A Detailed Overview: What is a Slip Ring Brush?

Introduction of Slip Ring Brushes

The brush (stator) and rotor are two extremely important components in a slip ring. The brush is used as a sliding contact body for importing and exporting current in the slip ring. It has a rectangular or cylindrical structure. It is installed on the slip ring stator through fixing devices such as brush holders and is pressed against the conductive ring channel under the action of a spring to conduct current.

Structure of Slip Ring Brushes

The brush is very small in size and mainly consists of the following parts:

Main Body

The main body of the brush is made of conductive materials such as gold, silver, or graphite. It is used to contact the conductive ring channel of the rotor part.

Lead Wire

There will be one or more wires led out from the top or side of the brush. These wires are used to connect the brush to the external circuit to transmit current and signals.

Fixing Structure

Sometimes, there are some fixing structures such as card slots and bosses at the bottom or side. The brush is connected and fixed through them to ensure that it does not move as much as possible during operation.

Materials of Slip Ring Brushes

The performance of the brush affects the operating state of the conductive slip ring, and this is mainly determined by its material. Brushes can be made of various types of materials, such as metal, graphite, optical fiber, mercury, etc. Here we introduce the three most commonly used ones:

Metal Wire

Metal brush wires are often made of thin metal sheets, such as gold, silver, copper, or silver alloys. These materials have very high conductivity and can transmit signals and electricity stably. In addition, gold and silver are not easily worn, and they have less power loss during long-term use. This also reduces noise during operation.
The standard brushes on the market are silver-copper alloy brushes containing 72% silver and 28% copper, and gold-to-gold brushes. Their outer diameter is usually 0.3mm, and the number of metal wires can be increased to carry a larger current.

Carbon Graphite Brush Block

The brush block is a contact point with a rectangular structure and has a large contact surface with the conductive ring. It is suitable for high-current conductive slip rings. Its main component is carbon. Due to its high conductivity and low price, it is widely used in heavy industry. Carbon graphite brush blocks will produce powder debris during operation. Therefore, they are often used in a separable slip ring structure for clean installation and easy maintenance.
It should be noted that graphite has self-lubricating properties and can better fit the surface of the slip ring, but this also makes it easy to wear. The good news is that they are easy to replace individually, so they are not particularly sensitive in terms of cost.

Contact Spring

Brush of contact spring type is a special structure of brush type. It is made of elastic metal contact springs or conductive materials with elastic structures. Its materials mainly include precious metals such as gold, silver and their alloys. As well as some copper-based composite materials and fiber-reinforced materials, such as carbon fiber, glass fiber, etc.
It is mostly used in disc slip rings. Because it has elasticity, it can achieve reliable contact while saving space. However, it is generally not used in sensitive signal transmission applications because the pressure of its contact spring may change during long-term use.

Their Contrast

	Metal Wire			Carbon Graphite Brush Block	Contact Spring
Material	Gold	Silver	Gold/silver alloy	Carbon & Graphite	special worn-resistance metal
Conductivity value	452000 S/m	6301000 S/m		100000 S/m	1000~2000S/m	Low conductivity
Applicable scenarios	High-precision signal transmission			High power	height limitaiton
Cost	comparitively high			comparatively low	depends (customization )

Applications of Different Slip Ring Brushes

(1) Metal Wire brush

Metal wire brushes are made of gold, silver and alloys. They have good electrical conductivity and low resistance. At the same time, these metals are not easily worn and can remain relatively quiet during operation. This is useful for some application scenarios that are sensitive to noise, such as medical equipment and laboratory instruments. For example:
High-Precision Electronics: Such as high-end communication equipment, precision instruments, etc. In these devices, slight signal changes may affect communication quality. The low resistance characteristic of metal wire brushes can reduce signal attenuation. This enables the signal to be accurately transmitted inside the device and avoids data errors caused by signal distortion.
Aerospace: Electronic equipment on aircraft and spacecraft needs to operate reliably in extreme environments. Metal wire brushes have high chemical stability and are not easily oxidated and corroded. Most importantly, they are maintenance-free designs and do not replace parts individually. This benefits for aerospace equipment that often performs tasks.
Medical Imaging Equipment: The structures of CT machines and nuclear magnetic resonance equipment are relatively small. They tend to be miniaturized and integrated, and the internal space is extremely limited. Metal wire brushes are usually relatively slender and can meet the installation requirements of narrow spaces and help these devices generate clear images.

