Dismounting bearings can cause damage to both the bearings and associated components, such as shafts, if incorrect tools and techniques are used. When shafts and bearings are damaged during bearing dismounting, there is often a cost penalty in both time and materials. In addition, the use of the wrong tools and techniques for dismounting can be hazardous to the operator, especially due to the high forces involved.
Individual installations may require mechanical, heat or hydraulic application methods for correct and efficient dismounting. Professional dismounting, using specialized tools and techniques, is another positive step towards achieving maximum machine uptime.
Dismounting bearings can be a hazardous and demanding task. Selecting the correct tool is therefore of utmost importance for reducing the risk of personal injuries.
Another reason for dismounting bearings is for maintenance or replacement of other machine's components. These dismounted bearings are mounted again, unless they are damaged during dismounting. To enable the reuse of the same bearing, care must be taken when selecting dismounting methods and tools.
Always wear protective clothing and goggles when dismounting bearings.
When dismounting bearings using mechanical pullers, make sure you select a suitable puller for the application with sufficient pulling force in order to help prevent puller overload. Overloading a puller can result in puller arm or beam breakage, causing injury to the operator. A safety blanket fitted around the puller and bearing helps reduce the risk on injury in case the puller' s arm or spindle breaks.
Remember that corrosion on the interference fit can easily require 50% higher dismounting force.
If the interference fit is corroded, use penetrating oil to dissolve/loosen the rust.
Always use heat resistant gloves when dismounting bearings using heat.
Do not strike the bearing directly with any hard object such as a hammer or chisel.
Remember that powerful forces may be involved when dismounting bearings and care must always be taken to avoid injury.
Keep the work area clean; good housekeeping reduces accidents.
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 removed from a shaft.
Figure 1. 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 dismounting is the removal of the bearing from the shaft and the bearing housing. It includes preparations, methods and tools.
In the discussion of bearing dismounting 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.
Choosing the right puller for the job is critical. Not only the puller type, but also its maximum withdrawal force (kN) is crucial for completing any dismounting job safely and easily. Make sure puller overload is avoided, as it can result in breakage of the puller’s arms and/or beam. This breakage can cause damage to the puller, shaft and bearing as well as personal injury. It is recommended to use a three-armed puller instead of a two-armed puller. It is preferred to grip the ring with the interference fit whenever possible.
Interference Fit on the Shaft: External Pull
Small and medium-size bearings mounted with an interference fit on the shaft can be dismounted using a puller. If possible let the puller grip the inner ring, then remove the bearing with a steady force until the bearing bore completely clears the entire length of the cylindrical seating. Apply the puller to the outer ring if it is not possible to grip the inner ring. However, this presents a risk of damaging the bearing. If the bearing has to be used again, the outer ring must be rotated during dismounting. This can be done by locking the spindle and turning the puller continuously until the bearing comes free. Another way to minimize this risk is to use a strong back puller.
The puller should be accurately centered during dismounting in order to help prevent seating damage. To eliminate that risk, use self-centering pullers.
Figure 3. Strong back puller.
Interference Fit in the Housing: Internal Pull
The use of a slide hammer assisted puller is recommended for easy and quick removal of bearings from housings. When the puller has to be applied to the inner ring and the bearings are to be re-used, the inner ring should be rotated during dismounting to minimize the risk of damage.
Figure 4. Internal bearing puller.
Interference Fit on Shaft and in the Housing: Blind Pull
For non-separable bearings with an interference fit both in the housing and on the shaft, the best method is to allow the bearing to be pressed out of the housing with the shaft. This technique helps ensure that no dismounting force is transmitted to the rolling elements. The opposite procedure, allowing the bearing to come off the shaft with the housing, can also be used.
Ball bearings can be removed with a special blind housing puller. This puller’s finger-shaped extensions grip between the rolling elements and on to the ring, allowing the bearing to be easily removed.
Figure 5. Blind housing puller.
