Diagnosis and Treatment of the Dahon Joule Dynamo Hub
Diagnosis and Treatment of the Dahon Joule Dynamo Hub
Steve Weeks, DDS
The Joule hub is a simple affair. It comprises a stator, a rotor, two bearings and the necessary structural parts to hold them together and allow the electricity produced to be conducted away to do its job. The “stator”, or stationary part, consists of a coil of wire would on a core of steel with many poles (Image 1). The core is directly connected to the axle, and does not rotate with the wheel. The “rotor”, the moving part, is the hub shell itself, which has four ceramic magnets mounted on its inside circumference (Image 2); these magnets constitute the “field”. This is the reverse of an “automotive” alternator, which has a stationary stator coil surrounding a rotor which serves as the field coil. The Joule hub contains no voltage regulator or rectifier; it is a simple alternator and, except for the bearings, has no parts that can wear out under normal conditions. There seem to be two basic types of problems that can occur with this level of simplicity: electrical conduction failure, and bearing failure. I am able to report on one failure of each type.
I have been using the Joule hub generator on my two Dahon folding bicycles for about five years. During that time, I have been generally well-pleased with the hubs’ performance. It was always a matter of curiosity what was inside one of these hubs: Are the bearings cartridge or cup-and-cone? Are they amenable to service for prevention or repair? The hub’s specifications suggest that it is not meant to be serviced.
My oldest Joule hub, an early production model, acquired in 2005, stopped working in early 2007. Luckily, I had acquired a third one as a “spare”- just in time, as it turned out. Here was what I was looking for: a no-lose opportunity to examine the inside of a dynamo hub and see if it was able to be serviced.
The hub began to be erratic providing enough electricity to light the Hella lamp. A few times the light went off by itself; in some of those cases, it came back on. Eventually, no light was produced. A voltmeter attached to the output of the hub would show voltage, but it was erratic and would not light the lamp. All the wires were checked and found to be OK; connecting the wires to a 6 volt battery resulted in normal performance of the light and on-off switch. I installed a spare wheel and brought the broken one in; on the bench, an ohmmeter showed an open circuit between the output terminals. Interestingly, there was also an open circuit between the “ground” terminal and all other metal parts on the hub, which was a clue.
Diagnosis: open (broken) circuit somewhere in the hub; the wire of the stator windings could be broken, but a defect in a connection seemed more likely.
It is not necessary to open the hub to address the connectors. The parts of the hub that are especially vulnerable to corrosion are all accessible with simple tools.
Remediation of Corroded Contact Parts:
1) Remove the 17mm locknut (Image 3), then gently pry up the metal (steel) washer on top of the connector assembly (image 4). This was a bit tough to get started because there is epoxy cement holding the parts together. I think a sharp knife worked under the edge of the connector would work better than the screwdriver I used. I stabilized the axle for this operation by using two axle nuts in a “jam” configuration on the non-output side of the hub and clamping the most outboard nut in a vise, as seen in the image. Note the scrap aluminum used to prevent scarring of the nut.
Underneath the washer is the “ground” connector ring. This is made of brass, and is supposed to be in good contact with the underside of the overlying steel washer. If these look like the parts shown in Image 5, you probably have found the problem.
2) Tease the ground connector and its ring out of the gray plastic shell, being careful not to bend it. Then it will be possible to gently pry out an inner plastic piece (Image 6) that separates the ground from the “hot” connector (the convenient “tab” is a good place to start). The “hot” connector is another brass piece which has a wire soldered to it (Image 7). The wire is the “hot” side of the hub’s windings; the other end is connected directly to the axle inside the hub (Image 8). Carefully unsolder the wire from the ring (Image 9). The wire may be straightened (Image 10), allowing the “hot” connector to be removed for cleaning.
The corrosion may be removed with Naval Jelly or other chemical means. A brass or stainless steel wire brush in a Dremel tool can be used cautiously to dress the surface. (image 11).
