Composites in machine structures o to increase machine ...

Composites in machine structures - How to increase machine performance, operating speeds and accuracyA CompoTech White Paper

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IntroductionIn automation and machine tool applications, carbon fibre reinforced composite materials can offer significant productivity, quality and lifecycle cost benefits. Yet the adoption of composites in the manufacturing sector has been relatively slow. In part, this can be attributed to design engineers that are unfamiliar with the properties of composites underestimating its benefits, while overestimating the cost and complexity of its application; and in part, there has been a natural reluctance to change from what is perceived as established methods of manufacture to a new way of machine design and construction.

The reality, however, is that composite parts can play a vital role in revolutionising machine performance, increasing operating speeds, accuracy and repeatability. This is largely due to a modern, highly automated manufacturing process – axial fibre placement – that offers particular advantages in the production of composite beams and tubes used in machine tool and automation systems. These can outperform their traditional metal counterparts across multiple dimensions, while remaining extremely cost-competitive.

Carbon fibre reinforced composites characteristics

The characteristics of carbon fibre reinforced composite materials are well known:

• High specific strength and stiffness allow the creation of parts that offer excellent structural properties and low mass

• Composite structures can be manufactured in complex shapes

• Alterations in material composition and lay-up allow the physical properties of the end product to be tuned to match the needs of the application.

These characteristics have led to the widespread adoption of carbon reinforced materials in applications including aerospace and defence, motorsport and high-end automotive products, and sporting goods. Increasingly, however, users in the industrial sector are also turning to carbon fibre materials as a route to improved machine performance.

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Machine tool performance requirementsManufacturers of industrial machine tools and automation systems face pressure from end users to continually improve capacity, quality, and throughput. Those requirements translate into a number of potential competing performance requirements:

• Larger machines, with moving elements that may be multiple meters in length

• Motion systems capable of high speeds and rapid levels of acceleration and deceleration

• A high degree of stability, accuracy and repeatability.

In addition to the challenges inherent in the provision of motion and control systems on these large, high performance machines, designers must also select suitable structural elements. These parts must be low in mass, while still offering sufficient strength and stiffness to support both static and dynamic loads with minimal deflection.

Whichever material a designer selects, the physical requirements of large, high performance machines call for hollow section beams with large cross sections and thin walls.

Structures of this type provide the required stiffness and low weight, but they can have other characteristics that adversely affect machine performance:

• Vibration. If the natural frequency of the member itself, or of its walls, is too close to the operating frequencies of the machine, the resulting resonance can significantly impair performance, leading to instability or inaccuracy in use

• Thermal stability. A machine may have to operate across a wide temperature range, due to changes in ambient conditions, heat from the workpiece or heat generated during cutting, machining or welding operations. Over a large span, the resulting thermal expansion can lead to significant distortion

• Machine integration. Regardless of their inherent characteristics, members must integrate effectively into the overall structure of the machine. That means they must accommodate appropriate fixing points for ancillary equipment, brackets for axles or bearings, or tracks for other moving components. Each of these elements must provide sufficient strength and dimensional stability for reliable operation.

Carbon fibre reinforced composites for machine applicationsAppropriately designed and manufactured carbon fibre reinforced composite components can address many of the major challenges facing designers of modern high performance machine tools. Carbon composite machine elements can be as little as a quarter of the mass of steel for the same strength. A unidirectional carbon fibre reinforced laminate can deliver twice the stiffness of steel in the direction of the fibres. In addition, they can be engineered to deliver vibration damping properties twenty times better than steel, or for zero thermal expansion in one dimension.

As few real-world applications are focussed on a single physical attribute, however, practical carbon composite machine members are precisely engineered to offer a suitable compromise between characteristics. This requires in-depth understanding of end-user requirements and of the strengths and limitations of different composite manufacturing technologies.

With a track-record of over 20 years designing and manufacturing structural composite tubes and beams, CompoTech has unparalleled experience in the development and production of carbon composite

parts for industrial applications. The company has also developed and refined proprietary manufacturing processes that enable it to offer both higher performance and more cost-competitive solutions.

