Category Archives: Turret Clock

Gravity arms for the double three-legged gravity escapement

Gravity arms

This is a quick first look at the gravity arms for the escapement. Unfortunately we have no direct plans for these arms, although the principal is explained in a number of book that we have along with generic designs. We also have limited access to another Smith clock with one, but the people of Duffield would probably not appreciate it if we stopped the clock and dismantled it to scribe around its parts. As a result these have been made from a combination of reference books, awkwardly taken part-measurements and careful study of our own photographs.

I’m not going to include any technical details yet because we don’t know if they will work! We hope this won’t become a process of trial and error but there might be some fine tuning to be done at least. I’ll post more pictures of our progress and details of any problems we find as we continue. Once we have it working I will, of course, provide proper details. These arms have been made from multiple parts welded together which I don’t imagine is how the originals were done, presumably they were cut from a single piece. However, this technique has made them a lot quicker and easier to produce (important since we might have to make more variations yet, if they don’t work) and the results look fine. Even easier would have been getting access to a water-jet cutter, then we could have just drawn them up on the computer and a few minutes later had a perfect set pop out of the machine.

Pendulum suspension

Pendulum suspension

A quick post to show the new pendulum suspension. This is a departure from the square version on the St. Alkmund’s church clock, but it’s been made to plans from Smith of Derby so it is an authentic design (if only we had plans for some of the more complicated bits, like the escapement!). There’s no way of knowing which design this clock would have had originally, so I’d be happy with either.

It’s surprising just how heavy this bit is, but it’s nothing compared to the weight of the whole pendulum. In actual fact we don’t know what the pendulum bob should weigh, but it’s a fair estimate to say it’ll weigh quite a bit. I really hope the new casting was well made, because all that weight is going to be hanging off it!

The current bolts are temporary ones and that is not spring steel connecting the two parts yet – we’re having trouble tacking down a source of suitably sized spring steel strip (at a sensible price). You will also notice gravity arms have been added, as well as the brass plates that hold them, but more on that in another post.

Escapement fly

Escapement fly, back view.

The double three-legged gravity escapement uses a fly to dissipate some of the excess energy in the system. When the escapement unlocks the legs of the escapement move 60 degrees before coming to a sudden stop again. The fly helps to reduce the acceleration of the escape wheel and, like the fly on the striking train, needs to be able to continue to move a little when the arbour stops suddenly. Unlike the bigger fly this one doesn’t use a ratchet, instead it uses a friction device. We need enough friction that it gets turned with the arbour starts to move, but when the arbour stops it should be able to slip and come to a natural stop. The fly was made from a drawing supplied by Smith of Derby when my Father visited them. It’s no coincidence that it looks just like the one on the St. Alkmund’s church clock.

The pictures below show the completed fly, with a bolt through the middle where the arbour would normally be – which is why it looks a little wonky in some of the pictures.

Turret clock restoration progress report

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I thought a progress report was in order for the turret clock we (my father and I) are restoring, after I realised we started this project way back in November 2014! As a refresher the clock is a 1922 cast iron flat bed turret clock made by John Smith & Sons of Derby. It was purchased in a poor state of repair, having been butchered in the 1950s to make it electric drive and later having much of the electrics stripped back off. The original parts removed during the motorisation would have been recycled at the time so were no longer available to be replaced, later removal of the motors (etc.) just left the clock in a non-working state.

Since November 2014 a lot has been achieved. We now have fully working striking and chiming trains and the going train finally feels like it’s well on its way to completion. I’ve tried to list out everything that has been made or sorted so far. Pictures of the current state of the clock are below, along with some pictures showing details of recent additions to the going train. The red parts are all new, as are quite a lot of brass parts and several steel arbours which haven’t been painted. Eventually the clock will be properly painted but in the mean time the red primer makes a nice contrast between the new and original parts. There are a few temporary supports and bolts in the pictures, but this is still very much work-in-progress.

Striking & chiming trains

Going train

Other

  • Removed 1974 plaque (covered maintaining power hole) and patched bolt holes
  • New maintaining power – arbour, front bush, pushing mechanism, arm and weight
  • New winding handle

Our first bell

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I’m already resigned to the fact that I’ll never own the set of 8 full size church bells intended for the chiming train. However, I do now have one so the clock will at least be able to strike the hour. This is a proper bell too, exactly like you would find in your local church. Admittedly it’s at the smaller end of the scale but it really looks the part. It was cast by John Taylor & Company Loughborough who, just like Smith of Derby who made the clock, are still going today. I bought the bell a couple of weeks ago from a retired GP near Tamworth via e-Bay. According to the listing it was “probably made around the turn of the 19th/20th century for a school in Norfolk possibly Stoke by Nayland”. It came with a substantial bracket and was intended to be wall mounted and rung by pulling a chain. The bell itself is 12.5 inches in diameter, 13.5 inches high and weighs 56 lbs.