Reassembly is the reverse of the previous process.
In the Spring of this year, 2010, another of my Joule hubs began to display a little looseness in the bearings; the hub could move a millimeter or two sideways along the axle. I don’t remember exactly how this came to my attention. The wheel seemed to turn freely, considering the magnets.
By the Summer, I had the feeling that the bike was “harder” to pedal. When I tried lifting the bike and spinning the wheels, the rear was fine (whew!), but the front would not spin even one full revolution. It was almost like the generator output was short-circuited, but the light worked normally. Fortunately, I had a spare Joule-equipped front wheel with normal bearings, so I installed it and found the bike much easier to ride. The old hub’s axle could not be turned by hand off the bike.
To my knowledge, there are no details available on the construction of the Joule hub. An exploded view is available on-line of a Shimano hub (DH-3N70-2N70) (Image 12) which bears a striking similarity to the Joule hub. One wonders if, in fact, these hubs share a common ancestor. One major difference is that, while the Shimano hub seen above has one cup-and-cone bearing and another that is not shown, the Joule hub has two identical cartridge bearings, type NBK 6001-2RS. The manufacturer is NBK (Chuang Hon Yi Axle Co., Ltd.)
Here is the sequence of operations used to dismantle the hub beyond steps 1 and 2 above:
3) Remove the jam nuts from the non-output side without clamping the output side axle in the vise unless you have an axle clamp. Being careful of the wire so as not to damage it, place the axle nuts on the output side and jam them.
4) Clamp the output side jam nut in the vise, then gently pry off the gray plastic nut cover on the non-output side. (Image 13)
5) Using a 17mm cone wrench and an adjustable (or 17mm) wrench, remove the lock nut on the non-output side. (Image 14)
6) Using a 17mm wrench, remove the bushing from the non-output side. (Image 15)
NOTE: the collection of removed parts should now look like Image 16. From left to right and top to bottom, the parts are in order from locknut on the output side to locknut on the non-output side. The thin, transparent plastic ring seen at the right end of the middle row was discarded.
7) Stabilize a 1-1/4″ (32mm) socket upside-down in a vise. I did this by setting the socket on a 1/2″ drive breaker bar clamped in a vise. (Image 17) The axle will interfere with the drive seating all the way into the socket’s square hole, but I was able to simply set the socket on the drive without clicking it on. A deep-well socket would make this simpler. Notice that the lip of the socket has been ground off to permit maximum engagement with the rather thin hexagonal “nut” which is part of the output side bearing holder. It is *not* necessary to cut a window in the side of the socket; that was done for my first Joule disassembly when I did not realize that the connector housing could be removed separately from the bearing holder.
8 ) Place the hub onto the socket, output side down engaging the bearing holder. Using the wheel’s rim, turn counter-clockwise to unscrew the bearing holder. The bearing holder and the stator coil assembly may now be removed from the hub. (Images
18 and 19)
On this particular hub, it is clear from Images 19 and 20 that it was the output-side bearing that failed. It looks as if water got into the bearing. This bearing cannot be turned by hand; the non-output bearing feels smooth. Compare Image 20 with Image 21, which is of a stator from a Joule hub that was not failing.
9) Pry off the hexagonal spacer (Image 22) that sits on the outside surface of the output-side bearing; it is held on with a bit of some form of adhesive, but should be able to be removed without difficulty. Do not turn it with a wrench! Doing so will wrap the output wire around the axle and possibly damage it. Should this occur, it may be possible to repair it; details further on.
10) Remove the stator assembly from the output-side bearing by supporting the bearing holder in a vise and tapping the axle through with a plastic mallet. Support the stator assembly from below to prevent it from falling. Image 23
shows the parts after this operation. Another view of these pieces is seen in Image 24; the hexagonal piece in question is identified by a red “1”. The bearing has been removed and the part of the stator assembly that is mounted inside the
bearing’s inner race is identified by the blue “2”. In this particular hub there was quite a bit of corrosion damage to the bearing and superficial corrosion on the stator as may be seen.