(2) Carbon Graphite Brush

Carbon graphite material supports high-current conduction and can provide the required current for the rotor. It also has good self-lubricating properties and can reduce the resistance between slip rings during operation. You can use it in the following industries:
Industrial Automation: In the field of industrial automation, industrial robots, conveyor belts, and automated mechanical arms all need to use carbon graphite brushes. It is installed at the joint connection part and transmits current from a fixed power source to the rotating motor to assist the equipment in performing movements such as handling, welding, and assembly.
Textile Machinery: When textile machinery is working, it needs to complete the weaving and sewing processes of large quantities of cloth at high speed. The motor is the core power component of textile machinery. Carbon graphite brushes drive various components to work in the motor.
Crane: Carbon graphite brushes are used in the motors, reducers, and control equipment of cranes. In the lifting mechanism, the motor transmits power to the drum through carbon graphite brushes, drives the drum to rotate, realizes the retraction and release of the wire rope, and thus completes the lifting and lowering of heavy objects. It is mainly made of carbon materials. Even if it is worn after long-term use, costs can be saved by replacing individual carbon brush blocks.

(3) Contact Spring Brush

The core characteristic of the contact spring brush is its elasticity. It always maintains close contact with the slip ring during use to keep the accuracy.
Motors: In DC motors, the applications of contact spring brushes include some small DC motors such as power tools and toy cars. In AC motors with commutation devices, it can also be used to assist in current transmission and commutation.
Electronic Devices: It is widely used in equipment such as audio systems, televisions, and electronic instruments. In these devices, the contact spring brush is in close contact with the surface of the slip ring to read digital information. By changing the contact position between the contact spring brush and the resistor body, continuous adjustment of the resistance value can be achieved to conduct media data more accurately.
Rail Transit Field: In rail transit vehicles such as trains and subways, contact spring brushes are used in current collection devices to transmit the electric energy of the overhead contact network to the electrical system of the vehicle. The elasticity of the contact spring brush can ensure good contact with the contact network, and it can stably obtain electric energy even if there is vibration and shaking during vehicle operation.

How to Choose Slip Ring Brushes

Material

The material of the brush is the fundamental factor determining its performance. If you want to use it for high-flow applications such as synchronous machines and AC wound-rotor motors. Then graphite brushes are very suitable for you. It has self-lubricating properties and high conductivity, and the cost is low. Even if it is worn, you can buy brush blocks separately from the manufacturer for replacement.

If your equipment has high requirements for brush life, you'd better choose metal wire brushes, such as gold, silver and their alloy brushes. Their cost is higher than that of graphite, but they have the best conductivity among all metals and are generally non-maintenance.

Current Carrying Capacity

The current carrying capacity of a brush is the maximum current value that it can safely carry under specific conditions. Take a brush with a width of 10 millimeters and a thickness of 5 millimeters and a brush with a width of 20 millimeters and a thickness of 10 millimeters as an example. The current carrying capacity of the latter may be more than twice that of the former. A large cross-sectional area can provide more conductive paths and increase the current carrying capacity.

According to the power, voltage and other parameters of the equipment, use the power formula P = UI (where P is power, U is voltage, and I is current) to accurately calculate the current size of the equipment under normal working conditions. You can choose a brush with a current carrying capacity 20% to 30% greater than the actual current demand of the equipment.

Voltage Level

Before choosing a brush, you need to understand the working voltage of the equipment to ensure that the voltage level of the brush can completely cover it. For example, for an industrial motor with a working voltage of 380 volts, a brush with a voltage level above 400 volts should be selected. Otherwise, the brush will be damaged due to excessive voltage.

Electrical Noise and Interference

The main reasons for generating electrical noise and interference include poor contact between the brush and the slip ring, uneven conduction of the brush, and friction and vibration during high-speed rotation. At this time, the brush of the contact spring type is the best choice. Because it has good elasticity and a smooth and flat surface. Therefore, it can maintain a stable contact pressure under different working conditions.

Rotation Speed

Before choosing a brush, understand the working speed range of the slip ring. You can determine the speed range of the equipment by consulting the equipment's technical manual, consulting the manufacturer, or conducting actual tests. For high-speed applications, we recommend that you choose carbon graphite brush blocks because the carbon component has self-lubricating properties and can reduce friction and wear.

