spring – Experimental Engineering

Posted on August 5, 2017August 3, 2017 by The Engineer — 3 Comments

Wheelchair Motor Service Part 1: Teardown & Inspection

Trolley Propulsion System: Wheelchair Motor Units

So it’s time to get the propulsion system underway for the trolley, a pair of wheelchair motors were sourced for this, from HacMan. Since I don’t know how many hours are on these units, or how they’ve been treated in the past, I’m going to do a full service on them to ensure reliability. I decided on wheelchair motors due to their extreme ruggedness & heavily built components – this project when complete is going to weigh in at about 150kg!
I suspected something was amiss with one of the motors from running them under no load: the left hand wheelchair motor was heating up to the point of being too hot to touch, so this one at the very least needed some investigation.

Motor Disassembly & Assessment

With the back cover removed from the motor the electromagnetic brake is revealed. This engages when power is removed to stop the motor freewheeling, which even though it’s a wormdrive box, it will do readily if backdriven.

The brake is rated 6.7W at 24v DC.

The brake disc is just visible between the plates of the brake here, with some green dust worn off the disc. When power is applied, the top disc, just under the magnet on top, is pulled upward against spring pressure away from the brake disc, which is attached to the motor armature.

Here’s the brake disc, removed from the motor. There’s only a little wear here, as I’d expect – these brakes don’t engage until the motors have come to a complete stop.

The steel disc above the magnet acts as one of the friction surfaces of the brake.

Finally, the solenoid is at the back, partially potted in resin. The strong coil spring in the centre applies the brakes when power is disconnected.

Removing the top of the gearbox reveals the state of the internals – There’s no wear at all on the gearset, but the lubricant is totally manky. The external oil seals have been leaking for some time, letting water in and grease out. The emulsified result is revolting! These gearboxes have a wormdrive first stage, the worm gear is underneath the left hand gearset. Steel spur gears then do the final gearing to the output shaft. The output gear is splined onto the output, and can slide along the shaft out of mesh – this is the freewheel clutch mechanism. At the moment it’s all obscured by the disgusting lubricant.

Here’s the failed seal on the left hand gearbox, the face damage was done by petrol immersion to clean everything up. (The seal is already compromised, so I’m not fussed about solvents eating the remaining rubber). The motor shaft is joined to the gearbox input by a rubber coupling.

The output shaft seals seem to be still OK, there has been some seepage past the collar that the shaft rides in, but nothing more. This can be resealed with some Loctite bearing sealant. The sleeve is held into the gearbox by the wheel hub when in operation, but this doesn’t seal the gap unfortunately. I don’t know why the manufacturer didn’t just machine the shaft to that larger diameter, instead of using an extra sleeve to accommodate the seal.

The bore seals covering the ends of the shafts are also fine, which is a good thing, since I can’t seem to find replacements for these anywhere. The input shaft seals will be replaced on both gearboxes though.

The oil seal must have been leaking for a long while! This is the gearbox end of the wheelchair motor frame, completely clogged with grease. Luckily only a small amount has made it down past the armature to the brushgear.

The commutator of this motor is badly damaged, and the brushes are very worn. This has been caused by the gearbox oil seal failing, and contaminating the motor internals with lubricant. The undercut between the segments is all but gone – filled with an abrasive mixture of brush dust, copper dust & old lubricant. Some repair work will be required here.

Here’s the brushgear removed from the second wheelchair motor, this one looks much more normal, and there’s not as much wear on the brushes or the commutator. Just the usual coating of brush dust.

Here’s both armatures together, with the contaminated one on the right, after some cleaning to remove most of the greasy old grok & brush dust from everything. The windings on the damaged left hand wheelchair motor haven’t darkened, which I would expect from severe overheating damage, so I’m hoping this armature is OK, and won’t require a rewind. Using an ohmmeter on these windings doesn’t tell me much – there’s only 7 turns of 0.86mm (20AWG) magnet wire in each coil, so they read as a dead short anyway. There was some leakage between the windings and the core before I cleaned things up – this was in the high (28+) megohms range, but this seems to have cleared now I’ve given things a real good cleaning.

More to come when new bearings & seals arrive!

Posted on January 20, 2017March 12, 2017 by The Engineer — 4 Comments

DIY Eberspacher Glowplug Screens: The Test Of Time

Some time ago I did a couple of posts on cheapening up the maintenance of Eberspacher hot air heaters by making the glow plug screens myself. Now one of my pieces of stainless mesh has been in the heater for nearly a year, and the heater is starting to get a bit smoky on a cold start. This is usually a sign that the screen isn’t allowing the fuel to vaporise quick enough for the glow plug to ignite the flame, because it’s becoming blocked. So far the heater has had about 150L of diesel through it with my DIY screen.

