What are the uses of carbon-carbon composites

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Carbon-Carbon (C/C) composites are a class of high-performance materials that stand at the pinnacle of engineering for extreme environments. Composed of carbon fibers embedded within a carbon matrix, they are renowned for one primary characteristic: their extraordinary ability to maintain and even increase their strength at extremely high temperatures.

Carbon-carbon Composite Features

High strength-to-weight ratio: They are incredibly strong yet lightweight.

Excellent high-temperature resistance: They can retain their mechanical properties at extremely high temperatures (up to 3000°C or even 3315°C in inert atmospheres).

High thermal conductivity: Efficiently dissipate heat.

Low thermal expansion coefficient: Resist changes in size with temperature fluctuations.

High fatigue resistance: Can withstand repeated stress without failure.

Biocompatibility: Suitable for use in the human body.

Chemical inertness: Resist corrosion from various chemicals.

Carbon-carbon Composite Applications

Carbon-carbon Composites

Due to these remarkable characteristics, carbon-carbon composites find applications in a wide range of demanding industries:

1. Aerospace and Defense: This is the primary application area for C/C composites.

High-performance braking systems: Used extensively in aircraft (commercial and military, like the Concorde and Airbus A320) and high-speed vehicles (Formula One cars, supercars like Bugatti Veyron, and many Bentleys, Ferraris, Lamborghinis, Porsches). They offer superior heat capacity, reduced weight, and longer service life compared to steel brakes.

Re-entry heat shields and nose cones: Essential for spacecraft (like the Space Shuttle orbiter’s nose cone and wing leading edges), missiles, and re-entry vehicles, protecting them from extreme temperatures generated during atmospheric re-entry.

Rocket nozzles and motor throats: Can withstand the immense heat and pressure of rocket propulsion.

Leading edges of high-performance aerospace vehicles: Areas subject to intense heat and friction.

Engine components: Parts of turbojet engines.

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What are the thermal properties of carbon-carbon composites and what factors affect the properties?

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Carbon-carbon (C-C) composites are renowned for their exceptional thermal properties, which make them ideal for high-temperature and demanding applications such as aerospace components (e.g., brake discs for aircraft, rocket nozzles, re-entry nose tips) and industrial furnaces. These properties are highly dependent on the manufacturing process, fiber type, orientation, and matrix density.

Carbon-carbon Composites Thermal Properties

Carbon-carbon Composites

High-Temperature Stability:

C-C composites retain their mechanical properties and dimensional stability at extremely high temperatures, often exceeding 2000℃(3632℉) and even up to 3000℃(5432℉) in non-oxidizing atmospheres. This is significantly higher than most metals and other advanced ceramics.

Their excellent thermal shock resistance is a major advantage, allowing them to withstand rapid and extreme temperature changes without significant damage.

Thermal Conductivity:

C-C composites can exhibit a wide range of thermal conductivities, from good insulators to excellent conductors, depending on their fabrication and microstructure. This “tailorable” conductivity is a significant advantage.

Anisotropy: Thermal conductivity is often anisotropic, meaning it varies with direction.

In-plane (along the fiber direction): Values can range from approximately $10 \text{ W/(m·K)}$ to $233 \text{ W/(m·K)}$. Graphitized fibers contribute to higher in-plane conductivity.

Through-the-thickness (perpendicular to the fiber direction): Values are typically lower, ranging from $2 \text{ W/(m·K)}$ to $21 \text{ W/(m·K)}$.

Temperature Dependence: Thermal conductivity generally decreases with increasing temperature, although values can be fairly stable at very high temperatures.

Compared to other materials: While a simple carbon fiber-epoxy composite might have thermal conductivity 40 times less than aluminum and 10 times less than steel, advanced C-C composites, particularly those with highly graphitized matrices, can achieve conductivities comparable to or even exceeding some metals.

Coefficient of Thermal Expansion (CTE):

C-C composites generally possess a very low coefficient of thermal expansion, meaning they exhibit minimal dimensional changes when subjected to temperature fluctuations. This contributes to their excellent dimensional stability and thermal shock resistance.

Anisotropy: Like thermal conductivity, CTE in C-C composites is often anisotropic.

In the fiber direction, carbon fibers can even have a negative CTE, meaning they slightly contract when heated. This unique property, combined with the matrix, can result in very low or even near-zero overall CTE in certain directions.

Typical ranges for linear CTE are from approximately −1× 10⁻⁶ K-1to +8×10⁻⁶ K-1

The CTE is influenced by the fiber type, weave architecture, matrix density, and the degree of graphitization.

