CONDITION OF THE SURFACE
OF COMMUTATORS AND RINGS-ROUGHNESS
TECHNICAL NOTE ■ STA BE 16-1 GB
It is often erroneously thought that after rectification of a commutator (or a ring), the surface should be brilliant and as
brightly polished as possible.
On brilliantly polished commutators, friction is higher, particularly at start, and is stabilised more slowly than on initially
unpolished commutators, as is shown in diagram 1.
coefficient of friction
time t
Diagram 1
1 - Commutator polished and bright R a : 0.2 µ 2 - Commutator unpolished R a : 1.5 µ
This difference in performance is due to two causes which to a certain extent are linked:
– When the contact between the friction surfaces tends to approach perfection the friction forces become much greater,
then vibrations and considerable heating are produced.
– On commutators which are polished and bright, the graphite, one of the essential constituents of the skin, is poorly
abraded from the brush or, it is leaves on the metal and fails to firmly adhere.
These disadvantages do not appear on unpolished commutators which are slightly rough: the skin forms normally and the
brushes rapidly establish a stable condition.
On the other hand, if the final surface is too rough, the commutator works rather like a grinding wheel and as a result
excessive wear of the brushes occurs.
From this, it will be seen that the roughness of commutators and rings must be confined to certain limits, and it is for this
reason that you should not hesitate to remove the polish from a commutator, if it is too smooth and bright (grinding
stones, fine abrasives,...).
The roughness of the surface can be measured without difficulty. Such measurements, however, are expressed in several
different ways, and the various terms should not be confused as each method has its own corresponding evaluation. It is,
therefore, important to keep in mind the definitions given hereafter.
ISO 9001: 2000 l ISO 14001
DEFINITION OF ROUGHNESS
Several definitions of roughness exist; the French standards NF E 05015 and E 05017 as well as the German standards
DIN 4762, DIN 4768 and DIN IEC 773 particularly treats this question in detail.
Rmax
hn
S1
Ra
S1
Rp
Rs
Rt
h3
S1
h1
h2
h4
S2 S2
h5
L
Diagram. 2
In diagram 2:
– The curve C represents the profile of a section of a rough, surface on a length L.
– The line D is such that the sum of the surface S 1 , situated between C and D above D should be equal to the sum of the
surface S 2 situated below D.
R t : total depth or total roughness variation is the distance between the highest crest and the deepest trough.
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R max : depth or maximum amplitude of roughness, is the maximum distance between a crest and the consecutive trough,
R s : Average geometrical variation is the effectual value of the variation of h between C and D.
1 L 2 h1 2 + h2 2 + ... + hn 2
Rs = h dL =
L o n
This value R s is used in the United States of America and is designated by RMS (Root Mean Square). It corresponds also to
the following designations: R g , R q , H rms , RMC, RMQ.
– R p : depth of imperfection is the distance between D and the highest point of the profile.
– R a : Average arithmetical variation between C and D is the average height h of the different heights h 1 , h 2 , h n , always
taken with the plus sign.
L h1 + h2 + ... + hn
Rs = 1 h dL =
L o n
Other designations AA: (Arithmetical Average) in the United States.
Ru and CLA: (Center Line Average) in Great Britain.
Measuring apparatus gives simultaneously R a , R s , R max and R p but R a is the one most commonly used, and which we have
retained for the measurement of roughness of rings and commutators.
The degree of roughness is expressed in microns (µ) but certain countries use the micro-inch (1 µm = 40 micro-inches).
VALUES WHICH ARE RECOMMENDED FOR THE DEGREE OF ROUGHNESS R A OF RINGS AND COMMUTATORS
Roughness below or equal to 0.2 µm should be avoided. It is necessary to remove the polish from the surfaces.
Roughness overpassing 2 µm, brings with it excessive wear of the brushes.
From our experience, we would recommend:
0.9 to 1.8 µm for the commutators of industrial machines.
0.5 to 1 µm for small commutators of machines with a capacity lower than 1 kW.
0.75 to 1.25 µm for steel or bronze rings.
The specifications or data here in contained are only given for indication, without any undertakings whatsoever. Their publication does not suggest that the matter is free of any rights whatsoever. Furthermore, due to constant evolution of technics
and norms, we reserve the right to modify, at any time, the characteristics and specifications contained in this document. CARBONE LORRAINE refuses all and any responsibility concerning their use whatever the purpose or the application. Any
copy, reproduction or information herei n contained, in whole or in part, made without CARBONE LORRAINE written consent, is forbidden according to the laws of France and particularly the law nr. 92-597 of July 1st 1992, relating to the copyright.
Head Office CARBONE LORRAINE
CARBONE LORRAINE APPLICATIONS ELECTRIQUES
BE 16-1 GB 7020
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