Pullies Vs Gears

Hi, yes it's the weekend and daft questions abound.

I'm trying to work out a problem with a clock I'm making. At the moment it requires 2.5kg to run for 1.5 days.

Now that's fine but I'm lazy. I want it to run for at least a week. There are two options mount the clock so high of the floor it would need to be upstairs with a hole in the floor. Not practical as I live in a bungalow.

Or use pullies to increase the distance the weight travels.

But that means I'd need a pulley ratio of 4:1 and at least 12.5kg of weight. Now as the clock is wooden anyway. To give me enough torque. That's a no go.

But and this is the big question. If I use a gear train with a ratio of at least 4.5:1. Would I get back the torque lost because of the pullies?

I realise some torque will be lost through friction etc. And normally the speed would be so slow as to be useless but the speed will be regulated from the clock not the weights. And so as long as the torque doesn't exceed the original tolerances would it work?

And if so what gearing should I use. A simple two gear train or more? And if not is there any other way of increasing the time between windings without needing steel reinforcement in the walls?

Sorry if my terminology or anything else is wrong but I'm just a daft old wooden top.

Thanks

Izack

use pullies to increase the distance the weight travels.

Bungalow? Bore a(deep) hole through the floor? Not sure that the weight will travel any further unless your pulley is in the roof!

Perhaps use some metal parts in your clock to reduce the friction losses? Best solution, if you are that lazy, is to get someone else to 'wind' it for you!

Hi,

I like your thinking and a deep hole would allow the weight to travel for longer but the house half buried below mine might get a bit miffed with a hole in their ceiling and a weight slowly descending once a week.

I'm already thinking of using pullies but that means a loss of three quarters of the torque and so not enough to drive the clock. Hence the need to regain the torque hopefully by a gear train.

Also Not done it yet, I live alone so no one else to wind it and the friction isn't a big issue it's just why I need a ratio of 4.5:1. On the gear train. To compensate. But I'm ok with pullies just not gears. As that's more of an engineering thing. As you don't get many gears trains in a chest of drawers... just gear changes.

deleted

Edited By Sam Longley 1 on 07/10/2016 08:26:48

Since a "multi purchase" block n tackle trades long pull at low force for short pull at high force...
You want a weight to fall say 4feet at 2.5 kg.
But really want 16 feet at 2.5 kg
thus a ten kg weight would in an ideal world suffice..
I would make the blocks allowing for say a 12 kg weight but adjustable down
A fourne purchase is modest...
Bear in mind the support doesn't need to be part of the clock...
Imagine hanging a weight on the wall from a block 'n' tackle ..with simple pully under clock redirecting fall end of cord....

The down side might mean a very large winfing drum in the clock.

Hi,

Your right about the idea of the pullies not needing to be attached but the problem is the size of the said weight as even with lead I'm looking at a block around a 300m cubed. I am hopeful of using a block and tackle that's what will give me the 4:1 ratio. But the torque I'd loose, 3/4 means the clock wouldn't have enough power to run. More like a breeze block than a sleek timepiece. I assume then that I can't use a gear train to compensate? As for the drum that's not a big issues as the design is such that there is plenty of space to show off all the gears so a big drum would be hidden.

My main confusion is clocks such as longcase clocks can run for up to a month. Yet they only have small weights perhaps 3kg and a drop of four feet. And I know they use pullies. So how do they do it. As John Harris made his clocks totally from wood even the shafts. So it's not friction that's the issue but some archaic secret it seems.

The energy (potential for doing work) available is the weight multiplied by the distance it drops.

If you want to get the clock to run for longer, you have to reduce the work required to operate it - any change of puliies/gears/driving arrangements won't get you round the basic physics or otherwise you would be on the way to perpetual motion.

So... the best you can do is look very critically at each moving part in your clock and see if it can be improved to reduce friction. Did John Harris use lignum vitae or some other very hard oily wood for pivot bushes? Do you have teeth that could be brought to a better finish? Could you incorporate a ball bearing in the most heavily loaded pivots?

