MINIATURE COUPLINGS IN MEDICINE
Fully automated production and manufacturing cells which in 24-hour-a-day operation do their work at a constantly high level, are found not only in the packaging industry. Highly dynamic gripper systems are a building block of automation solutions. High-speed articulated robots are compact in design and feature great freedom of movement for the described tasks. A wide range of industries and applications will benefit from these robots by virtue of the programming and control of up to 6 axes, speeds of over 7.2 m/s and a repeatability of up to + /- 0.02 mm.
Short throughput times and consistently high output - robot grippers contribute to secure and efficient processes in many areas of production. Increased automation is an actively sponsored and central part of the "Industry 4.0" project for the future initiated by the German government.
Packaging systems as an example: Faster cycle times and more complex forms of product packaging place increasing demands on the packaging equipment manufacturer. The machines must offer maximum flexibility to pack a wide range of products and at the same time provide high productivity. In addition to the cycle times of the machines, provision and feeding of the required packaging material constitute an important interface. The material flow then goes on: The packaged goods are ready for removal and palletizing. Practice shows that modern industrial robots fulfil these requirements at fast cycle times
The different taks of the handling systems require increasingly specialised solutions for the gripper systems, so that the goods reach precisely the relevant programmed transfer point – with handling either by vacuum suction cup, positive engagement or with a simple clamping mechanism. Naturally the speed with which the product is moved plays a decisive role for the productivity of the entire plant. Therefore such automation systems are designed primarily with a view to speed and acceleration. The drive components installed must have very good repeatability. The requirements on the couplings used are correspondingly high. The precision couplings from the R+W company are designed to meet these special requirements and transmit the torque reliably.
Vibration-damping properties of elastomer couplings
The company offers the elastomer couplings of the EK series (Fig. 1) for up to 25,000 Nm in many applications, e.g. also for industrial robots. The elastomer materials, elastically deformable plastics, allow high flexibility with good strength. They dampen rotational movements especially in areas in which vibrations or impact loads occur. In addition, industry uses these as compensating element for insufficiently aligned shafts.
The elastomer coupling is similar in construction to a claw coupling, whereby a plastic segment is located between the two hub halves, which are available in different hub designs and mounting possibilities. This elastomer insert is usually made of polyurethane and, because of its spring-elastic properties, it is able to compensate for axial misalignment, vibration and impacts. For optimum running, the insert must be fitted closely into highly accurately manufactured hubs. Polyurethanes have become the material of choice for elastomer inserts. The polyurethane types based mainly on polyester have very good mechanical properties and are largely resistant to many influences. Their good vibration damping behaviour has a positive effect on the life of the connected units in operation in plants with frequent impact, alternating or vibrational loads.
Calculation and design
Just like metal bellows couplings, elastomer couplings are designed according to DIN 740. This standard includes safety factors, which are of essential importance for the design of the coupling. As a rule, the behaviour of elastomer couplings used in the drive line cannot be described by linear transfer functions, since elastomer materials have properties depending on operating parameters such as load, starting and temperature factor The temperature factor describes the behaviour of the elastomer insert in different temperature zones. Because of their construction, the elastomer inserts are very temperature-dependent and thus act directly on the torque transmission in the drive line. The temperature factor must therefore be selected very carefully. The starting factor provides information about the frequency of the starting processes within one hour. As the coupling is additionally loaded by the moment of inertia of the input side on starting, the coupling must be designed correspondingly larger to avoid impairment of its service life. R+W takes account of surges, impacts and direction changes acting on the coupling by considering the load factor, also name the impact factor. Here the company differentiates between three groups of impacts or loads similar to impacts: regular harmonious loads, irregular, periodic loads and high dynamic loads with frequent changes of direction of rotation. These three parameters can clearly reduce the life of a coupling, so that R+W also takes account of these in the design of the elastomer coupling, especially of the coupling inserts.
To obtain the correct coupling for every application, designers can select from among elastomer inserts with different Shore hardnesses (Fig. 2). In most cases a good compromise can be found between damping and required torsional stiffness as well as compensation for misalignment and torque transmission. The elastomer insert with Shore hardness of 80 Sh A (yellow) has excellent damping characteristics and is used mainly in applications characterised by severe vibrations or impacts. This can also be used in environments with temperature ranges from -30°C to +100°C. Elastomer inserts with Shore hardness 98 Sh A (magenta) cover the same temperature range, but they have lower damping properties than those with hardness 80 Sh A. The third group is elastomer inserts with a Shore A hardness of 64 Sh D (green), high torsional stiffness and low damping capacity. These can be used in a temperature range between -30°C and +120°C. Couplings for ATEX applications with a special electrically conductive version in 65 Sh D (black) can also be offered. If extreme requirements in terms of strength and temperature range exist, R+W can offer elastomer inserts manufactured from Hytrel material (beige). These are designed in 64 Sh D and can be used in a range from -50°C to +150°C.
Metal bellows couplings: If positioning accuracy is required
If positioning accuracy is required, absolutely backlash-free bellow couplings with torsionally stiff metal bellows can be used. Here as well R+W offers with its series MK, BK (Fig. 3) and BX precision couplings up to 100,000 Nm for the most diverse applications. Just like elastomer couplings, bellows couplings must also be designed and dimensioned for the different areas of operation and therefore the type of load. At the heart of every bellows coupling is the metal bellows made from thin-walled stainless steel (Fig. 4). The design of such a coupling, for example according to the torque to be transmitted or in respect of compensation for misalignment, depends on the requirements in practice. Similarly to elastomer couplings, parameters such as acceleration torque, resonant frequency or torsional deflection also play a decisive role. These are taken into account by the engineers at R+W when selecting or designing a coupling, just as much as requirements on mass moment of inertia, installation friendliness or cost effectiveness.
Standard and special solutions from R+W
The main area of application of zero backlash precision coupling is industries in general machinery, plant and equipment construction. Apart from automation systems, the high-precision drive elements are also used, for example, in machine tools, packaging machinery or printing presses. Depending on the application, R+W offers standard products or custom-developed special solutions.
Author: Dirk Hasenstab