Pendulum question

[Rocker]

The movement I have got for my grandfather clock is driven by weights and regulated by a pendulum. But the pendulum I have seems not to be long enough, even when adjusted to its maximum length. The clock gains about 5 minutes every hour. The pendulum bob, unlike on antique clocks, where the bob is usually heavy, is hollow on mine and very light.

My question is: If I added weight to the bob, thus lowering the centre of gravity of the pendulum, would that help me to attain an effectively longer pendulum, so that the clock would keep correct time?

Rocker

[outback]

Where is the adjustment? Is it where the pendulum attaches at the top, or is there adjustment at the bob? Maybe lengthening at the top if possible.

I don't think extra weight will help, I thought the bob was only to maintain momentum and ensure the pendulum kept swinging. The length is the critical factor in it keeping accurate time.

OK, someone tell me I'm a goose and have no idea what I'm on about.

[ubeaut]

Personally I'd be going straight back to the supplier or manufacturer. Those movements aren't cheap and are supposed to work brilliantly

Seems that maybe they have given you the wrong one or there is a problem with weights or similar. Maybe it's just a dud.

You paid out good money for it so you should expect it to work.

Yep..... Go back to the supplier/manufacturer.

Cheers - Neil

[Greg Q]

The period of the pendulum's swing is independant of mass, I'm afraid. You will have to find a way to lengthen the arm.

Greg

[Greg Q]

I just found this on a google search...looks like it has a calculator to find the needed length:

http://www.school-for-champions.com/...e/pendulum.htm

Greg

[KevM]

Love the clock and the workmanship is A1, a greenie despatched.

Over the years I have done some maintenance on my mother's grand father clock to keep it going. Don't know what it is called but the pendulum hangs off 2 short straps consisting of thin steel (like feeler gauges) with brass (from memory) folded over each end to hang off the mechanism and allow the pendulum to be hung.

These are available in various lengths and the pendulum is adjusted up & down. Replaced it at one stage with one of incorrect length and could not get the clock running accurately. From memory they cost between $15.00 - $20.00.


Kev M

[Rocker]

The period of the pendulum's swing is independant of mass, I'm afraid. You will have to find a way to lengthen the arm.

Greg

Greg,

I am aware that the the period of the pendulum is dependent solely on its length; but how do you determine the effective length? Surely it must be the distance from the pivot point to the centre of gravity. A theoretical pendulum has negligible mass except in its bob. However mine has a light bob and a fair amount of mass in the 7 metal rods that connect the bob to the pivot point at the top. I would contend therefore that adding weigth to the bob would effectively lengthen it. Obviously length is not the only factor. if I added 300 mm of fuse wire to the length, it would lenthen it substantially, but I doubt if it would have any effect on altering the period of the pendulum.

Outback,

The only adjustment for the length is at the bottom end of the bob.

I have taken Neil's advice and e-mailed Hermle. I suspect that Carbatec supplied me with the wrong pendulum.

Rocker

[Greg Q]

Hello R.

I didn't mean to imply, if I did, that you didn't know the physics-it was a hurried post on my part. Is there a convenient way to increase the mass of the bob? I suspect that it would change the period, based on your description of the mechanism.

On the other hand, a return-to-sender is probably going to be easiest.

Greg

[zenwood]

The period (in seconds per cycle) of an ideal pendulum is T = 2π√(l/g), where l is the length (m), and g the acceleration due to gravity (9.8 ms^-2).

Ideal here means that all the mass is concentrated at the end of a weightless rod (and that the displacement of the weight is small, but that wouldn't come in here). You're right, Rocker, that increasing the weight of the end relative to the weight of the rods, will slightly more closely approximate the ideal pendulum, but you rapidly approach diminishing returns: even if you put half a ton of weight there, the formula will still represent the slowest that your pendulum can swing, limited by its actual length.

Adding 300 mm of fuse wire:
Assume that the initial length of the ideal pendulum is 500mm. The period is 1.42 s. Adding 300mm to the length will increase the period to 1.80 s. I would be adding the fuse wire: simple, and very effective.

PS Most bobs I've seen have a little screw to adjust its height, and hence the length of the pendulum.

PPS For a non-ideal pendulum, I think the length would be determined by the position of the centre of mass (centre of 'gravity') of the pendulum.

PPPS Just seen Neil's post: I agree, a commercial unit should be expected to have enough adjustment to get it working accurately. Does it have temperature compensation? (Otherwise the length will change with temperature and the clock will keep terrible time.)

[outback]

Neil's answer is of course the correct one.

I answered with some trepidation, as I have no experience with long case clocks, but like Zenwood, the only bobs I have seen have a built in winder doover to lengthen and unlengthen the pendulum length.

Sounds like Fred just picked up the wrong pendulum for your movement. Send it all to me, and you make another clock with a new movement so there are no mistakes this time. :eek:

[q9]

I have taken Neil's advice and e-mailed Hermle. I suspect that Carbatec supplied me with the wrong pendulum.

That sounds likely. http://www.cmi-hermle.com/

[ozwinner]

Hi
I put a Hermle in mine, on the back of the movement is a mesurement of the lenth of pendulam required.
The bob on mine is hollow, weight has little to do with it, its the length that matters..
I think the shop has given you the wrong size.

I calibrated mine when I made it, it took about 2 weeks to get it accurate.
Since then, you could set your watch by it.

And who said size doesnt count.

Al

[Rocker]

Mystery solved. Carbatec admit they have sent me the wrong pendulum. They will pay for the return postage, so I am not too unhappy.

Rocker

[Rocker]

My pendulum woes are well and truly over, and I am surprised and delighted at the accuracy of the Hermle movement of my clock. After making adjustments to the length of the pendulum over the past few days, I have got it regulated, using the World Clock, so that my clock is within 2 seconds of the correct time after 24 hours' running.

When I installed the movement, I oiled all the bearings with the oil supplied; however, I did not oil the cogs or the escapement teeth. Should I have oiled them, or not?

Rocker

[ozwinner]

My pendulum woes are well and truly over, and I am surprised and delighted at the accuracy of the Hermle movement of my clock. After making adjustments to the length of the pendulum over the past few days, I have got it regulated, using the World Clock, so that my clock is within 2 seconds of the correct time after 24 hours' running.

When I installed the movement, I oiled all the bearings with the oil supplied; however, I did not oil the cogs or the escapement teeth. Should I have oiled them, or not?

Rocker

Hi Rocker, yep Im sure you will be happy with the Hermle movement.

I didnt oil mine, I think it would allow dust to stick, and as most dust is abrasive it will wear it out quicker.

Al