SECTION 3 - BRUSH THEORY AND CHARACTERISTICS
Question 3:9 What Is Mechanical Wear?
Any surface, even though it may appear polished, when viewed under a microscope, is in fact a series of peaks and valleys and the points of the peaks may represent as low as 1% of the nominal surface area. It is these points that initially make physical contact. Which result in a very high force per area. Because the friction at these points is very high the instantaneous local temperature approaches the melting point of the commutator or slipring material and softens it causing the polishing effect. It is these points that initially make physical contact.
A brush is basically made up of compacted grains of carbon, these high points do break away. This causes a number of rolling contacts rapidly moving over the surface of the brush face.
As well as providing graphite as a partial lubrication, something like dragging a box over a concrete floor with fine gravel on it, these highly concentrated current points also cause electrical burning and arcs which erode the carbon. This is the electrical part of the wear. This electrical arcing also produces the temperature to create another important part of the lubrication which is water vapour.
This water vapour or molecules of water come from two places:
1 The film of air on the surface of the commutator.
In some dry and cold atmospheres such as high altitude water vapour or other additives have to be artificially introduced to give good brush performance.
2 The moisture in the brush material.
In an ideal situation quite literally the brushes are riding on a cushion of water vapour without direct contact between the two surfaces.
Apart from inherent frication there are other direct and indirect factors affecting mechanical wear:
1 Commutator condition including
Patina or skin condition
Surface machining
Gases and dust etc
High temperature and humidity
2
Mechanical
Vibration
3
Excessive spring pressure
Question 3:10 What
Is Electrical Wear?
The information has been drawn from experience of Morganite Taiwan Limited Application Specialists combined with a wide range of published information from major carbon supplier and motor manufacture. However, the material can not be downloaded, copied or used without written permission.
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