Jump to content

electric powder clutches

Recommended Posts

I have just extracted two electromagnetic clutches from my BRDM's turret elevation gearbox.


As far as I can tell they are powder clutches. I think they may have got some moisture inside as they are dragging a lot when not engaged.


The hold great when powered.


The seem to be sealed units so I wondered if anyone has any ideas about freeing them up?


Maybe a bake in the oven or a few sharp taps with a hammer?


I tried putting one onto my battery drill but it didn't seem to free up much.


I think these are pretty exotic parts and it will be well nigh on impossible to get replacements...

Link to comment
Share on other sites

What is the problem with them in normal use? These powder (sometimes lead shot) couplings are normally used as 'soft starts' i.e. they will allow an initial slippage but once turning will rapidly lock up, thereby taking the initial start-up stress off the motor / drive.


Is this the application for which they are being used in the BRDM? Sounds like they are working correctly from your description :-)



Having said that, from your description - if they are electromagnetic clutches then that does the soft start thing anyway, so I can't work out why a powder coupling element is needed -- unless the clutches work by energising magnetic powder to solidify it thus creating a drive ??

Edited by N.O.S.
Link to comment
Share on other sites

These sound similar to those used in the Abbot traverse gearbox. They aren't a soft start, rather a way (with appropriate gearing) of reversing the direction of an electric motor that only turns in one direction. Bearing in mind that we're talking about a dc motor it's always seemed to me to be a rather complicated way of doing it.


I'd suggest that long application of gentle heat is the best approach. Something like leaving them on top of a radiator/boiler/Aga or in the airing cupboard next to the immersion? I think that an oven may cook the ferro-magnetic powder and lose its magnetism.



Link to comment
Share on other sites

Have you any Silca Gel packets? Warm them and place on the box. If you can then wrap shrink wrap or something round and leave for twenty four hours, that should take any moisture out with no damage to anything else. I bought a dehumidifier because of damp problems in the flat, it has also found a secondary use for drying vehicles out and supplying soft water for cooling systems, not that cheap though.

Link to comment
Share on other sites

After some careful thought, I now realize the function of the clutches.


The turret traverse is a servo system which tracks the gunsight position sensor. The servo drive works by having an electric motor which runs at full power (torque) in one direction and two inter-meshed clutches. when both are disengaged, the turret is stationary - the motor is just free wheeling. As the error signal between the turret position and the gunsight position grows (by either getting moved) the appropriate clutch is engaged - by the same amount as the error signal so the drive starts to run in the correct direction to reduce the error. (this is a closed loop feedback control system)


The clutches allow maximum torque transfer at low speeds without the need for a complex motor controller.


I might try sealing one up with a load of silica gel and see what happens. If the moisture got in, it should migrate out too. Gentle heating and cooling in a dry atmosphere should do the trick.


It's weird that initially the drive was ceased as both clutches were jammed, but I freed them up and I operate the drive manually with the manual handle, but after getting the electrics going, the clutches ceased up again. Maybe repeated manual driving and powering up might loosen up everything inside...

Link to comment
Share on other sites

Well I have wrapped one up in clingfilm with all the silica gel bags I could find! (dried them first in the microwave)


We'll see what happens tomorrow.


Ideally I'd like to get one open to see what's inside but it looks like the mounting plate holding the main bearing has been staked into place so it would be a bit of a bugger to open up... I need to get the mounting plate off to expose the screws that hold the lid on the powder chamber.


There are lots of rust marks on the screw heads but not much actual corrosion. I would assume that if the powder is iron based, then moisture would cause havoc as it would go lumpy. The actual drive just feels really tight to different extents around a full 360 rotation.


It still locks up ok when you energise the electromagnet which means the powder must still be loose enough. I'm thinking that there is an area around the edge where it's caked up with rust making the drive tight and the rest is still loose powder.


It's interesting that the Russians don't use o-ring or lip seals on these parts.


My experience at work showed me that any enclosure that is nearly airtight will cause a lot more problems with moisture ingress than one which has a big hole in! You get a pumping effect as the air pressure goes up and down with temperature.

Link to comment
Share on other sites

More progress!


Today I managed to open up the guts of the clutch. I had to drill out a couple of stake marks, undo a retaining ring, take out another load of tiny screws and press out the shaft and hey presto loads of black powder!


Closer inspection showed hard caked areas - more cake than Mr Kiplings larder....


I emptied the loose powder out and then scraped out all the caked stuff.


Surprisingly little powder actually inside the unit - maybe two teaspoons.


Next I put the mixture into a little mortar and pestle and ground it up - lumps and all.


Once back in powder form all that remained was to re-assemble....


Not as easy as I had hoped...


The clearances between the rotating parts are very very tight. If you just pile the powder in and try and close the container, you can't. Even shaking it out to the bottom didn't work - it just jammed the central shaft.


I noticed a single screw in the lid of the container so I guessed that the unit has to be assembled and then the powder is injected while the unit is spinning to distribute it evenly.


That's were I'm up to now!

Link to comment
Share on other sites

Yeah, I might try an Estes model rocket!


I think the Russian stuff is actually very clever. They make up for having virtually no electronics by having very ingenious mechanical and optical systems.


The launcher has spring loaded rams that eject the empty tubes and the rams are automatically re-cocked when the turret retracts to reload. New rockets are even automatically loaded onto the launcher so it's ready to fire again in seconds.


The guidance system is automatic and uses some basic transistor circuits, but the tracking is done by passing the IR beacon at the back of the rocket through spinning slotted disks. The faster the light pulses, the further away from the cross hair it is.


In the 9P148, there is only one optical sight and guidance system, it's the one in the portable launcher! So you take the sight from the portable launcher and fit it to the gunners turret and dock the rest of the portable launcher with a big cable which connects into the vehicle systems.


I've found working it all out fascinating.

Link to comment
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

  • Create New...