11) Once the stator is removed, the bearing itself may be removed by supporting the holder in the 32mm socket used in step 7, and driving it through (toward the *outside*) with a socket about the same size as the inner race struck by the plastic mallet. Image 25 shows an intact output-side bearing holder, bearing installed, and another holder with the bearing out.
12) The non-output side bearing may be removed by putting a large drift into the inner race and driving the bearing out while the hub is supported on a padded surface. A socket whose diameter is the same as the inner race may be used instead of a drift; a plastic mallet is recommended. Note that this is *not* the way to install the new bearings; they will have to be pressed in using pressure on the race with the tight (“interference”) fit to avoid damage to the bearing balls and races. This will be illustrated presently.
13) Once the bearings have been removed, the hub shell may be cleaned as needed. One of the stators in this series of images was clean, and the other was corroded. The corroded one was cleaned up using a wire brush in a Dremel tool. Note that this operation, and any other that may generate bits, flakes or filings of a ferrous metal should be done well away from the hub shell to avoid the magnets becoming contaminated with metal fragments. The magnets are strong, and small pieces of iron or steel will be very difficult to remove. Another potential concern is the loss of magnetic field strength if the stator is left out of the hub for a prolonged time period. In the past, magnets required the use of a “keeper” to prevent this, and the stator would serve this
function here. The magnets in the Joule hub are “ceramic” magnets, which may not be as susceptible to this strength loss. It is recommended to keep the stator inside the rotor anyway; better to be safe than sorry.
14) Now begins the reassembly of the hub! The output side bearing holder has been cleaned of all the old grease and corrosion, then the bearing seat area is coated with a bit of grease to facilitate installation. Image 26 shows the holder and the new bearing mounted in a vise. The vise is lined with a thin aluminum sheet to protect the holder. The bearing is about to be pressed into the holder with a Craftsman 13/16″ spark plug socket, which has an outer diameter slightly less than the bearing’s outer race. This is necessary to prevent damage to the bearing seat, while permitting the bearing to be fully seated. Image 27 shows the bearing fully seated… it can’t be seen! Note that no pressure has been placed on the inner race, which is unsupported at this point. This is important to avoid damage to the bearing. Image 28 shows the completed holder/bearing assembly.
15) Image 29 is the tool fabricated from metric hardware (from Ace!); the threaded rod just fits through the bearing’s inner race. All this hardware set me back less than 5 bucks. Image 30 shows the non-output side bearing with the tool in place. Note the washer that engages only the outer race of the bearing, since it is the outer race that is being pressed into the hub. Image 31 illustrates the output side with the tool in place. Note the bearing holder has been screwed in (just finger tight), and the washers transmit the load to the holder, not the previously-installed bearing. Image 32 shows the bearing being installed. There is not a lot of force required on the wrench, but the fit is very snug. A view of the installed non-output-side bearing through the rotor magnet assembly is seen in Image 33.
A few words of explanation are in order at this point. When I was learning (“trial and error”) how to dismantle the stator assembly from the output-side bearing holder, I used a wrench on the hexagonal spacer mentioned in step 9 above. This turned out to be a bad idea, because the spacer is not threaded but just held in place with adhesive. When I turned it the wire was sheared off too close to the stator to reach out to the connector; it would have to be spliced. As luck would have it, I had a roll of the right gauge of “magnet wire” on hand. The easiest way to strip insulation from this type of wire (insulated by a thin layer of varnish) is to hold a match under the part to be soldered until the wire glows. This burns off the varnish, and a light scraping with a knife easily removes any oxides that might interfere with soldering. A bit of flux also helps. Image 34 shows a length
of magnet wire soldered to the stub of wire from the stator. A short length of blue insulation removed from another wire has been slipped over the stub from the stator coil. Image 35 shows a length of red heat-shrink tubing placed over the new wire extension. The joint is covered with a short length of blue heat-shrink tubing (Image 36), and the wire is positioned in a slot in a bushing on the axle; this slot allows the wire to pass through the inner race of the bearing cartridge. Note the hexagonal spacer, which (for demonstration purposes) has been placed over the axle; the bearing will be located between this spacer and the stator coil.
