Custom polymer bearings replace non-lubricated sheaves

Established in 1873, Belimed Technik GmbH specialises in cleaning, disinfecting and sterilisation equipment and services for the pharmaceutical industry, healthcare establishments and laboratories. The company recently launched a new range of cleaning machines, the PH 8xx.2 type, partly in response to requests from pharmaceutical firms for machines with greater loading capacity within limited external dimensions.

In Belimed's latest design, the slide-out racks that hold the equipment to be cleaned originally rested on plain, non-lubricated bearing sheaves that rolled on rails, as in earlier models. However, some customers found that when the racks were heavily loaded, they became difficult - or impossible - to slide into and out of the cleaning chamber.

Lubrication of the sheaves was impractical because grease could leak out from the bearing into the super-clean environment of the washing chamber, unless the bearings were hermetically sealed, which would have required more space than was available.

Bearing requirements

Jurgen Althammer, who heads development at Belimed Technik, explains the situation: "We needed new sheaves that had exactly the same dimensions and geometry as the former unsatisfactory ones but with much lower rolling resistance. They had to retain this feature even at high temperatures, because our machines wash at 85degC and the drying process is at 120degC. The bearings had to be either hermetically sealed or easy to rinse out, and the whole part had to withstand exposure to aggressive chemicals including nitric and sulphuric acids and alkalis such as caustic soda."

Faced with this set of constraints, SKF developed a customised product. To reach the maximum load-carrying capacity, together with axial guidance in the very limited headroom available, SKF designed a cageless polymer track-runner ball bearing with two full-complement, asymmetric sets of stainless steel balls. The rings are made of polyetheretherketone (PEEK). As polymers have a low coefficient of friction, the bearings do not need lubrication, while, as there is no cage, the bearings are easy to rinse out. Finally, the device did not require any other changes in the equipment's design.

Jurgen Althammer comments: "SKF gave us a perfect, function-integrated solution that required absolutely no modifications to the existing machines, and they were remarkably fast in developing it. Our first meeting to discuss the problem was in the final days of 2005; within four months we were testing the first sheaves with SKF's polymer bearings, and a month later they were being fitted on our machines in pharmaceutical establishments. What is more, we have not had a single comment since then about hard-to-slide equipment racks."