Medium-size bearings with an interference fit on the shaft often require considerable dismounting force. In these cases, a hydraulically assisted puller facilitates quick and effortless dismounting. A self-centering hydraulic puller can be used for forces up to 500 kN (50 ton).
Figure 6. Hydraulic assisted jaw puller.
Dismounting with 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 (dis)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.
By injecting oil of a certain viscosity between two shrink fitted surfaces, the mating surfaces will be separated by a thin oil film. If the surfaces are cylindrical, the fitted components can be more easily removed. The force required by a puller is greatly reduced.
Figure 7. Oil injection method applied in dismounting from cylindrical shaft.
Referring to Figure 7, the steps are:
A. During manufacture, the shafts are prepared with oil ducts and grooves. SKF can advise on how to prepare the shafts.
B. Bearings are mounted for example by heating them with an induction heater.
C. Dismounting the bearing is made easy by pumping oil under pressure between the mating surfaces. Once the oil pressure has built up, the component can be removed from the shaft with a minimum of effort.
When flangeless inner rings of cylindrical roller bearings, or those with only one flange, have to be removed infrequently, heating rings can sometimes be used. The inside diameter of the ring is the same as the raceway diameter of the inner ring. The ring is heated using a hot plate or naked flame to approximately 280 °C and then placed over the inner ring and clamped using the handles. The heat is transferred from the heating ring to the bearing inner ring, causing the bearing inner ring to expand. Once the bearing inner ring expansion has overcome the interference fit, then it is easy to remove the bearing inner ring.
Figure 8. SKF heating rings.
SKF has developed special induction heaters for dismounting the inner rings of cylindrical roller bearings having no flanges or only one flange. They heat the inner ring rapidly without heating the shaft to any degree, so that the expanded ring can easily be removed.
The use of such induction heaters becomes economic when cylindrical roller bearings are frequently mounted and dismounted, e.g. axle box bearings or rolling mill bearings. SKF heaters are available in adjustable and fixed types depending on application.
Figure 9. Adjustable induction heater.
Small bearings on tapered seating can be dismounted using a puller. Since the bearing is mounted with an interference fit on the shaft, the puller should, if possible, grip the inner ring. The same tools as those used for mechanically removing bearings on cylindrical shafts can be used.
Take care when dismounting bearings from tapered seatings using a puller. Always use a lock nut to prevent the bearing (and puller) from flying off the shaft!
With the oil injection method, oil under high pressure is injected between the mating surfaces. An oil film is formed, which separates the mating surfaces and appreciably reduces the friction between them. The method is mainly used when dismounting bearings fitted directly on tapered shafts, but is also used to dismount 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 being supplied to 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.
Injecting the oil between two tapered surfaces will create a reaction force that could be quite substantial as the oil will also act as a "hydraulic cylinder" which can push the outer component off.
Referring to Figure 10, the steps are: B. Bearings are mounted by pushing them up the shaft with the aid of a hydraulic nut.
A. During manufacture, the shafts are prepared with oil ducts and grooves.
C. Bearings are dismounted by injecting oil between the mating surfaces and when sufficient pressure is reached, the bearing will be pushed off. A nut is required to keep the bearing from sliding off the shaft.
Hydraulic Methods Used in Practice
Depending on the bearing mounting arrangement and the size of the bearing, the figures below show the most common methods of using hydraulics with tapered shafts, adapter and withdrawal sleeves.
The dismounting of large bearings from tapered journals, adapter or withdrawal sleeves is greatly eased if both a hydraulic nut and the oil injection method are used. After injecting pressurized oil between the mating surfaces, the bearing will separate suddenly from its seating and therefore some form of stop must be provided, for example a shaft nut or end plate to limit the axial movement of the bearing.
Adapter and Withdrawal Sleeves for Oil Injection
The larger sleeves have oil supply ducts and distribution grooves, enabling the user to inject oil between the sleeve and bearing bore and between the sleeve and the shaft. This oil reduces friction and force necessary for mounting and dismounting, particularly when mounting in a dry state. The use of sleeves reduces the risk of damage to shaft and bearings.