16) Image 37: The stator assembly, showing a bit of epoxy stabilizing the joint in the wire, and a bit of silicone rubber paste where the output wire goes through the groove in the bushing. The epoxy is just to make sure the wire doesn’t migrate under impact loads, as there are rotating parts not at all far away; unless the wire needs to be spliced, the epoxy is not necessary. The silicone is to seal the groove; there were remnants of a similar material when the hub was dismantled. Image 38 shows the bearing cap installed. Fortunately, it was not necessary to figure out a way to press the bearing on. It is a “slip fit”; the silicone
that was there originally was probably why I had to use gentle persuasion to get the bushing and stator out of the bearing. Image 39: Shows the output-side cap being tightened. Note: the non-output side must be *loose* during this operation
to avoid displacing or damaging the non-output-side bearing. The notch in the socket, though unnecessary for disassembly as described in step 7, is handy here to avoid damage to the wire; however, with care damage to the wire may be avoided. Image 40: The installed cap and the non-threaded spacer in place. A small amount of silicone was placed between the spacer and the axle.
17) Image 41: The non-output-side bushing (also a slip fit in the bearing) is now fully tightened. Since these are cartridge bearings, there is no adjustment for bearing play. Image 42 shows the non-output-side lock-nut installed and tightened over the bushing.
This completes the reassembly of the hub proper. All that remains is replacing the output connector parts, and the hardest part of that will be soldering the wire back on to its conductor ring.
18) Image 43: The output wire is passed through the gray plastic connector housing and bent around to see how long it needs to be (in this case, because the wire is new). Image 44: The wire has been trimmed, stripped and soldered to the “hot” connector lead. The red insulation seen is a piece of small-diameter heat-shrink tubing; this was cut with a normal wire-stripping tool. Image 45: The plastic insulating washer in place. Image 46: The “cold” (“ground”) connector in place. Image 47: The outermost washer, protecting the connectors, in place. This washer is steel and will rust (badly) if exposed to salt and water (read: “Chicago Winter”). In corrosive environments, I recommend coating this end of the hub with silicone grease. Image 48: After tightening the output-side lock-nut, hub assembly is complete!
Miscellaneous follow-up observations:
The axle has no play in it, and it spins quite freely except for the normal interaction between the magnets and the stator. It is like new, except now I probably should replace the rim, which is quite worn. Still, not counting my time (which is my own!), the cost of overhauling this hub is well under $10. Not too shabby. Hopefully, this is well-enough documented that anyone else can do this repair if they so desire.
As far as testing goes, I plugged in a connector and hooked it to an AC voltmeter. Spinning the wheel, I get 10 volts easily. This is without any load; with a bulb connected the voltage would be less. Also, in case it is of interest, the electrical resistance of the stator coil is about 2 ohms, measured from the output connector. Both of my stators measure the same, so I suspect it is right. The important thing is that it is neither *zero* (short circuit) or *infinity* (open circuit).
Here are some images of the bearings. Image 49 shows the inner aspect of the output-side bearing in its holder; the bearing identification markings are clearly seen (NBK 6001-2RS). Image 50 shows the shipment of new bearings received from “SkateBearings.com” for $1.56 each, plus shipping. The failed output-side bearing, with its seals removed, is seen in Image 51. The last image (Image 52) is the bearing after being dissected… lots of rust there. This hub made a dry rubbing sort of sound intermittently since it was new. I wonder if the bearing was partly corroded even then.