Around 16% of all premature bearing failures are caused by poor fitting, usually using brute force, and being unaware of the availability of the correct mounting tools and methods.
Figure 1. Damaged raceway due to forced mounting.
Individual installations may require mechanical, heat or hydraulic application methods for correct and efficient mounting. Professional fitting, using specialized tools and techniques, is another positive step towards achieving maximum machine uptime.
Rolling bearings should be stored in a cool, clean, low humidity environment, free of dust, shocks and vibrations. Storing bearings directly on the floor should therefore be avoided. If stocks of bearings are kept, it is important keep it rotating. SKF bearings are preserved and packaged to ensure that when complying to these storage circumstances the storage life will be years.
The simplest and most effective bearing maintenance is to keep the bearing clean. Contamination will shorten the life of any bearing and the importance of cleanliness cannot be overstressed. The following considerations are important for bearing maintenance:
All components should be clean and dry.
Cleaning material should be readily available.
Cotton waste should not be used to clean or dry bearings.
Clean hands and tools when.
Before mounting the bearing, the following should always be observed:
Make sure that housing and shaft are clean and not damaged.
Make sure that the new bearing is identical to the one being replaced.
Ensure that the lubricant to be used is clean and of the correct specification.
Make sure the necessary tools and equipment are at hand.
Keep the work area clean.
When mounting the bearing the following should always be observed:
Do not remove the bearing from its wrapping until the last moment.
Do not try to wash the bearing. The preservative used is compatible with mineral based oils and greases and should only be removed from the bore and the outside diameter of the bearing's outer ring. A lint-free cloth dampened with a cleaning agent is suitable for this purpose. Ensure that the mounting forces are only applied to the bearing ring with the interference fit.
Use a "minimum force with maximum control" method.
Lightly coat the mating surfaces with oil.
SKF bearings are made of high quality, hardened steel. For optimum safety:
Always wear protective clothing and goggles when mounting rolling element bearings.
Always use heat resistant gloves when mounting heated bearings.
For your own safety and to prevent bearing damage, do not strike the bearing directly with any hard object such as a hammer or chisel. Medium and large bearings can be difficult to handle. Bearing handling tools allows a bearing to be lifted from the floor and to be safely placed onto a shaft.
Figure 2. Bearing handling tools.
During its service life, a bearing has to be mounted on and dismounted from its shaft and housing at least once. Bearing mounting is the fixing of the bearing on the shaft and into the bearing housing. It includes preparations, methods and tools.
Figure 2. Bearing handling tools.
Most bearings are fitted to their shaft or housing with one component having an interference fit or in some cases both. For determining the correct fit, refer to the SKF General Catalogue, the SKF Maintenance Handbook or consult an SKF application engineer.
In the discussion of bearing mounting methods, two configurations are generally distinguished:
1. Cylindrical seating – A cylindrical seating is just a cylindrical shaft or housing, although special arrangements can exist. Bearings mounted on a cylindrical seating can have an interference or a loose fit, depending on the application.
2. Tapered seating – Tapered seatings include taper shafts, adapters and withdrawal sleeves. Bearings mounted on a tapered seating normally have an interference fit.
In the remaining part of this paper, various detailed techniques are described. Appendix A shows an overview of SKF methods and tools.
Mechanical mounting is generally suitable for small bearings. Mounting force can be applied to the bearing by placing a fitting tool impact ring and sleeve against the inner ring and using a press or hammer to advance the bearing to its proper location on the shaft. Be sure the correct size impact ring and sleeve are selected from a bearing fitting tool kit. The bearing should be exactly at the right angle to the shaft before beginning and the shaft lightly lubricated.
When bearings are mounted cold, care must be taken to ensure that the forces are applied to the ring with the interference fit. Damage and a resulting bearing failure can occur if the mounting force is transmitted through the rolling elements causing damage to the raceways. Do not apply a sleeve to the outer raceway when mounting on a shaft, or to the inner raceway when mounting in a housing. NEVER mount by striking the bearing directly with a hammer.
