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- Oct 30, 2019
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- 225
Someone reached out to me asking if I could do bolt extraction and repair on the rear wheel of a 1978 Ducati motorcycle (Darmah maybe?)
This is their "Cushion drive" design, where there is a rubber damper (not unlike a lovejoy coupling) between two parts of the of the driven sprocket assembly, and the inner casting of that is then bolted to the hub of the wheel casting.
Apparently the earlier years of this design relied solely on the 5-bolt pattern to translate the torque to the wheel, leading to a common failure mode where the bolts are sheared. Newer designs have dowels or roll pins added, to pin the parts together and help translate the torque.
The person had that failure mode on an earlier revision design and requested:
And make it look like this example from someone else:
Looking at the broken fasteners, 4 of the 5 were broken off 2-4 threads below the face of the hub, causing significant thread damage as the broken fasteners mashed around (still translating torque to the driven wheel), in some cases visibly ovallizing the hole
They did try drilling out one of the fasteners himself, but the bit unsurprisingly started wandering and they aborted before causing damage.
I tried the usual escalation, since these were sheared off and not expected to be corroded in or over-tightened and distorted:
Since the exposed threads above the broken fasteners were already damaged, I saw no harm in stacking tacks on with my small MIG to then be able to weld a nut on. This successfully removed all 5 nuts. The deeper ones took a few tries, I needed to crank the heat up to get good fusion into the broken fastener, at least partially due to the giant aluminum heatsink that was involved here.
No collateral damage on the wheel at all, just some smoke marks under the nuts that wiped right off (after the pic below):
The holes turned out to be through-holes into the hollow hub of the wheel casting, with only 3-5 good threads behind the damaged threads. We decided to do a helical-insert thread repair on each hole rather than push our luck just with longer fasteners or drilling an entirely-new hole pattern.
Nothing special for the thread repair - bolted this to the drill press table and did the repair there. To align each hole, I chucked up a 6.8mm drill (tap drill for M8), and ran it down through the good threads and moved into alignment until there was no deviation in the bit. The holes were not ovallized badly enough that I needed to use an end mill to get a concentric hole first.
The fasteners used are the usual metric flat-head socket drive with a 90 degree countersink angle. I didn't have any of these around in M8 and the right length, but also I needed to think about how to align the part to the mill to place the roll pins where I wanted.
So instead of waiting to order screws, and still needing an alignment solution - I turned some tapered spacers on the lathe
As you can see on the one in the foreground, I also used a bit of prussian blue to test and clean up the damaged countersink with some small files. The countersink is counterbored a bit in the part, and I didn't have a mix of tools to counterbore deeper and fit a countersink tool inside of that bore - so it was better to clean the existing countersink up until it made good-enough contact.
The extra height on these tapered spacers also gave me something I could indicate off of. The goal was to try and locate these pins as close as possible to what looked like the factory (or commonly accepted) solution - 2 pins on the centerline of the wheel/hub/drive assembly, on the 100mm bolt circle that the other 5 fasteners are on.
This meant getting these cast, round parts located in the mill such that the pattern is aligned to the X or Y axis.
To do this, we used a dial indicator to sweep across the two spacers of the same diameter so that it was aligned to X . Then an edge finder to center the axis on the two, which should also center on the axle axis of the entire assembly. Then edge-find off of other machined features in the hub to center the other axis
These are probably normally 8mm holes and pins, but I used a clean 3/16 drill bit and 3/16 roll pins (.003" smaller than 8mm), for a few reasons:
Additionally, the goal was to be close to factory based on the pictures available - not actually expecting this to interoperate with factory parts.
The back wall of the drive housing is nominally 10mm thick, and the wheel's hub seemed to be around 15mm thick. 25mm/ 1" long pins should sit flush with the inside of the drive (to avoid interfering with the rubber insert, etc), and in theory the drill would just need to break through into the inner housing but not fully clean the hole up -- that way the pin shouldn't ever be at risk of being pushed into the inside of the wheel, requiring bearing removal to get it out.
That ended up working perfectly. The bit broke through right at the depth the pin would fully be installed, leaving a cone of material below the pin that it would bottom out against when installed.
Pulled everything apart, chamfered the holes in the wheel and the back of the housing to make for easier pin installation and future reassembly
Pressed the pins in with the arbor press -- it was just able to reach far enough over the wheel to drive these in, but a c-clamp or kant twist would have worked too. I pressed them in. I pressed them in until 10.5mm was protruding above the machined surface of the wheel, they can always be pushed in further if that extra 0.5mm interferes with anything inside (unlikely)
Test assembly - worked great. Had to use the bolts to draw the parts together
Cleaned up and ready for reassembly
A few other thoughts along the way:
Thanks for reading
This is their "Cushion drive" design, where there is a rubber damper (not unlike a lovejoy coupling) between two parts of the of the driven sprocket assembly, and the inner casting of that is then bolted to the hub of the wheel casting.
Apparently the earlier years of this design relied solely on the 5-bolt pattern to translate the torque to the wheel, leading to a common failure mode where the bolts are sheared. Newer designs have dowels or roll pins added, to pin the parts together and help translate the torque.
The person had that failure mode on an earlier revision design and requested:
- Extract 5x broken bolts
- Thread repair as-needed
- Clean up a damaged countersink on the inner housing of the drive
- Install two pins to mimic the newer design
And make it look like this example from someone else:
Looking at the broken fasteners, 4 of the 5 were broken off 2-4 threads below the face of the hub, causing significant thread damage as the broken fasteners mashed around (still translating torque to the driven wheel), in some cases visibly ovallizing the hole
They did try drilling out one of the fasteners himself, but the bit unsurprisingly started wandering and they aborted before causing damage.
