complimentary bearing analysis service

DP&A Sales offers free bearing analysis services. You can ship your bearings to us and we will provide a customized bearing analysis report indicating if electrical, mechanical or both types of damage are present along with appropriate recommendations. It is best to call us first prior to removing the bearing so we can instruct you how best to remove and mark the bearing(s). This important step will ensure the bearing analysis report to be as thorough as possible. This free service is designed first and foremost to educate the field regarding bearing damage and its potential fixes including motor shaft grounding, if appropriate.

The following is a recent bearing analysis report to give you an idea of this important service. It was conducted for a large sugar manufacturing plant that had been experiencing frequent bearing failure.

BEARING ANALYSIS REPORT

Motor: Teco-Westinghouse, 700hp, 4-pole, s/n BU.AD092057-1

Bearings Received: 1) Drive end roller bearing complete with rollers, cage, inner race and outer race that was cut in half. 2) Non-drive end insulated bearing complete with balls still in cage, uncut inner race and outer race cut in half. Both bearings came with a grease sample.

DRIVE END (LOAD SIDE)

Visual inspection without the aid of optics was performed revealing significant streaking patterns in the direction of travel on the roller balls as well as the inner and outer races. This streaking does not appear to be mechanical in nature and cannot be felt to the touch. These streaks are mostly brown to yellow-brown in color. A shadowing effect can also be seen running in the axial direction of the shaft.

Shadowing effect can be seen and presents as a series of up and down marks in the direction of the shaft and is present along the entire outer diameter race.

Using a microscope the very early stages of electrical bearing damage can be seen and is charaterized by a classical fluting pattern on the outer race.

Bearing Grease:

The drive end bearing grease was blackish in color and smelled a bit burnt. Under magnification significant foreign material can be seen as reflective specks scattered throughout the grease sample.

NON-DRIVE END (OPPOSITE LOAD SIDE)

Visual inspection of the insulated bearing’s outer race without the use of magnification reveals a light and largely uniform frosted streak around the entire diameter and in the direction of ball travel. Also present are what appear to be false brinelling marks.

Below fleck marks are enlarged for easier viewing. These fleck marks are present along the entire frosted area of the outer race.

Below is the insulated bearing’s ball. Each ball had a distinct orbital wear pattern indicative of a ball that cannot move freely within its cage. The top half of the photo consists of the frosted area. The wear pattern averaged 0.20” wide and is frosted in nature and does not appear to be mechanical damage, but rather electrical in nature.

Bearing Grease:

Bearing grease for the insulated bearing appeared to be in better condition than the drive end grease, although, flecks of foreign material are still visible.

IMPRESSIONS

Both the drive end and non-drive end bearings show early signs of electrically induced bearing damage.

The non-drive end insulated bearing shows early signs of a frosted pattern along the outer race. Each ball also shows a frosted pattern associated with electrically induced bearing damage. In addition, the balls appear to have been restricted in movement which led to a distinct orbital track around the ball that averaged 0.20” in width.

The drive end bearing shows significant streaking on the outer and inner races as well as each roller ball. Roller ball bearings will exhibit electrical bearing damage in the manner described. In addition, under significant magnification, a classical electrical fluting pattern emerges. This fluting pattern will develop overtime and become very visible and will actually cause a washboard effect on the bearing race.

RECOMMENDATIONS

For a motor of this frame size we recommended both ends of the motor be grounded with a shaft grounding device. Motors between 100-200hp at 1200 or less RPM or motors above 200hp regardless of RPM should have both ends grounded to eliminate both capacitive discharge and possibly present high frequency circulating currents inherent to the motor. Our work with OEM motor manufactures and the field suggest the incident rate of circulating currents in larger frame motors to be between 25-35%. Possible reasons for circulating currents include asymmetries in the motor, a magnetized shaft, or air pockets in the insulation.

We do not recommend using insulated bearings and/or insulated shafts as our field studies and feedback over the last three decades have shown insulated bearings can electrically fail. The insulated bearing assessed in this report has likely failed due to electrical discharge machining. Reasons for an insulated bearing failure include, but are not limited to, damaged ceramic coating or grease working its way around the bearing shoulder that causes a path for current to flow through the dielectric and then to the bearing.

Also recommended is ensuring the motor is properly aligned and the new bearings being installed do not show signs of damage caused by false brinelling. Mechanical damage related to misalignment, etc., can hasten electrically induced bearing damage despite having shaft grounding installed. If an insulated bearing is going to be used, then do not grease the sides of the bearing in an attempt to make bearing installation easier. Greasing the sides of the bearing can cause current to flow around the bearing shoulder and may lead to electrically induced bearing failure.

We hope this analysis is useful. Please contact us if you have questions or need clarification.