The field of products in which an increase in speed and stiffness or energy savings can be achieved through a reduction in mass is very large. This area is not only limited to applications where lightweight construction has traditionally played an important role, such as in aerospace, but also includes production machinery such as industrial robots, machine tools, and hand-guided machines.
This is why the safety couplings from R+W follow the trend in general machine and plant construction to produce ever-higher quality with the same level of precision at ever lower costs. The selection of new high-tech materials in combination with innovative coatings creates lower moments of inertia; this results in an enormous reduction in weight with a simultaneous increase in disengaging torques.
New materials for safety couplings
Thanks to the proven spring-loaded ball-locking principle, the compact coupling series operates without any backlash. These innovations and developments were completed within only two years in conjunction with universities. Realizing that the goal of a 50% weight reduction could only be achieved with a completely new development, R+W did not limit itself to additional developments or optimizations, but rather designed, constructed and manufactured a completely new series.
The SL coupling series is manufactured from high-tech materials combined with unique coatings. This enables a weight reduction of up to 60% compared to the standard series. A coupling for a torque limitation of up to 160 Nm has a deadweight of just 370 grams and a mass moment of inertia of 0.8 * 10-3 kgm2. The coupling series is made up of four sizes. Torques ranging from 10 Nm up to 700 Nm can be safely limited. In addition to the choice of material, weight reduction was also achieved by compressing the individual components, which of course has no impact on durability or accuracy. The lightweight SL series safety couplings can achieve cycle numbers of up to 10,000 or more disengagements in use. The use of disc springs, which were newly developed especially for R+W, in combination with a further development of the ball-locking principle creates a torque increase of up to 40%.
Use in aviation – Scissor lift
Lighter, more compact, more efficient: These trends have revolutionized the aerospace industry in recent years. Intelligent lightweight construction with high-tech materials and the latest production technologies is the guiding principle for new designs and further developments. With this in mind, R + W worked on new high-tech components for the aviation industry. With its compact and powerful safety couplings, the company now offers high precision for the smallest installation spaces.
A leading manufacturer of aircraft and aviation equipment required a safety coupling for an intelligent lifting device in mobile cabins for transport and service work. The goal was to reduce mass moments of inertia in a compact design. Depending on the aircraft type, the design had to integrate a disengagement torque of between 40 and 135 Nm with a maximum overall length of 30 mm and a maximum overall weight of 200 g into a scissor lift in the smallest possible space. The company opted for the lightweight safety coupling here. The SLP Series 30 safety coupling has been further adapted to customer requirements: with overload protection in a sealed design, protection against liquids and dirt, and the possibility to use it in any weather.
The use of high-tech materials enables a weight reduction of up to 60%. For example, the SLP 30 has a torque limitation of up to 135 Nm, a deadweight of just 200 g, and a mass moment of inertia of 0.1 * 10-3 kgm². Thanks to the special gearing of the base body and mounting flange, the safety coupling reacts within milliseconds with a repeat accuracy of +/- 5%.
Use in electric vehicles
Situations that require emergency braking may also occur when driving an electric car. However, the braking energy that is released when doing so transmits high torque to the transmission – a higher torque than it can tolerate. Its protection is therefore ensured with a safety coupling.
Mechanically switched safety couplings protect systems and machines in the event of a collision or incorrect operation. However, these couplings can do more: with some adjustments, they can also be used in electric vehicles. The engineers here at R+W developed a mechanical safety coupling for a pilot project. The coupling is located between the two drive shafts on the transmission and the brakes. During emergency braking, the resulting force is differently distributed to the brakes. Since the braking energy applies a higher torque than the transmission allows, the transmission must be protected from the brakes’ reaction torque.
The customer’s brief was to develop a mechanical safety coupling with a maximum weight of 1.5 kilograms that could handle an overload of more than 600 Nm at 1,100 RPM. This could be fulfilled with the TÜV-certified SLN series 300 model range. It separates the input and output sides within milliseconds. When doing so, the safety coupling works in a way that avoids any backlash. The SL series is manufactured from alloys – new in coupling technology – in combination with coatings. A coupling that provides a torque limitation of 600 Nm, as could be used in an electric vehicle, has a deadweight of just 1.5 kilograms and a mass moment of inertia of 3•10 kgm².
