sarboy w****r
18th Oct 2012, 11:04
Hi All,
There are a number of instruments which have a mechanical-type display of numbers, e.g. the pressure settings (mb) dial on an altimeter and the altitude drums on an altimeter, or the DME distance on an HSI. Old-style devices use mechanical counters, whereas newer instruments use LEDs (or EFIS!). I'm interested in how the mechanical devices work. Many of the counters look like this:
http://www.roymech.co.uk/images7/mechanism_28.gif
Each dial is about 20mm in diameter, as each number (0 - 9) is about 6mm high.
These are really simple for when the device just counts in a decimal fashion, i.e. each dial moves 1/10th of a revolution when the preceding dial has moved a full revolution. For a course display (degrees from 000 - 359), it's slightly different. From what I can see, the units dial moves as per normal, which then drives the next dial one step per revolution of the units dial, but this next dial has two digits on it (00, 01,…. 33, 34, 35) and then rolls over to 00 again, i.e. there are 36 separate numbers, each having 2 digits.
The problem I have is that if each number is about 6mm high and there are 36 numbers around a dial then the total circumference of the dial is 216mm (36 x 6). This means the diameter of the dial is approx 68.75mm (diameter = circumference / pi). This means that the centre of the numbers (i.e. centre of the dial) has to be at least ~35mm (the radius) away from the edge of the instrument so that the dial doesn't extend beyond the edge of the instrument. However, from the instruments I've seen, the numbers are much closer to the edge of the device than this. For example:
http://www.candyparty.com/ST/cockpit/HSI.jpg
From the dimensions I have for this particular instrument, I believe the centre of the course numbers (352) is below the outside of the instrument case by about 20-25mm, yet the numbers are bigger than would be possible if they were mounted on the circumference of a dial that fits in this space.
So my question is, have you ever seen how these dials work?
An alternative would be to use a belt (like a metal/rubber/silicon/mylar timing belt) on which the numbers are printed on the outside of the belt (and so the belt could be bent internally to fit whatever space is available), but this seems like an unusually complicated solution, but I could be wrong….
Any insights into how these work (or even better, a picture of the internals of one) would be very gratefully received.
Many thanks.
There are a number of instruments which have a mechanical-type display of numbers, e.g. the pressure settings (mb) dial on an altimeter and the altitude drums on an altimeter, or the DME distance on an HSI. Old-style devices use mechanical counters, whereas newer instruments use LEDs (or EFIS!). I'm interested in how the mechanical devices work. Many of the counters look like this:
http://www.roymech.co.uk/images7/mechanism_28.gif
Each dial is about 20mm in diameter, as each number (0 - 9) is about 6mm high.
These are really simple for when the device just counts in a decimal fashion, i.e. each dial moves 1/10th of a revolution when the preceding dial has moved a full revolution. For a course display (degrees from 000 - 359), it's slightly different. From what I can see, the units dial moves as per normal, which then drives the next dial one step per revolution of the units dial, but this next dial has two digits on it (00, 01,…. 33, 34, 35) and then rolls over to 00 again, i.e. there are 36 separate numbers, each having 2 digits.
The problem I have is that if each number is about 6mm high and there are 36 numbers around a dial then the total circumference of the dial is 216mm (36 x 6). This means the diameter of the dial is approx 68.75mm (diameter = circumference / pi). This means that the centre of the numbers (i.e. centre of the dial) has to be at least ~35mm (the radius) away from the edge of the instrument so that the dial doesn't extend beyond the edge of the instrument. However, from the instruments I've seen, the numbers are much closer to the edge of the device than this. For example:
http://www.candyparty.com/ST/cockpit/HSI.jpg
From the dimensions I have for this particular instrument, I believe the centre of the course numbers (352) is below the outside of the instrument case by about 20-25mm, yet the numbers are bigger than would be possible if they were mounted on the circumference of a dial that fits in this space.
So my question is, have you ever seen how these dials work?
An alternative would be to use a belt (like a metal/rubber/silicon/mylar timing belt) on which the numbers are printed on the outside of the belt (and so the belt could be bent internally to fit whatever space is available), but this seems like an unusually complicated solution, but I could be wrong….
Any insights into how these work (or even better, a picture of the internals of one) would be very gratefully received.
Many thanks.