Frequently asked questions - FAQ

1. Frequently asked questions - FAQ Frequently asked questions

2. Content • General • Brush manufacturing, design and application • Theory • Brush holders • Maintenance • Common problems Frequently asked questions

3. 1. General Frequently asked questions

4. 2. Brush manufacturing, design and application Frequently asked questions

5. 3. Theory and characteristics Frequently asked questions

6. 4. Brush holders Frequently asked questions

7. 5. Maintenance Frequently asked questions

8. 6. Common problems Frequently asked questions

9. Chapter 1 General Frequently asked questions

10. 1. Why are they called brushes ? • The inventors of rotating electrical machines were faced with the requirement to transfer current from a stationary position to a rotating object. They initially solved this problem with bundles of copper wire assembled like a paint brush rubbing against the rotating current collector. • The term "Brush" correctly described the item they used but because it had high friction and wear it wasn't long before the bristle brush was replaced and carbon blocks were used as a much better alternative. • The name "Brush" however has remained to the present days • Approx.1870 copper brushes • 1885 – first patent for carbon for sliding contacts Frequently asked questions

11. 1.2. Why carbon ? • Carbon has some unique properties which makes it the preferablematerial for electrical sliding contacts • Good electrical and thermal conductivity • Low shear strength of graphite crystal • Low friction coefficient • Low modulus of elasticity • Retains moderate strength at high temperature • No melting point, passing from solid to vapour at 3500°C • No welding between carbon and counter material • Wide band of phys. Characteristics by means of - Raw materials - Process - Design Frequently asked questions

12. 1.3. How is carbon made ? The basic raw materials of coke, graphite, carbon black or lamp black are combined with a variety of special additives which have been formulated in various ways. A binder is added and this mixture is then baked. In the case of electro-graphite grades a further process of passing electric current through the blocks or by inductive methods changes the crystalline structure of the material. In the first case the process is called “electro graphitisation“ or Acheson-graphiti-sation. The porous structure of the materials enables various after-treatments to modify the materials properties. The total process can take up to 6 months to complete. Frequently asked questions

13. 1.4 What additives are used All manufactured carbon is porous, to some degree, making later treatments possible. Special operating conditions like low humidity, bad ambient conditions etc. sometimes require the introduction of additives into the brush material to counteract any adverse effects of such conditions and to help in the control of commutator patina or skin formation.  Additives like paraffin, resin, special oils and inorganic additives can improve the performance and brush life under certain circumstances. . Frequently asked questions

14. 1.5. How many brush grades are there? There are literally thousands of different grades. Each manufacturer has its own series. Published lists are usually of the most common grades but others are developed according to the needs of industry and applications. Most manufactures have grades which will perform equivalent duties but subtle differences can make one particular grade perform better for a specific application.  Frequently asked questions

15. 1.6. What general groups of brush grades do exist ? 1. Hard Carbon – Carbon Graphite From the point of view of material characteristics, this material is between carbon (hard carbon) and electro-graphite. The use of hard carbon is restricted to low speed and low current density but some of this group are used for flush Mica commutators, others for collectors and some carbon contacts. It thus has a certain abrasive or polishing capability, but this is slight enough that flush commutator insulation cannot be abraded. Its main field of application is in universal motors with undercut inter-segment insulation. In the industrial carbon brush fields, the material is only used in special cases, where for example, electrographite brushes do not have a sufficient cleaning capacity, but hard carbon cannot be used, e. g. because of excessive friction. Frequently asked questions

16. 1.6. What general groups of brush grades do exist ? 2. Graphite Depending on the raw material used, this group contains a greater or lesser proportion of very finely distributed inorganic impurities, which give the natural graphite a certain abrasive property as well as good frictional performance. On the one hand, this makes the material suitable for operation on steel rings at high running speeds, while on the other, it can be used in the form of so-calIed cleaning brushes as supplementary equipment, for example to remove slight burn marks or to counteract excessive film formation. Because of the particular structure of natural graphite, the material feels extremely soft and smooth. Natural graphite brushes can be loaded continuously up to 10 A/cm², but will also withstand short-time current peaks up to 20 A/cm² Frequently asked questions

