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DO MAG-COUPLINGS SLIP?

DO MAG-COUPLINGS SLIP?

DO MAG-COUPLINGS SLIP?

(OP)
I am working at a gold mine in Alaska. We have recently installed several mag-couplings on certain pumps which pump processing chemicals, gland water, potable water. They are installed on pumps with vfd's and without. My question is very general in nature; Does the mag-coupling have a certain amount of slippage on the output to the pump or is this "rock solid" technology" and if you know of a reference I could study on these I would appreciate that as well.
Thank You.  

RE: DO MAG-COUPLINGS SLIP?

Without question, there is an amount of slip in a magnetic coupling.

The amount of slip will increase with torque through the coupling.

I will search for something for your reference.

Adrian

RE: DO MAG-COUPLINGS SLIP?

Are you referring to the magnetic couplings that are designed to be variable speed?  In which case, yes.  If you are referring to a magnetic drive style centrifugal pump, it is my understanding that they are synchronous, and there is no slip.

RE: DO MAG-COUPLINGS SLIP?

(OP)
Tenpenny,
These couplings actually have an air gap, between the faces on the coupling halves.

RE: DO MAG-COUPLINGS SLIP?

Oversized magnets can be used to reduce the slip in synchronous drive conditions, but there is no physical connection so there will be some amount of slip. When load is applied the increased torque will cause increased slip.

In variable applications where the magnets are pulled away from the induction ring, the slip increase is non-linear.  

RE: DO MAG-COUPLINGS SLIP?

TenPenny is correct, and that's an important distinction.  Mag-drives for seal-less pumps and agitators are permanent magnet devices which are synchronous and do not slip.  Instead, at high torques, they completely de-couple.

RE: DO MAG-COUPLINGS SLIP?

Agree with moltenmetal - there's no slip in a magnetic coupling. It is a synchronous device, right up until breakaway torque.

An eddy current clutch / eddy current coupling has slip as a fundamental part of its operating principle. The eddy current devices are sometimes used as variable speed controls and can put up with outrageous levels of abuse, but are very lossy and run at blistering temperatures.
  

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If we learn from our mistakes I'm getting a great education!
 

RE: DO MAG-COUPLINGS SLIP?

(OP)
I appreciate all responses, The "Mag-Coupling" we have purchased and installed replaces a rigid coupling or a flex coupling. It was intended to help get more life out of our mechanical seals, keep in mind, we are pumping slurry at various speeds, flows, density, pipe stress,  cavitation, etc... Despite laser alignments within tolerance, mechanical seals weren't lasting as long as they should. The Mag-coupling does not transfer vibration, thermal growth or pipe strain, which is no longer an issue. the service life of our mechanical seals has at least doubled.

There is no physical connection or mechanical connection between pump and motor. a magnet floats between two circular faces which is slipped on the end of each shaft. There is an air gap between the magnet and the faces. In some cases I have noticed some slight reduced flow. I am wondering if I decrease the air gap would that in fact increase my torque?   

RE: DO MAG-COUPLINGS SLIP?

Reduced flow, reduced under what?  The flowrates you had with rigid coupling.

We are more connected to everyone in the world than we've ever been before, except to the person sitting next to us.  Lisa Gansky

RE: DO MAG-COUPLINGS SLIP?

(OP)
BigInch,
Yes, I noticed a 45 gpm loss in a water circuit that treats underground mine water. It is pumped  through a sand filter and discharged to the river. Sand filtering is just one portion of the system. The filters go through several back flush cycles and the sand gets  periodically changed. At this point in the process we are dealing with very clean water anyway, however the filters pose a restriction of flow and when I reinstalled the rigid coupling the flow came back up @45 gpm. I am pumping about 400 gpm. I strobed the two shafts and recently found a 181 rpm difference between motor and pump shafts. Without any literature at all on this coupling I was wondering if reducing the air gap would be worth my efforts to decrease the slippage.

RE: DO MAG-COUPLINGS SLIP?

I think that if you reduce the air gap, you would get an improvement.  It seems logical to me, but I have never played with mag couplings, so I'll let somebody else answer to that.