The previous owner of the bell did say that he’d had the bell checked out and that the length of the ringing tone was slightly shorted than it should be and so there could be a crack. It rings well to my ear and there is certainly no visible defect so I’m happy with. Don’t forget that Big Ben has been cracked pretty ever much since it was installed, mind you Big Ben doesn’t sound particularly good so that’s not saying much. The funny thing here is that before this bell went on eBay the previous owner offered it to UK Architectural Antiques in Staffordshire, but they weren’t interested because of this potential defect. This is the same company from whom I purchased the clock which, it should be fairly clear from this blog, was a long way from being in mint condition!

Positioning a new arbour

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Now we have our new wheel we need another new lantern pinion to engage with it and that pinion needs to be mounted on another new arbour. The position of the arbour needs to be chosen carefully to ensure a good mesh between wheel and pinion. Our horizontal position is fixed – it needs to sit central to the rest of the escapement position. However, we can choose the vertical position, up to a point, to ensure the correct mesh. The vertical position of the remaining escapement parts (still to be made) will then follow.

To aid the positioning of the arbour Dad made a little tool which can have the wheel and pinion mounted on it allowing the mesh to be perfected, before scribing the arbour position (where it crosses the horizontal midline). I’ve attached pictures showing the tool for this clock and a smaller version he used previously when making a couple of long case clock movements. The new tool (that’s the bigger one, in case you hadn’t guessed) isn’t quite finished as you can see compared to the original. From the pictures you can see how the mesh is adjusted with the wheel and pinion mounted on the tool. Then the tool’s wheel arbour can be placed in the bearing on the clock and the tool rotated to see where the pinion’s arbour crosses the horizontal midline and the position scribed.

Cutting a new wheel

Test cut wheels in plastic and brass.

So far we have needed to create several new lantern pinions but no wheels, until now. The next missing part of the going train is a wheel and we have already worked out how many teeth it needs (72 based on a 1.25 second swing of the pendulum). Based on the 72 teeth, and the dimensions of the arch made based on measurements from St. Alkmund’s clock, we ended up with a diameter of 4.522 inches. The pitch circle diameter of the 9 pin pinion it engages with is 0.478 inches.

Cutting the wheel should be easy in theory, with a milling machine and a dividing head, although finding the right cutter isn’t. These wheels should have epicycloidal teeth of a decent depth, to engage with a lantern pinion, but you just can’t seem to get suitable cutters any more. One option would have been to make a fly cutter, but we managed to avoid the need to do that. A test wheel cut with involute teeth and deepened slots seemed to work pretty well so we decided to go with that. The first test was cut in a bit of plastic fascia board, then a section was to be test cut in brass on a blank made from an old engraved sign. Dad got a bit carried away and cut the entire thing and after spending so many hours on it we decided to keep it, despite the remnants of engraved lettering on one side, at least for the time being. After the cut-outs to form the spokes there won’t be a lot of the lettering left and a little metal filler, maybe even a touch of brass coloured paint, and the last traces will disappear. If not we can always replace it later, but in the mean time we can move on to the next part.

The dividing head was from my Dad’s Myford ML7, but was set up on an unbranded ‘Micro Mill’ because it gave a better view at the point of cutting.

Freshly cast pendulum support

Newly cast pendulum support.

The pendulum support has been cast and we’ve just got it back from the foundry. It’s not quite finished yet though. It needs a bit of light sanding to remove some of the casting marks on the edges. Then it needs the bottom of the two feet flattening so it sits nicely on the clock’s frame, plus bolt holes drilling. Recessed areas needs to be milled to mount brass plates on the front and back. Finally, a v-shaped groove needs to be cut across the top for a small bar to sit in,  from which the pendulum actually hangs. This is another significant chunk (literally) of the clock done, or almost done, and allows further progress to be made on building the new escapement.

Pendulum support casting

Pendulum casting pattern, front view

This is the first update for a while on the clock, but we are still working in it. It was a bit cold for spending long in the workshop over winter. Then, as things picked up in spring, there were plenty of other priorities. As well as creating the pattern for the pendulum support casting we now have all the key measurements and positions worked out for the escapement. We have also started work on another missing wheel (but more on that in another post to come).

