How To Break the Famous Labophot-2 Plastic Focus Gear

[lothman]

(I am always puzzled by the "module" idea. I realise it's a Standard, but it seems to be a way to guarantee, that gears of different diameters will not work together )

Chris,

the module or modulus describes the dimension of the teeth. Both gear must have the same module. See Wikipedia, mxPi=p (module x Pi = circular pitch of one teeth). By the way it is not so easy to get the right module from the worn/broken plastic gear by measuring its diameter and counting the teeth. It is much more accurate to measure the outer diameter of the large metal gear, count its teeth and calculate the module. The outer diameter of a gear should be D = (z+2)xm (Outer diameter = (number of teeth +2) x module). So you can measure, count the teeth, calculate the module and search the replacement gear by number of teeth with the corresponding module.

[rjlittlefield]

If you are on the mechanical dead end of the focus block you can create at lot of torque due to the high gear transmission ratio of the fine focus.
...
Or that they did not use a safety clutch instead of a braking plastic gear for preventing further damage of the focus mechanism ;-)

But Nikon did include such a clutch! It's the one on the coarse focus knob.

As explained by Microscope Solutions, who provides replacement metal gears:

Some are curious if other gears in the focus system will break because of the strength of this metal gear, but because of the design of the gear system, no damage can occur. Even when the fine focus is turned at full stop, the coarse focus will reverse to take up the movement. The only torque that is placed on the fine focus system would be old sticky grease, this is typically why the plastic gear would split.

So in fact you can not damage the block simply by continuing to turn fine focus when the block is at end of travel. When the focus slider runs up against any stop, the main drive gear stops rotating. Continued rotation of the fine focus knob then rotates the whole reduction gear assembly instead, by overcoming friction in the coarse focus clutch.

The only conditions under which which the plastic gear serves as a sacrificial part are when:
1) lubricant in the gear system has turned into glue, or
2) some user has tightened up the clutch to an impossibly high level, or
3) some user turns the two fine focus knobs against each other.

--Rik

[soldevilla]

what was the ultimate benefit of making this part out of plastic? a couple of dollars at most, in savings and those were very orderly dollars!

As an engineer I've had tough talks with the economics of the business, trying to keep that they save two cents € changing a metal washer with a plastic one...

The manual is explicit on this point: there are three small gears, one each in chrome plated steel, brass, and black plastic, plus the white plastic one that was formed around the shaft. Why there are four different materials, I could only guess.

In high speed reductions is usual to mount a plastic gear in order to avoid noise. in this case, I think is only to make easier the assembly to the workers.

Rik, if your gear has only one cut but is preserved as a whole piece, your cheapest solution is a small hollow metal (very inexpensive to produce on a lathe) cylinder placed under pressure by closing the gear.

[rjlittlefield]

@Rik
I think Nikon also uses module 0.3 on your focus block, then you could get for e few $ a 16 teeth pinion with part number RBS0323 from Lemo Solar. I did not search for trader in USA, but I can help on getting these pinions and drill them to 3mm.

Thanks, Lothar. This may be very helpful for other people, or if I happen to encounter other broken gears in the future. As I mentioned earlier, this particular gear was only cracked, so it was easily made workable again by simply banding it to close the crack, drilling it out to remove the D section, then drilling it just a hair more to fix the shrinkage problems mentioned by soldevilla several posts above. The resulting gear fits smoothly around the shaft and is splined only to the focus knob, so it both fixes the problem and prevents any recurrence. Twisting the fine focus knobs against each other now just causes the right knob to turn on the shaft, since there it is held in place only by friction against the shaft. The operation is now silky smooth, and the gearing is protected by what are effectively two clutches.



The copper wire is not as formal as the small metal cylinder mentioned by soldevilla, but in this case it works equally well and was even quicker to do.

--Rik

[ChrisR]

I'd read the Wikipedia article, and other references which express the meaning slightly differently.

I don't see what "module" tells anyone, that other more fundamental measurements don't already. With Module alone, you still don't know for sure that the teeth will work together? "Pitch" would be equally useful/inaccurate. Neither tells you how deep the teeth are cut - unless you depend on additional fundamental dimensions/angles.


"D = (z+2)xm (Outer diameter = (number of teeth +2) x module)"
Why is it 2?

[lothman]

"D = (z+2)xm (Outer diameter = (number of teeth +2) x module)"
Why is it 2?