Specific Heat Capacity:

The specific heat capacity of carbon-carbon composites is generally comparable to that of graphite, especially at temperatures above 340 K.

Typical values are around$760 \text{ J/(kg·K)}$ ($0.18 \text{ BTU/(lb·^\circ F)}$) at room temperature, which can increase with temperature.

Factors Influencing Thermal Properties:

Carbon-carbon Composites

The thermal properties of C-C composites are not fixed values but vary significantly based on:

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What is the difference between die-molded graphite and isostatic graphite?

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The graphite stands out for its exceptional properties, including high thermal and electrical conductivity, excellent thermal shock resistance, and chemical inertness. Within the realm of synthetic graphite, two prominent types are die-molded graphite and isostatic graphite. While both are manufactured from carbonaceous raw materials and undergo high-temperature processing to achieve their graphitic structure, their fundamental difference lies in their shaping process, which profoundly impacts their resulting material properties and suitability for diverse applications.

Difference Between Die-molded Graphite and Isostatic Graphite

Die-molded Graphite

1. Manufacturing Process:

Die-Molded Graphite:

Produced by compressing a mixture of graphite powder and a binder into a mold using uniaxial (single-direction) or sometimes bidirectional pressure.

This process can involve “pressed to size” (PTS) technology, allowing for near-net-shape production, which can be cost-effective for high-volume, complex parts.Can be done via cold or hot molding.

Isostatic Graphite:

Manufactured using a cold isostatic pressing (CIP) process.

The raw material mixture is placed in a flexible mold and subjected to uniform, high pressure from all directions by a fluid medium (liquid or gas) in a sealed chamber. This is based on Pascal’s law, ensuring even compression.

This method is generally considered more advanced and can also include warm or hot isostatic pressing.

Isostatic Graphite

2. Material Properties:

Die-Molded Graphite:

Anisotropic properties: Its characteristics (strength, thermal conductivity, electrical conductivity) can vary depending on the direction of measurement, particularly if formed by extrusion (which is a form of molding through a die). Some die-molded graphites can also show high anisotropy due to the axial forming process.

Can have a fine to ultrafine granulation.

Properties can be adjusted for specific tribological or electrical needs.

Isostatic Graphite:

Isotropic properties: This is the most significant difference. Due to the uniform pressure from all directions during manufacturing, isostatic graphite exhibits consistent properties (strength, density, thermal and electrical conductivity, thermal expansion) in all directions.

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What is the difference between carbon composite and graphite?

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Graphite is the raw substance. A carbon composite is the high-tech, engineered final product that uses a special fibrous form of graphite to achieve its incredible performance.While both carbon composites and graphite are made of carbon, they are fundamentally different in their structure, properties, and applications.

Difference Between Carbon Composites and Graphite

Graphite

Here’s a breakdown of the key distinctions:

Graphite:

Definition: Graphite is a naturally occurring crystalline allotrope (form) of the element carbon. It’s one of the most stable forms of carbon under standard conditions.

Structure: It has a layered atomic structure. Each layer consists of carbon atoms arranged in a hexagonal lattice (like a honeycomb). These individual layers are called graphene. The layers are held together by weak van der Waals forces, allowing them to slide easily over each other.

Properties:

Softness: Due to the weak bonding between layers, graphite is very soft (Mohs hardness of 1-2) and has lubricating properties (which is why it’s used in pencils).

Conductivity: It’s an excellent conductor of both heat and electricity, especially along the planes of its layers.

High Temperature Resistance: It can withstand very high temperatures (sublimates around 3,600°C) in inert atmospheres.

Density: Relatively low density.

Applications:

Lubricants

Pencil lead

Electrodes in batteries and industrial furnaces

Refractory materials (materials resistant to high temperatures)

Thermal management (heat sinks)

Carbon Composite (often Carbon Fiber Reinforced Polymer – CFRP):

Carbon Composite

Definition: A carbon composite is an engineered material created by combining strong carbon fibers (a form of carbon) with a binding matrix material, typically a polymer resin (like epoxy). It’s a type of composite material, meaning it’s made from two or more distinct materials that, when combined, produce properties superior to the individual components.

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Precision Crossed Roller Bearing Lubrication Guide

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Precision crossed roller bearings are critical components in applications requiring high rigidity, accuracy, and the ability to handle combined loads (radial, axial, and moment loads). Proper lubrication is paramount to their performance, longevity, and overall system reliability.

Crossed Roller Bearing Lubrication

Precision crossed roller bearings

1. Importance of Lubrication:

Reduces Friction and Wear: Creates a lubricating film between rolling elements and raceways, minimizing direct metal-to-metal contact.