Figure 4. Applying mounting forces in the correct way.
Raceway damage incurred when a bearing is • Incorrect sized shafts and housings, i.e., incorrectly mounted can result in premature too loose or too tight. bearing failure. Typical problems, which can cause premature failure, are:
Loose retaining lock nuts during operation.
Damage caused during mounting.
Burred and damaged shaft and housing seats and shoulders.
Figure 5. Damaged raceway due to wrong application of mounting forces.
The force needed to mount a bearing increases rapidly with bearing size. Because of the mounting force required, larger bearings cannot easily be pressed onto a shaft or into a housing. Therefore, the bearing, or the housing, is heated before mounting.
The temperature difference between the bearing and seating depends on the magnitude of the interference fit and the bearing size. Normally a bearing temperature of 80 to 90 °C (144 to 162 °F) above that of the shaft is sufficient for mounting. Never heat a bearing to a temperature greater than 125 °C (257 °F), unless otherwise specified. Extreme heat can cause the material to change metallurgically and produce alterations in diameter, hardness, etc. Local overheating must also be avoided and in particular, never heat a bearing using an open flame.
Wear clean protective gloves when mounting a hot bearing. Lifting (hoisting) gear can facilitate mounting. Push the bearing along the shaft as far as the abutment and hold the bearing in position, pressing until a tight fit is obtained.
An induction heater can be compared to a transformer using the principle of a primary coil with a large number of windings, and a secondary coil with a few windings, on a mutual iron core. The input/output voltage ratio is equal to the ratio of the windings, while the energy remains the same. Consequently, the secondary coil will provide a low voltage at high amperage. In the case of an SKF induction heater, the bearing is a short circuited, single turn, secondary coil through which a low AC voltage flows at high amperage, thus generating high heat. The heater itself, including the yoke, remains at ambient temperature.
Figure 7. Principle of induction heating.
As this type of heating induces an electric current, the bearing will become magnetized. It is important to ensure that the bearing is then demagnetized so that it will not attract metal particles during operation. All SKF induction heaters are equipped with automatic demagnetizing cycles.
Figure 8. Example induction heater.
There are no restrictive guidelines to follow when choosing a bearing induction heater. Your choice will depend upon the type and geometrical dimensions of the components you want to heat. SKF can help select the correct heater for any bearing application.
Bearings mounted on tapered shafts, adapter or withdrawal sleeves achieve their interference fit by being driven-up the tapered seating. Care should be taken to ensure that the bearing is not driven up too far as all the internal bearing clearance may be removed, which can result again in bearing damage.
The inner rings of bearings with a tapered bore are always mounted with an interference fit. The degree of interference in this case is not determined by the chosen shaft tolerance, as with bearings having a cylindrical bore, but by how far the bearing is driven up onto the tapered seating. The original radial internal clearance is reduced in the process and this reduction gives an indication on the interference fit obtained. For small and medium size bearings, the reduction in radial internal clearance can be measured. Before mounting the bearing, the radial internal clearance is measured. During drive-up, the reduction in radial internal clearance is checked, until the requisite value is obtained. To measure the clearance, feeler gauges having blades with a thickness of 0,03 mm and above should be used.
Figure 9. Measurement with feeler gauges.
Small Spherical Roller Bearings
Correct adjustment of spherical roller bearings is determined by measuring the residual internal clearance in the bearing or by the amount of axial drive-up, see Figure 10:
Case A – Correctly mounted: Bearing driven up the correct distance and the right clearance is achieved.
Case B – Incorrectly mounted: Bearing is driven up too far and all clearance removed. Damage is possible.
Case C – Before adjustment.
Case D – After adjustment by driving up by a distance S.
Figure 10.Drive-up method applied to spherical roller bearing.
Self-Aligning Ball Bearings
Adjustment of double row, self-aligning ball bearings to obtain a correct interference fit is more difficult to achieve than spherical roller bearings, because the feeler gauge method cannot be used.
