Beogram 4002 (5513): Rebuilding DC Motor with Oil Infused Oilite Bearings

After replacing the RPM relay and the RPM trimmers of the Beogram 4002 (5513) with modern components, it was time to give the DC motor some TLC. The DC motor can be the most tricky part of a 4002 restoration and it is virtually almost always necessary to do it. The main difficulty with the motor restoration are the Oilite oil infused brass sleeve bearings that are used in this motor. They come with 'oil built in' when the motor is new. Within the typical consumer engineering time horizon of 10-15 years, such bearings can be regarded as 'maintenance-free'. However, on the 'Beolove Time Scale' (BTS) this is of course not a long enough period, and so the oil needs to be replenished. This can be done by removing the bearings and immersing them in oil under vacuum. The vacuum serves to expand the air that over time replaced the oil in the bearings. This draws it out to be replaced with fresh oil.

The first step is to disassemble the motor. This shows the motor opened up:

Once the motor is open, the bearings need to be extracted, and this means to pretty much completely dismantle the motor:

Once the bearings are out, it is time to do the vacuum infusion process. This shows the bearings in a mason jar under SAE30 oil immediately after pulling a vacuum with a FoodSaver pump:

Air bubbles emerge from the bearings. The larger top bearing (left) usually emits more than the bottom bearing (right). I think this has to do with the fact that the top bearing sees more torque over its lifetime due to the belt bearing on it directly. And that probably causes more oil to be extracted from the bearing due to the closer contact.

After 12-24 hrs or so the process more or less stops

And that is the sign that the oil has been replenished. Now the bearings can be reinserted:

The top bearing is held in place by a lock ring that presses the bearing mount up against the motor housing. This ring needs to be pressed firmly against the motor housing in order to re-bend the tabs that clamp it in place. I made a 3D printed mounting tool that allows the use of adjustable pliers to apply even pressure across the ring and in the same time bend the tabs back into place one by one. 

This shows the top bearing re-inserted with the tabs bent back:

The bearing needs to be snugly in place. Test it with a tooth pick. If it can be moved, but there is some noticeable resistance, then it is correctly installed.

The next step is to put the motor back together in a way that it can turn freely and runs in the right direction. The direction depends on the orientation of the bottom plate. The screws that hold the bottom plate to the main enclosure allow three orientations. Only one is right (hence, it is a good idea to make a mark before taking the motor apart...;-). Once the motor turns in the right direction one needs to make sure that it spins at the lowest friction possible. I came to the conclusion that measuring the current while testing it is a good way to assert that everything is o.k. The lowest current achievable is the best alignment of the motor housing relative to the rotor. I usually can achieve ~20-30 mA at 5V when the motor spins freely:

If the current is higher, then the bottom plate needs to be adjusted by playing with the three mounting screws. I usually torque them in small steps in sequence while going around a few times, as you would tighten a vacuum flange.

Once the screws are tightened, the second enclosure can be put back on and then it is time to test the performance of the motor with my BeoloverRPM device for 24 hrs to make sure that everything is really Beolovely! 

Here is the test result for this motor:

The black curve was measured before the motor rebuild. Not too bad as far as original condition DC motors go these days. The casual listener would probably even miss the RPM drop spikes. Usually, these are much more pronounced when people complain about varying RPM in Beogram 4002s. But of course the presence of these spikes means that the bearings are close to the end of their lifetime and that bigger drops would have surely started to occur in the near future.

The red curve was measured after the bearings procedure and also reflects the new RPM relay and trimmers. No more spikes. Of course there is still some variation over time, but those mainly have to do with temperature drift...the price of having a 1970s analog control system.