Cyndi Nyberg
Former EASA Technical Support Specialist

Editor's Note: This article is similar to a February 2010 Currents article titled "Preloading roller bearing motors for no-load run testing." These two articles complement and supplement each other.

Ball and sleeve bearing motors can always be test run without any type of external load on the motor and bearings. 

However, when repairing a motor equipped with roller bearings that is used in an application with a radial load, such as a belted load, it is not advisable to perform the standard no-load test run for any length of time. Yet the no-load test run is a crucial step in the repair process to ensure proper operation. Without that radial load, the bearings can be damaged. This article will describe two ways to put a load on the shaft of a motor and therefore the roller bearing, so that it can be test run to ensure that it has been properly repaired. 

Minimum load on a roller bearing 
Without a certain minimum radial load on the roller bearing (Figure 1), the rollers will skid. Once they begin to skid, they will make noise, or “squeal.” Unfortunately, when bearings become noisy, what do we instinctively do? We add grease to the bearing, which silences it. The fact is that the bearing will already have sustained damage by the time it starts to make noise. 

By adding grease to it, we are only masking the damage to the bearing that will only get worse the longer it runs without a load. 
While the bearing may not fail after a short time on the test stand, any damage will certainly reduce its life in service. That motor may come back to the service center in a very short period of time, probably within the warranty period, with a failed drive end bearing. 

The best way to test-run a motor with a roller bearing is to apply at least the minimum radial load to the bearing. Every bearing has a certain minimum load requirement, based on the type, size and speed, which can be determined by calculation or by contacting your bearing supplier. Loading a roller bearing can be accomplished in a number of ways. 

Calculation of minimum load 
The minimum load requirement for a roller bearing will depend on the design, but it will be a fairly small percentage of the bearing’s static load rating. 

The minimum load on a roller bearing can be calculated by using the following equation: 
Frm = kr x (6 + 4n/nr) x (dm/100)² 
where 
Frm = minimum radial load, N 
kr = minimum load factor = 100 for bearings of series 10 = 150 for bearings of series 2XX, 3XX and 4XX (e.g. 215, 318, etc.) = 200 for bearings of series 22XX = 250 for bearings of series 23XX 
n = operating speed, rpm 
nr = speed rating for oil lubrication, rpm, from bearing manufacturer’s tables 
dm = mean diameter of bearing = 0.5 x (d + D), in mm 

Example: 
Calculate the minimum load required for an 1800 rpm machine with a 315 roller bearing: 
kr = 150 
n = 1800 
nr = 4000 
dm = 0.5 x (75+160) = 117.5 
Frm = kr x (6 + 4n/nr) x (dm/100)² 
Frm = 150 x (6 + (4 x 1800/4000)) x (117.5/100)² 
Frm = 1615N 

To convert the Newtons calculated to pounds or kilograms, use the following equations: 
Pounds = Frm x 0.2248 
Kilograms = Frm x 0.1019 
For this example, determine the force required for the minimum bearing load: 
Frm = 1619N 
1619N x 0.2248 = 363 lb. 
1619N x 0.1019 = 165 kg 

Method 1. Bottle jack on shaft

After the minimum load requirement has been calculated, now the question is how is that load applied to the shaft? One method to apply load to a shaft with roller bearings is to set
up a bottle jack with a pair of non-marring rollers (Figure 2). Make sure that the load capability for the pair of rollers exceeds the load you are placing onto the shaft. It is also a good idea to use rollers that have a larger diameter than the motor shaft, so the peripheral speed of the rollers will be lower than the motor shaft. 

To set up the arrangement, mount the rollers on a plate, then set it up so that the bottle jack is applying upward force on the plate. (See Figure 3.) 

Simply press up a little on the shaft by engaging the bottle jack with enough force to provide the necessary minimum load. That way, you can test run the motor long enough to get “no-load” current, speed and vibration readings, without potentially damag­ing the bearings. 

For safety, make certain that the output shaft has a guard to prevent accidental contact. 

Since the actual force applied to the shaft will not be known, here is a useful tip: Use a scale under the rollers. That way, as the bottle jack force is applied upward, the down­ward force of the shaft on the rollers will give a reasonably accurate reading on the scale. Industrial scales are available in a wide range of maximum weight. A regular bath­room scale will probably only read up to 300 pounds, but the assembly can be set up to take advantage of physics to determine the total weight. 

The easy way to do this using a single scale is to set up the assembly with the bottle jack mounted on a on one end and just a base on the other. This is illustrated in Figure 3. Using simple physics and statics, we know that if the bottle jack is exactly centered between the scale and the other end of the channel iron, both the scale and the base will carry equal load. So the scale reading would be half the applied load. 

This principle can be used for any position of the bottle jack on the channel iron. If the applied minimum load should be 900 pounds and your scale only reads up to 300 pounds, then if you position the jack one third the distance across the channel iron, a load of 900 pounds will read 300 pounds on the scale. 

For example, if the channel iron is 3 feet long, and the bottle jack is two feet from the scale, and one foot from the other support, then the scale will read 1/3 of the total force downward 

Method 2. Use belts and second motor to load the bearing 
The second easy way to test run a motor with roller bearings is to use a second motor, and connect the two shafts with a v-belt, as shown in Figure 4. 

By tightening the belt, a radial load will be applied to the bearing. To tension the v-belt, position a pair of bottle jacks between the motors. 

Again, a bottle jack will work well for this. You won’t need to use a pulley with this arrangement, but the shafts must be parallel to prevent the v-belt from walking off the shaft; and the belt must be guarded for safety. 

Place two bottle jacks horizontally to push the two motors apart. Use the bottle jack to tension the v-belt and load the shaft of the motor being tested. 

With this method, you also won’t know for sure how much load you are actually applying. Since the force is being exerted against the two motors, the motors do not have to be bolted down. 

The second motor serves as an idler pulley, and should be large enough that we do not exceed the radial load capacity of its DE bearing. 

Categories: Bearings, Roller bearings, Repair procedures & tips, Testing, Motor testing

Tags: Motor testing Roller bearings

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