Fine leveling adjustment means?

Dave, not familiar with those coins but judging from the copper, that must be the budget model leveller?

Good point about wear on knife edges under repeated to-ing and fro-ing of pendulums. . I wonder if using something like hardened and ground round bar such as linear slide rails, would be more durable yet still minimal friction? Or is the knife edge essential for the latter point.

I do wonder if on a clock with active impulse compensation reducing frictional losses is actually helpful. In order to exert control over the pendulum you actually need a certain amount of amplitude decay as impulsing can only work to increase amplitude. If you imagine the limiting condition where there are no losses there is nothing for the amplitude control to work against.

Very small impulses require driving currents with hard to achieve signal to noise ratios so the frictional losses so long as they are not huge and are constant help to keep a decent signal to noise ratio on the impulsing cuts.

I recently designed a Cryostat to maintain liquid ethane at a constant temperature. The device was a copper cup in a bath of liquid nitrogen with insulation between the two. Two copper ‘ears’ of 2mm wire were attached between the cup and the nitrogen to ensure a reasonable cooling of the cup. The cup has a heater and a temperature sensor and the whole is driven by a PWM PID controller. Without the extra losses provided by the ears the controller had little ‘command’ of the ethane temperature.

I can understand the desire to minimise friction in a totally mechanical timepiece but with impulse compensation I don’t think it’s relevant and may well be counter productive.

regards Martin

Yes, the knife edge is essential, but for a different reason: A round pivot would result in the hinge of the pendulum rocking back and forth too, disturbing its performance. Of course, even sharp knife edges have a non-zero radius, and changing knives could result in changes in performance as well.

Edited By S K on 02/02/2023 13:24:41

You don't have to apply an impulse each swing, you can wait for a certain amount of decay and then impulse. The classic Hipp toggle does this, for example.

In another thread, there's been a bit of a debate about whether it's better to impulse with each swing or more occasionally. I'm not sure it's been completely settled.

As I say I’m not suggesting this for mechanical clocks and that would include the Hipp toggle which really isn’t in the realms of SOD’s electronic compensation being just a fixed impulse triggered by the decay constant of the pendulum.

regards Martin

Since the levelling needs to "Micrometer" accuracy, am surprised that no one has suggested taking the easy way by using either ME 40 tpi threads or 1/8 BSW, both os which are 40 tpi

EWith a large enough diamter knurled adjuster, sub 1/1000" should be obtainable.

Howard

.

As you say, Howard … ‘Micrometer’ accuracy is easily achieved in the adjuster

What I suspect will be the non-trivial part of the exercise, is making the mount both adjustable and suitably stiff.
[ I avoid using the word rigid because that is reserved for the hypothetical, and this is a practical exercise where the devil will be in the detail ]

MichaelG.

Which is why I think pendulums avoid hinges of any type

A round pivot causes friction proportional to the bearing area, and making and maintaining an ultra-low friction bearing isn't for the faint hearted, especially when the bearing supports a heavy weight. Surfaces have to be highly polished, low viscosity oil kept in place, and dirt kept out. Plain bearings are unsuitable because they have high starting stiction. Roller bearings reduce stiction, but aren't as smooth running. Jewelled bearings are a possibility, but they tend to be tiny. Air cushion bearings need a pump. Rotating lighthouse lamps were floated in a large tank of Mercury, but the geometry is wrong for supporting a pendulum.

A sharp edge is low friction, but the enormous pressure on the edge tends to cause it to cut into the bearing and to go blunt. Bearing and edge both have to be as hard as possible, but wear is inevitable. Friction rises as the edge cuts and blunts, so performance inevitably falls. How long depends on bob weight and materials, but I guess a well-made clock would run for a few years. The effect of increasing friction could be partly compensated, but I think the pendulum's period would gradually become noisier, eventually forcing the owner to fit a new bearing.

Spring suspensions mostly dodge friction by eliminating metal on metal contact. Instead of grinding on a bearing, a suspension spring stores energy by bending, and returns most of it to the pendulum on the backstroke. (Anyone have any figures for frictional losses in springs? I believe in a thin pendulum suspension spring, the losses are less than those due to the bob stirring air.)

Just a thought, but today's super-magnets might be a good alternative. As well as being more powerful than traditional types, they retain magnetism extremely well. Might be good enough to make a low-friction pendulum bearing. Anyone tried?

Dave

Edited By SillyOldDuffer on 03/02/2023 11:33:36

Knife edge suspensions are a bit more common on French clocks, not necessarily precision ones. And they do seem to last fairly well.