Temperature Stability

The brush will generate heat during operation, and its performance will be affected by temperature. In some high-temperature furnaces and heat treatment equipment, metal and graphite brushes perform well. At the same time, we'd better choose brushes with heat dissipation measures, such as heat sinks and ventilation designs.

Overall Cost

When choosing a brush, you cannot only consider the performance of the brush, but also should choose a brush with high cost performance. For example, graphite brushes have a lower cost but need to be replaced regularly and are suitable for occasions that are sensitive to cost. While metal wire brushes have a higher initial cost, but have higher transmission accuracy and do not need to be replaced. They are more suitable for the field of high-quality signal transmission.

Maintenance and Replacement of Slip Ring Brushes

Maintenance Tips for Brushes

Daily Cleaning
Open the end cover of the slip ring and carefully check after removing the sealing ring. If there is wear or breakage in the brush part, it needs to be repaired in time. Use absolute ethanol as a cleaning agent to clean up dirt such as carbon-containing powder generated by the brush sliding on the slideway. After cleaning the slideway, pay attention to observe whether there is powder around the carbon brush and wipe it clean.

Routine Inspection
(1) Appearance: Check whether the brush has obvious damage, such as fracture, breakage, deformation, etc.
(2) Color: Check whether the brush turns blue. If the brush turns blue, it means that it has been shocked and its ability to transmit signals will be greatly reduced.
(3) Maintain the Oxide Film State: When the motor is running, the surface of the slip ring should maintain a bright brown oxide film. This is beneficial to stabilizing the contact voltage of the brush and reducing the friction coefficient. For brushes used in special environments, brushes that have been properly impregnated should be selected.
(4) Check Brush Pressure: The spring pressure applied on the brush should be as uniform as possible, especially for brushes used in parallel. Otherwise, it will lead to uneven load on each brush.
(5) Avoid Using Lubricants: All kinds of brushes have self-lubricating properties. It is strictly prohibited to apply oil, paraffin and other lubricants in the slip ring.

Signs That The Brush Needs To Be Replaced

(1) Severe Wear: When the length of the brush is worn to 1/3 or 2/3 of its original length, it should be replaced.
(2) Severe Damage: If the brush is severely damaged such as fracture, breakage, and deformation, it will not work properly.
(3) Poor Contact: When the contact area between the brush and the slip ring is less than 70%, the contact resistance will increase. The higher the speed of the equipment, the greater the pressure generated and the larger the wear area of the brush. We need to adjust according to performance.
(4) Declining Performance: If the brush is used for too long, it may experience unstable signal transmission, excessive sparks and other situations. At this time, even if the appearance of the brush is not damaged, it needs to be replaced.

How To Replace Slip Ring Brushes

(1) First, we use tools to replace the worn and broken brushes inside the slip ring.
(2) Install the brush into the brush holder and confirm that it can move up and down freely. The gap between the side of the brush and the inner wall of the brush holder is between 0.1 and 0.3 millimeters.
(3) The lower edge of the brush holder is within the range of 2 to 3 millimeters from the surface of the slip ring.
(4) Grind the arc surface of the brush with glass sandpaper, cover it on the slip ring, apply appropriate pressure on the brush, and grind along the rotation direction.
(5) Make its arc surface fit the slip ring.
(6) Remove the surrounding powder and it can be used when the brush and the slip ring are in good contact.

Precautions

(1) Check that the pressure difference of each brush holder should not exceed 10% of the average pressure. During the operation of the slip ring, as the brush wears, the brush holder spring should be able to automatically adjust and maintain the carbon brush pressure unchanged.
(2) In addition, pay attention to safety when replacing brushes. We should ensure that the equipment is powered off and take corresponding safety measures to avoid electric shock.
(3) The brush is a relatively delicate part. Handle it with care during disassembly and installation to avoid dropping it.
(4) After replacing the brush, a test run should be performed first. This can help us check the working condition of the brush and the performance of the equipment to determine whether the equipment can operate normally.

Conclusion

In this blog, we have deeply explored all aspects of slip ring brushes. We learned about its types, structures and materials, their application scenarios and how to choose, and so on. As a leading slip ring manufacturer in the industry, ByTune will also provide you with more professional knowledge about slip rings. For example, our article "Use of Slip Rings in AC generators" aims to help you understand the use of slip rings in motors.