are heated as well. Temperature should not be higher than 125°C and necessary temperature differences can be determined using bearing and shafts (housing) measurements in diagram 13. Local overheating of the bearing should not be permitted, so if it is heated on a hot plate it has to be moved and turned around often. Heating can also be done using oil, heating ring, (heating tool or induction).
Fig. 23 Mounting of bearings with cylindrical bore

 

Fig. 23 Mounting of bearings with cylindrical bore

 

Diagram 13. Necessary temperatures for bearing mounting
Diagram 13. Necessary temperatures for bearing mounting

 

Mounting bearings with tapered bore

Fitting doesn't depend on the size of the bore and shaft, but on the length of pulling it on the tapered seating. Bearing clearance is reduced that way. Clearance reduction explains the fitting.

In the case of spherical bearings, either the clearance reduction or the axial displacement along the tapered seating are measured. Guidelines can be found in the introductory text, prior to tables.

Smaller bearings are mounted using the sleeve nut and an appropriate key. Large sized bearings are mounted using hydraulic nuts.

 

MAINTAINING OF BEARINGS

The following information and recommendations relate to bearings without integral seals or shields. FKL bearings and bearing units with integral seals and shields at both sides are supplied greased. The standard greases used by FKL for these products have operating temperature ranges and other properties to suit the intended application areas and filling grades appropriate to bearing size. The service life of the grease often exceeds bearing life so that, with some exceptions, no provision is made for relubrication. The possibility of relubrication is accomplished with Y bearings and bearing units supplied with the lubricants and corresponding grease channels. FKL sealed bearings are filled with lithium-grease consistency 2 and cinematic viscosity of basic, mineral oil about 90 mn2/s; temperature range of application from -30 up to +120°C.

 
1.13.1 Grease lubrication
About 90% of all bearing arrangements are lubricated with grease. Grease has the advantage over oil that it is more easily retained in the bearing arrangement, particularly where shafts are inclined or vertical, and it also contributes to sealing the arrangement against contaminants, moisture or water. However, the shortcoming is the lower speeds than with the oil lubrication. With the bearings with higher speeds, the excess of lubricant will cause the operating temperature to rise rapidly. As a general rule, therefore only the bearing should be completely filled, whilst the free space in the housing should be between 30 and 50% filled with grease. Where bearings are to operate at very low speeds and must be well protected against corrosion, it is advisable to fill the housing completely with grease.

1.13.1.1 Lubricating greases
Lubricating greases consist of a mineral or synthetic oil combined with a thickener. The thickeners are usually metallic soaps. Additives can also be included to enhance certain properties of the grease. The consistency of the grease depends largely on the type and concentration of the thickener used. When selecting grease, the viscosity of the base oil, the consistency, operating temperature range and the load carrying ability are the most important factors to be considered.
Base oil viscosity
The importance of the oil viscosity greases for the bearing life is dealt with under the heading "Rating life". The base oil viscosity of the greases normally used for rolling bearings lies between 15 and 500 mm2/s at 40°C. Greases based on oils having higher viscosities than this bleed oil so slowly that the bearing will not be adequately lubricated. Therefore, if a very high viscosity is required because of low speeds, oil lubrication will generally be found more reliable. The base oil viscosity also governs the maximum permissible speed at which given grease can be used for bearing lubrication. For applications operating at very high speeds, the most suitable greases are those incorporating diester oils of low viscosity. The permissible operating speed for grease is also influenced by the shear strength of the grease, which is determined by the thickener.
A is speed factor A= n . dm is often quoted by grease manufacturers to indicate the speed capability; n is the rotational speed and dm is the bearing mean diameter - dm=0, 5(d+D).
Consistency
Greases are divided into various consistency classes (DIN 51 818), according to the National Lubricating Grease Institute (NLGI) Scale. The consistency of greases used for bearing lubrication should not change unduly with temperature within the operating temperature range or with mechanical working. Greases that soften at elevated temperatures may leak from the bearing arrangement. Those that stiffen at low temperatures may restrict rotation of the bearing. Metallic soap thickened greases of consistency 1, 2 or 3 are those normally used for rolling bearings. The consistency 3 greases are primarily recommended for bearing arrangements with vertical shafts.

Protection against corrosion
The rust inhibiting properties of grease are mainly determined by the rust inhibitors which are added to the grease and its thickener. Grease should provide protection to the bearing against corrosion and should not be washed out of the bearing in cases of water penetration. These two features have lithium and calcium base greases containing lead base additives. However, because of environmental and health reasons such additives are being replaced by other combinations of additives which do not always supply the lubricant with so good features.

