Chuck Yung
EASA Senior Technical Support Specialist

There are times when an application calls for a motor to carry a radial load for which sleeve bearings are not suitable. In cases such as low rpm, unusual frames, etc., it may be desirable to convert a customer's existing sleeve bearing motor rather than obtaining a ball/roller replacement motor. This article contains suggested procedures as well as cautions about potential problems with such conversions.  

First, inspect the end brackets to ensure they are mechanically rigid enough to support the load and suppress vibration (see Figure 1). If the end brackets lack rigidity, it may be  necessary to use complete fabricated replacements using thicker material. In other cases, gusseting can be used to stiffen the existing bracket.

Modifying the Bracket
The general procedure is to machine away the sleeve bearing housing and oil cavity and then modify the bracket in one of two ways. One method relies on flange-mount bearings, while the other requires fabrication of a conventional bearing housing. Of these options, a conventional bearing housing is the preferred choice.  

The flange-mounted ball/roller bearing requires less machine work, although there is a greater chance of error in aligning the shaft centerline and bearing housings. The vibration levels are apt to be higher than for a conventional bearing housing design. The type of bearing (ball or roller) that can be used with this arrangement is varied. If this modification is being considered for a radial load application, the shaft diameter must be sufficient to accept a suitable roller bearing. If a roller bearing is desired, shaft replacement (to a diameter adequate for the roller bearing) may be required.  

The conventional bearing housing modification requires more machine work, but it offers greater rigidity and more consistent alignment of both housings when done correctly.  

Conventional Conversion
For a conventional bearing housing fabrication conversion, the shaft size will constrain the choice of bearing sizes. Once the bearing size has been chosen, the bearing OD (outside diameter) dictates the housing size. The conventional housing fabrication conversion can be simplified by boring the end bracket to accept a large enough donut of steel plate of sufficient size and thickness to accommodate the bearing choice. When possible, the rough housing should be axially centered in the end bracket so that it protrudes far enough to be welded on both sides. Heavy fillet welds are essential. Depending on the end bracket shape, gussets should also be added to increase the housing's stiffness.

The end bracket, if steel, should be stress-relieved in a heat-treating oven after welding. This prevents warping during the machining process. Bearing retainer caps can then be machined; there should be some allowance for spacers so that endplay can be adjusted.   

Ideally, the bearing housings should be machined while installed on the frame. This allows the machinist to align on the stator bore, install the brackets and machine the housings concentric to the bore. When size constraints prevent this, it is normally acceptable to machine each bearing housing in the same way a normal housing would be bored and sleeved. 

Measuring and Sketching
Careful measurement and a sketch of the rotor – shaft – bearing journal, relative to the stator iron and modified end bracket, permits accurate positioning of the required bearing shoulders in both housings. The geometry of a sleeve bearing shaft with relatively small diameter journals and large diameter shoulders could make it impractical if not impossible to modify for a rolling element bearing.  There should be some text cautioning about the potential pitfalls of trying to convert from sleeve bearing to rolling bearing. By using both inner and outer bearing caps, the motor can be partially assembled before the bearing caps are machined, so that the desired shoulder locations can be determined accurately. 

A shoulder on the shaft to accept the bearing and provide axial positioning is necessary for the conventional housing method. Non-shouldered bearings are likely to result in higher axial vibration levels and less certainty of the rotor's axial position relative to the stator each time the machine is dismantled and reassembled.  

The housings should also be fitted with both inner and outer bearing caps (retainers) see Figure 2. The bearing caps prevent grease from reaching the windings. They are also needed so that the drive end (DE) can be located (i.e., held captive) to prevent the shaft's axial movement. The opposite drive end (ODE) bearing must have room for thermal expansion of the shaft as the motor heats up during operation. The shaft thermal growth is greater than that of the frame since the shaft and rotor temperature are higher than that of the frame in operation. 

A Reasonable Safety Margin
The rule of thumb is to allow 0.010” per foot (0.78mm / meter) of distance between shaft bearing journals for thermal expansion. This is more than what is necessary for normal operating temperatures; it includes a reasonable safety margin.  

It is essential that the inner bearing cap does not apply pressure to the back of the bearing. This ensures that the bearing caps do not pull the bearing away from the inside shaft shoulder.  

Cautions
Finally, you might encounter someone else’s attempt to perform this modification, so here are some of the things someone else might have done wrong: 

• The flange-mount ball/roller styles are more likely to be misaligned. Bearing life would be decreased. If badly misaligned, there may be evidence of uneven tracking in the ball path.  
• The use of tapered roller style bearings is not a good idea for electric motors. Used in tandem (with one at each end of the motor), tapered roller/cone bearings require some axial preload to prevent the shaft from pivoting. As the shaft expands thermally, the preload increases. Depending on bearing size, operating speed and other factors, it may be impossible to set the axial preload to keep the minimum and maximum preload within the rating of the bearings. When a double-thrust (dual cup) tapered roller is used on the DE, adjusting the ODE bearing preload can be difficult. 

There is considerable opportunity for errors that can damage a bearing in short order. If you run into a modification using tapered roller bearings, plan to change it to a conventional bearing arrangement.

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