Bearings with a tapered bore
For bearings having a tapered bore, inner rings are always mounted with an interference fit. The degree of interference 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 shaft seat, or onto the adapter or withdrawal sleeve. As the bearing is driven up the tapered seat, its radial internal clearance is reduced. This reduction can be measured to determine the degree of interference and the proper fit.
When mounting self-aligning ball bearings, CARB toroidal roller bearings, spherical roller bearings, as well as high-precision cylindrical roller bearings with a tapered bore, either the reduction in radial internal clearance or the axial drive-up onto the tapered seat is determined and used as a measure of the degree of interference. Guideline values of clearance reduction and axial drive-up are provided in the relevant product sections.
Small bearingsSmall bearings may be driven up onto a tapered seat using a nut. In the case of adapter sleeves the sleeve nut is used. Small withdrawal sleeves may be driven into the bearing bore using a nut. A hook or impact spanner can be used to tighten the nut. The seat surfaces of the shaft and sleeve should be lightly oiled with thin oil before mounting is started.
Medium and large sized bearingsFor larger bearings, considerably more force is required and
- SKF hydraulic nuts should be used and/or
- the oil injection method should be employed.
In either case, the mounting process will be considerably easier. The oil injection equipment required for both, operating the hydraulic nut as well as for applying the oil injection method, is available from SKF. More information about these products can be found in the section Maintenance products.
When using an SKF hydraulic nut for mounting it has to be positioned onto a threaded section of the journal or onto the thread of the sleeve 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. Pumping oil into the hydraulic nut displaces the piston axially with the force needed for accurate and safe mounting. Mounting of a spherical roller bearing with the aid of a hydraulic nut on
With the oil injection method, oil under high pressure is injected between the bearing and bearing seat to form an oil film. This oil film separates the mating surfaces and appreciably reduces the friction between them. This method is typically used when mounting bearings directly on tapered journals (fig 4), but is also used to mount bearings on adapter and withdrawal sleeves that have been prepared for the oil injection method. A pump or oil injector produces the requisite pressure, the oil is injected between the mating surfaces via ducts and distributor grooves in the shaft or sleeve. The necessary ducts and grooves in the shaft must be considered when designing the bearing arrangement. A spherical roller bearing mounted on a withdrawal sleeve with oil ducts is shown in fig 5. The withdrawal sleeve is pressed into the bearing bore by injecting oil between the mating surfaces and tightening the screws in turn.
Determination of the interference fitBearings with a tapered bore are always mounted with an interference fit. The reduction in radial internal clearance, or the axial displacement of the inner ring on its tapered seat is used to determine and measure the degree of interference.
Different methods can be used to measure the degree of interference:
- Measuring the clearance reduction with a feeler gauge.
- Measuring the lock nut tightening angle.
- Measuring the axial drive-up.
- Measuring the inner ring expansion.
A brief description of these four different methods is provided in the following. More detailed information about these methods can be found in the relevant product sections.
Measuring clearance reduction with a feeler gaugeThe method using feeler gauges for measuring the radial internal clearance before and after mounting bearings is applicable for medium and large-sized spherical and toroidal roller bearings. The clearance should preferably be measured between the outer ring and an unloaded roller (fig 6).
Measuring the lock nut tightening angleMeasuring the lock nut tightening angle is a proven method to determine the correct degree of interference in small to medium-sized bearings on tapered seats (fig 7). Guideline values for the tightening angle a have been established, providing accurate positioning of the bearing on its tapered seat.
Measuring the axial drive-upMounting bearings with a tapered bore can be done by measuring the axial drive-up of the inner ring on its seat. Guideline values for the required axial drive-up are given in the relevant product sections.
However, a more suitable method in this case is the "SKF Drive-up method". This mounting method provides a reliable and easy way to determine the degree of interference. The correct fit is achieved by controlling the axial displacement of the bearing from a predetermined position. The method incorporates the use of an SKF hydraulic nut fitted with a dial indicator, and a specially calibrated digital gauge mounted on a selected pump (fig 8). Determined values of the requisite oil pressure and the axial displacement for the individual bearings provide accurate positioning of the bearings. Values can be found in the product sections for spherical roller bearings and CARB bearings but more detailed information is obtained at skf.com/mount
Measuring the inner ring expansionMeasuring inner ring expansion is a simple and very accurate method to determine the correct position of large-size spherical and toroidal roller bearings on their seats.
For this kind of measurement the SensorMount® is available, using a sensor, integrated with the bearing inner ring, a dedicated hand-held indicator and common hydraulic mounting tools (fig 9). Aspects such as bearing size, shaft smoothness, material or design - solid or hollow - do not need to be considered.