A Closer Look at a Questionable Claim
A quick literature search of common shaft grounding devices, including conductive microfiber grounding devices, revealed that some make the strong claim of being maintenance-free. The following is an analysis of claimed performance relative to a maintenance-free claim. Remember, these devices are meant for industrial level performance. Having industrial level performance means the device must operate in often dirty environments in a 24/7 capacity while performing as claimed. In this case, performance means being able to eliminate electrically induced bearing damage for the entire claimed wear life of the device.
A large manufacturer of conductive microfiber grounding devices makes the following claim:
“Based on wear of less than 0.001” [0.025mm] during 10,000 hours of testing, proven to withstand over 200,000 hours of continuous operation.” The degree of performance is claimed to be maintenance-free.
Let’s begin to apply logic to this claim to figure out in more depth what is being asserted.
This conductive microfiber technology is dependent on the microfibers contacting a rotating surface. That rotating surface creates friction. Friction will always result in wear and tear. Based on their wear rate claims, the grounding device is expected to last for over 22 years of continuous operation. To show in more detail what their claim entails, let’s do some basic math:
200,000 hours divided by 8,760 hours per year = 22.83 years of claimed, maintenance-free performance.
Putting the 200,000 hours of continuous, maintenance-free operation claim to further scrutiny, each conductive fiber experiences a degree of travel relative to the rotating shaft. Stated differently, each conductive microfiber experiences a degree of travel that is relative to the motor’s rpm and shaft diameter. A smaller shaft diameter will subject the conductive microfibers to less travel than a larger shaft diameter. Here is the math:
Common electric motor shaft size is 3-3/8” diameter.
Circumference of a circle is C = 2(π)r
Circumference of a 3-3/8” shaft diameter is 10.55”.
Each microfiber filament experiences 10.55” of travel for every revolution of the shaft. Factoring in a typical 1800rpm motor, the math is as follows:
10.55” x 1800 revolutions per minute equals 18,990” of travel per minute or 1,582 ft/min of travel. To determine out how much distance each conductive microfiber is subjected to over the 22.83-year claim by this manufacturer:
1,582 ft/min x 1,440 min/day = 2,278,800 ft/day.
Converting to miles/day: 2,278,800 ft/day divided by 5,280 ft/mile = 431.59 miles of travel per day each microfiber is subjected to.
Let’s expand out the calculation to the claimed 22.83 years of continuous operation:
431.59 miles/day x 8,340 days = 3,599,460 miles of travel each conductive microfiber will experience over the 22.83 years of continuous operation. That’s almost 3.6 million miles of wearing friction the microfibers will experience over the claimed maintenance-free service life of this shaft grounding device.
Math shows the maintenance free claim to be dubious at best. The motors and bearings these devices are tasked to protect typically have a run life of 10 years, but that run life is very highly dependent on regular maintenance being performed. Can an industrial device subjected to that much friction over that much time be maintenance-free while claiming to prevent electrically induced bearing damage?
Let’s look at SGS® shaft grounding systems, particularly the non-drive end CR Series shaft grounding system. We make the claim that the SGSTM CR Series shaft grounding system is expected to perform for 10 years at 1800rpm prior to needing a brush change while eliminating electrically induced bearing damage for the entire wear life of the brush. Although not 22.83 years, our 10-year performance claim is still quite bold. Imbedded in that claim is the acknowledgement that these grounding systems require infrequent maintenance. Firstly, let’s apply the same logic to SGSTM as we did to the conductive microfiber devices and do a little math.
SGSTM non-drive end shaft grounding systems are end-of-shaft applications where the running surface is not on the outside diameter of the shaft, but rather on a patented rotor assembly attached to the end-of-shaft. For a 3-3/8” shaft diameter, the distance of travel the brush is subjected to is as follows:
SGSTM brushes are subjected to approximately 1.25” of travel per shaft revolution and remains the same no matter the shaft size.The brush is 0.4 inch in length and the end of shaft rotates on the 0.4” wide brush.
Distance traveled in one revolution is:
C= π x diameter
Diameter = 0.4”
3.14 x 0.4 = 1.25 inches
Rotational travel the brush experiences is 1.25” per shaft revolution.
TAKEAWAY POINT: For every revolution of a shaft with 3-3/8” diameter the conductive microfiber device is exposed to 10.55 inches of travel whereas SGSTM is exposed to a fraction of that distance at 1.57 inches. SGSTM non-drive end devices experience only approximately 10% of the travel in this situation and therefore inherently are subjected to far less wear over time.
Knowing that maintenance is a critical aspect of any process operation, we designed a system that possesses class leading maintainability. Because of its patented and proprietary designs, infrequent SGS® CR Series shaft grounding system maintenance can be done while the motor is running and in less than a couple of minutes. There is no need to shut down the motor as brush checks/changes can be done on the fly. Our 10-year performance claim is backed up by actual 10-year long field studies that assessed wear rates and grounding system performance with the help of third-party end users.
The primary take away is maintainability should be a core principle of any process operation. Claims asserting process operations are maintenance-free should always be looked at with a healthy degree of skepticism.