CAUTION --- When disassembling the Nikon Labophot 2 and similar focus blocks, you MUST remove the fine focus knobs by pulling the knob STRAIGHT ALONG THE SHAFT.
DO NOT APPLY TORQUE UNDER ANY CIRCUMSTANCES, no matter how much axial pull is required to remove the knob!
Certain models of Nikon microscopes are famous for having an easily cracked plastic gear on the fine focus shaft. I've read about it for years.
But I now have first-hand knowledge of the problem.
Indeed, the short story is that I received a Labophot-2 arm in good condition, started to disassemble it just far enough to measure the focus shaft diameter, and in the process of disassembly managed to crack that famous plastic gear.
The damage was annoying, of course, but that lasted only until I started to understand what all went wrong. Then it became quite fascinating, and I'm glad I had the experience. I will enjoy sharing it with you!
To begin with the cracked gear, this is what that looks like:
Cracking like this is traditionally attributed to "old sticky grease", which is supposed to gum up the works so badly that forcing the fine focus knob will cause the gear to split.
However, it turns out that the gear is easily broken even when the grease is in perfect condition.
The problem is due to an intrinsic weakness in the design, which allows the gear to be cracked in an instant by simply twisting the two fine focus knobs in opposite directions.
Let me show you how that works...
This is a Labophot-2 fine focus shaft, with all parts attached. The assembly consists of two knobs, a retaining collar, and a plastic gear. The two knobs are retained on the shaft by what the repair manual calls "handle center nuts", which are slightly recessed into the ends of the knobs and are not visible in this image.
Nothing in the above photo would suggest that there's anything to complain about.
The design weakness appears only when the parts are disassembled. Consider now these photographs of components at the left and right ends of the shaft.
In words, the chain of physical connections in the device goes like this:
- Left fine focus knob is locked to a collar using pins in slots.
- Collar is set-screwed to the focus shaft.
- Focus shaft is locked to the plastic gear by a D-shaped cross-section cut into the shaft, around and into which the plastic gear is formed.
- Plastic gear is locked to the right fine focus knob using pins in slots.
- Right fine focus knob is retained by a threaded nut, but only friction prevents it from turning on the shaft.
Apply very much torque and the shaft turns inside the gear, the D-section acts like a cam, and the gear splits. No involvement of other gears or their lubrication is required.
The trick for successful disassembly is to realize that the left focus knob must be pulled straight off -- no twisting permitted!
This would be no problem, except that on the unit I received, there was a thin ring of plastic inside the bore of each fine focus knob, which interfered with the threads of the shaft so severely that the fine focus knob apparently could not be pulled straight off.
Failing in my attempt to pull it straight off, I tried twisting it, and that's when the gear split.
Now, the standard explanation for split gears is that the grease dried up and somebody forced the fine focus knob. After first-hand experience with the mechanism, I now suspect a different explanation. It looks to me like the gear train, with an intact plastic gear, could apply more force than needed to overcome all but the most egregious cases of dried lubricant.
Instead, I'd place a modest bet that many (most?) split gears actually happen like mine did -- somebody goes to disassemble the mechanism, makes the same mistake of applying torque between the two knobs, and there goes the gear. Dried lubricant is still important in the scenario, but only as motivation to disassemble.
After I did get things fully disassembled and figured out what had happened, I went back to see what more I could learn about the details. The result leaves me better informed but still quite puzzled.
Here is a closeup stereo view of the plastic ring inside one of the knobs. The ring is that section of plastic that looks like it has threads torn in it, sitting at the bottom of the hole.
The reason for the threaded appearance is that the way I finally removed the knobs was to "unscrew" them by forcing them to turn on the shaft. Pulling them off axially seemed impossible.
After removing and reinstalling the knobs in this incorrect way several times, I finally decided to measure the force that would have been necessary to pull them off axially, by simply applying enough force to drag the threaded part of the shaft through the ring.
That measurement -- after the ring had been degraded by repeated manipulation -- still took 9 pounds for one knob and 10 pounds for the other. I suspect that in its initial condition, even more force would have been required. That's a lot of force, particularly because there's no obvious way to grab hold of the tapered knob when it is fully assembled on the scope. (See image below, shown with the retaining nut still in place.)
Stepping back and thinking about the whole experience, there are two aspects that puzzle me.
1. I don't understand why that little ring of plastic was inside the knobs in the first place. It serves no useful purpose that I can see, so being an intended part of the design seems unlikely. It seems much too thick to be flashing around the parting line of a mold. I considered the possibility that it formed during or after assembly, by deforming the surrounding plastic. But that makes no sense given the shapes of other mating parts and the fact that the hole is clean for the rest of its depth. So I am mystified. In any case, requiring 10 pounds (or more) to pull the knobs off axially seems far beyond what the designer would have intended, particularly given the fragility of the rest of the design.
2. As for "fragility of the rest of the design", I don't understand that either. It would make perfect sense for the gear to be locked to the shaft, if both knobs were then locked only to the shaft and not directly to the gear. Or it would make perfect sense for the gear to be locked to one knob, if the knob were locked to the shaft and the gear was not. But to have the gear be locked to both the shaft and the knob, thus creating a chain of connections in which the left knob is locked to the right knob only through this fragile plastic sleeve that also serves as a gear, that just seems like a bad idea that somehow slipped through design review.
If anybody knows more about the history and design of this mechanism, I would be very interested to hear.
In the meantime, I'll draw comfort from the thought that perhaps this posting will keep somebody else from having the same difficulty.
I hope at the very least that you find this interesting!