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bobhare

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  1. This shows the Knife edge bearing and the lower impulse face narrower than the upper. View of escapement and pendulum arrangement. This shows the knife edge in it's fitting. A point i forgot to mention earlier. It seems the pendulem is not getting enough energy from the escapemant so i increased the weight driving the wheel but this only seems to have the effect of making the movement clunky. The wheel rotaes faster and the pallets don't have time for the locking faces to enter properly.
  2. I'm building a clock with woden gears and have set up the escapemant with which I'm having some problems. The clock is pendulum driven and the mechanism is driven through a chain wheel. I have been testing the escapemant without the gear assembly to eliminate any issues from that. I've used the design provided in Brittens Watch and Clock Handbook for the Dead beat escapement with 30 teeth on the wheel. The pallet is solid and made from steel whilst the wheel is made from aircraft grade aluminium. (I tried making it from brass but the teeth profiles were poor). They are both CNC machined and are to a good tolerance. However, the pallet, when manufactured initially had a small amount of asymettry and this was corrected by grinding one of the locking faces. As a consequence one of the inpulse faces is slightly narrower than the other. The bearings supporting the shaft were initailly plain PTFE bearings but in order to reduce friction I changed them to knife edge bearings with with the edge of the knife on the centreline of the shaft. This the nreplicates exactly the rotatin of the shaft. The knife edge sits in a 3D printed fitting and i can adjust the distance between the wheel and pallet shaft by printing different fittings. The pendulum bob weighs about 110grams and is supported by a 2.5 mm steel rod via a pendulum spring to the pallet shaft. For testing I have wound a piece of string around the second/escapement wheel shaft with a small weight on it. The weight is 6gr on a 6mm shaft. If I swing the pendulum and release the weight wrapped round the shaft all goes well. There is good engagement of the lock faces and the tik/tok sounds balanced. However, with time, the amount of lock decreases and themechanism stops. I have varied the seperation of the two shafts and the size of the weight driving the second wheel with little effect. I'll post some images when i've worked out how to do it. Any help would be appreciated.
  3. Update. I've now made the following changes and things have significantly improved; more later. I remade all the gears using cycloidal tooth profiles. There is much more freedom in them and hence any meshing friction is minimised. I fixed all the gears on their own pinions and placed their bearings in the plates. I'm still using PTFE bearings but as one of them is threaded I can adjust the end float. I have not polished or lubricated the teeth I had the pinion for the second hand far too long and it projected from the front plate by about 100mm. As it was made from steel it applied differential loads on the bearings in the two plates. I shortened it and made it from wood. With the initial design it required 3.5kg on the chain to turd the gears but with the above modifications it requires just over .5kg. I believe the main problem was 4. above. 1 unit of frictional torque on the second shaft requires 1500 units of torque on the chain wheel to overcome it. nevenbekriev; without you i would still have been swimming around in a sea of uncertainty. A big thanks. I'm now setting up the esacape mechanism and whilst i'm nearly there i'm having some problems which I'll post sepearately under "Dead beat escapement problem". If you could help on that i'd be grateful.
  4. I think that i understand your reasoning. Gear 3 currently runs on the shaft to which pinion 1 is fixed and whilst they both run in the same direction they run at different speeds. I can fix the shaft on which they currently run and only gear 3 will run on this shaft. I will then have to put pinion 1, fixed to the escape wheel,running on a new seperate shaft. Is this correct? Driving the gear box low to high was a concern early in the design but i could see no way around this; hence making the plastic assembly. I used involute gears because i thought they would be the best but from what you say obviously not. What gear form would you recommend? Thanks once more for your help. I've taken pinion one off the shaft on which it is currently fixed and had it driven by wheel 2 whilst on a seperate shaft and it seems to work well. I just held the shaft so the gear engaged but will have to make a better trial. I'll keep you posted. Loss of the second hand is no problem.
