Jim Bryan
EASA Technical Support Specialist

Vertical motors can be identified by their capacity for an external thrust load. One common application with this factor is vertical pumps. The thrust load applied is the sum of the weight of the pump line shaft, the column of water in the casing that is being lifted, the weight of the pump impeller and the thrust produced by the pump’s volutes which forces the water to move upward.  The distance between the motor and the impeller can be very short such as pumping from a tank at or above ground level to lifting water from a subsurface aquifer several hundred feet deep. In the lat­ter example, the combined weight of the thrust load can be thousands of pounds (kilograms). The motor must then have a bearing design capable of these loads utilizing thrust bearings.

Bearing life
Options includ­ing angular contact ball, spherical roller and tilting pad bear­ings are capable of handling progres­sively larger thrust loads. This is de­scribed further in the Currents article, “Vertical Motors: Bearing Configura­tions and Oil Leaks,” published in June 2012. The goal is to achieve a specified bearing life based on the application. This life can be quantified as the L10 life or the length of time in hours of operation in which we expect that 10% of a large population of bear­ings will fail. Many factors determine the bearing life: load, speed and the bearing dynamic load rating. The equation in Figure 1 calculates this life for angular contact thrust bearings. Other factors also contribute but are difficult to quantify such as lubrica­tion, environment and contamination. Because of these, the L_¹⁰ life is only an estimate and should be used for comparative studies only.

For example, if the rotor weight is 1000 lbs (455 kg), the thrust is 10,900 lbs (4955 kg), the speed is 1800 rpm and the bearing is a 7226 angular contact ball bearing with a dynamic load rating of 41,800 lbs (19,000 kg) substituting in the formula:

8800 hours is approximately one year at full time operation, and for the example consider that a 20,000 hour minimum life is desired. The 8400 hours is not acceptable. In this case we could add a second bearing in tandem to increase the capacity by 60-75%, depending on the source of the information.  If we use the more conser­vative 60%, the equation becomes:

If we go to a spherical roller thrust bearing such as a 29426EJ, the dynamic load rating is 350,700 pounds (159400 kg).  Adjusting the formula, including using 10/3 as the exponent for roller:

These bearings also have a mini­mum load requirement, especially important for roller bearing designs (see Table 1). If the expectation is to increase the L10 life, using a bearing with more capacity may not be the right thing to do. An example is that if we calculate 20,000 hours of life for an angular contact ball bearing, we could use a spherical roller bearing and achieve >2.1x10⁷ hours based on the equation. This fails to recognize that the spherical roller bearing has a minimum load requirement that ex­ceeds the projected thrust load. If the roller bearing does not have adequate load, the rollers will not roll properly but will skid resulting in damage and premature failure of the bearing.

Bearing cage retainers
The bearing cage retainer is very important. There are four types in com­mon use: molded plastic, pressed steel, pressed bronze and machined bronze. Molded plastic and pressed steel are the least expensive but may be noisy. Larger bearings greater than or equal to 3000 rpm usually have ma­chined bronze cages.  The bearing cage should be ball riding as opposed to land riding, so that the lubrication flow is not impeded. See Figure 2. With the cage riding on either race, lubricant cannot easily pass to provide lubricant between the rolling element and the race upon which the cage is riding.

It is acceptable to mount motors with deep groove ball bearings (Con­rad) vertically with the shaft up or down because they are capable of a thrust or axial load albeit much less than an angular contact ball or spherical roller thrust bear­ing. The formula in Figure 3 defines this relationship.  The X&Y factors are de­termined by the ratio of the axial load to the basic static load rating given by the bearing manufac­turer. The value for Y in the table is typically 2-4 times greater than X so the axial load contributes to the equivalent dynamic bearing load at an order of magnitude greater than the radial load.

Therefore it is acceptable to carry a moderate axial load such as the weight of a belt sheave or a fan if the equiva­lent dynamic bearing load meets the requirement for bearing life. Only one of the bearings in the motor will carry this axial load because the other bearing must allow space for shaft thermal growth. In spe­cial cases such as where the drive end bearing is a roller bearing that is not capable of thrust loading, the opposite drive end bearing should be designed for the axial load. The thrust might be applied in either direction depending on the mounting position of the motor. If the motor is mounted with the shaft up, the axial load will be on the shaft bearing shoulder and the end bracket so the bearing location is secure. If the mounting is shaft down, a means of securing the bearing (e.g., a snap ring) so that it cannot work its way off the shaft is required; and the bearing back cap must be capable of supporting the weight of the rotor and the thrust load.

A thrust bearing should not be used on the opposite drive end when a roller bearing is used on the drive end. The radial force, with the roller bearing acting as a fulcrum, will displace the angular contact thrust bearing radially resulting in damage to the bearing.

Conclusion
Proper design will optimize the life of the bearings in a motor.  If redesign is being considered, all factors should be weighed carefully or the new design may not provide the improvement required.

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Categories: AC motors, Bearings

Tags: Bearings Vertical motors Vertical pumps

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