Cirkut Gear Selector

There seem to be continuing online questions about how to select the correct gears for Cirkut cameras, depending on focal length and focal distance.
​By inspecting the scales on the bed of my Cirkut #8, I find that there is a simple straight-line logarithmic chart for each distance and all focal lengths.

Note that the x-axis scale is logarithmic and the y-axis is linear. However there is no need to worry about any of that. Choose your lens focal length (in inches) along the x-axis, then the y-axis shows the tooth count of the required gear at 25, 50 or 100 ft. I guess infinity uses the 100 ft setting. A larger PDF version is downloadable from the link below.
It would be a useful exercise to check the data for other Cirkut models, but I don't have those so leave it to others - who may hopefully to publish the results!

cirkut_8_gear_chart.pdf
File Size:	766 kb
File Type:	pdf

Download File

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Using modern gears
There is much debate and concern in various online forums about how to make or find new gears for a Cirkut camera which doesn't have a full set. The industrial standards for gears have changed over the years, in particular the use of the Module System which has largely replaced the earlier Diametral Pitch standard, and the Pressure Angle which for original Cirkut gears was 14.5° and for standard off the shelf gears is usually now 20°. How much of all this matters? Remember we are enjoined in the instructions only to half-engage the gears anyway, so we are not talking about a high-performance gearbox.
I have made and tested modern gears for my Cirkut #8 outfit, and concluded that it is perfectly OK to use new off the shelf gears, all you will need to do is make the shaft on which they rotate, which screws into the motor of the camera. The specification is Mod 0.8 brass spur gears of whatever tooth count you need. Or even plastic ones if you don't mind the appearance. The screw on the end of the shaft is #8 UNC, so you will need a die for cutting that.
I have used both commercial modern gears and home-made ones (which are a bit less precise), and both work. The illustrations show three different gears on the left, engaged with an original Cirkut gear on the right.

Two original Cirkut gears

Home-made + Cirkut gears

Commercial + Cirkut gears

Gear Design
If you don't have a gear to use as a model, here is a basic drawing. Any small lathe will be suitable.

Dimensions:
D1 - turn to 4.1mm diameter then form a screw thread #8 UNC over the full length.
L1 - 5.5mm
D2 - 6.3mm nominal, but should be a sliding fit inside the gear socket of your motor.
L2 - 7.3mm
​D3 - 9.5mm in the original, but can also be any convenient larger diameter. The right face of this section positions the gear the correct distance below the motor.
L3 - 11.5mm in the original but see below
D4 - this is the gear itself, so the diameter will vary by tooth count. Use Module 0.8, PA20° (which is the default)
L4 - 3.75mm in the original.

L4 is the thickness of the actual gear blank, and the gear you buy or make may be different, typically thicker. 3 - 5mm thickness should be no problem, if more it is probably best to face it thinner. The shaft and gear will be made separately, so the shaft will have a projection to the left to fit the bore of the gear, typically 6mm. If the gear comes with a boss, the diameter of the boss will displace a good part of D3/L3. If there is no boss, be extra careful to get the gear truly square to the shaft when you attach it. In the original I think the shaft is a press fit to the gear, with the left end peened over to be sure. I find that with a good fit, soft solder or a modern adhesive are both fine - the gear is not highly stressed in service.
The key to getting the gear at the right position against the big toothed wheel of the tripod head is getting the total of L3+L4 correct. These dimensions are as original for my Cirkut #8 but I have another original Cirkut gear whose shaft was about 9.5mm longer before I cut it down to fit this camera, so it's clear that there is more than one standard size.