Project 2128, analysis
Idler Wheel Condition:
Measuring the inside diameter of the idler wheel bushing for evidence of wear. The instrument is a Mahr (split ball) bore gage. The meter is in "inches" and the graduations are to the nearest .0001 inch. The gage has been set to read zero at 5mm (.1969 inches)
The bushing bore measures .1984 inches in diameter at the uppermost part of the opening then tapers to .1979 in the lower area where the photo shows the reading. There is.0005 inches of taper top to bottom.
Roundness of the bore checks good with no observed deviation.
hint: click on thumbnail to view image full size.
A close-up detail of the idler shaft. There is some visual evidence of a
wear pattern in the area where the idler bushing spins. Note also the
washers at the base. These function as a vertical thrust to the spinning
idler wheel and also as a means of adjusting height of the idler wheel via shim.
Measuring the idler shaft . Even though a wear pattern is visible the
shaft mic's uniformly the same size up, down and around. Size: .1965
inches. Running clearance between our worn idler wheel bushing and this
shaft is .0014 to .0019 inches, noting the taper in the bushing. That's
getting a bit sloppy. The wear is happening in the sintered bronze idler
wheel bushing. These bushings are very soft and tend to wear out much
faster than the shafts they ride against. In this case we will want to
make sure that the replacement idler wheel has a new bushing installed so we can
tighten up on the running clearances here.
Platter bearing.
This component is critical to the overall good sound quality of the deck. Correct running clearance between bushing and shaft must be restored.
Yikes!. Talk about visible wear patterns. There is no doubt as to
the exact location of the bushings around this shaft. It appears that the
platter bearing was allowed to run dry. Click on thumbnail image to view it full
size. In this case the wear pattern is not only visible to the eye but
quite apparent as one runs a fingernail over the surface.
Note: This is an early TD124....as evidenced by its low serial number. Early models used a nylon bushing within the bearing housing and then lubricated that with a silicon based lube. It is not unusual to see these early models with such wear patterns on the main spindle shafts due to having been run dry within the nylon bushings.
Thorens later changed over to sintered bronze bushings and a straight 20 wt. turbine oil. We'll do that.
Measuring the affected area with a precision micrometer. Note the readout
on the gage.
Now measuring in the area of the shaft that runs between the oilite
bushings. This area should measure within factory tolerance since it will
be unworn. The weird part of it is that the unworn area actually measures
a couple of 'tenths smaller than it does at the bushing area. Conclusion,
there has been a transfer of materials between bushing and shaft, with bushing
material being added to the diameter of the shaft. This was caused by
allowing the shaft to run dry against a dry bushing. Bad ju-ju. More
about lube further within.
After polishing with 3m compound and cotton sling. The wear pattern is still visible but considerably smoother.
After polish the shaft measures .5506 inches. Before polish it measured
.5507 inches. And it is still .0001 inches larger in diameter than in the
area where the bushings don't run. It means I need to do more
polishing. I will, but not tonight. Getting tired.
Ok. It's tomorrow. The shaft material (stainless steel alloy) is hard and
takes some considerable amount of polishing before any change in size can be
recorded. Here's a shot after more polish. Evidence of the former
condition is almost erased except for some traces. That's where I'll leave
it.... with some trace evidence left to indicate that I haven't taken any more
than was necessary off.
Why all this fussing about size? Because the goal is to maintain a specific running clearance between shaft od and bushing id. We'll discuss this more when it comes to replacing the bushings.
Here's a shot of the finished shaft after reworking the outer surface by
polish. The abrasives used were a combination of 600 wet/dry emery cloth
followed by 3M polishing compound applied with a soft cotton sling. Size
after polish in the affected areas: .5506. Size in the center area between
bushing lands: .5505. I could take more but choose to leave it here.
Roundness is still good with no deviation in measure 360 deg. around at any
level.
before
after
Notice there is no ball retainer clip in this bearing shaft. That is
typical of the early models. Regardless of clip or no clip.... a 60 deg. center-drilled shaft end cradles the thrust ball. Because this socket
offers greater contact area against the ball, the ball remains fixed within the
shaft and spins with it against the stock nylon thrust pad in the base of the
bearing housing. More about thrust balls and pads further within.
About the Oilite bushings used in the bearing housing. Oilite is a product name for a bushing made from a sintered bronze material. Sintered bronze is a kind of powdered metal bronze alloy that is cast by a unique process to produce a bushing bronze with sponge-like properties. The product is very porous. It will drink lubricant like a sponge and then retain it over a very long period....while lubricating the shaft running against it.
The Oilite bushing material is very soft and easily deformed. This is good because it is the bushing that wears rather than the shaft running within it. When the time comes again, just press out the old and press new Oilite bushings into the bearing housing.
Next: assembly