 Load carrying ability
For heavily loaded bearings, e.g. rolling mill bearings, it has been customary to recommend the use of greases containing EP additives, since these additives increase the load carrying ability of the lubricant film. Originally, most EP additives were lead-based compounds and there was evidence to suggest that these were beneficial in extending bearing life where lubrication was otherwise poor i.e. without elastic-hydrodynamic lubricant film, e.g. when <1 (see the section "Rating life").
Miscibility
Some greases are incompatible and if they are mixed together the consistency can change dramatically and the allowed operating temperature as well. Greases having the same thickener and similar base oils can generally be mixed without any consequences. Lithium and calcium base greases are generally miscible with each other but not with sodium base greases. However, mixtures of compatible greases may have a consistency which is less than either of the component greases, although the lubricating properties are not necessarily impaired. In bearing arrangements where a low consistency might lead to grease escaping from the arrangement, the next relubrication should involve complete replacement of the grease rather than replenishment.

 1.13.1.2 Relubrication
Rolling bearings have to be relubricated if the service life of the grease used is shorter than the expected service life of the bearing. Relubrication should be undertaken at a time when the lubrication of the bearing is still satisfactory. The time at which relubrication should be undertaken depends on many factors which are related in a complex manner. These include bearing type and size, speed, operating temperature, grease type, space around the bearing and the bearing environment. The information given in the following is based on long-term tests in various applications but does not apply to applications where water and /or solid contaminants can penetrate the bearing arrangement. In such cases it is recommended that the grease is frequently renewed in order to remove contaminants from the bearing.

Relubrication intervals
The relubrication intervals t1, for normal operating conditions can be read off as a function of bearing speed n and bore diameter d of a certain bearing type from Diagram 10. The diagram is valid for bearings on horizontal shafts in stationary machines under normal loads. It applies to good quality lithium base greases at a temperature not exceeding 70°C. To take account of the accelerated ageing of the grease with increasing temperature it is recommended that the intervals obtained from the diagram are halved for every 15°C increase in bearing temperature above 70°C. The intervals may be extended at temperatures lower than 70°C but as operating temperatures decrease the grease will bleed oil less readily and at low temperatures and extension of the intervals by more than two times is not recommended. For bearings on vertical shafts the intervals obtained from the diagram (t1,) should be halved. For large roller bearings having d of 300 mm and above, the high specific loads in the bearing mean that adequate lubrication will be obtained only if the bearing is more frequently relubricated than indicated by the diagram, and the lines are therefore broken. It is recommended in such cases that continuous lubrication is practised for technical and economic reasons. The grease quantity to be supplied can be obtained from the equation below:

 Formula Gk

where are:

Gk grease quantity to be continuously supplied, g/h
D bearing outside diameter, mm
B total bearing width (for thrust bearings use total height H), mm

 

Standard procedure for relubrication

1. Advice is to relubricate according to tf value:
If the period of relubrication is up to 6 months, than on every 0.5 tf refill the grease, and after 3 refillings the grease needs to be replaced completely. Volume of refilling grease can be determined acc. to folllowing form:

Formula Gp

where are:
Gp grease quantity to be added when replenishing, g
D bearing outside diameter, mm
B total bearing width (for thrust bearings use total height H), mm

 

Diagram 10. Relubrication interval
Diagram 10. Relubrication interval

Scale a: deep groove ball bearings
Scale b: cylindrical roller bearings, needle bearings
Scale c: spherical, taper roller bearings, thrust ball bearings
roller bearings - full complement (0,2 t1),
cross-roller bearings with cage (0,3t1)
thrust roller, needle, spherical bearings (0,5t1,)

 

2. When the lubrication intervals are longer than 6 months it is recommended that all used grease be removed from the bearing arrangement and replaced by fresh grease.
All of this is a rough guideline recommendation if there are no other recommendations by the manufacturer or maintenance service. To facilitate the supply of grease using a grease gun, a grease nipple should be provided on the housing. It is also necessary to provide an exit hole for the grease so that excessive amounts will not collect in the space surrounding the bearing. This might otherwise cause a permanent increase in bearing temperature.

However, as soon as the equilibrium temperature has been reached following a relubrication, the exit hole should be plugged or covered so that the oil bled by the grease will remain at the bearing position. The danger of excess grease collecting in the space surrounding the bearing and causing temperature peaking, with its detrimental effect on the grease as well as the bearing, is most pronounced when bearing operate at high speeds. In such cases it is advisable to use a grease escape valve rather than an exit hole. A grease escape valve consists basically of a disc which rotates with the shaft and which forms a narrow gap together with the housing end cover. Excess and used grease is thrown out by the disc into an annular cavity and leaves the housing through an opening on the underside of the end cover.
To ensure that the fresh grease actually reaches the bearing and replaces the old grease, the lubrication duct in the housing should either feed the grease adjacent to the outer ringside face or, into the bearing tracks. Generally, the care must be taken to see that the grease has adequate pumpability and that it does not remain within the bearing.