After removing the plug, here’s what’s left of the screen. The bottom end has completely disintegrated, but this is to be expected – OEM screens do the same thing as this end is exposed to the most heat in the burner. There’s quite a bit of coke buildup on the top end of the screen around the fuel nozzle, again this isn’t surprising, as this is the coolest part of the heater not all the heavier fractions of the diesel fuel have the chance to vaporise.

Looking further down into the mixing tube of the main burner, everything looks good. There’s a coating of soot in there, but no tar-like build up that would tell me the unit isn’t burning properly. Another advantage of making my own screens is that they’re much easier to extract from the hole once they’ve been in there for months. The OEM screens have a stainless ring spot welded to the mesh itself to hold it’s shape, and once there’s enough fuel residue built up the entire mess seizes in place, requiring some sharp pokey tools & some colourful language to remove. The single loop of mesh held in place by it’s own spring pressure is much easier to remove as it collapses easily.

I’ve decided to change the mesh size of the screen while I’m in here, in this case to 80 mesh, which is much closer to the OEM screen size. There doesn’t seem to be much of a difference so far in either the starting or running capability of the heater, although the thicker wire of this screen might last longer before disintegrating at the burner end.

He-Ne Laser Safety

As with *any* laser, proper precautions must be taken to avoid any possibility of damage to vision. The types of He-Ne lasers mostly dealt with in this document are rated Class II, IIIa, or the low end of IIIb (see the section: Laser Safety Classifications. For most of these, common sense (don’t stare into the beam) and fairly basic precautions suffice since the reflected or scattered light will not cause instantaneous injury and is not a fire hazard.

However, unlike those for laser diodes, He-Ne power supplies utilize high voltage (several kV) and some designs may be potentially lethal. This is particularly true of AC line powered units since the power transformer may be capable of much more current than is actually required by the He-Ne laser tube – especially if it is home built using the transformer from some other piece of equipment (like an old tube type console TV or that utility pole transformer you found along the curb) which may have a much higher current rating.

The high quality capacitors in a typical power supply will hold enough charge to wake you up – for quite a while even after the supply has been switched off and unplugged. Depending on design, there may be up to 10 to 15 kV or more (but on very small capacitors) if the power supply was operated without a He-Ne tube attached or it did not start for some reason. There will likely be a lower voltage – perhaps 1 to 3 kV – on somewhat larger capacitors. Unless significantly oversized, the amount of stored energy isn’t likely to be enough to be lethal but it can still be quite a jolt. The He-Ne tube itself also acts as a small HV capacitor so even touching it should it become disconnected from the power supply may give you a tingle. This probably won’t really hurt you physically but your ego may be bruised if you then drop the tube and it then shatters on the floor!

However, should you be dealing with a much larger He-Ne laser, its power supply is going to be correspondingly more dangerous as well. For example, a 35 mW He-Ne tube typically requires about 8 mA at 5 to 6 kV. That current may not sound like much but the power supply is likely capable of providing much more if you are the destination instead of the laser head (especially if it is a home-made unit using grossly oversized parts)! It doesn’t take much more under the wrong conditions to kill.

After powering off, use a well insulated 1M resistor made from a string of ten 100K, 2 W metal film resistors in a glass or plastic tube to drain the charge – and confirm with a voltmeter before touching anything. (Don’t use carbon resistors as I have seen them behave funny around high voltages. And, don’t use the old screwdriver trick – shorting the output of the power supply directly to ground – as this may damage it internally.)

And only change electrical connections or plug/unplug connectors with power OFF, being aware of the potential for stored charge. In particular, the aluminium cylinder of some HeNe laser heads is the negative return for the tube current via a spring contact inside the rear end-cap. So, pulling off the rear end-cap while the laser is powered will likely make YOU the negative return instead! You will probably then bounce off the ceiling while the laser bounces off the floor, which can easily ruin your entire day in more ways than one. 🙁 🙂 This connection scheme is known to be true for most JDS Uniphase and many Melles Griot laser heads, but may apply to others as well.

Now, for some first-hand experience:

(From: Doug (dulmage@skypoint.com).)

Well, here’s where I embarrass myself, but hopefully save a life…

I’ve worked on medium and large frame lasers since about 1980 (Spectra-Physics 168’s, 171’s, Innova 90’s, 100’s and 200’s – high voltage, high current, no line isolation, multi-kV igniters, etc.). Never in all that time did I ever get hurt other than getting a few retinal burns (that’s bad enough, but at least I never fell across a tube or igniter at startup). Anyway, the one laser that almost did kill me was also the smallest that I ever worked on.

