BRECO Timing belts in continuous operation 24/7
BRECO® timing belt drives, with a huge centre-to-centre distance of 27 m, are the connecting and central drive element in a modular assembly line for dialysis sets. Positioning accuracy with high loads in start-stop operation sets the highest requirements.
BRECO Timing belts in medical manufacturing
Dialysis is used as a life-saving measure if the kidneys fail. This is a process that removes toxins from the blood, supplies important substances such as sodium and calcium to the body, and balances the potassium levels. The dialysis set, including tubing, clamps, cannulas, connectors and bags, forms a life-saving connection between the patient and the dialysis machine. Only with automated production and very short cycle times in continuous operation 24/7 is it possible to produce a large quantity of these dialysis sets and meet the extremely stringent requirements regarding cleanliness and leak-tightness.
In particular, the handling of the flexible tubing requires a lot of experience. The special machine manufacturer Knoll Feinmechanik from Umkirch has been specialising in the automation of assembly lines for flexible parts since 1982. They have mastered all the necessary handling steps such as winding, unwinding, cutting, application of packaging bands, positioning of components on tubing, leak-tight bonding using their own dosing system, ultrasonic welding and printing. “Very few companies attempt this task, particularly not with the speed, precision and repeatability with which we can do it,” says Thomas Karotsch, project manager at Knoll. The engineer explains why 100% quality control is so important: “If a dialysis set develops a leak or contains foreign particles, this can cause serious damage to the health of the patient. We must ensure that the customer and the patient can have a very high degree of confidence in this product.”
Automation with modular linear indexing systems
The engineers in Umkirch developed linear indexing systems with very short cycle times for this complex automation task. The system shown is nearly 30 m long, about 15 m wide and divided into individually functioning modules. This makes commissioning considerably faster and facilitates de-installation, transport and re-installation of the system in the customer’s factory. It is also possible for the customer to completely exchange individual modules, for example, for a change of product.
More than 60 product carriers are used in one cycle in the linear indexing system. The product carriers – large anodised aluminium plates with a weight of nearly 30 kg – are cycled from one station to the next with a cycle length of 1 m. The timing belt drives on the longitudinal sides with a huge centre-to-centre distance of 27 m form the connecting and central drive element of the modular system. They transport the product carriers from station to station for assembly, welding, gluing, filling and inspection.
The torque of the large geared motor is transmitted by two parallel timing belts from pulleys measuring almost 500 mm to two welded-on belt cams. These are bolted to a stainless steel bracket and an aluminium cross support and are not visible when fitted.
Quality feature: welded-on timing belt cams
The manufacturer of the heavy duty timing belts is BRECO Antriebstechnik from Porta Westfalica. The customer-specific cams are equipped in the factory with precisely drilled holes for the screw connection between the stainless steel brackets and cross supports. Each product carrier is supported “floating” between two cross supports. At the assembly stations, two bottom vertically moveable centring pins align the product carriers to a tenth of a millimetre. The precise and repeatable precision of the position is necessary to allow the robot assembly of the flexible hoses onto the ports.
Due to the high and very frequent acceleration and braking in conjunction with the high weight of the product carriers, the welded joints of the cams are subject to high loads. Engineer René Preßler from Mulco’s sales partner, Hilger u. Kern in Mannheim, who is overseeing this project for the automation specialist from Knoll, adds: “The cam is welded over only one tooth so as not to change the flexibility of the timing belt. This means that there is a limited area for welding. However the load on this cam is still within the permissible range of 190 N/cm line load. We could have chosen a screw connection as an alternative. However, we had to take into consideration that a screw connection could come loose during continuous operation 24/7. As long as the permissible load is not exceeded, the welded variant is a very durable and reliable solution.”
Maximum pitch accuracy from one batch
When two timing belt drives are connected in parallel, their pitch tolerance must be as close as possible. Otherwise the system could become jammed or deformed. Therefore, Breco supplies both polyurethane timing belts of the type BRECO® AT20 for this application from one manufacturing batch. René Preßler continues: “The entire drive and, in particular, the timing belt has to keep the position precisely at all stations along the 27 m so that all product carriers stop at the same time. Therefore the timing belt has to be very low stretch. After all, nearly 1.5 tonnes have to be accelerated and braked in less than one second.”
For transport systems that carry light loads the timing belt normally runs on a sliding rail that deflects the weight forces of the goods to be transported via the belt to the frame. This is a tried and tested solution.
Due to the high loads in this application, Knoll chose an elegant alternative: The cross supports are bolted onto stainless steel plates so that they run on sliding rails.
Ingenious load bearing solution
In this way, Knoll was able to achieve a better tribological pairing, reducing friction and wear, which is particularly important in the clean room. If the permissible number of particles in the air is exceeded, the entire production batch has to be disposed of for safety reasons due to the risk of contamination.
Regarding the reliability of the timing belts, design engineer, Thomas Reiser, adds: “One of our customers has been operating a machine line 24/7 since 1998 and not a single timing belt has yet had to be replaced.” This proves that the belt is correctly designed and the quality is how it should be.
Welding or timing belt lock
For this application the tensile forces in the timing belt are so high that a timing belt lock had to be chosen in preference to welding. Knoll Feinmechanik developed their own timing belt lock years ago and have constantly optimised it. This very sturdy and stiff lock cannot be used for a different application. During fitting it is important that the first and last tooth are precisely within the pitch tolerance of the tensioned timing belt. René Preßler explains: “The tensile forces in this machine line are so high that they can only be transmitted by an endless manufactured timing belt. Therefore the lock is an important component in this application; without it the drive would not function properly.”
You can specify and purchase BRECO belts from TransDev. More information can be viewed here.