1.13.2 Oil lubrication
Oil is generally used for rolling bearing lubrication only when high speeds or operating temperatures preclude the use of grease, when frictional or applied heat has to be removed from the bearing position or when adjacent components (gears etc.) are lubricated with oil. In such cases there are different methods of oil lubrication:
Fig. 19 Oil bath
Fig. 19 Oil bath


1. Oil bath (Fig.19)

The simplest method of oil lubrication is the oil bath. The oil level should be such that it almost reaches the centre of the lowest rolling element when the bearing is stationary. Speed ratings for oil lubrication given in the tables apply for oil bath lubrication
Fig. 20. Circulating oil lubrication

 Fig. 20. Circulating oil lubrication

2. Circulating oil lubrication (Fig.20.)

Operating at high speeds will cause the operating temperature to increase and will accelerate ageing of the oil. To avoid frequent oil changes circulating oil lubrication is generally preferred; in that way the oil is cooled before being returned to the bearing.

Fig 21. Oil jet

 Fig 21. Oil jet

 
3. Oil jet (Fig 21.)

When speed is especially high, oil is injected under high pressure, in order to provide needed quantity without overheating. The velocity the oil jet must be higher than 15 m/s to penetrate the turbulence around the bearing

4. Oil-air (Fig 22)
Air oiled by precisely dosaged oil quantity is brought up to each individual bearing. This minimum quantity enables bearings to operate at the highest speeds and the lowest operating temperatures. Air stream not only cools bearings, but also prevents dirt penetration into arrangement.

 

 Fig 22 Oil-air lubrication

 

1.13.2.1. Lubricating oils
Primarily unalloyed mineral oils are used. Alloyed mineral oils containing antiwear additives or additives for enduring pressure are applied only in special cases. Synthetic oils can be used in extreme situations, when operating temperatures are very high or very low.

Selection of lubricating oils
Viscosity of mineral oils depends on the temperature, and it decreases as the temperature rises. This ratio is expressed through viscosity index. High index VI means that the temperature caused a small change of viscosity. With bearings, oils with high viscosity index are used (VI=85).
On the operating temperature of a bearing, oil must have certain minimal viscosity so that lubricating film could be formed. Diagram 11 is used to determine the necessary kinematic viscosity 1 for mineral oils whose index of viscosity is VI=85. When the operating temperature is known, referential viscosity v for temperature of 40°C (as well as for some other temperatures for examining viscosity, e.g. 20°C) can read from diagram 12. Because of its duration, it is recommended to select an oil of viscosity on operating temperatures higher than 1. However, since bearing temperature rises together with the increase of viscosity, this improvement is of limited effect.
When the viscosity ratio = / 1 is less than 1, oil with EP additives is recommended, and if is less than 0,4 an oil with EP additives must be used. With medium and large roller bearings, when >1, that type of oil can make certain improvements.

Example:
Bearing 6314 operates at a speed 3000 r/min and its operating temperature is 80°C. Calculate oil viscosity when temperature is 40°C.
From the bearing table: d=70mm, D=150mm so that dm=0,5(d+D) = 110mm. From diagram 2, for dm=110, when n=3000min-1, the necessary viscosity on operating temperature is 1 =7mm2/s. When operating temperature is 80°C, diagram 12 shows that on 40°C the oil viscosity should be =18mm2/s.

Oil change
Operating conditions and the quantity of oil are the factors that determine how often it should be changed. With oil baths this change should be made once a year, if operating temperatures were never higher than 50°C and unless it is not impure. When temperatures are increased, oil should be changed more often, e.g. every three months if operating temperature is 100°C.

 

Dijagram: Potrebna kinematska viskoznost
Diagram 11 Estimation of the rated viscosity 1

 

1.13.3. Inspection and cleaning of bearing arrangement
Bearing arrangement should be inspected and cleaned, from time to time. Between the regular detailed inspections of the entire machinery, noisiness and lubricants are controlled, temperature is measured. These inspections are more frequent with heavily loaded machines, e.g. whenever a roller, which is a part of a rolling mill equipment, is changed, the bearing is dismounted and inspected as well.
After an arrangement is cleaned and washed (e.g. by using benzene, paraffin, chlorinated carbon-hydrogen, alkali means), it should be well dried and oiled or greased in order to prevent corrosion, especially if the machine is not immediately used.

1.13.4. Bearing storage
When bearings are stored in their original packaging, they are corrosion protected for several years. Humidity in the storeroom should not exceed 60%. In the case of sealed bearings, if they are kept in the storeroom for a long period of time, grease can solidify so that after a bearing is mounted, its moment of friction is higher than in the case of a new bearing. Therefore, this should be taken into consideration.


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