Tiny magnets for Hall effect tach - sources, adhesives?

Since you just need splinters, why not shatter a larger magnet (steal one from the fridge...) by brute force? Shouldn't be too hard to do, given a solid surface and a sledge hammer ;->

I'm not at all certain, but IMHO epoxy and cyan-acrylate ("super-glue") should both work out fine. Preferrably avoid using very much of either, ie have the mounting place fit the shape of the magnet as closely as possible.

Which glue is better would depend on what you want to glue it on: super-glue works best between two brittle, inflexible items, becaust it's quite brittle itself. Epox is somewhat more flexible.

--
Hans-Bernhard Broeker (broeker@physik.rwth-aachen.de)
Even if all the snow were burnt, ashes would remain.

There are people on

that sel the small 1/16" and 1/8" cylindrical or square magnets from time to time. That is where I got the ones I use. Also some of the small flexible magnets they use for advertising andsticking to things like the refridgerators, may work OK. You can cut these with a pair of sissors into small peices and stack them up even. But some of the hall effect sensors may not be sinsitive enough for these.

For this purpose, I use Samarium Cobalt magnets from:

My application is for outdoor use so I use acrylic adhesives such as Loctite Depend or 3M Dp-810. These work on a prettty broad range of substrates.

I have tried cutting a very thin piece off a flexible fridge magnet, and also tried to saw a piece off a neodymium magnet out of a hard disk, but the resulting speck is not strong enough to actuate the Hall effect sensor. So I think I need to buy it pre-manufactured in the right size.

Yeah, this is why I wanted to use the flexible magnet :) If it falls off, the system reverts to an open-loop control system, and this is going to lead to big inaccuracies. The principal application is direct-drive of a boat propeller. I have no way of measuring the speed of the boat directly most of the time (because it is submerged, can't use GPS), and I found that PWM duty cycle vs. speed of vehicle isn't well correlated (because there are other variables, esp. water depth).

The magnet will be mounted on the shaft of the motor itself.

Excellent response, thanks.

My application is also outdoor, and underwater :) In other words, I'm close to testing a "non-flight-qualified" version of my submarine. I've tested parts of it, with mostly satisfactory results, but I still have yet to solve the problem of preventing water ingress at the propeller and plane glands. The electronics and software are really simple compared to these horrid (and EXPENSIVE) mechanical engineering problems. I doubt I will have a fully demonstrable prototype by the time my book has to hit the publisher, but at least the subsystems work individually [more or less ;)] and it all works together on the breadboard.

With New York the way it is right now (you can see the Empire State Building from my back porch, which means I have military aircraft and police helicopters buzzing around all the time), it seems an inopportune time for field tests, anyway.

This is Usenet so, Without knowing any of the details of your application, I'll leap in with something that's probably obviously not workable if one's actually holding the application, but ...

Why not an LED/photo-transistor detector pair? I'd imagine that painting alternating white and black areas around the shaft circumference would be a whole lot simpler than reliably mounting tiny magnetic chips.

Or, if a Hall effect really is indicated, tear apart a muffin fan of the appropriate diameter (if it's a small fan and your app is a large motor). I sacrificed one recently (broken blade) to salvage the Hall sensor and found that the permanent magnet on the rotor was nothing more than a "refrigerator-style" magnet molded into a circle to fit inside the hub. You would have only two poles but would gain a much larger surface area for the adhesive.

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Rich Webb   Norfolk, VA

Could you accept that some leakage is inevitable and install a bilge pump system? A small amount of leakage may even be desirable in that it will help to lubricate the shaft packing. Perhaps a small shield just inboard of the gland(s) to collect the leakage and pipe it to a bilge tank. Adds another variable to the buoyancy issue, of course, but designing to be leak-tollerant up to some defined rate (depending on depth and pump capacity) may be a Good Thing.

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Rich Webb   Norfolk, VA

How about pressurizing the hull?

You could try

as they have a range of rare earth magnets in some quite small sizes.

Mike Anton

For small magnets try

.

I recently got some small (.0635 dia. X .03125 thick $1.25/10) for wheel encoders for a miniature robot I'm building.

Jay

-- .