Neil

Hi Neil

I realise that I can't get back all the energy hence the reference to torque not speed. As I would be sacrificing speed to balance the equation and so it's not a perpetual motion device. Yes Harrison used Lignum Vitae for his bushings as well as some very complicated gear construction to compensate for humidity. Such as putting odd shaped pieces of different wood at angle to the growth rings. The original plans that have it running for just 1.5 days uses bearings and such likebut to still at its limit of power input to torque output. Once the power has gone through all the gears as they have to gear down the power as it's the number of teeth that's important. Not the torque as with a normal gear box.

My thought was as the speed is regulated by the mechanism not the drive force then the speed of the primary drive wouldn't matter so long as its faster than the minimum rpm the clock creates. Rather than the drive setting the speed.

I'm still baffled as to why gear can't compensate the loss of torque. As they seem to increase in power when geared up but loose speed. Which is fine. And would negate the problem of the loss of power of the increased travel of the weight on the pullies.

(I haven't read most of this thread btw)

If you can't do up or down then how about sideways, a weight on a lever, i.e. a couple of breeze blocks on a steel bar

Glad I could help

The lead I've got 75% fills a box 300 x 250 x 450 mm and that only weight 9.5kg. So I must have been sold a pup and it's not lead. But it looks like lead and it weighs the same and melts the same. So...

I don't know about using breeze block but it would be a very unusual clock with the weights like that

Izack,

Sorry to be blunt, but; I think you will struggle to get this working.

As an alternative approach: Please have a look at my post of 29-September, on the 'Basic Clock' thread: **LINK**

http://www.model-engineer.co.uk/forums/postings.asp?th=116604&p=16

Huygens' endless rope [or chain] was a masterpiece of ingenuity; and works very well, whether hand-wound or motor assisted.

Ask yourself if you really need a wooden clock to run unattended for a week, or if you could live with pulling the rope occasionally, when you pass.

MichaelG.

Don't understand why you would be losing so much torque (i presume you really mean power?) with pulleys? You might lose a bit through friction if you were using v-belts, but flat belts or round belts that are not relying on the wedge effect of a v-belt to transmit the power would have much lower losses. Bicycle chain is even more efficient, and a suitable set of bicycle cogs having the required number of teeth to provide the ratio you need would not only be a low-friction and efficient power transmission system but would also have no problem with the weights you are suggesting. They'll support my 85kg no problem.

To work out the ratio you need, measure the maximum length of drop you can accommodate for the weight, then measure the circumference of the last gear which actually operates the clock movement. Multiply the gear wheel circumference by the number of revolutions it will need to make for you 1-week operation. Divide this number by the distance your weight can fall and that is your overall ratio.

It's not a torque issue! What matters is the energy used. If you geared down to recover the torque, the output end of the gears would run 4 times slower, so the weight would have to fall four times further for the same running time. So you would still need 4 times the drop if you stay with a 2.5 kg weight. As has been said you really need to reduce the energy used by the clock, dissipated in friction. There are several designs for wooden clocks around that used ball races for critical pivots. Neither gears nor pulleys "increase power" they only lose energy, but they can change torque and speed. Power delivered is (loosely) torque x speed.

By the way, Harrison used LV for bushes, which are self lubricating. Work on ballraces for clocks suggests though that they are better than jewelled bearings but should not be lubricated as the oil/grease eventually dries up, so the races should be washed throughh in several changes of clean white spirit and allowed to dry. Don't clean in an ultrasonic bath as it damages them. They may then be better than LV too, but probably no clock using ball races has run long enough to find out!

If you really want to be lazy.

Fit a pulley to the floor and another to the ceiling. Run the clock line down to the floor round the pulley up to the ceiling and thus to the weight. You then have the full hight of the room for the drop.

regards Martin

Lead weighs 11.34 g/cm³ so 11.34 x 30 x 25 x 45 x 0.75 = 287,000 g, about a quarter of a ton! How did you lift it?

Russell.