In addition to the choice of materials, the developers achieved weight reduction by compressing the individual components – without any negative effect on the service life or accuracy. These lightweight safety couplings can achieve cycle numbers of up to 10,000 or more disengagements in use.
The use of disc springs, which were specially developed for R+W, in combination with an optimization of the ball-locking mechanism creates a torque increase of up to 50%. The disc springs with special characteristic curves enable precise torque limitation and short switching times. Only after exceeding the set disengagement torque does the coupling start to move. The balls leave their conical countersinks, and the torque flow is interrupted. The spring force decreases towards zero. The balls cannot move about in their countersinks within the limit range of the set disengagement torque. Wear to the coupling during operation below the disengaging torque is thus avoided. The reduced spring force during switching and the lightweight switch ring help to reduce the force and keep wear caused during switching to a minimum.
Safety coupling for the International Space Station (ISS)
The International Space Station (ISS) is one of the biggest technology projects of modern times. Since the year 2000, two or three astronauts have been conducting research on key topics such as life sciences and materials science there. German scientists are specifically studying the human equilibrium system, basic physics (plasma research), and questions around radiation biology.
It is a well-known fact that the astronauts lose muscle and bone mass during their stay – which can last up to six months — as the human body changes under the effects of weightlessness. Aerospace medicine specialists also know that a longer stay in space weakens the immune system, changes the blood sugar system, and causes bone and muscle loss due to the reduced level of use.
The ESA and OHB System have developed a special training device to prevent muscle loss. Similar to a rowing bench, astronauts can use it to train specific muscle groups. A specially weight-optimized and sealed version of the R+W SK1 safety coupling made of stainless steel provides a safety function in this training device. As an overload unit with an angular synchronous design, the sensitive load unit is protected if the astronaut applies too much force. The re-engagement after 360° allows the exact position of the rowing bench to be reinstated. After a flight lasting 15 days, the training device, which requires no maintenance, was permanently integrated into the space station and has since been used by each crew to build muscle.
The design of the special safety coupling deviates from the standard design in many areas of the coupling. The coupling manufacturer fulfilled the requirement for a primarily “light” and specially sealed safety component by reducing the weight of the existing safety coupling series and drawing upon many years of experience in the field of vacuum applications. The selection of special materials and the compression of the used components — of course, without any impact on service life or accuracy – were decisive factors for weight reduction.
Overload components based on the ball-locking principle require a lubricant. In continuous operation, this prevents the components from corroding or, in the event of overload (disengagement of the coupling), excessive stress being placed on the components. The coupling heats up during repeated disengagement processes due to the disc springs’ residual friction, which makes the greaseless viscous. Excess grease or lubricant leaks out. This is precisely the behavior that had to be avoided with the safety coupling in space. Special seals on all of the coupling’s parts and components including an extended switch ring hermetically seal the grease in the coupling. This prevents any possible leakage of the superfluous grease in the event of repeated disengagement.
Furthermore, the ESA requires that all used materials are approved for use in the space industry. The safety coupling’s components had to be tested beforehand, especially in the area of the heavily stressed mechanical parts. After the successful TÜV testing of R+W safety couplings for standard applications, an internal test was carried out according to similar criteria. The space industry’s high requirements were fulfilled with a final documentation/listing of all components. And with that, R+W not only develops, designs, and manufactures safety components “for” the space industry but also for “real” use in space. This shows the manufacturer’s expertise and professionalism when it comes to out-of-the-ordinary requirements.
Conclusion: The combination of the changes in the ball-locking principle and the use of new materials enabled R+W to develop the backlash-free TORQLIGHT SL series of safety couplings. This gave the market a lighter, more compact safety coupling with a higher power density.