 The Clarinet BBoard
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Author: Wes
Date: 2002-04-11 03:47
Does anyone have a recommendation for repair of a plastic bearing on a pivot screw on the upper joint link mechanism of a 1999 LeBlanc Concerto? This joint has some play in it which, on most clarinets, could be fixed by swedging. Usually this is a metal to metal bearing but, in this case, there is a plastic insert in the hole in the key rod. Would you put in epoxy and let it dry or what? This kind of clarinet construction has not been seen before by me. Thank you for possible suggestions for fixing it.
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Author: Jeff
Date: 2002-04-11 09:42
Wes,
Not being familiar with the plastic, nylon or whatever bearing surface insert...is the fix possibly the replacement of the insert?
If not and IF I understand your question...you might try countersinking the area of the post that the head of the pivot screw rests in. If you countersink just a tad too far..you got a different problem.
It might be Tech time, shouldn't be a difficult or lengthy repair.
Jeff
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Author: Gordon (NZ)
Date: 2002-04-11 11:40
It sounds a silly design to me!
I cannot think of a polymer tough enough to take what this pivot has to during relatively normal handling - which, no doubt, is why you are having trouble with it.
Although counterboring the post to achieve a deeper screw seat is appropriate for very rare adjustments for metal to metal, I would not interfere with it isn this case, because you may have to continue a long way down this road, finishing up with no thread left in the post!
It seems the appropriate cure is to pull the insert out with a very small crotchet hook and replace it. I guess you then have to handle this clarinet with a lot more care than most of us are used to.
A permanent cure would be to fill the insert's cavity with silver-solder and redrill/shape the hole to fit the screw, as per a conventional clarinet.
It seems that the traditional method of robust construction is now too difficult for the countenance of the Le Blanc accountant. Shame!
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Author: Wes
Date: 2002-04-11 17:13
Thank you, Gordon, for your sage comments. I later looked at the same bearing on a Vito clarinet of the same vintage and found the traditional(but off-center) metal to metal bearing.
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Author: Gordon (NZ)
Date: 2002-04-12 03:31
It seems that very few changes that are made to instruments are for the better, except for the reduction of manufacturing costs.
It is strange that most of these undesirable changes seem to appear in above-student-range instruments.
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Author: JMcAulay
Date: 2002-04-12 16:44
On the contrary, I can think of a polymer that is extraordinarily well-suited for this application: it is nylon. Nylon is suitable in a great many bearing applications, including those which must withstand pressures of several tons. In slow-moving applications, it provides much smoother operation than metal-to-metal contact, better in many cases than ball bearings. A nylon insert should last longer and operate more easily with less wear than a metal-to-metal interface. I can think of only one advantage of a metal-to-metal interface: lower cost.
But this one wore out? Well, sometimes things wear out. But that doesn't necessarily mean it is a stupid design. Maybe the part was defective from the beginning. Take a look on the same instrument at other pivots, and see what shape they're in.
By the way, I would consider no polymer ecept nylon for such an application. Some others may appear promising, but they don't have the long successful history that nylon has in bearing applications.
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Author: Gordon (NZ)
Date: 2002-04-13 02:11
JMcAulay. You mention pressure of several tons. That is a measure of force, not pressure, and therefore irrelevant. Many tons over a very large area represents quite a low pressure. (Remember that stilleto heels do more damage to a floor than an elephant walking over it)
I thinjk the issoue is one of pressure though. I don't think wear is particularly significant, certainluy over a few years only.
During the handling of a clarinet (especially by children, and during assembly), or during relatively minor knocks, it is easy to exert significant force on a pivot screw via a key. This force is exerted over a very small area on a pivot screw, and this area becomes far smaller if there is a slight flexing. Flexing ALWAYS happens and it is entirely possible that it is significantly large to be significant.
A significant force over a very small area can represent a very large pressure over that area.
One measure of the strength of materials is how much pressure it can take before distortion, i.e. compressive strength.
The compressive strength of nylon is around 2500 psi.
The compressive strength of nickel silve is probably higher than its tensile strength which is around 80000 psi.
Also, be aware that when a technician exerts force on a key to correct key cup and ring alignment, or adjust linkages, the force is FAR higher than that a player exerts. Misalignment is extremely common ex-manufacture, but also occurs frequently from mishandling.
An instrument needs to be made strong enough to cope with normal repair and servicing techniques, not just showroom activity.
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Author: JMcAulay
Date: 2002-04-13 17:55
Gordon, you have said that my statement "pressure of several tons" is irrelevant. Actually, it is simply a bad statement, as I left one thig out completely. I should have written "tons per square inch," but you have also corrected me in that the compressive strength of nylon is not substantially more than a single ton per square inch. Which still makes it, however, pretty strong stuff.
I also would hope that you, or any other technician, when adjusting any alignment on an instrument, would not exert great lateral force on a pivot screw. To do so would be, in my view, an improper practice.
Other than those items, I am in complete agreement with your last post.
I wonder if Leblanc suggests oiling their nylon bearings? That might be not a very good idea. I'm curious to learn about their recommendation.
Regards,
John
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Author: Gordon (NZ)
Date: 2002-04-15 03:30
Oiling nylon....
Some oils probably damage it, perhaps making it soft, or brittle.
Some nylons, I believe, have lubricating appendages on their molecules, or have lubricating matter suspended in the mix.
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Author: Gordon (NZ)
Date: 2002-04-15 11:32
John wrote <I also would hope that you, or any other technician, when adjusting any alignment on an instrument, would not exert great lateral force on a pivot screw. To do so would be, in my view, an improper practice.>
I would hope for the same in an ideal world. And of course it all depends on what the word 'great' means. A force less than what would bring any associated materials to their respective elastic limit causes no damage, and is a perfectly acceptable repair technique. If nylon is incorporated in the bearing all it means is a safe force is reduced to about 1/30 of what it was without the nylon, and hence certain servicing techniques have to be replaced with others which will probably take considerably longer, hence be more expensive.
Most changes in construction of instruments are to shorten manufacturing time and hence reduce manufacturing costs and increase profits. But very frequently they greatly lengthen servicing time. In the long term they are often more expensive for the owner.
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