I was doing some testing of AO devices along with some small cylindrical HeNe tubes from Siemens. These little coax tubes had clips for attaching the anode and cathode connections. Well, I was going through a few boxes of these things a day doing various tests. Just slap them on the bench, fire them up, discharge the supplies and then disconnect and try another one. They ran off a 9 VDC power supply.

At the end of one long day, I called it quits early and just shut the laser supply off and left the tube in place as I was just going to put on a new tube in the morning. That next morning, I came and incorrectly assumed that the power supply would have discharged on it own overnight. So, with each hand I stupidly grab one clip each on the laser to disconnect it. YeeHaaaaaaaaa!!!!. I felt like I had been hid across my temples with a two by four. It felt like I swallowed my tongue and then I kind of blacked out. One of the guys came and helped me up, but I was weak in the knees, and very disoriented.

I stumbled around for about 15 minutes and then out of nowhere it was just like I got another shock! This cycle of stuff went on for about 3 hours, then stopped once I got to the hospital. I can’t even remember what they did to me there. Anyway, how embarrassing to almost get killed by a HeNe laser after all that other high power stuff that I did. I think that’s called ‘irony’.

Comments on HeNe Laser Safety Issues

(Portions from: Robert Savas (jondrew@mail.ao.net).)

A 10 mw HeNe laser certainly presents an eye hazard.

According to American National Standard, ANSI Z136.1-1993, table 4 Simplified Method for Selecting Laser Eye Protection for Intrabeam Viewing, protective eyewear with an attenuation factor of 10 (Optical Density 1) is required for a HeNe with a 10 milliwatt output. This assumes an exposure duration of 0.25 to 10 seconds, the time in which they eye would blink or change viewing direction due the uncomfortable illumination level of the laser. Eyeware with an attenuation factor of 10 is roughly comparable to a good pair of sunglasses (this is NOT intended as a rigorous safety analysis, and I take no responsibility for anyone foolish enough to stare at a laser beam under any circumstances). This calculation also assumes the entire 10 milliwatts are contained in a beam small enough to enter a 7 millimeter aperture (the pupil of the eye). Beyond a few meters the beam has spread out enough so that only a small fraction of the total optical power could possible enter the eye.

Posted on May 29, 2016 by The Engineer — Leave a comment

Mini USB Soldering Iron

Here’s a novel little gadget, a USB powered soldering iron. The heating tip on these is very small & might be useful for very small SMD work. Bigger joints not so much, as it’s only rated at 8W. (Still breaks the USB standard of 2.5W from a single port).

These irons aren’t actually too bad to use, as long as the limitations in power are respected. Since nearly everything has a USB power port these days, it could make for a handy emergency soldering iron.

The heater & soldering bit are a single unit, not designed to be replaced separately. (I’ve not managed to find replacement elements, but at £3 for the entire iron, it would be pretty pointless).
Above is the socket where the heater plugs in, safely isolating the plastic body from any stray heat.

The DC input is a 3.5mm audio jack, a non-standard USB to 3.5mm jack cable is supplied. Such non-standard cables have the potential to damage equipment that isn’t expecting to see 5v on an audio input if it’s used incorrectly.

There isn’t actually a switch on this unit for power management, but a clever arrangement of a touch button & vibration switch. The vertical spring in the photo above makes contact with a steel ball bearing pressed into the plastic housing, forming the touch contact.

The large MOSFET here is switching the main heater current, the silver cylinder in front is the vibration switch, connected in parallel with the touch button.

The main controller is very simple. It’s a 555 timer configured in monostable mode. Below is a schematic showing the basic circuit.

Big Clive also did a teardown & review of this iron. Head over to YouTube to watch.

Posted on December 26, 2015 by The Engineer — Leave a comment

Quickie Teardown – ShopGuard Anti-Theft Tag

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.

Posted on August 24, 2015 by The Engineer — Leave a comment

GY561 Frequency & Power Meter LiPo Conversion

From the factory, the GY561 meter uses alkaline AAA cells for power. As these are not rechargable, and I don’t carry any other devices that take such batteries, I figured I’d replace them with a single Lithium Polymer cell that I can charge via USB.

Here’s the battery compartment, with the original spring terminals removed.
I searched eBay for a suitable sized cell, and settled on a 1000mAh type, with dimensions of 47mm x 28mm x 7mm.

This size cell required a small amount of modification to the battery compartment to make it fit properly with the associated charge & protection circuitry.

Here’s the modifications made to the compartment, I’ve ground away the plastic to make the bottom flat, and the plastic tabs that retained the original spring terminals.

After grinding away the original battery spring holders with a dremel, the cell fits perfectly in the available space. The small PCB on the top of the cell is the USB charger & protection.

The charger is located in a slot cut in the bottom of the casing, so the USB port is accessible from outside the compartment.