Composite Fibre Type Glass Aramide Carbon HS Graphite Steel Aluminium

Longitudinal Modulus EL 45,000 Mpa 85,000 Mpa 134,000 Mpa370,000 Mpa

410,000 Mpa210,000 Mpa 70,000 Mpa

Transverse Modulus ET 12,000 Mpa 5,600 Mpa 7,000 Mpa 6,000 Mpa 210,000 Mpa 70,000 Mpa

Shear Modulus GLT 4,500 Mpa 2,100 Mpa 4,200 Mpa 3,800 Mpa 80,000 Mpa 26,000 Mpa

Ult. Tension Strength UCL 1,250 Mpa 1,410 Mpa 1,700 Mpa 1,200 Mpa400 Mpa

1,200 Mpa

130 Mpa

460 Mpa

Ult. Compression Strength UCL 600 Mpa 280 Mpa 1,250 Mpa 560 Mpa400 Mpa

1,200 Mpa

130 Mpa

460 Mpa

Density 2,500 kg/m3 1,500 kg/m3 1,600 kg/m3 1,900 kg/m3 7,800 kg/m3 2,400 kg/m3

Uni-directional Composite vs Steel and Aluminium Properties

Matrix E Mod.=3Gpa | Composite E Mod. =60% Fibre + 40% Matrix =60% x 640 Gpa + 40% x 3 Gp =385.2 Gpa

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Axial fibre placement technologyCarbon composite tubes and beams are commonly manufactured using a filament winding process. In this technique, fibres are coated in resin then wrapped under tension around a shaped mandrel. Once the resin is cured, the mandrel is removed to leave a hollow composite part.

A key limitation of conventional filament winding is the requirement for all fibres to be placed at an angle to the longitudinal axis of the tube. Since carbon fibres only provide strength and stiffness in their direction of the fibre, this means that conventional filament wound structures sacrifice some performance in bending.

For machine members and other applications where bending loads are critical, CompoTech uses a process known axial fibre placement, which allows fibres to be aligned along the length of the tube or beam. CompoTech has been developing axial fibre placement technology since 1994. The company has designed and built its own bespoke production machinery, along with the related processing and control software.

In the axial fibre placement process, an array of radial pins is fitted to each end of the mandrel. Fibres are wrapped around these pins, laid along the length the mandrel, then wrapped around the corresponding pin at the other end. Repeating this process around each pin allows a complete layer of longitudinal fibres to be laid over the surface of the part.

As well as providing ideal fibre alignment for bending loads, axial fibre placement allows the creation of composite parts with a very high percentage of fibres within their structure. And because the fibres are perfectly straight, they are optimally loaded, further improving the mechanical characteristics of the finished part. Compared to conventional filament winding techniques, axial fibre placement can produce beams and tubes that are 10 to 15% stiffer in the direction of the fibre and offer 50% greater strength in bending.

As most machine components experience a combination of loads, additional layers of fibres can be wrapped around the longitudinal fibres to produce a part with an ideal combination of characteristics.

After the fibres are laid, parts are transferred to a press, which applies controlled heat and pressure to the structure during curing. This step precisely controls the surface finish and external dimensions of the finished beam.

Fibre selectionIn many machine applications using carbon fibre materials, stiffness rather than ultimate strength is the critical design parameter. To maximise stiffness, CompoTech uses Graphite Carbon fibres that are made from pitch, rather than the more common polyacrylonitrile (PAN) fibres. Differences in their microstructures mean pitch-derived Graphite Carbon fibres offer a higher Youngs Modulus than their PAN counterparts, making them ideal for machine applications.

CompoTech creates solutions that are very structurally and technologically different from the current or most common solutions made by the classic method of metallic materials. This is where the technical benefits of composites lie. Improvements in properties and parameters can be as high as tens of percent and the cost recovery can be very fast. Companies embracing such solutions make them innovators and enhance their reputations as leaders in their field.