I tried the usual escalation, since these were sheared off and not expected to be corroded in or over-tightened and distorted:
- Left-hand drill bit
- Chisel/punch
Since the exposed threads above the broken fasteners were already damaged, I saw no harm in stacking tacks on with my small MIG to then be able to weld a nut on. This successfully removed all 5 nuts. The deeper ones took a few tries, I needed to crank the heat up to get good fusion into the broken fastener, at least partially due to the giant aluminum heatsink that was involved here.
No collateral damage on the wheel at all, just some smoke marks under the nuts that wiped right off (after the pic below):
The holes turned out to be through-holes into the hollow hub of the wheel casting, with only 3-5 good threads behind the damaged threads. We decided to do a helical-insert thread repair on each hole rather than push our luck just with longer fasteners or drilling an entirely-new hole pattern.
Nothing special for the thread repair - bolted this to the drill press table and did the repair there. To align each hole, I chucked up a 6.8mm drill (tap drill for M8), and ran it down through the good threads and moved into alignment until there was no deviation in the bit. The holes were not ovallized badly enough that I needed to use an end mill to get a concentric hole first.
The fasteners used are the usual metric flat-head socket drive with a 90 degree countersink angle. I didn't have any of these around in M8 and the right length, but also I needed to think about how to align the part to the mill to place the roll pins where I wanted.
So instead of waiting to order screws, and still needing an alignment solution - I turned some tapered spacers on the lathe
As you can see on the one in the foreground, I also used a bit of prussian blue to test and clean up the damaged countersink with some small files. The countersink is counterbored a bit in the part, and I didn't have a mix of tools to counterbore deeper and fit a countersink tool inside of that bore - so it was better to clean the existing countersink up until it made good-enough contact.
The extra height on these tapered spacers also gave me something I could indicate off of. The goal was to try and locate these pins as close as possible to what looked like the factory (or commonly accepted) solution - 2 pins on the centerline of the wheel/hub/drive assembly, on the 100mm bolt circle that the other 5 fasteners are on.
This meant getting these cast, round parts located in the mill such that the pattern is aligned to the X or Y axis.
To do this, we used a dial indicator to sweep across the two spacers of the same diameter so that it was aligned to X . Then an edge finder to center the axis on the two, which should also center on the axle axis of the entire assembly. Then edge-find off of other machined features in the hub to center the other axis
These are probably normally 8mm holes and pins, but I used a clean 3/16 drill bit and 3/16 roll pins (.003" smaller than 8mm), for a few reasons:
- The range of sizes for stainless spring pins covers both 8mm and 3/16" holes
- Slightly smaller is better here. If this ever wallers out, in theory it could be reamed to 8mm or slightly larger to clean it up and an 8mm pin would give it more life with a slightly tighter fit (if 8mm pins are even actually bigger, who knows if 3/16 and 8mm pins are effectively identical)
- The pins I have are 3/16, it didn't seem worth buying 8mm pins for religion when this would interoperate with 8mm parts or future changes just fine.
Additionally, the goal was to be close to factory based on the pictures available - not actually expecting this to interoperate with factory parts.
The back wall of the drive housing is nominally 10mm thick, and the wheel's hub seemed to be around 15mm thick. 25mm/ 1" long pins should sit flush with the inside of the drive (to avoid interfering with the rubber insert, etc), and in theory the drill would just need to break through into the inner housing but not fully clean the hole up -- that way the pin shouldn't ever be at risk of being pushed into the inside of the wheel, requiring bearing removal to get it out.
That ended up working perfectly. The bit broke through right at the depth the pin would fully be installed, leaving a cone of material below the pin that it would bottom out against when installed.
Pulled everything apart, chamfered the holes in the wheel and the back of the housing to make for easier pin installation and future reassembly
Pressed the pins in with the arbor press -- it was just able to reach far enough over the wheel to drive these in, but a c-clamp or kant twist would have worked too. I pressed them in. I pressed them in until 10.5mm was protruding above the machined surface of the wheel, they can always be pushed in further if that extra 0.5mm interferes with anything inside (unlikely)
Test assembly - worked great. Had to use the bolts to draw the parts together
Cleaned up and ready for reassembly
A few other thoughts along the way:
- Pin diameter and placement was based on the pictures found online of other peoples' wheels with pins - they appeared to be 8mm and on the 100mm bolt circle
- Longer pins would be ideal to ensure they stay put after many load/unload cycles, but since the drive hub is hollow they would do no good here. If the hub was solid aluminum, then yeah a 30, 40, or 50 mm pin would have been nice to give it way more meat in the hub to spread the load out
- The front face of the wheel hub and back face of the drive housing are galled quite badly, they clearly rotated against one another when the fasteners finally fully let go. I put this together and ran it around with an indicator, the runout were better than I expected and I opted not to try and clean those up. Not only would the wheel not fit on my lathe (so I would need to take the brake rotor off and do this on the rotary table at the mill), but removing more than the bare minimum of raised galled material would risk impacting the sprocket alignment. Not worth the risk even though it looks ugly The owner can clean this up with coarse , manual/hand wet sanding if they feel the need.
- The only question I had at the end is whether the rotation of the spring pins matters. I oriented them with the opening perpendicular to the direction they would be loaded/unloaded. I was thinking that having the opening to the "side" would reduce the chance that the cyclical loading/unloading would eventually cause the spring pin to chew into the aluminum pieces. Facing it forward/backward in one part would be the opposite in the other, so I figure it is more likely to flex open/close and whittle into one piece or the other... not sure. If this ever was to come loose, sooner than expected, then maybe a solid pin red-locktited in would be better. Wouldn't be hard to turn some stainless 8mm plus-ize dowels to fit into the same hole with a touch of red locktite. Maybe with a threaded hole in the middle should they ever need to be extracted.
Thanks for reading
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