A more effective method to adjust this type of bearing correctly is to use the SKF TMHN 7 lock nut spanner set. This tool ensures mounting the bearings with the correct radial clearance, see Figure 13.
Figure 13.TMNH 7 lock nut spanner; after tightening the nut using hand-pressure only, the shaft is marked, and the nut is tightened using a handle until the shaft marking corresponds to the end of the orange semicircular marking.
For larger bearings, considerably more force is required to drive them up a tapered seating.
A practical solution is to use a hydraulic nut, which uses hydraulic power to provide the drive-up force. Oil is pumped into the nut and the piston is pushed out with a force that is sufficient for mounting the bearing.
If the oil is injected between the mating surfaces, then the force required can be further reduced. The hydraulic nut is screwed onto a threaded section of the journal or sleeve thread, so that its annular piston abuts the inner ring of the bearing, a nut on the shaft or a disc attached to the end of the shaft.
Figure 16. Hydraulic nut for driving the bearing onto an adapter sleeve.
Figure 17. Hydraulic nut screwed onto the shaft for driving in a withdrawal sleeve.
Figure 18. Hydraulic nut for driving the bearing onto tapered seating.
SKF Drive-up Method
It can be quite difficult or even impossible to measure the radial internal clearance of a bearing, and measuring the radial internal clearance requires a high degree of skill. For this purpose the "SKF drive-up method" has been developed. This method has proven to be very reliable, easy to use and giving consistently accurate results. The correct fit is achieved by controlling the axial drive-up of the bearing from a predetermined position. The method incorporates the use of a hydraulic nut fitted with a dial indicator, and a highly accurate digital gauge mounted on a selected pump. Tables have been developed, showing the pressure required to drive the bearing to the starting position and the axial drive-up distance to the final position for each bearing type.
Figure 19. SKF drive-up method.
SKF SensorMount® Method
A recently introduced method is the SKF SensorMount® method. Special series of SKF bearings are equipped with a sensor and dedicated hand-held indicator. The normal drive up methods are used, but the SensorMount® system indicates exactly how much the inner ring expands, thus ensuring that the interference fit between the bearing and the shaft can be accurately achieved.
Example case – Consider a compactor that is compacting potash from a fine powder to a hard board form. The compactor has two rollers with 500mm diameter and 2000mm length, pressed with 200 tonnes force between each other. The compactors rolls are both driven by a 950 kW electric motor via a double output gearbox with an output speed 18 RPM. The compactor is working in a very harsh environment, due to the abrasive potash as well as the high-elevated ambient temperature, reaching 50 degrees C during the summer. High quality bearings, with an easy, fast and very accurate mounting procedure to avoid very costly bearing failure due to wrong mounting are desired. The company suffered sudden bearing failure after 6,000 hours operation instead of 25,000 hours average bearing life. The SKF SensorMount method provided them with high accurate way of bearing installation, avoiding early bearing life deterioration due to wrong mounting. A detailed story will be featured in SKF Evolution during 2004.
SKF Oil Injection Method
Developed by SKF in the 1940s, the "Oil Injection Method" allows bearings and other components with an interference fit to be fitted and removed in a safe, controllable and rapid manner. When using the SKF Oil Injection Method the mating surfaces are separated by a thin film of oil injected under high pressure, thereby virtually eliminating the friction between them. Consequently, the mounting force required is considerably reduced. With the oil injection method, high-pressure oil is injected between the mating surfaces. The oil film formed separates the mating surfaces and appreciably reduces the friction between them.
The method is mainly used when mounting bearings directly on tapered journals, but is also used to mount bearings on adapter and withdrawal sleeves, which have been prepared for the oil injection method. A pump or oil injector produces the required pressure, the oil being supplied to the mating surfaces via ducts and distribution grooves in the shaft or sleeve. The necessary ducts and grooves in the shaft must be considered when designing the bearing arrangement.