Bearing handling

 

 1 Precautions for proper handling of bearings

Rolling bearings are high precision machine parts, they must be handled accordingly. Even if high quality bearings are used, their expected performance can not be achieved if they are not handled properly. The main precautions to be observed are as follows:

Keep bearings and surrounding area clean

Dust and dirt, even invisible to the naked eye, have harmful effects on the beatings. It is necessary to prevent the entry of dust and dirt by keeping the bearings and their environment as clean as possible.

Carful handing

Heavy shocks during handing may cause bearings to be scratched or otherwise damaged possibly resulting in their failure. Excessively strong impacts may cause brinelling, breaking, or cracking.

   (3) Use proper tools

Always use the proper equipment when handling bearings and avoid general purpose tools.

   (4) Prevent corrosion

  Since perspiration on the hands and various other contaminants may cause corrosion, keep the hands clean when handling bearings. Wear gloves if possible. Pay attention to rust of bearing caused by corrosive gasses.

2  Mounting

The method of mounting rstrongly affects their accuracy, life and performance. Their characteristics should first be thoroughly studied, and then they should be mounted in the proper manner. The standards is established with respect to the following items:

Cleaning the bearings and related components

Checking the dimension and accuracy

Mounting

Inspection after mounting

Supply of lubricants

Bearings should not be unpacked until immediately before mounting. When using ordinary grease lubrication, the grease should be packed in the bearings without first cleaning them. Even in the case of ordinary oil lubrication, cleaning the bears is not required, however bearing for instruments or for high speed operation must be cleaned with clean oil in order to remove the anti-corrosion agent. After the bearings are cleaned with filtered oil, they should be protected to prevent corrosion.

Prelubricated bearings must be used without cleaning.

Bearing mounting methods depend on the bearing type, type fit and conditions, as the bearings are usually used on rotating shafts, the inner rings require a tight fit. Bearings with cylindrical bores are usually mounted by pressing them one the shafts (press fit) or heating them to expand their diameter(shrink fit).Bearings with tapered bores can be mounted directly on tapered shafts or cylindrical shafts using tapered sleeves.

Bearings are usually mounted in housing with a loose fit. However, in cases where the outer ring has an interference fit, a press may be used. Bearings can be interference-fitted by cooling them before mounting using dry ice. In this case, a rust preventive treatment must be applied to the bearing because moisture in the air condenses on its surface.

 

 

2.1  Mounting of bearings with cylindrical bores

Press fits

Fitting with a press is widely used for small bearings. Just as shown in Fig 1, a mounting tool is placed on the inner ring and the bearing is slowly pressed on the shaft with a press until the side of the inner ring rests against the shoulder of the shaft. The mounting tool must not be placed on the outer ring for press mounting, since the bearing may be damaged.

 

Fig.1  Press fitting inner ring

Before mounting, applying oil to the fitted shaft surface is recommended for smooth insertion. Any time a hammer is used a mounting tool must be placed on the inner ring.The mounting method using a hammer should only be used for small ball bearings with minimally tight fits and when a press is not available. In the case of tight interference fits or for medium and large bearings, this method should not be used.

When both the inner ring and outer rings of non-separable bearings, such as deep groove ball bearings, require tight-fit, a mounting tool is placed on both rings as show in Fig.2, and both rings are fitted at the same time using a screw or hydraulic press. Since the outer ring of self-aligning ball bearings may deflect, a mounting tool such as that shown in Fig.2 should always be used for mounting them.

Fig.2  Simultaneous press fitting of inner and outer rings

 

In the case of separable bearings, such as cylindrical roller bearings and tapered roller bearings, the inner and outer rings may be mounted separately. Assembly of the inner and outer rings, which were previously mounted separately, should be done carefully to align the inner and outer rings correctly. Careless or forced assembly may cause scratches on the rolling contact surfaces.

（2） Shrink fits

Since press fitting large bearings requires a large force, a shrink fit is widely used: The bearings are first heated in oil to expand them before mounting. This method prevents an excessive force from being imposed on the bearing and allows mounting them in a short time.

The expansion of the inner ring for various temperature differences and bearing sizes is shown in Fig.3. The precautions to follow when making shrink fits are as follows:

(a)  Do not heat bearings to more than 120℃.

(b) Put the bearings on a wire net or suspend them in a tank in order to prevent them from touching the bottom of oil tank.

 (c) Heat the bearings to a temperature 20-30℃ higher than the lowest temperature required for mounting without interference since the inner ring will cool a little during mounting.

 (d) After mounting, the bearings will shrink in the axial direction and radial direction while cooling. Therefore, press the bearing firmly against the shaft shoulder using locating methods to avoid a clearance between the bearing and shoulders.

 

Fig. 3 Temperature and thermal expansion of inner ring

 

KMR bearing induction heaters

Besides heating in oil, the bearing induction heaters, which use electromagnetic induction to heat bearings, are widely uses. In KMR induction heaters, electricity in a coil produces a magnetic field that induces a current inside the bearing that generates heat. Consequently, without using flames or oil uniform heating in a short time is possible, making bearing shrink fitting efficient and clean.

In the case of relatively frequent mounting and dismounting such as rolling bears, tapered roller bearing used for railway vehicle axle box, induction heaters should be used for mounting and dismounting inner rings.