High-precision over 370 million cycles: Axis/plain bearing system in the artificial heart

Profile

• What was needed: Special plain bearing and shaft for the drive unit that deflects the diaphragm in the pump chambers
• Requirements: Maintenance-free, compact, low clearance, resistant to chemicals, moisture and repeated torsional and alternating axial loads, service life over 370 million cycles, lightweight
• Industry: Medical technology, medical devices
• Success for the customer: With an iglidur J special plain bearing and matching drylin aluminium shaft, ReinHeart TAH GmbH was able to develop a precise, maintenance-free and durable drive system that works reliably despite high loads and minimal installation space and maximises the efficiency of the artificial heart.

The artificial heart consists of a drive unit and two pump chambers that work as positive displacement pumps. The pump chambers have a stiff housing and flexible membranes that are alternately deflected by a patented electromechanical linear drive. This movement causes the artificial heart valves to generate a directed, pulsatile blood flow.

Within the drive unit, an axis-plain bearing system is used to guide the coils of the actuator with high precision through the magnetic field of the stator. In order to maximise the performance efficiency of the drive, there is only a minimal air gap between the coils of the actuator and the magnets. Position sensors on the bearing precisely monitor the movement of the axis and only allow a few tenths of a millimetre of offset and a few degrees of rotation.

The technical challenges are numerous: the system must be maintenance-free and guarantee permanent precision under fluctuating loads. The plain bearing must withstand repeated torsional loads over a service life of five years (around 370 million cycles) as well as changing axial loads, depending on the patient's position and movements.

A compact installation size, lightweight feature as well as, chemical resistance and low moisture absorption are further decisive factors. The plain bearing is located inside the drive unit, where there is no direct contact with the body's own fluids. However, due to the permeability of the plastics, moisture must be expected inside the drive and therefore on the bearing in the long term. In addition, potentially dangerous metal abrasion residues must not be produced.

In the first attempt, a two-axis system was tested, but it was difficult to move due to manufacturing tolerances that were difficult to maintain and even led to the breakage of one axis. In addition, the space requirement of the solution was significantly higher due to two plain bearings.

Because no standard solution was available for the extensive requirement profile, the axis-plain bearing system was specially conceived and designed by ReinHeart and realised in collaboration with igus. The specially moulded bearing consists of the material iglidur J, which is ideally suited for this application due to its chemical resistance, low moisture absorption and freedom from lubrication. Since there is no risk of metal particle abrasion, safe operation is guaranteed. The axis is a drylin AWMP aluminium shaft, an ideal mating partner for iglidur J, which ensures good running behaviour and minimal wear over the entire service life.

The bearing is space-saving, lightweight and has recesses that allow the integration of position sensors for precise monitoring of the axis movement. The bearing and axis have an anti-rotation feature, are geometrically matched to each other and are closely toleranced in terms of shape and position. A core feature of the axis-bearing system is a planned "burn-in" phase: when the axis is first brought together, it is deliberately clamped in the bearing and can only be moved by applying a specific force. In the first phase of operation, the contour of the bearing harmonises with the axis to ensure optimum guidance.

The combination of axis, bearing and sensor system fulfils the requirements for minimal air gaps, precise guidance and robustness under changing transverse and torsional loads. This results in precise guidance that remains maintenance-free over a service life of five years.

After several years of development, the axis-bearing system is now in its second generation. For ReinHeart TAH GmbH, the result is very satisfactory and must now prove itself in endurance tests. Together with igus, test concepts were developed for the plain bearings, in which igus is also breaking new ground.