Here’s the rest of the wiring completed, with the power wires going through holes in the bottom of the battery compartment to join onto the PCB where the original terminals were located. I have insulated the solder joints on the control PCB with some Kapton tape to prevent any shorts against the lithium cell.

A small cutout was also required in the battery cover to allow the USB connector to poke out. This was easy to do on the soft plastic with a Dremel tool.

With the battery cover installed, the USB port is nicely recessed into the edge.

The indicator LEDs on the charging & control board show nicely through the plastic, here’s the unit on charge. When the charge is complete, another LED lights as shown below.

Posted on July 22, 2015July 28, 2015 by The Engineer — Leave a comment

Solderless N-Type Connector Fitting

I thought I’d detail the process I use to fit an N-Type connector to a coax cable, as I don’t usually solder these connectors.

Before stripping, fit the backnut, washer & rubber seal onto the cable.

The cable is first stripped back to reveal the shield. This cable has a foil tape as well as the usual copper braid.

Once the inner core has been revealed, the shield washer is fitted. This has a knife edge on the inner diameter, to fit between the outer sheath & the shield, this makes the electrical connection.

With the shield washer fitted, the inner insulation can be cut back, it should be just about level with the final washer when you’re done, this allows the connector to fit together properly.

Trim the center conductor to about double the length required, to allow it to be folded over, as shown. This allows the copper to spring back against the center pin of the connector when it’s fitted, to allow a good connection.

Here the final washer is fitted over the shield washer. The center insulation should be at the same level to allow the center pin to fit properly.

Finally, the center pin is pushed over the inner conductor of the cable, with it’s insulating spacer. Soldering these usually results in the plastic melting and a ruined connector.

Finished plug. Make sure the backnut is tightened fully home, without twisting the connector body itself. After I’m done with the termination, I use self-amalgamating tape to form a strain relief on the cable. This prevents it from breaking at the point where it enters the backnut.

I’ve been terminating these connectors this way for a long time & have not had any issues with SWR or bad connections, dispite the fact that I don’t solder them. This also has the advantage that fewer tools are required for the job & the connectors can easily be reused should the cable wear out.

Posted on February 16, 2013 by The Engineer — Leave a comment

Amano PIX 3000x Timeclock

This is a late 90’s business timeclock, used for maintaining records of staff working times, by printing the time when used on a sheet of card.

Here is the top cover removed, which is normally locked in place to stop tampering. The unit is programmed with the 3 buttons & the row of DIP switches along the top edge.

Closeup of the settings panel, with all the various DIP switch options.

Cover plate removed from the top, showing the LCD & CPU board, the backup battery normally fits behind this. The CPU is a 4-bit microcontroller from NEC, with built in LCD driver.

Power Supply & prinhead drivers. This board is fitted with several NPN Darlington transistor arrays for driving the dox matrix printhead.

Printhead assembly itself. The print ribbon fits over the top of the head & over the pins at the bottom. The drive hammers & solenoids are housed in the circular top of the unit.

Bottom of the print head showing the row of impact pins used to create the printout.

Bottom of the solenoid assembly with the ribbon cable for power. There are 9 solenoids, to operate the 9 pins in the head.

Top layer of the printhead assembly, showing the leaf spring used to hold the hammers in the correct positions.

Hammer assembly. The fingers on the ends of the arms push on the pins to strike through the ribbon onto the card.

The ring of solenoids at the centre of the assembly. These are driven with 3A darlington power arrays on the PSU board.

There is only a single drive motor in the entire unit, that both clamps the card for printing & moves the printhead laterally across the card. Through a rack & pinion this also advances the ribbon with each print.

Posted on November 11, 2010December 10, 2010 by The Engineer — Leave a comment

Motorised Valve

This is the internals of a motorised valve for central heating systems. Here the top is removed showing the motor & microswitch.

Left side of the valve, showing the gearing under the motor, & the valve body under the powerhead.

Right side of the valve, showing the sprung mechanism of the valve quadrant.

Here the motor has been removed from the powerhead, showing the microswitch & the sprung quadrant gear. This spring keeps the valve closed until the motor is energized. The motor remains energized to hold the valve open.

Here the valve body has been opened showing the internal components. The rubber valve rotates on the shaft, blocking the lower port of the valve when in operation.

The motor’s protective cap has been removed here showing the rotor. This is a synchronous motor, of a special type for use in motorised valves. As the windings need to be continuously energized to hold the valve open, it is designed not to burn out under this load. 240v AC 50Hz, 5RPM.

Posted on August 2, 2010 by The Engineer — 1 Comment

ICL Barcode Scanner

An ICL barcode scanner from the 80s is shown here. This is the top of the unit with cover on.

Plastic cover removed from the unit showing internal components. Main PSU on left, scan assembly in center. Laser PSU & Cooling fan on right. Laser tube at top.