17. 1.6. What general groups of brush grades do exist ? 3. Electrographite Electrographite is the material with the widest field of application and therefore the most widely used material for carbon brushes. Electrographite is used, within certain limits, both on commutators and on slip-rings. Because of its high purity, electrographite protects the material on which it runs and, because of its crystal structure, it has very good frictional properties. Depending on the material structure and the operating conditions, the coefficients of friction normally are in the range µ = 0.1 to 0.25. Some electro-graphites can be used up to relatively high peripheral speeds of 50 to 60 m/s, and in special cases, when special grades are used, up to 80 m/s.It is always possible, by varying the raw materials and the production process, to lay particular emphasis on individual properties and, for example, to produce materials with good current distributing capability and high overload capability, carbon brushes with high strength for severe mechanical stresses and brushes with high commutation capabilities. Depending on the field of application and the cooling conditions, the nominal current capacity of electrographite brushes lies between 12 and 16 A/cm² (77- 103 A/in²). Depending on the duration and type of material, peak loadings up to 60 A/cm² (387 A/in²) are possible. Frequently asked questions

18. 1.6. What general groups of carbon brushes do exist ? 4. Metal-Graphite These materials are composed of graphite and metal powders, preferably copper, and thus have a relatively high electrical conductivity. Depending on the proportion of meta1 and the structure, the specific e1ectrica1 resistance of meta1-graphite is in the region of O.1 to 10 µΩm. This results in low contact resistance and voltage drop. Its hardness is relatively low. The graphite incorporated in the material gives the good friction properties which are necessary for satisfactory operation. With a high proportion of metal, metal-graphite brushes have a noticeably greater mass than metal-free carbon brushes, so that it sometimes becomes necessary to provide a greater contact pressure for these grades. The maximum permissible peripheral speed lies in the range of 30m/s. Depending an the metal content, current loadings up to 25 A/cm² (161 A/in²) are possible in continuous operation. The main field of application for metal-graphite brushes is in low-voltage machines with high current densities and commutation conditions which are not too extreme, and on slip-rings with high brush current densities Frequently asked questions

19. 1.6. What general groups of brush grades do exist ? 5.Metal-impregnated Graphite The grades in this class have its porous structure impregnated with a metal. The essential character of the base material is relatively unaffected by the comparatively small proportion of metal. However, the increase in mechanical strength given by this metal reinforcement, plus an increased thermal and electrical conductivity, has substantially extended the uses for electro-graphite based carbon. The grades are used for brushes on high current duty machines such as low voltage motors for battery driven vehicles and current collectors (for example pantographs for trains and trams).  Carbon bearings with white metal, antimony and other impregnations are used in mechanical applications. 6. Resin Bonded  Graphite As a result of the resin bonding, this materia1 has a relatively high intrinsic resistivity (of the order of100 to 350 µΩm) and also a high ratio of transverse to longitudinal resistance. The latter is due to the laminar structure of the graphite used. In conjunction with a high contact voltage, the material is therefore capable of greatly attenuating short circuit currents between segments bridged by the carbon brushes. It is therefore particularly suitable for three-phase commutator machines. But due to the improved load capacity at the present day, carbon brushes of this kind have also proved themselves increasingly on small and some medium sized D.C. machines and are used in relatively large quantities. Through the resin bonding, however, the lead capacity and especially the overload capacity is still low in comparison with electrographite brushes. Specific continuous current densities of 8 -10 A/ cm² (51-64A/in²) should not be exceeded for long periods. On D.C. machines, short-term peak values up to 12 A/cm² (77A/in²) are permissible. Frequently asked questions

20. 1.6. Are slip ring and commutator brushes different ? Slip-ring and commutator brushes can be distinguished in their application. Slip-ring brushes have only to transfer the current to a ring. They generally have the wider dimension tangential to the shaft with the appropriate number of brushes per ring based on the size and grade of carbon necessary to carry the required current. Low resistance electrographite brushes can be used for lower currents and metal graphite grades (metal content up to 90% metal) are used for higher currents. Brushes with 50%-75% metal content are the most common brushes used on slip-rings of induction machines. The construction of a slip ring brush will usually a solid block brush with the cable number and size to carry the relatively high currents. Commutator brushes generally have their widest dimension axially along the length of the commutator segments with the number of brushes per arm according to the required current and type of carbon used. Commutator brushes can be of the copper graphite type for less than 48 volt DC supply because of the high current involved. However, by far the bulk of commutator brushes will be made of electrographite with medium to high resistance depending on the load, the application and machine design. The construction of commutator brushes can be varied from a block brush to the most complex multi-wafer type with grades and other features from a large range of possibilities.　 Frequently asked questions

