I wrote a few weeks ago about replacing the hot water circulating pump on the boat with a new one, and mentioned that we’d been through several pumps over the years. After every replacement, autopsy of the pump has revealed the failure mode: the first pump failed due to old age & limited life of carbon brushes. The second failed due to thermal shock from an airlock in the system causing the boiler to go a bit nuts through lack of water flow. The ceramic rotor in this one just cracked.
The last pump though, was mechanically worn, the pump bearings nicely polished down just enough to cause the rotor to stick. This is caused by sediment in the system, which comes from corrosion in the various components of the system. Radiators & skin tanks are steel, engine block cast iron, back boiler stainless steel, Webasto heat exchanger aluminium, along with various bits of copper pipe & hose tying the system together.
The use of dissimilar metals in a system is not particularly advisable, but in the case of the boat, it’s unavoidable. The antifreeze in the water does have anti-corrosive additives, but we were still left with the problem of all the various oxides of iron floating around the system acting like an abrasive. To solve this problem without having to go to the trouble of doing a full system flush, we fitted a magnetic filter:
This is just an empty container, with a powerful NdFeB magnet inserted into the centre. As the water flows in a spiral around the magnetic core, aided by the offset pipe connections, the magnet pulls all the magnetic oxides out of the water. it’s fitted into the circuit at the last radiator, where it’s accessible for the mandatory maintenance.
Now the filter has been in about a month, I decided it would be a good time to see how much muck had been pulled out of the circuit. I was rather surprised to see a 1/2″ thick layer of sludge coating the magnetic core! The disgusting water in the bowl below was what drained out of the filter before the top was pulled. (The general colour of the water in the circuit isn’t this colour, I knocked some loose from the core of the filter while isolating it).
If all goes well, the level of sludge in the system will over time be reduced to a very low level, with the corrosion inhibitor helping things along. This should result in much fewer expensive pump replacements!
For a while I’ve wondered how these pancake type (AKA “Shaftless”) vibration motors operate, so I figured I’d mutilate one to find out.
These vibrators are found in all kinds of mobile devices as a haptic feedback device, unlike older versions, which were just micro-sized DC motors with an offset weight attached to the shaft, these don’t have any visible moving parts.
These devices are crimped together, so some gentle attack with a pair of snips was required to get the top cover off.
It turns out these are still a standard rotary DC motor, in this case specifically designed for the purpose. The rotor itself is the offset weight, just visible under the steel half-moon shaped section are the armature coils.
The armature lifts off the centre shaft, the coils can clearly be seen peeking out from under the counterweight.
The underside of the armature reveals the commutator, which in this device is just etched onto the PCB substrate, the connections to the pair of coils can be seen either side of the commutator segments.
The base of the motor holds the brushes in the centre, the outer ring is the stationary permanent magnet. These brushes are absolutely tiny, the whole motor is no more than 6mm in diameter.
Everyone at some stage must have seen these EAS security tags in shops, usually attached to clothing with a steel pin. As some of this year’s presents had been left with the tags attached, I had to forcibly remove them before wrapping could commence.
These are just a plastic disc about 50mm in diameter, with an internal locking mechanism & RF tag inside.
After some careful attack with a saw around the glue seam, the tag comes apart into it’s halves. The RF coil & it’s ceramic capacitor can be seen wrapped around the outside of the tag. The capacitor in this case isn’t even epoxy dipped to save that extra 0.0001p on the manufacturing price. In the top centre is the pin locking mechanism, enclosed in a small plastic pill.
Popping off the back cap of the lock shows it’s internals.
The lock itself is very simple. The centre section, held in place by a spring, carries 3 small ball bearings. The outer metal frame of the lock is conical in shape.
When the pin is pushed into the tag, the conical shape of the lock chamber causes the ball bearings to grab onto it, helped by the action of the spring that pushes the ball bearing carrier further into the cone.
This also means that any attempt to force the mechanism causes it to lock tighter onto the pin.
In normal operation, removal is achieved by a strong magnet that pulls the ball bearing carrier back slightly against it’s spring, allowing the pin to disengage & be pulled out.
This design is incredibly simple & cheap to make, and gains it’s locking strength from friction alone.
I would consider the RF coil being around the outer edge of the device a bit of a security risk – a quick chop with a sharp pair of wire cutters would disable the tag’s alarm functionality instantly. Making the coil slightly smaller & keeping it out of reach of the edge of the tag would help in this regard.
If you’ve found my content useful, please consider making a donation via the link below!
All collected funds go towards new content & the costs of keeping the servers online.