Worm-Gear advice sought

If I recall from my days of bell ringing, the main chandelier in the nave of St Paul's church in Durban has a ex Wellington bomber bomb winch mounted in the ringing chamber to lower the massive array when lamps needed to be changed. It is a hand wound contraption mounted on an ali bracket with lots of lightening holes, apparently salvaged from a crashed Wimpey at the end of WW2. Not sure what the chandelier weighed but the bomb winch handled up to 500lb bombs. It has a plate attached with its rating!

I've just had a thought [a dangerous occurrence, I know]

Given the infrequent use proposed for this device; might it be possible to make the wormwheels in the form of a 'lantern pinion' ? ... with the trundles skewed at the helix-angle of the worm.

This would lend itself to manufacture on the Mill , and should be economical.

MichaelG.

I posted as shot of the arrangement in the other thread on epicyclic - I had time to take a shot when I posted in that one. The web page it's on has gone but the same info is available on the wayback machine here

**LINK**

While multistart worms increase efficiency I believe that twin start worms are often used in motor gearboxes to increase their load carrying capacity. I doubt of a low pitch 2 start worm would drive backwards but as you have no lathe you would need to buy a tap to hob a worm wheel - can't say as I have ever seen any 2 start taps.

I think Michael posted and idea that could be switched around a bit. A lantern wheel with a huge number of pins driven by a pinion. I suggest that the pinion would need to have 15 teeth minimum to get several acting at the same time. It could be hardened and tempered to increase it's strength. For the lantern wheel I would have thought that 3/16 dia silver steel as it comes would be fine for the pins providing it's supported at each end - sort of wheel with a groove around the periphery. Even 1/8 dia should do it providing there is very little unsupported length at each end. I'll leave stress calcs to some one else. The pitch would need to be greater than the dia suggests to provide clearance for simple teeth on the pinion. The flanks cut radially with a round or cycloid type end. This is the type of tooth form usually used on the gear rather than on the pinion.

The pins to go with that would have a dia of 0.9 x the gap between the teeth, They would mesh when an arc drawn from the centre of the pinion passed through the centre of the tooth rad and the centre of the pins. The motor could drive the pinion via a couple of commercial bevel gears.

John

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PS Thanks for a copy of the pdf Keith.

Edited By Ajohnw on 16/05/2016 10:46:44

Edited By Ajohnw on 16/05/2016 10:49:02

That's windmill technology but the design is after Harrison's great wheels. The proportions should be correct but I can't be 100% sure of the depthing but that can be found after the parts are made.

John

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It used to be quite common for amteur telescope makers to produce their own worms and wheels.
Traditionally you make the worm leaving plenty to produce a hob at one end wich is later discarded.

**LINK**