21. 1.7. General principle for grade selection ? For practical reasons the answer is limited to slip ring drives, DC Machines. The OEM of the machine will generally select a grade appropriate for the design of the machine assuming it will operate at full load. However this is often not correct for the actual load. To avoid problems a brush specialist should be contacted with all relevant information like name plate data and actual load data. 1. Slip rings Low resistance electrographite grades are used for current density less than 10A/cm² (65 A/in²) and effective cooling conditions. Grades from the metal graphite class with 50%-75% metal are used for current densities up to 15A/cm² (97A/in²). Very high current welding jigs etc. require metal graphite up to 90% metal. 2 Low Voltage DC Motor (Up to 48V) Battery powered vehicles, starter motors etc. use metal graphite brushes with a percentage of 25%-75% metal. Generally the higher the voltage, the less metal percentage is required depending on the brush configuration. 3.General Industrial DC Motors Voltages from 350 to 500V will require electrographite brushes of medium to high resistance. Lower load can permit the use of higher resistance brushes. As a general rule, the brush grade with the lowest resistance which will achieve minimum arcing but still generate enough heat to permit good film formation, should be used. Frequently asked questions

22. 1.8. Is there any shelf live for carbon brushes ? Electro-graphite grades are manufactured at a temperature of 3000°C and are more or less “dead material”. Some additives may lose some of their effectiveness. However, this should not greatly affect the general operation, though the benefit of the additive may be reduced or lost. Metal graphite brushes or the cooper flexible connection may be subject to corrosion if stored in an unsuitable atmosphere. If corrosion is present, a possibility may be to order brushes with tinned shunts which will assist in protecting the leads against corrosion if stored for long periods Frequently asked questions

23. 1.9. Are any safety measures necessary while cleaning motors ? Carbon itself is not toxic but appropriate breathing protection is recommended when cleaning electrical machines from carbon dust, particularly with metal graphite brushes. Persons with existing respiratory conditions may experience irritation from breathing high concentrations of dust. The material safety data sheets (MSDS) are available for full details. Frequently asked questions

24. Chapter 2 Brush manufacturing Frequently asked questions

25. 2.1. How are brushes made ? Some metal graphite and resin bonded brushes for automotive applications and FHP motors are pressed to size and come out as a fully formed brush complete with the flexible connection lead. The bulk of brushes however are cut from blocks of raw material. Dimensions and the features of the brush are produced in manufacturing plants specialized for that purpose. The flexible connection (shunts or pigtails) are connected to the brush body by tamping or rivetting. 　 Frequently asked questions

26. 2.2. How is the connection fixed ? 1 Tamped Connections The tamped connection is mechanically strong and of low electrical resistivity. A copper wire is placed in a hole and fine powder is compacted around the flexible with special machines, which guarantee a virtually solid bond between the brush body and the copper lead. The top of the tamped connection is sealed to prevent corrosion. This is the preferred and most effective method of wire connection for normal applications. 2 Riveted Connections This type of connection is also widely used. It is mainly applicable when brush proportions are not suitable for the tamped version or for soft carbon grades. The flexible lead is looped around a copper rivet fitted into a prepared recess and the rivet flared over to hold pressure between the wire and the carbon surface. The same rivet can be used to secure a metal top when fitted. However, this dual use of the rivet is not recommended. Frequently asked questions

27. 2.3. What information is needed by a brush manufacturer ? • In order to recommend a suitable brush grade the carbon brush manufacturer need the following data: • OEM of the motor- Power kW/HP • - Voltage V- Nominal current A- Actual current A- Peripheral speed m/s- Number of Poles- Brushes / Pole- Brush dimensions • Application • In case of problems additional information is necessary. • Description of the problem • Present brush grade • Actual number of brushes Frequently asked questions

28. 2.4. How may I select the right grade? There is no general rule for grade selection, but it requires a lot of experience. Much of the knowledge of a brush supplier on brush design, construction and operation is the result of correlation of reports on the behavior of brushes in service received from customers and from field engineers in all parts of the world. Results from the laboratories of the brush suppliers support grade selection. In order to make the right choice some basic information like OEM of the machine, application, actual load data and grade presently in use are helpful (see here…) Grade selection is always a compromise, since there are no “super grades” existing. Different parameters, sometimes mutually contradictory, have to be taken into consideration. Some trials, patience and time can be required to find the best grade for a particular application. Frequently asked questions

