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Monday, March 21, 2016

Beogram 4002 (5513): Infusing Oilite Brass Bearings of DC Motor with Oil Under Vacuum

One of the challenges with restoring DC motor Beogram 4002/4 Types (551x and 552x) is the restoration of the motor itself. Most of the DC motors present themselves with loud running noise and/or strong RPM variations at this point in time. It turns out that simply taking the motor apart and lubricating the brass sleeve bearings with grease is only a temporary solution.
The bearings used in these motors are 'oilite' brass bearings (see a recent thread on Beoworld where Dillen hinted at this). Such bearings are infused with oil after manufacturing. The brass is porous, and therefore oil can be 'inserted' into the metal itself via a vacuum based infusion process. This results in light duty bearings that can run without the need for external lubrication. However, over time the oil is slowly depleted from the bearing metal (and probably distributed inside the motor). Once the oil is gone and the bearing dry the noise issues and RPM variations start occurring.
The restoration of the bearings requires to infuse fresh oil into the metal. Unfortunately, one has to completely disassemble the motor to get to the bearings (one can force them out without disassembly, but it is impossible to get them back in...).

Here are a few impressions of the process. This shows the motor disassembled:
The two bearings are actually different:
The bottom bearing is shown right with its plastic support washer next to it. The top bearing is on the left. It is slightly bigger.
The infusion process can be done with SAE 30 motor oil and one needs to pull a vacuum in order to suck the air out of the pores of the brass volume to make room for the oil that it can diffuse in by means of Brown's motion. I did this with a FoodSaver vacuum sealer unit that came with an attachment for Ball mason jars. Here is an impression of the bearings in the oil under vacuum in the jar:
As soon as the vacuum is applied small air bubbles arise from the bearings. They accumulate on the surface of the oil as seen in the photo. It is interesting to note that the top bearing produced considerably more bubbles than the bottom one. This is probably a result of the higher friction seen by the top bearing due to the belt torque applied to it. After about six hours the bubbling pretty much stops and the bearings are ready for re-installation.
This is where it gets a bit difficult since the top bearing is held in place with a metal ring that is fixed in place with tabs that needed to be bent up to get the bearing out. It can be difficult to get the tabs bent back far enough to ensure a snug fit of the bearing in its fixture.

I designed a 3D printed tool to simplify this process:
The red cylinder goes inside the motor housing to hold the ring in place:
Once the ring is pushed up all the way the tabs can be bend slightly with pliers:
the final press fit of the tabs into the small recesses is then done with the top part of the tool and adjustable pliers against the red bottom part:
This results is a pretty tight fit almost as good as before disassembly:
I am planning to make a video about this process. But for now it is time to test this motor for 24 hrs using my BeoloverRPM device:
33.33! So far so good! Let's see how this motor feels about things tomorrow!

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