When the clock was converted to electric drive it was became little more than a gear train from the motor to the drive shaft. It no longer used a pendulum or escapement and unfortunately these were not kept with the clock. The pendulum would have been mounted on a large cast iron wishbone-shaped casting at the back of the clock. The arms of the escapement were also attached to this structure.

The pattern for the casting was made out of a couple of chunks of pine joined together. The basic shape was cut out with a band-saw. A small amount of hand carving was required for fine details and wax fillets were applied to the recessed front edge. Dimensions for the arch were taken from the clock of St. Alkmund’s Church, Duffield. The measurements were taken in situ, from the top of a ladder, while the clock was going. As a result they may not be quite 100% accurate, but very close to the original.

My father approached a couple of local foundries about casting the pattern. I was surprised there were any local foundries left, let alone a choice, but then Derbyshire was the birthplace of the industrial revolution! Neither of us really knew how much it was likely to cost but we were disappointed with the first quote of over £160. In the end we managed to get it for £55, with a 3 week turnaround. We’re still waiting and I’ll post pictures once we get the finished article. It’s going to need a little machining to complete. The base needs to be made flat and a v-shaped groove cut across the top for the pendulum to sit in. It also needs to accommodate two brass plates, one on the front and one on the back, to mount the arms of the escapement.

Going train revisited

Diagram of the going train

After examining the St. Alkmund’s clock we are able to start making some progress again. However, as the going train isn’t exactly the same as ours, we still don’t have all the information we need. Some details still need to be worked out. It’s also worth noting that even if it had had the same as the parts that we currently have, there is no guarantee that our missing parts would have matched – the engineer at Smith of Derby said there was a lot of variation between clocks. We may never know exactly what the original train was on this clock, so we will have to go with something sensible.

Yesterday, my father discovered something interesting we had never noticed before. The missing front arbour support, for the missing arbour, was attached to the frame by two bolts. The same support on the clock at St. Alkmund’s is attached by one bolt. That may not sound very important, and it’s possible we are reading too much into it, but of all the pictures of Smith flat bed turret clocks of this vintage we can find there is only one other that has two bolts for this support. The other is the clock at Trinity Collage, Cambridge. That clock has an extra dial at the end of this arbour and a second hand – something else the Smith engineer mentioned our clock could/ should have had. It is the presence and position of the extra dial that requires the two side bolts for fixing the support, rather than a single bolt through a front ”foot”. (I’ve previously mentioned how similar I though our clock was to the Trinity clock (except for their unique fourth train) and I did contact the keeper for more details about the clock, but unfortunately never received a reply.)

So, if we assume from this observation that our clock had a second hand then that fixes the rotation speed of the missing arbour to one rpm. As the lantern pinion at the front end of this arbour must turn the 120 tooth wheel at a rate of one tooth every seven and a half seconds then that pinion must have eight trundles. Suddenly we have two less unknowns in the train!

I previously suggested some options for the missing parts of the train, but if we take the above (60 second rotation, 8 pin pinion) as correct we can improve the list of options:

  • 1.5 second swing, 60:9 ratio at escapement.
  • 1.25 second swing, 72:9 ratio at escapement.
  • 1.125 second swing, 80:9 ratio at escapement.
  • 1 second swing, 90:9 ratio at escapement.

The engineer at Smith’s did suggest a 90:9 ratio, but I find it hard to imagine this clock only had a one second pendulum. That’s only about a metre, the same as a typical long case (“grandfather”) clock. The pendulum at St. Alkmund’s is 1.5 seconds or about 2.24m. The Trinity clock pendulum is described as being “2 metres” but it’s not clear how they are measuring it. The length of a pendulum is measured from the pivot to the centre of mass and so the overall physical length of the pendulum may be quite a bit longer. 2m is not a standard/ common length so I suspect it’s an approximation of the physical length rather than an accurate “functional length”.

As we’ll never know what this clock originally had we can choose one of the above options, keeping in mind the practicalities of the pendulum length. Only the 1 second pendulum would fit the clock on its current stand and would be the most convenient length, but that feels to short to me. The 1.5 second pendulum is far to long to be practical, so our current intention is to make it with a 1.25 second pendulum and 72:9 ratio. We just need to make sure that actually fits around the rest of the escapement mechanism.