Extends Fatigue Life: A proper lubricant film reduces stress concentrations and prevents surface damage, thereby prolonging the bearing’s operational life.

Dissipates Heat: Helps to carry away heat generated by friction, preventing overheating and material degradation.

Prevents Corrosion: Forms a protective barrier against moisture, contaminants, and corrosive agents.

Damping and Noise Reduction: The oil film can absorb some energy, contributing to smoother operation and reduced noise.

Sealing: Grease, in particular, can act as a barrier to prevent the ingress of dust, dirt, and moisture.

2. Types of Lubricants

The two main types of lubricants used for precision crossed roller bearings are grease and oil. The choice depends heavily on the specific application’s operating conditions (speed, load, temperature, environment).

Grease Lubrication:

Advantages: Adheres well to surfaces, lasts longer, provides good sealing against contaminants, and is often preferred for applications where easy access for re-lubrication is limited.

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What are the advantages of precision crossed roller bearings

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Precision crossed roller bearings are highly specialized components, and their unique design offers a powerful set of advantages, particularly in applications demanding high performance in a limited space.Precision crossed roller bearings offer a multitude of advantages that make them ideal for high-precision applications.

Precision Crossed Roller Bearings Advantages

Precision crossed roller bearings

High Precision and Accuracy: The unique design, with cylindrical rollers alternately crossed at 90 degrees, ensures excellent rotational accuracy and minimal deflection under load. This precise arrangement provides a full line of contact between the rollers and raceways, leading to greater stiffness and less elastic deformation compared to point-contact ball bearings. They are crucial for tasks requiring fine motor skills, precise positioning, and smooth, consistent movement.

Exceptional Load Capacity and Rigidity: Crossed roller bearings can handle heavy and complex loads simultaneously, including radial, axial, and moment loads from any direction. This versatile load capacity contributes significantly to the rigidity and stability of the system, which is essential in applications where positional accuracy is paramount (e.g., precision assembly or robotic surgery).

Compact Design and Space Saving: Their orthogonal roller arrangement allows for a highly compact and space-saving design. This is particularly beneficial in applications with limited installation space, such as modern robotics, where miniaturization is a key design goal.

Low Friction and High Efficiency: Crossed roller bearings typically exhibit a low coefficient of friction. This reduces energy consumption, heat generation, and minimizes the difference between static and dynamic torque, contributing to greater overall efficiency and potentially extending battery life in automated systems.

Smooth and Quiet Operation: The non-recirculating design of crossed roller bearings results in extremely smooth motion, as they don’t suffer from the pulsations experienced by recirculating bearings. This also significantly reduces noise generation, making them noticeably quieter.

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Large Equipment Slewing Bearing Replacement Strategy

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Slewing bearings are critical components in large equipment like cranes, excavators, and wind turbines, enabling rotation and supporting significant loads. A robust replacement strategy is essential to minimize downtime, ensure safety, and optimize operational costs. This strategy involves a combination of proactive maintenance, condition monitoring, and a well-planned replacement or repair process.

Slewing Bearing Replacement Strategy for Large Equipment

Slewing bearings

I. Proactive Maintenance and Inspection (Preventive & Predictive)

The goal is to extend bearing life and predict failure before it happens.

Regular Lubrication:

Frequency: Follow manufacturer guidelines. This typically ranges from every 50-100 hours of operation for slow-moving equipment to every 8 hours for continuously rotating equipment.

Method: Add grease slowly while rotating the bearing to ensure even distribution and purge old, contaminated grease. Sufficient grease is applied when it overflows from the seal.

Type of Grease: Use heavy-duty, extreme-pressure (EP) grease as recommended by the manufacturer.

Gear Lubrication: If the slewing bearing has integrated gears, lubricate them separately as their requirements differ from the raceways. Apply small amounts of grease at the point of mesh.

Bolt Torque Checks:

Initial Check: After 100 hours of initial operation, re-check bolt torque.

Subsequent Checks: Every 300-500 hours, and more frequently in harsh conditions (vibration, shock).

Procedure: Wipe bolts clean, apply thread-locking adhesive if replacing, and tighten to the manufacturer’s specified pre-tension (often 70% of the bolt material’s yield limit). Use Q&T (quenched and tempered) washers; spring washers are prohibited.

Importance: Loose bolts can lead to uneven load distribution, localized stress, and premature failure.

Visual Inspections:

Frequency: At least weekly, or before each operation.

What to look for:

Cracks or damage on the slewing ring.

Signs of biting, gnawing, or surface peeling on gear teeth.

Integrity of seals: replace damaged seals promptly and reset any that have fallen off. Seals prevent contamination of the raceways.