29. 2.5. Why do brushes have top or bottom angles ? The most common purpose of a top bevel is to promote side thrust on one brush face and thereby give stability of location on the brush holder. 　 A bevel in the contact face is applicable to brushes running in a trailing or reaction position relative to the collector. The values for bevel angles are important to the mechanical relationship of brush and holder. The reasons and supposed benefits of "trailing" holders and "reaction" holders and the appropriate angles of setting have been the subject of much differing opinion by machine designers. 　 Frequently asked questions

30. 2.6. What is the purpose of a rubber pad on the top? Rubbers and rubber/plastic composites can act as a damper in case of vibrations. They are known in a bonded and a loose design. Even in the case of block brushes the rubber/fibre pad absorbs some of the vibrations in the brush like a shock absorber which gives better brush face contact. The electrical insulation of brush top from the holder pressure device is a supplementary advantage. If loose dampers are used, care has to be taken that the pressure device of the brush holder is fitted correctly on the brush top. Otherwise it might happen, that the brush top hangs itself up on the upper end of the brush box. Frequently asked questions

31. 2.7. Why do some brushes have saw cuts in the face ? • These saw cut, which generally is only applied to slip ring brushes, serves two functions.  • It collects and expels dust to the side. • The cut interrupts the air cushion in the contact face, which develops at high speeds and might lift individual brushes from the collector. Thus the cut avoids the so called “aero-planning” of brushes and guarantees more uniform current distribution. Frequently asked questions

32. 2.8. Why do some brushes have grooves in the side or internal face ? These are grooves which minimize the risk of carbon dust building up on the brush faces which can result in the brush sticking in the holder. The grooves tend to clear the dust assisted by air movement through the grooves. Dust grooves are most commonly applied to slip ring brushes where the copper dust is more prone to building up inside the holders. DC motor brushes can also have grooves on the outside or inside faces. Low voltage forklift motor and traction motor brushes often have diagonal grooves across the outer faces. Frequently asked questions

33. 2.9. How is the cable size and number determined ? 1 Flexible Length The length of the shunt is measured from the top of the brush to the centre of the terminal.　 The cable must be long enough to allow full travel of the brush to its shortest position in the brush holder. If the cables are excessively long there is the possibility that they could foul in rotating parts, particularly in the case of Schrage type motors where the leads could catch in the moving gears. Another problem with long flexibles can be that in motors with high velocity cooling air the turbulence may cause some leads to move excessively and to get damaged by this movement. Placing a plastic band or a metal clip approximately midway along the length provides additional support. In some traction applications the copper wired can incorporate some steel strands to strengthen it. If the flexibles are relatively small in diameter placing a single insulation over two leads also gives additional support and keeps the leads tidy. : Frequently asked questions

34. 2.9. How is the cable size and number determined ? 2 Flexible Diameter  The cable diameter is generally selected in relation to the maximum current that the particular brush can handle considering its dimension and the type of material used. For high current carrying metal graphite brushes the cables would be much larger than for an electrographite brush of the same dimension. Another consideration of the shunt diameter is potential short term overload. The brush material can stand overload to varying degrees, however the lead may be the limiting factor, particularly with high starting currents that can occur with, for example, traction motors or electric forklift motors. In these cases the shunt is best dimensioned to the largest cable practical to fit in the brush, taking into consideration how the flexibility of the lead my affect the free movement of the brush up and down in the brush holder. 3 Flexible Insulation Though wire insulation prevents the cable shorting to earth or other live components nearby, it is better not to fit it as a standard option, if it is not necessary. This is because it adds to the cost and decreases the radiation and dissipation of heat arising in the shunt. Frequently asked questions

35. 2.10. Are there any alarm devices to indicate short brushes ? There are some type of brush holders which have so called micro-switches built into the holder which can be combined with external circuits to indicate individual brushes or rows of brushes which require attention. The switches can be incorporated into the protective control system and be sued to raise an alarm or shut the machine down. Detectors can also be embedded within each brush which, combined with appropriate external circuits, can positively indicate brushes of critical length. If every brush has to be monitored the amount of cables in the machine is a major disadvantage of this method. Adequate short circuit protection must be used where these indicating circuits are directly connected to the brush holders as armature voltages and high prospective fault currents are present. Power tool brushes can have lift-off devices fitted into the brush which consist of a spring loaded pin which releases when the brush wears to the predetermined position. Frequently asked questions