  5. Thanks for your reply;much appreciated. As you have probably realised i know very little about clocks but suddenly decided to make one. Some images of the clock here: https://www.flickr.com/photos/188874329@N06/? The minute hand is on gear4 and there are two other gears between this and pinion 1, the second hand. Unfortunately the second had rotates anticlockwise so there is another gear to reverse the rotation. Between gear4 and the winding drum there are a further two gears which drive wheel 7 which is locked to the winding gear. There are then another set of gears to drive the hour hand. I could send the spreadsheet with the details but i don't think it will add any value. The gears range from about 25mm to 140mm diameter with modules of between 2 and 4. All the shafts are 6mm silver steel finished to a mirror finish. I used PTFE as it has the fhird lowest coefficient of friction of all solid materials. PTFE if one of the few plastics that remains stable and does not shrink with time or moisture. Under no load the gears spin freely on the shafts and i'm bacoming convinced that the problem lies in the friction between the gear teeth. I first assembled the gears with untreated gears but it was even more difficult to spin the gears. The lubricating wax is by Liberon and is used for reducing friction on cutting tools used on woodworking machines. I have tried the the dry PTFE lubricant on a piece of wood and it doesn't seem to reduce the friction. I think the gears just reach a critical point were the force applied to the winding gear is such that the friction on the bearings and gear faces increases non-linearly to the point were the gears lock, or nearly. I could reduce the gearing between the winding gear and the minute hand by four which would make it a 2 day clock but i'd rather find a solution to the arrangement i have. Any ideas on reducing gear friction would be welcome.
  6. There are 18 gear wheels; being 7 pinion/wheel combinations and four separate pinions. The shafts rotate in plain bearings with the steel shafts running in PTFE plain bearings. After using 1000 grit paper on the shafts i further polished them with brasso, a mild abbrasive used for polishing metal. Could you explain what you mean by, "Their size is very important"? I'll upload some photos when i have mastered the technology. Thanks.
  7. I've designed and built a clock with wooden gears but without applying excessive torque on the chain wheel the gear train will not run. A little bit of background. I wanted the clock to have a pendulum, have a second finger and require winding every eight days. The escapement wheel has 30 teeth and is on the shaft for the second hand. An arrangement of gears with a ratio of 1:60 drives the minute hand and a pair of gears with a ratio of 1:12 drives th hour hand. In order to achieve the 8 days between winds a further set of gears with a ratio of about 1:24 drives the hour hand. This makes the gear ratio between the chain wheel and escapement wheel 1:1440; quite a ratio to drive! With all the gears in place the chain wheel torque to drive the mechanism is too high and would require too much weight on the chain. However, if I lift the final pinion on the escapemant shaft so it is disengaged from the gear train the gears run smoothly with virtually no torque on the chain wheel. With the exception of the connection of the gear train to the escapement wheel shaft all the other gears are in place and run as required. I just don't understand why engaging this one gear almost stops the mechanism so dramatically. There are a total of 18 gear wheels and pinions and adding one more element of friction shouldn't have the effect it does. I realise the last straw brakes the camel's back I'm using PTFE bearings on silver steel shafts. (I originally used oilite bushes and changed to PTFE to reduce friction but there has been little benefit.) I have polished the shafts with 1000 grit wet and dry and brasso and all individual gears on their shafts spin extremely freely. I have used a lubricating wax on the gear teeth and polished them. All the involute gears and the support frames are CNC machined and are therfore pretty accurate. My next step is to strip the wax from the gears and replace it with a dry PTFE lubricant. Interestingly, I made a proof of concept mechanism using a 3D printer. I wasn't too bothered about accuracy so the bearing fits are loose and the PLA runs on wooden shafts but it runs quite freely. I sprayed the teeth with furniture polish but PLA is quite slippy anyway. So, I conclude that if i can reduce bearing or gear friction all should be well but any advice would be welcome. I've spent a few hyndred manhours getting to this stage and wouldn't like to see them wasted. A few images would have helped me describe the problem better but I can't find a way of uploading them.
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