---------------------------------------------------------------------------- "I'm pullin' for you; we're all in this together", Red Green

I do accept that some leakage is inevitable, and the electronics is on a raised platform to keep it dry as long as possible. I'm getting more leakage than I can deal with, though. So far the best solution I can come up with is to run the prop shaft through a piece of metal tube of just slightly larger bore than the shaft, and pack the space with viscous grease. It doesn't work very well.

Due to expense and danger (I had a couple of narrow escapes from being shot by pieces of bursting fittings), I had to abandon my original plan for active depth control, which means I don't have an internal source of compressed air - so there's only so much pumping I can do.

I can't help but think that this problem was solved about 80 years ago, and not by a bunch of EE's (:

I am not familiar with propeller shafts, but I have seen the packing on old steam engines. They generally have something called a stuffing box or a packing gland, where grease-covered flax or cotton string is forced between the shaft and the box, sealing the joint but still allowing shaft motion.

This link might be of help:

What about a variation on a labyrinth seal? Should be manufacturable with a drill press/hole saw rig and some sheets of teflon.

================= []#o#o#o#[] []#|#|#|#[] _____|_|_|_____ inboard outboard side_____ _ _ _____side | | | []#|#|#|#[] []#o#o#o#[] =================

Where the # are teflon (or brass?) plates with a hole through the center slightly larger than the shaft and the o are o-rings that seal between adjacent plates.

The | are teflon sheets that interdigitate with the above but are affixed to the shaft.

The [] are the walls of the stuffing tube held together by a bolt circle right through, inboard to outboard.

--
Rich Webb   Norfolk, VA

In this part of the world attempting to acquire nitric or muriatic (hydrochloric) acid is likely to bring down the Ashcroft/Bushian forces and result in incarceration in the Gitmo archipelego with no further communication with the outside world, lawyers, etc.

--
Chuck F (cbfalconer@yahoo.com) (cbfalconer@worldnet.att.net)
   Available for consulting/temporary embedded and systems.
     USE worldnet address!

Last time I was in the building supply store, they still sold gallon jugs of muriatic acid without an ID check. It may not be reagant grade, but it is strong enough to etch concrete very rapidly (its intended purpose).

Getting nitric acid of a purity and strength suitable for the manufacture of nitrate-based explosives is probably more difficult. None of the procurement problems are anything that couldn't be solved by an attentive student of a college freshman course in chemistry. And you can get the same education in a lot of places without professors.

Mark Borgerson

Your problem is that you have to seal around your driveshaft, because you put your motor in the dry space.

Have you considered going to motors that were DESIGNED for unprotected submersion, putting them OUTSIDE the dry space, and just running non-moving electrical connections?

There was a do-it-yourself dive scooter project many years ago that used a salvaged trolling motor just this way.

Hi John,

I thought about it, but they're really expensive. Also, I need stepper motors as well as straight DC motors - search for "waterproof stepper motor" and see just how incredibly expensive they are!

Besides, this engineering problem has solutions that work - I just haven't found one that lies within my mechanical skills yet.

I have here on my desk some sealed bearings that, if mated with an exactly fitting shaft, perform very effective water exclusion up to

2atm, but unfortunately I need 3atm - and I would prefer to have an actual "rated for xxxkPa" part rather than just testing some scrounged parts. Oh, well. I'm just about to order the "chassis" (skeleton) for the preflight version, I don't have to make final decisions on all of this until a little later - there's plenty of room to add extra sealing hardware, or to bolt on outboard motors, if necessary.

I've seen a drawing of a sealed mechanism to transfer rotary power through a diaphram (sheet of rubber, such as from a bicycle inner tube). The shaft going through it doesn't turn (and is thus perfectly sealed), but moves along the surface of a cone with the apex at the diaphram. A standard bearing on each side converts the motion to that of a regular turning shaft. Another approach is magnetic coupling. Torque can be transferred by a rotating magnet on each side of a sealing non-magnetic plate. You can get small, powerful Neodynm (SP?) magnets out of old hard disk drives as well as the usual sources such as ebay.

It seems like the things I described above should be available somewhere as off-the-shelf parts. I presume you have a Grainger and Mcmaster-Carr catalogs, which should have that sort of thing.

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In case no one has mentioned it yet,

are a useful source of magnets large and small.

Declan Hughes (please remove abc if replying by email)