36. Chapter 3 Theory Frequently asked questions

37. 3.1. What is commutation ? The closed-circuit armature winding of a commutator machine must be regarded, in conjunction with the carbon brushes, as being built up from individua1 branches. The transition from one armature branch to the other takes place in each case at the point where the winding current is fed in or out through the commutator segments. As the armature rotates, the current in a coil of the armature winding must change its direction, when it changes from one armature branch to the next. This change of direction is called current reversal or commutation Welsch & Partner, scientific media Frequently asked questions

38. 3.1. What is commutation ? Switching-off Switching-on + IA ½ + IA ½ is 0 is 0 - IA ½ - IA ½ 2 IA ½ 2 IA ½ VK VK - IA ½ + IA ½ + IA ½ - IA ½ is is Frequently asked questions

39. 3.1. What is commutation ? • When the coil is powered a magnetic field is generated around the armature. The left side of the armature is pushed away from the left magnet and drawn toward the right The armature continues to Rotate Frequently asked questions

40. 3.1. What is commutation ? • When the armature becomes horizontally aligned the commutator reveres the direction of current through the coil, reversing the magnetic field. The process then repeats. Frequently asked questions

41. 3.2. What role does resistivity play in a brush ? Obviously the resistance of the material differs from one type to another depending on its ingredients and its production process, especially the final temperature treatment. Although carbon is unique among the non-metals in being a fairly good conductor of electricity, it is a poor conductor when compared with metals. The low resistivity electrographite materials are of the order of 10 µΩm. The corresponding figure for copper is 0.0178 µΩm. Thus the resisitivity of electrographite material is over 450 times that of copper while hard carbon grades can be ten times higher than this factor. Even so, the energy loss arising from this resistance of the brush material is only 10% of that which is dissipated at the brush contact surface due to contact-resistance and friction. Contact resistance, coefficient of friction and thermal conductivity of the brush have a much greater influence than the resistivity of the material. The measurement of resistivity is however a useful quality control test in production. Frequently asked questions

42. 3.3. How important is brush hardness? The Rockwell Hardness is well proven as hardness measurement of carbon ceramic materials due to its accuracy and repeatability. Brinell-Hardness and Shore-Hardness are used by some of our competitors, but in our opinion less suitable due to the worse repeatability. It is particularly valid for the hardness: There is no correlation of hardness and wear. The wear rate of carbon brushes or other carbon contacts like carbon strips is determined by various surrounding effects and the electrical and mechanical stability of the contact points in the contact surface of brush and strip, This stability cannot be described with the macroscopic value hardness. The most impressive example is a "soft" carbon brush grade , which gives a much better performance e.g. on large mill or fast running machines than "hard" brush grades. Cleaning and polishing action is also not determined by the hardness of a material. The ingredients of the material do have a much bigger influence. Commutator attack is not triggered by "hard" materials, but by the ambient conditions, uneven current distribution or electrical overload. The same is valid for the catenary wear caused by carbon strips. The catenary wear is particularly triggered by abrasive mineral ingredients as impressively. Appropriate information can be given on request. Frequently asked questions

43. 3.4. What is contact drop or voltage drop ? U1 U U2 U3 When contact is made between a carbon brush and a collector, an electrically conductive contact is made only at a limited number of points. This small number of contact points causes a reduction of the cross sectional area of the brush, causing an increase. In the electrical resistance. This resistance is called the constriction resistance. This, together with the resistance of the patina, forms the contact resistance, to which must also be added the resistance of the brush itself plus pigtail and terminal. The sum of al1 these resistances is cal1ed the contact resistance of a carbon sliding contact The voltage drop due to the contact resistance of two carbon brushes series-connected across a short circuited commutator or slip-ring is called the contact voltage. (In accordance with IEC Publication 276, voltage drop for two brushes in series) This is an important quantity for the user of carbon brushes since it inf1uences commutation and ohmic losses. The voltage drop is made up of the component voltage drops shown. The contact voltage drop ∆U3of an electrographite brush grade of average resistivity amounts to about 80 -85% of the tota1 voltage drop ∆u. The percentage is still higher n the case of metal-carbon grades. It is therefore permissible to use the contact voltage as a measure of the contact conditions in the practical application and evaluation of the quality of a carbon brush Frequently asked questions