This is a French clock dated to about 1790 which is suspended from a knife edge suspension at the top. The whole clock acts as the pendulum. The suspension is ( I think) steel on steel.

Oh dear, not paying attention in physics class? Friction is independent of area, it is just the force between the surfaces * the coeff of friction. The reason small diameter axles are easier to turn is as follows. Imagine a wheel on and axle diameter d with a coeff of friction u and a force F applied to the wheel. The friction force on the axle is F*u, so the work done in rotating the wheel through one rev is

the circumference of the axle * F * u = pi*d * F * u. clearly this is smaller for smaller values of d, so the friction loss is smaller.

If a knife edge pivot is actually sliding rather than rocking, then a sharp edge is as small an effective diameter as you can get. One disadvantage of having a knife edge bearing down on a vee slot is that the vee gets full of dust and any wear debris just sits there. A way to avoid this is to have the knife edge facing up and the vee facing down. Bit more difficult to construct.

Now to spring suspension, bending a spring to and fro absorbs energy due to mechanical hysteresis. In the extreme IC engine valve springs can get hot from it, but these are a fairly extreme case. The aim should be for the spring to be very thin, to reduce bending stresses, and made from hard material. Mine is made from something I couldn't drill, had to make a punch. The best clocks, Scott, Fedchenko, Reifler use spring suspension. This appears to be a case of simplest is also best.

Edited By duncan webster on 03/02/2023 14:34:11

Just a thought, but today's super-magnets might be a good alternative. As well as being more powerful than traditional types, they retain magnetism extremely well. Might be good enough to make a low-friction pendulum bearing. Anyone tried?

Alas, it is impossible to make a stable fully constrained bearing using static magnetic fields because of something called Earnshaw's Theorem. You can suspend diamagnetic materials, such as a frog, but they are weakly magnetic and unable to take a large load. It is possible to make a bearing which has one mechanical constraint, for example a knife edge suspended from a strong magnet pole, which works just like a knife edge but can have much reduced load on the knife itself. I recall seeing a balancing device that had a ground steel shaft with a concentric point at each end, which was "suspended" horizontally between the poles of a strong magnet so that one point was sitting on a pole face and the other held in mid-air by the field.

But anyway, I think it is true that the energy dissipated in the suspension is orders of magnitude less than air resistance for normal arcs. I have done some run-down measurements that suggested that pendulum Q increases slowly as the arc decreases, then drops quite quickly for very low amplitude, which might be because the spring loss is becoming dominant.

That's me in a nutshell!

The suspension of Citroen 2CV cars used a “knife edge pivot pin”, though in practice it was a rolling contact. A pull rod with a large round eye, maybe 16mm diameter, connected to the spring, and a triangular pin on the suspension arm with about 3mm corner radius was in contact, finding its own point of operation. It worked very well, with little or no wear over many years.

Gas centrifuges used to enrich uranium for nuclear power stations use a magnetic bearing which takes a lot of the vertical load and provides sideways location at the top. See ME 14 Aug 1998. As JH says, it can't take all the vertical load or it would be unstable. I'm tempted to use this idea for a horizontal balance wheel, but my knowledge of magnetic forces is woefully small.

There are clocks which suspend the pendulum on a loop of silk. Probably rather prone to humidity variations but more modern materials may be suitable. Carbon nano tubes perhaps?

regards Martin

If you want adjustability with no backlash then a trap a pair of thrust bearings either side of the part pre-loaded with a belville stack. You can adjust the force of the pre-load to suit your rigidity requirements (and need to prevent it self-adjusting) and you'll be able to adjust it for level with no need for locking nuts etc.

I've settled on the following plan:

• A brass plate acts as a platform for the assembly.
• The pivot knives (square tool steel blanks) are inset into V-grooves in a brass bar.
• Both the bar and plate have book-matched V-grooves near one end, in which a ground stainless rod rides and acts as a pivot.
• A knob loaded by a compression spring clamps the bar and plate together near that end.
• At the other end of the bar, a micrometer head rides on a short stainless rod, also sitting in a V-groove in the plate only, to set the angle.
• Another spring-loaded knob applies pressure at that end as well. (I need about 3" of clear space in the middle, so two knobs near the ends are used.)

If the compression springs don't allow for enough stiffness, I can delete the springs and clamp down directly after its been adjusted. The ones I have are pretty strong and should suffice, though.

Thank you all for your suggestions.