Progress in composite materials and technology is set to grow very fast. I greatly admire the innovation, creativity and technical leadership of CompoTech. With each new product that replaces a traditional metallic solution, a progressive idea emerges that leads to excellent performance in operational applications. Professor Milan Ruzicka, FEng.Head of the Department Mechanics, Biomechanics and Mechatronics, Faculty of Mechanical Engineering, Czech Technical University, Prague. Professor Ruzicka also specialises in the field of research and application of fibre polymer composites.

Axial fibre placement is a highly automated process, suitable for the production of composite parts in volumes from single prototypes up to many thousands.

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Return on investmentAs a raw material, carbon fibre is significantly more expensive than steel or aluminium. That fact alone is enough to stop some machine designers from considering composite parts. It is important, however, to take a holistic approach when comparing the cost of the composite parts. The ability of modern composite manufacturing methods to produce accurate components with little requirement for post processing operations can significantly narrow the cost gap by the time a part is prepared for installation on a machine. In some cases, composites can actually be a lower cost option than their metal counterparts.

Even when, as is more common, the cost of the composite beam is slightly greater than the alternative, it can often generate significant savings elsewhere. Reducing the weight of major moving components can allow designers to select smaller bearings, motors and other motion components. Once a machine goes into service, the reduction in energy costs provided by weight reduction can pay back the extra cost well within the lifetime of a machine.

In most applications, however, the largest benefits of carbon fibre reinforced composite machine parts arise from improvements in speed, throughput and quality. High performance machines make money for their users, and the annual productivity payback achieved through the use of composite parts can be tens or hundreds of times greater than any one-off additional cost.

Press Line Transfer BeamsThe development of composite transfer beams was partnered in a project with Schuler. The concept evolved from a basic square section beam to a 3Dc hybrid composite version of a steel T-slot beam. The result was a beam that weighed ¼ of the original section with a modulus of 400GPa.

The reduction in weight and the increase in stiffness meant the natural frequencyincreased while the dynamic response decreased. The limiting factor on the press line speed is amplitude of beam displacement. The composite beam allowed machine strokes/min to go up from 20 to 32, giving an average output increase of 40%.

The development of the t-slot built into the beam meant standard fastenings could beplaced anywhere along the beam and can be transferable. This has added the functional element that steel and aluminium profiles have. Having developed the technology for t-slot profile means that other profiles are easily achievable, widening the scope of use.

Improving vibration stabilityThe low mass and high stiffness of Graphite Carbon composite parts already provides good vibration and vibration-damping characteristics compared with steel or aluminium alternatives.

Vibration is so important in many machine applications, however, that it requires special attention during design. Vibration behaviour can be optimised in a number of ways. The dimensions and wall thickness of the part can be adjusted to tune its natural frequencies, for example. Damping materials, including rubber and cork fillers, can be incorporated into the structure. Internal foam reinforcements may be included to improve the vibration stability of the walls of large-section parts.

IntegrationTo facilitate the integration of the composite part into the machine structure, appropriate fixing points can be built into the structure of the part during manufacture. These might include additional layers of machinable material to allow for the drilling of holes, included metal inserts, or pads to support tracks and brackets.

Application benefitsThe benefits of composite structures can be demonstrated by a recent project. Fibre laser machine manufacturer, Eagle, was seeking a solution that reduced weight and deflection in a 3.1m long Y-axis transverse beam used in a laser cutter. It replaced the existing steel part with a thin-walled composite design reinforced with foam cores. The change reduced the weight of the beam by 44%, while increasing its stiffness.

Eagle was able to take advantage of those improvements to double the peak acceleration of the beam from 3g to 6g. That in turn, reduced the time required to cut a sheet of material in the machine by up to 30%. Furthermore, the extra stiffness and improved damping characteristics of the part resulted in accuracy improvements of up to 50%.

Reducing the weight of major moving components can allow designers to select smaller bearings, motors and other motion components.

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