Unusual noise or impact during rotation.

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How to improve the life of precision crossed roller bearings

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The “life” of a precision crossed roller bearing, like any rolling element bearing, generally refers to its fatigue life. This is the number of revolutions or operating hours a bearing can achieve before the first signs of material fatigue, such as spalling or flaking, appear on its raceways or rolling elements.

Precision crossed roller bearings are critical components in high-accuracy applications, and their longevity is paramount. Here’s a comprehensive guide to improving their life, based on best practices.

How to improve the life of precision crossed roller bearings

precision crossed roller bearing

1. Proper Selection and Design:

Accurate Load Calculation: This is the foundational step. Understand the magnitude, direction (radial, axial, moment), and combination of loads the bearing will experience. Overloading is a major cause of premature failure.

Environmental Conditions: Consider temperature, humidity, dust, and exposure to contaminants. Choose bearings with appropriate materials, seals (if applicable in the surrounding design, as crossed roller bearings themselves typically don’t have integrated seals), and lubricants for the operating environment.

Precision Requirements: Select the appropriate precision class for your application. Higher precision bearings often require more precise mounting and handling.

Stiffness and Preload: Crossed roller bearings offer high stiffness. Proper preloading during design and assembly is crucial. Too little preload can lead to clearance and vibration, while excessive preload can generate heat and reduce life.

Anti-Creep Mechanisms: For applications with high acceleration/deceleration, uneven loading, or vertical/inclined strokes, consider bearings with anti-creep mechanisms to prevent roller slippage and wear.

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Jaw Crusher Troubleshooting Guide: Common Issues & Fixes

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Jaw crushers are robust machines essential for primary crushing in various industries, but like any heavy equipment, they can experience issues. Regular maintenance and prompt troubleshooting are key to minimizing downtime and ensuring efficient operation.

Common Jaw Crusher Issues and Their Solutions

Jaw Crusher

1. Main Engine Suddenly Stops or Fails to Start

Possible Causes:

Crushing chamber/discharge port blockage: Material buildup can halt the machine.

V-belt issues: Loose, broken, or slipping V-belts.

Eccentric shaft bushing problems: Loose bushing can cause the eccentric shaft to get stuck.

Low voltage/insufficient motor power: Electrical issues can prevent the motor from driving the sheave.

Damaged bearings: Worn or damaged bearings can cause the machine to seize.

Fixes:

Clear blockages: Stop the crusher and remove any material blocking the discharge port or crushing chamber.

Check and adjust V-belts: Tighten loose V-belts or replace broken ones.

Reinstall/replace bushing: If the eccentric shaft bushing is loose, reinstall or replace it.

Adjust voltage: Ensure the working voltage meets the motor’s requirements.

Replace bearings: If bearings are damaged, replace them.

2. Reduced Crushing Capacity / Output Does Not Meet Standards

Possible Causes:

Incorrect feed size: Material fed into the crusher is too large, causing clogging.

Worn jaw plates: Worn or unevenly worn jaw plates reduce crushing efficiency.

Improper settings: Incorrect eccentric speed, stroke length, or discharge opening settings.

Clogged discharge chute: Material buildup in the discharge chute can cause back-pressure.

Incorrect relative position of jaw plates: The tooth grooves of the movable and fixed jaw plates are misaligned.

Voltage too low: Insufficient power to the motor.

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What are the factors affecting the capacity of jaw crusher?

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The capacity of a jaw crusher is usually measured in tons per hour (tph), but this is not a fixed figure. It is a dynamic variable and is affected by a combination of factors related to the material crushed, crusher design and operating parameters, and maintenance practices.Understanding these factors is critical to optimizing crusher performance and overall plant efficiency.

Jaw Crusher Capacity Influencing Factors

Jaw Crusher

I. Material Characteristics:

Hardness and Abrasiveness:

Hardness: Harder materials require more energy to crush and can significantly reduce the crushing speed, thus lowering capacity.

Abrasiveness: Highly abrasive materials cause faster wear on jaw plates and other crushing parts. Increased wear leads to reduced efficiency and necessitates more frequent replacements, resulting in downtime and lower overall capacity.

Humidity/Moisture Content:

Materials with high moisture content (especially “inner moisture” absorbed by the rock, not just surface water) can become sticky or “clay-like,” leading to clogging and bridging in the crushing chamber. This impedes material flow and reduces throughput.

Composition and Particle Size Distribution of Feed:

Fines Content: Excessive fines (powder) in the raw material can hinder crushing by filling voids in the chamber, reducing the efficiency of the crushing action, and potentially causing adhesion issues. Screening out fines before crushing can improve capacity.

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