Rejuvenating a Chronometric Speedo - PHASE 5

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Re: Rejuvenating a Chronometric Speedo

Postby Pharisee » Wed Nov 07, 2018 7:08 pm

39speedtwin wrote:Also my repairer states that mounting a Chronometric on rubber increases the vibrations.


Hmmm... interesting. Is there any evidence to show that this is so?

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Re: Rejuvenating a Chronometric Speedo

Postby clanger9 » Wed Nov 07, 2018 7:24 pm

Not sure I believe that.
I initially mounted my Chronometric straight onto the bracket. After the face plate came loose, the needle fell off and one of the mounting nuts undid itself (3 separate incidents...), I realised I needed to rubber-mount it. It's been fine ever since...
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Re: Rejuvenating a Chronometric Speedo

Postby clive » Wed Nov 07, 2018 8:21 pm

The heavyweight headlight mounted speedos should be rubber mounted with a shroud the fits under the flange and extends slightly round the body.
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Re: Rejuvenating a Chronometric Speedo - PHASE 2

Postby Group Leader » Fri Oct 04, 2019 12:48 pm

I noticed the speedo was making an unusual clicking sound the other evening when I returned from a ride, it's done about 2500 miles since I originally rejuvinated it.

After dismantling it (so much easier this time!) it became clear that the main gear drive gear mounted on the input drive spindle was heavily worn as evidenced by the brass fillings scattered round about and a distinct lack of teeth. It looked clean and dry inside, maybe I heeded the warnings of "over-lubrication" a little too enthusiastically or maybe just coincidence? Looking carefully at the gear and how it meshed with the next one in the train (on the rocking gear shaft) it was clear that the driven gear was riding up over the top of the remaining teeth on the drive gear and in effect forcing it and the input shaft down and out of the case. This is the exact opposite of the normal problem created by using the incorrect speedo cable which is too long that forces the shaft up and into the case I understand.

Worn Gear 1.jpg


Worn Gear 2.jpg


After some discussion with Clive (as in the Adverts Policeman :D ) I had another close look at the mechansim and found the probable cause of the wear was in fact excessive freeplay and thus movement of the driven gear. This gear is retained in position by a small steel top hat bearing that is a good, peened fit into the lower brass baseplate. In one direction there was about 1mm sided to side movement of the gear whilst at 90 degrees there was none. This bearing also provides the restraint for the lower end of the rocking shaft and thus this must be removed in order to gain access to the top hat bearing.

I'm still not brave enough to completely disassemble the speedo mechanism and so I just loosened off the top plate sufficiently to allow the rocking shaft bearing top arm enough upward movement to enable the shaft to be extracted. As soon as it was out the assembly was closed up and the screws tightened so nothing else could escape. With the rocking shaft removed access was provided to enable the top hat bearing to be drifted out.

Worn Bearing Removal.jpg
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Re: Rejuvenating a Chronometric Speedo - PHASE 2

Postby Group Leader » Fri Oct 04, 2019 1:11 pm

And here is the worn out bearing, sorry about the quality of the phootgraph it was taken on my phone and it's very small! However, you can clearly see the undercut in the bearing on one side.

Worn Bearing.jpg


With Clive's assistance, I was able to source a replacement drive gear but no luck with finding a top hat bearing. As I have a small clock/model makers lathe I set to measure up the old one and turn up a replacement.

New Bearing.jpg


This was turned to be a good running fit in the gear and almost a very light interference fit in the brass lower plate. Once the gear was mounted on the bearing it was pushed into place in the lower plate and then peened in position using a four cornered tool (a small, sharp Phillips driver bit).

Here is the new bearing and gear fitted but before the rocking gear was replaced.

New Bearing - Bearing Installed.jpg


Once the bearing and gear was fitted the rocking gear was replaced at which point the opportunity was taken to re-align the finger springs with the speed recording wheels. This was done by slackening off the two mounting screws, adjusting the springs so the fingers sat nicely over the centre of each gear and then nipping up the screws.

The worn main shaft driving gear was pushed off the shaft by mounting it loosely in the lathe chuck and then using the tail stock to gently push the gear along and off the splines. The replacement was then pushed into position using a similar method which ensured it went on nice and square.
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Last edited by Group Leader on Fri Oct 04, 2019 1:41 pm, edited 2 times in total.
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Re: Rejuvenating a Chronometric Speedo - PHASE 2

Postby Group Leader » Fri Oct 04, 2019 1:33 pm

Once assembled, it needed testing so a little test box was rustled up using a small, geared, dc motor which was used to propel model aeroplanes in a former existence with the speedo mounted on a bar attached with some 6mm studding. The flexible drive between the geared motor shaft and a bit of square section brass tube (representing the speedo cable end) was a bit of silicone tube used as fuel pipes on model aircraft but also used for air pipes on fish tanks etc.

Test Box.jpg


It seems to work and this time I've been slightly more generous with the lubrication. Time will tell if it's a long term fix but it has been a most interesting exercise.

Here are a few videos of it working but before replacement of the odemeter and tripmeter drive mechanism.

The first is a general view of the device in operation, the second shows the rocking shaft driving the (lower) speed sampling wheel. If you look carefully you can see the (middle) recorder wheel being pushed round by the sampling wheel as each sample is taken. The third video shows the cams that operate the finger springs that hold the recorder and indicator wheels in position and the escapement (hence 'clock') that regulates the movement of the rocking shaft and thus the sampling frequency.

What an amazing bit of kit the chronometric instrument is!

Many thanks to Clive for his assistance and sharing his knowledge.

Hopefully this will be of interest and possibly use to others.

Alan

https://www.youtube.com/watch?v=TemYEpwDEyI&t=9s

https://www.youtube.com/watch?v=PNyC-lz0CJU

https://www.youtube.com/watch?v=eKfLTUb-5u4
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Re: Rejuvenating a Chronometric Speedo - PHASE 2

Postby Pharisee » Fri Oct 04, 2019 1:47 pm

Well done... I'm not sure I would have tackled that. Then again... maybe I would!! :D
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Re: Rejuvenating a Chronometric Speedo - PHASE 2

Postby 39speedtwin » Fri Oct 04, 2019 2:13 pm

Normally that gear gets worn because the cable imput is too long.

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Re: Rejuvenating a Chronometric Speedo - PHASE 2

Postby SPRIDDLER » Fri Oct 04, 2019 4:37 pm

Such skill! I wouldn't have the courage to fiddle with one.
Great Youtube clips too which convinced me that out of all the engineering on our old bikes the chrono speedo is far and away the most complex, precise and sophisticated part. You can't restore one o' them with a Helicoil or a pair of Mole grips. :(
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Re: Rejuvenating a Chronometric Speedo - PHASE 2

Postby clive » Fri Oct 04, 2019 5:52 pm

39speedtwin wrote:Normally that gear gets worn because the cable imput is too long.

I would agree with this but usually when this happens the slot in the worm drive gets damaged and the plate that holds it in place is worn on the underside. Alan reported neither damaged in thus case.

Well done Alan.
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if it ain't broke don't fix

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