Anyone who has worked in the field of electromechanical repair has likely come across one or more pieces of German engineered equipment. It doesn’t take long to figure out the Germans have a unique way of engineering their products and for the most part, are not repair friendly. Recently our shop repaired a Knodler CMG gearbox that was designed and built in Germany. The design of the CMG gearbox is to solve a problem of needing a large amount of torque in a small package, and this gear- box does that well. CMG is an acronym for “compact motor gear” and Its construction is complicated but works great at what it is designed to do. This particular gearbox is used in a plastic extrusion facility and pro- vides the torque needed to get the job done while having a minimal footprint.
We received two of these gearboxes, both not working, with the goal of making one good one using parts from both units. These CMG gearboxes are unique as they are a planetary like and are powered by 3 internal AC motors. All the gears and 3 motors are constructed into one unit. Both units were dismantled and each component was tested to determine what parts were reusable and which were not. Fortunately, we had enough reusable parts and with the help of our vendors, we felt we were able to quote rebuilding one unit in a reasonable amount of time. After the job was released for repair, we ordered bearings, parts cleaning and ordering special gaskets to seal the water jackets in the motor/ gearbox. Assembly was slow and steady and was going well until we hit a snag with the pressure test. This unit has separate passages for oil and water. The oil lubricates the gears and bearings while the water runs through adjacent passages to keep the oil cool. The oil channels passed the pressure test without any issues, but the water passage leaked during the first attempt.
We tried several sealants on the special gaskets we ordered and nothing seemed to adhere to the gasket material so we con- tacted the vendor for their suggestions. The vendor told us that the material the gaskets were made from was Buna -N and they had a special glue for this material. The special glue provided by our gasket vendor did the trick! We had to apply the glue to one side of the gasket then let it dry before applying glue to the other side. Once both sides of the gaskets had dried glue on them, we were able to seal the water passages in the gearbox airtight. This was a moment of relief as it took several tries with several products to get the gearbox to seal properly. Once we had the gearbox sealed we were ready to test all 3 of the motors in the gearbox. We tested each motor individually and all ran well and smooth. We felt a major sense of accomplishment once we had the final test completed on this gearbox.
This gearbox was one of the more challenging gearboxes we have repaired here at the Springfield Division. We are fortunate to have a great crew of mechanics and winders that work together as a team to accomplish any task presented to them. Sometimes, we have to get out of our comfort zone to expand our capabilities.