I hope I find the right English words. You can imagine two gears by two cylinders rolling on each other. What makes the transmission ratio is the ratio of the cylinder circle (or pitch circle on gears).
So the diameter of the pitch circle is "module x Pi" and "a" the addendum is normally 1xm and on diameter it makes that double

picture is screenshot from:
http://www.hzpt.com/tech/engrdroit.html



and yes in reality everything is little bit more complex. If you only use standard gears without addendum modification than the distance of two axes can only be in steps of m, let's say 25m, 26m, 27m... So with addendum modification the distance can be chosen as desired, a great advantage of involute tooth system. But at the correct distance of the axes each gear with the same module works.

[rjlittlefield]

I don't see what "module" tells anyone, that other more fundamental measurements don't already. With Module alone, you still don't know for sure that the teeth will work together? "Pitch" would be equally useful/inaccurate. Neither tells you how deep the teeth are cut - unless you depend on additional fundamental dimensions/angles.

I have the same concern.

From only a little reading, it seems like the tooth shape is determined by a combination of pitch, pressure angle, and tooth depth.

Is the situation that, in these small gears, everything is so standardized that there's nothing left to specify except for module/pitch and number of teeth?

--Rik

[rjlittlefield]

but I can help on getting these pinions and drill them to 3mm.

A small but important correction: for Labophot 2, the shaft diameter is 4 mm, not 3 mm.

--Rik

[lothman]

Is the situation that, in these small gears, everything is so standardized that there's nothing left to specify except for module/pitch and number of teeth?

--Rik

That's why we engineers make these standards

Pressure angle is standardised to 20° and with that you can build a transmission not only with small gears, also with higher module.

For a variable dimension between axes you need addendum modification (and there is no reason why a microscope manufacturer should do this), otherwise the distance of the axes can only be a multiple of "m".

And usually catalogue gears do not have addendum modification, therefore the easy formulas are working.

[lothman]

but I can help on getting these pinions and drill them to 3mm.

A small but important correction: for Labophot 2, the shaft diameter is 4 mm, not 3 mm.

--Rik

Hmh with 16 tooth and module 0,3 (proofed in my Labophot 1) the basecircle (bottom of the teeth) would be (16-2x1,4)xm = 13,6x0,3 = 4,08mm. So there would be no chance to get a ø4 bore.

Are you sure about 4mm diameter, if so then probably a higher module (0,4 ?) was used. With 16 teeth and module 0,5 the gear should have 9,5mm outer diameter and 7,2mm with module 0,4.

Edit: If the bore on your picture is 4mm then the outer diameter is 7,47mm so that looks like module 0,4.

[rjlittlefield]

Hmh with 16 tooth and module 0,3 (proofed in my Labophot 1) the basecircle (bottom of the teeth) would be (16-2x1,4)xm = 13,6x0,3 = 4,08mm. So there would be no chance to get a ø4 bore.

Are you sure about 4mm diameter, I doubt.

Yeah, pretty sure. My digital caliper said 4.00 mm, but perhaps this picture is more convincing.



Outer diameter measures about 7.22 mm, slight variation from one tooth to another.

--Rik

[lothman]

Outer diameter measures about 7.22 mm, slight variation from one tooth to another.

--Rik

so this would be a spare part, but you would have to drill it to 4mm.
http://sdp-si.com/products/Miniature-PD ... 002011.pdf

they also have a solution with radial fixation by small worm screws, but not for module 0,4 and 4mm bore.

http://sdp-si.com/products/Miniature-PD ... 002003.pdf

That's why microscopesolutions can charge 140$ ;-)

[rjlittlefield]

so this would be a spare part, but you would have to drill it to 4mm.
http://sdp-si.com/products/Miniature-PD ... 002011.pdf

In brass, this is part number A1B2MY04016, currently listed at $12.77 plus shipping.

they also have a solution with radial fixation by small worm screws, but not for module 0,4 and 4mm bore.

Not even for module 0.4 and 16 teeth, if I'm reading the table correctly.

That's why microscopesolutions can charge 140$ ;-)

Given the limited number they must sell, minus fabrication and handling costs, I doubt that they're getting rich from these. On the consumer end, I wouldn't consider it a bad deal to get a purpose-made replacement part for that price, versus buying a generic replacement and then having to mod it myself.

Of course it's better to avoid the need in the first place -- buy only a block that feels OK to start, then don't damage it yourself like I did.

--Rik

[lothman]

16 Teeth, brass, module 0,4 for 0,20 $

Amazon

with radial screw

[rjlittlefield]

For US based workers, I'll point out that 4 mm = 0.1575" is just a comfortable hair smaller than a #21 drill, nominal 0.159" = 4.04 mm.

If your 4.0 mm drill doesn't make quite a big enough hole (as mine did not), then #21 is the next step larger.

--Rik