44. 3.5. How does the voltage drop influence commutation ? In the case of dynamic current loading such as exists with commutation between carbon brushes and segments, the individua1 contact points have insufficient time to adapt themselves to the current densities which are constantly changing at relatively high frequency. The trend of the voltage drop which forms the criterion for the current reversal is therefore determined by the maximum current density which occurs. If the coil resistance and the inductance of the commutation circuit are predominant in a machine, the influence of the voltage drop becomes rather less important. The factor of contact stability with high current densities, such as occur during commutation at the edges of the brushes, becomes more important. This is also the reason why relatively low-resistance materials without a particularly wide commutation band but with good contact stability ( coke-based materials) have better performance on large machines than high-resistance materials which are less capable of withstanding surge loadings (carbon-black based materials). Things are somewhat different, f the commutation is not only influenced as a result of inductances, hut additional induced voltages are present in the commutation circuit. This is more or less the case with D. C. supplies with a relatively high harmonic content in the supply voltage. Here it sometimes becomes necessary to use high-resistance material, right up to resin-bonded graphite or even sandwich carbon brushes, with a high voltage drop in order to reduce the transverse currents caused by the induced vo1tages. If a machine shows mechanical difficu1ties, the commutation characteristic of a carbon brush as such can only be influenced to a slight degree, in order to improve the commutation, by the use of a different material. The greatest successes are obtained here by changing the design of the carbon brushes, e. g., with twin brushes etc. Frequently asked questions

45. 3.5. What is current density Current density is the value of the current passing through a particular brush in relation to its contact area and is expressed as Amps per cm² or Amps per inch². The actual current a brush can carry is widely influenced by operating conditions such as type of ventilation, continuous or intermittent duty, speed and other factors. The published data sheet ratings for electrographite brushes are generally conservative, some allowance having been made for short term overloads above those listed in the published data. The current carrying capacity of a brush depends ultimately on the operating temperature. On well-ventilated machines having small brushes with larger surface area in proportion to their volume and where brushes cover only a small percentage of the commutator or ring surface, conventional current densities for electrographite grades can often be doubled without seriously jeopardising their performance. On the other hand, increasing the current density without making provisions for maintaining a suitable low brush temperature may reduce the brush life dramatically. In practice low current density in a machine caused by running a machine below full rated load is potentially more damaging than a moderate overload. For good operating temperature and performance as a general rule, the actual operating current density should be not lower than 60% of the published rated current density. Frequently asked questions

46. 3.7. How is current density calculated ? DC Motors 1000 A – 6 pole 5 cb’s each, i.e. 30 brushes,i.e. calculation with 15 brushes 20 x 32 x 50 mm³ 1000A - 4 pole 5 ea. Tandem Brushes , i.e. 20 brushes, calculation with 10 brushes 12,5 x 32 x 50mm³ Tandem Brushes i.e. total dimension - t – is 25mm Frequently asked questions

47. 3.7. How is current density calculated? Slip ring drives Asynchronous-Slip-Ring Drive500A - 3 rings 5 cb’s, i.e. calculation with 5 brushes – 40 x 20 x 40 mm³ Turbogenerator1000A - 2 rings with 10 cb’s each , i.e. calculation with 10 brushes – 32 x32 x64mm³ Frequently asked questions

48. 3.8. What is the reason for low load problems and what are remedies ? Carbon brush Contact point Patina Collector Plasma Contact point Carbon brush Collector Patina Carbon brush Collector Standard conditions Low load • The film consists of Graphite and copper oxides • copper oxide is a semiconductor • High el. resistance at low temperatures • current goes via some frit bridges only Frequently asked questions

49. 3.8. What is the reason for low load problems and what are remedies ? Carbon brush Collector • Mechanical breakage of copper particles out of the surface • Hard copper particle in the contact surface • grooving • Remedial action • reduce number of brushes track wise • use preheated cooling air • reduce cooling air (ask OEM first) • use a low load resistant brush grade Frequently asked questions

50. 3.9. What causes brush wear ? There are many variables which influence brush wear, but individual influencing factors can not be calculated nor evaluated separately. It is only possible to specify approximate values for brush wear. Peripheral speed Brush wear is normally specified in mm/1000h for industrial drives or mm/10.000km for traction machines. The wear is determined by the distance travelled by the brush on the surface of the collector External effects Probably error in measurementPossibly collector wear Frequently asked questions