Linear motion - how to decrease (shear) pull force 1

[free_flyer]

My design has a linear, sliding 'cassette' [purple] which slides by pulling on the teflon cable [yellow]

In the cassette are magnets (x15) which attract to a mating part (not shown) with the same number of magnets but with reversed poles.

When the cassette is in the closed position, the magnets attract and hold the mating part.

When the cassette is in the open position, the magnets repel and eject the mating part.

In terms of magnetics, when the cassette is in the closed position the force required to slide the cassette open is known as the 'shear' force, whereas the force between the mating parts is known as the 'axial' force..

I have a working prototype, but I am trying to reduce the shear force required to slide the cassette to the open position (i.e. to eject the mating part)

Currently (when the cassette is in the closed position) the shear force to slide open the cassette is around 6 to 7 kg and the axial force between the mating parts is about 10kg.

Ideally I want the axial force to be as large as possible (when the cassette is in the closed position) yet the shear force to be as small as possible.

Below is a top view showing the cassette in the closed position...

Below is a top view showing the cassette in the open position...

The distance the cassette moves is around 12mm (i.e. between the open and closed position).

I've been trying to think of a way to decrease the shear force to slide the cassette open, without having a detrimental effect on the axial force (such as creating an air gap between the mating magnets).

Originally the cassette ran on linear bearings and rails, but these made little or no difference to the shear force so I removed them.

I then looked into something like a rack and pinion (does this even give a gearing advantage?)and other kinds of gear and cam mechanisms, but I've made no progress.

How can I reduce the shear force to slide the cassette open and preferably without increasing the size of the design too much ?

 

[MintJulep]

Slippery stuff.

UHMW, Teflon, etc.

 

[hydtools]

Increase the axial magnet to magnet distance.

Ted

 

[desertfox]

Hi

Not sure you can have it both ways, if you increase the axial force gap by Means of say a Teflon coating between the faces then the axial force will be reduced following a squared law. Whilst this coating will reduce the shear force it defeats the outcome you require. I guess you are viewing it the same way as I am ie simple friction law where the axial force times the coefficient of friction gives the shear force as you call it, I realise it’s not quite as simple as this because of the magnetics involved but I do know the force of attraction decreases by a squared law in magnets as the air gap is increased so I doubt you will achieve the desired result without reducing both the axial and shear force.

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

 

[rb1957]

how about a little grip handle that engages a mechanical retention device ?

it could be hinged to deploy automatically on loading, then retracted (with a squeeze grip in the handle) to remove.

another day in paradise, or is paradise one day closer ?

 

[free_flyer]

Thanks for the replies

What if I was to fit a spring that was compressed when the cassette is in the closed position...

If the force required to pull the cassette is currently 6 to 7kg, then if each spring applied a few kg of force when compressed would this reduce the pull force required using the cable ?

 

[free_flyer]

 

[itsmoked]

The springs would probably work.

I'm not seeing how you engage the system. You aren't doing it by 'pushing a cable' are you? Adding the springs would make that a bigger problem.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[BUGGAR]

I suggest an over-center spring/mechanism.

 

[IRstuff]

Move the magnets apart before sliding; that's the conventional approach. 10 kg normal force is a huge amount to overcome.

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert!

https://www.youtube.com/watch?v=BKorP55Aqvg

faq731-376 forum1529 Entire Forum list

http://www.eng-tips.com/forumlist.cfm

 

[desertfox]

Hi free_flyer

Yes the springs would probably work however how do you propose to compress the springs when closing the Teflon cable into the cassette? Also isn’t the yellow cable meant to be pulled and move the cassette as a single unit to some other position before the thing is opened? If that is the case then when the cassette is moved with whatever it is loaded with then you need to ensure the yellow cable doesn’t just go to the open position with the reduced shear load, in short we need to know what else this cassette is doing when in the closed position.

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

 

[dvd]

Please show a section view. I think I understand what your point is, but not certain.

If the axial "force" is 10 kg (assuming on earth), it must be reacting against something - what?

If this gizmo is made of plastics, it may be very susceptible to temperature swings.

What is the point of a teflon cable? Plastics and teflon have a much higher coefficient of friction than rolling element bearings.

I would try to incorporate bearings and not some alleged slippery material(s).

Bearings need to provide enough restraint, but not too much. Were your linear bearings fighting each other?

That section view would aid in understanding how this thing works.

 

[itsmoked]

With those disk magnets not being closed loop magnets let's hope this thing is no where near a compass.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[IRstuff]

Are the magnets actually contacting? If so, then you've made a very difficult situation. Any physical space will make the job infinitely easier

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert!

https://www.youtube.com/watch?v=BKorP55Aqvg

faq731-376 forum1529 Entire Forum list

http://www.eng-tips.com/forumlist.cfm

 

[free_flyer]

Thanks for everyone's input, I've added some more images below with answers to the questions asked.

The bottom part will be fitted to a helmet and the top part will be fitted with a GoPro camera. The idea is to jettison the camera in the event of an emergency, this is assisted when the cassette is pulled to the open position because the magnets will oppose each other causing the top to eject.

Magnets in top mating part...

Magnets in bottom mating part...

Top and bottom parts mated together...

There is no air gap between the bottom and top mating parts.

Cross section with cassette closed...

Cross section with cassette open...

Exploded view - note there is a 2mm steel plate for the magnets in the top and bottom parts (does this make it closed loop?)...

Below is the original design with the bearings and rails...

I realise that if springs are fitted then the cassette cannot be closed with the flexible teflon cable, I would need to add some feature to assist in closing the cassette (I am thinking of some kind of feature in the top of the cassette itself)

I was wondering about using magnets instead of springs, so that the magnets oppose each other when the cassette is in the closed position.

Cassette closed...

Cassette open...

 

[itsmoked]

So a camera that would typically be fitted while two hands are available in a static situation. I'd expect handling the camera itself or some small raised finger posts would work for installing and the cable pull ignored during install. Still looks good for springs.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[rb1957]

in an "emergency" is someone going to have time, and hands, to do this ?

It'd be easier to have the camera protected (a roll cage ?) and to have a frangible piece (a structural fuze) that'll get overloaded before the roll cage breaks

another day in paradise, or is paradise one day closer ?

 

[free_flyer]

The application is for skydiving and its usually a requirement to have some form of cut away system when using a camera.

The following video shows exactly why such a system is required...

Link

The cut away system in the video has a cable on the chin strap, which allowed the skydiver to remove the entire helmet.

This is a common system found on open face helmets, however, this system is not available on some full face helmets (which have a visor like a motorbike helmet).

For these types of helmets there are other systems...

- This system disconnects the mount from the helmet, but this mount requires drilling holes in the helmet which voids the warranty and weakens the helmet...

Link

- This system mounts the camera to the chin of the helmet and the camera is attached using magnets, but this mount can impede seeing the emergency handles (located at the chest area)...

Link

 

[desertfox]

Hi free flyer

If you use magnets instead of springs it’s means when you insert the cord the magnets will try to repel the cord so how will line up the magnets to get the axial load when the shear force magnets are resisting cord insertion.

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

 

[itsmoked]

desert; Not following you! How do the magnets affect Teflon cordage?

Free flyer; That first video... man. Scary. But those guys all kept their cool so well.

I see what you're after. Makes sense.

Would seem like an FAA kind of problem though letting solid objects fall (bomb) to the ground.

Springs for the win.

Keith Cress
kcress -

http://www.flaminsystems.com