Chuck Yung
EASA Senior Technical Support Specialist

There are applications where the end float inherent to a sleeve bearing machine is not desirable, and some means of limiting the axial movement is needed. This is usually accomplished by selecting an appropriate coupling and relying on the driven equipment to prevent axial movement of the motor shaft. 

The gear-hub style of coupling can be end-float limited by installing a “hockey-puck” spacer. The grid-style coupling can be limited by spacers inserted on both sides. 

Regardless of coupling style, unless the driven equipment has some internal means to limit end float, there are circumstances where some external means of preventing axial movement is needed.

One such application is the cement industry, where an expensive air clutch would be damaged by axial movement. Other examples include certain pumps and blowers, any driven equipment that lacks thrust capacity, and the rare case where the motor itself cannot maintain magnetic center.

Solutions to consider
A solution, first used for the cement industry, lends itself to each of these scenarios. Installing a stub shaft (see Figure 1, green) on the non-drive end (NDE) to mount a ball bearing and adding a housing to axially capture that bearing allows the service center to axially locate (i.e., prevent axial movement of) the shaft. While the ball bearing inner race must be concentric to the main shaft, the housing should be oversized by at least 2-6 mm (0.080"-0.240") so that the ball bearing does not carry any radial load. The ball bearing only serves to prevent axial shaft movement.

The bearing outside diameter (OD) has to be large enough that it can be held captive against the outside surface of the sleeve bearing housing; for convenience, select a bearing approximately the diameter of the labyrinth seal on that end. In the Figure 1 example, the housing dimensions dictated the use of a 6318 ball bearing, but typical modifications range from a 313 to 320 ball bearing. Because the inner race fit is all that holds the bearing on the stub shaft, the wider 6300 series bearing is preferable to a 6200 series bearing.

The stub shaft to be bolted to the motor shaft must be concentric to the shaft; therefore, a rabbet fit should be machined on the end of the motor shaft with a corresponding locating fit on the back of the stub shaft. It’s convenient to design the stub shaft for four retaining bolts that are equally spaced. But if the motor shaft has drilled and tapped holes for some other purpose, those can be used instead. At any rate, a locating fit is essential to keep the stub shaft concentric to the motor shaft. Size the stub shaft so that it is at least twice as long as the bearing width, and size it properly for the inner race tolerance.

Bolt the stub shaft in place and spin the motor shaft using a dial indicator to confirm that the stub shaft runout does not exceed 0.002” (0.05 mm). Thrust the shaft and position it on magnetic center while the shaft coasts to a stop.

Determine required housing dimensions
Measure the relative position of the stub shaft and housing (or labyrinth seal fit) and determine the required housing dimensions. If the motor is fitted with a bolt-on labyrinth seal, it may be convenient to install all-thread (screw stock) into the mounting holes and install nuts to retain the labyrinth seal. If so, the all-thread should be long enough to pass through and secure the ball bearing housing and the outer bearing cap.

If the motor does not have a bolt-on labyrinth seal, it will be necessary to machine the end of the housing flat and provide a locating fit for the bolt-on ball bearing housing (see Figure 2).

Install the stub shaft (see Figure 2, green) and the ball bearing onto the stub shaft. The back of the ball bearing must be further away from the face of the housing where the extension housing (red) will mount to permit installation of a spacer (see Figure 2, C-shaped piece) behind the outer race of the bearing.

Spacer requirements
With the motor shaft positioned on magnetic center, measure the gap behind the outer race of the ball bearing to determine how much spacer is required. Rather than removing the ball bearing, it is convenient to fabricate a “C” shaped spacer (see Figure 2) with the opening of the “C” just large enough to clear the stub shaft extension. The OD of the spacer should fit loosely inside the housing (see Figure 2, dark red cylinder), but the housing bore should be approximately 2-6 mm (0.080"-0.240") larger than the OD of the ball bearing. This is to prevent the ball bearing from carrying any radial load.

Now use calipers or a depth micrometer to measure the distance from the outside edge of the ball bearing to the face of the housing. Machine a bearing cap (See Figure 2, at far left) with a lip to take up that space; bolt it to the bearing housing. Even though this ball bearing carries no radial load, the bearing should be lightly packed with grease.

This modification can be helpful to end users who need to restrict axial movement of an electric motor and the driven equipment. This may be done to protect the driven equipment, the motor bearings, or even to maintain alignment of the stator and rotor vent ducts for cooling.

Note: The purpose of this modification is only to limit axial movement of the assembled motor.

Categories: AC motors, Bearings, Sleeve bearings, Miscellaneous, Couplings, Repair procedures and tips

Tags: Sleeve bearings End float End play

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