I had a feeling that you wern't getting the whole truth about mag couplings not slipping.  Every liquid, solid and gas is compressible, everything deflects under load and ... everything slips.  It might not be much, but you know that it has to.  Nothing is perfect.  In fact I believe that you have almost proven as much with the 181 rpm difference between the pump and motor rpm that you have observed.  If the motor speed was the same with both the mag and rigid couplings, we can find out how much slip there acually was.  What were the motor speeds and the pump speeds with each type of coupling?

 

We are more connected to everyone in the world than we've ever been before, except to the person sitting next to us.  Lisa Gansky

RE: DO MAG-COUPLINGS SLIP?

I'm going to say it again...

Magnetic couplings have slip!!!

We have a patented pump design for a variable flow pump. There is a magnet array on one side of a membrane and an induction ring on the  other. The induction ring is attached to the impeller.

We vary the flow by moving the magnets away from the induction ring. The bigger the air gap, the slower the impeller.

Even when the air gap is reduced to its minimum, there is about 6-8% slip.

The neodymium magnets are sized to allows an amount of torque. If that torque level is exceeded, the slip increases.

So, even in the synchronous position...magnetic couplings do slip.

Adrian
  

RE: DO MAG-COUPLINGS SLIP?

(OP)
Hydromech,
Yes, it is obvious there is slip. I am truly getting familiar with what applications I can run these couplings and not have it affect certain pump circuits. The information you supplied me will help to research this further.

Thank You,
Scott

RE: DO MAG-COUPLINGS SLIP?

Adrian,

SOME magnetic couplings slip. Some do not.

Google magnetic coupling synchronous which will bring up a multitude of synchronous types.
  

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If we learn from our mistakes I'm getting a great education!
 

RE: DO MAG-COUPLINGS SLIP?

Even the permanent magnets do have a slip - with every slight change in pressure or conditions of the pumped fluid or with every slight speed change of the motor.
To prove this it will be probably be enough to monitor the temperature of the pump in the magnetic coupling area because the slippage generates heat  

RE: DO MAG-COUPLINGS SLIP?

There are two types of magnetic coupling, the most common being a synchronous drive coupling which will have absolutely no slip - in fact if it does de-couple, it will completely de couple until both drive and driven magnets completely stop rotation, as I said before these are now the most common couplings on the Market for everyday applications.

The other type, and an older design, is the 'slip ring' type of which were developed as the very first couplings, they are still available, butvtend tonne used for special applications such as hot liquids which become more viscous when cold which start up with slip to the impeller to ease the start up process.  These are still quite rare in the current Market and very few manufacturers still make this type due to the very high costs and reducing marketplace.

Hope this clarifies things ?
 

Ash Fenn

www.cdrpumps.co.uk

RE: DO MAG-COUPLINGS SLIP?

Hysteresis or Eddy current type...or whatever type, there is no physical link between the couplings. If the resistance is high enough, the coupling will have some amount of slip.

So, with no mention in the original question of load or type of coupling, we can assume that we are all correct..can't we??

RE: DO MAG-COUPLINGS SLIP?

Even with a "rigid" physical link, a drive shaft and coupling, torsional displacement of the shaft under load and pin tolerance, or whatever play remaining from manufacturing tolerance in "rigid" coupling will force at least a minute amount of slip.

Perhaps the EEs out there see things through more of a binary mask.  

Only put off until tomorrow what you are willing to die having left undone. - Pablo Picasso

RE: DO MAG-COUPLINGS SLIP?

Possibly..?

Do we need to go back to basics and put a definition on "slip"?

For our magnetic clutch, 5% slip is good considering the amount of neodymium iron boron we use. More would reduce the slip, but its not cheap.

I guess for some 0.0001% may be excessive.

RE: DO MAG-COUPLINGS SLIP?

"Slip" is a difference in rotating speeds, not a minor and limited displacement.

Magnetic couplings which consist of pairs of permanent magnets DO NOT SLIP.  They rotate at the same speed until they de-couple.

RE: DO MAG-COUPLINGS SLIP?

I think we all know what slip means. Thanks to moltnemetal for clearing that up!!!

So what does de-couple mean? If the flux breaks down between the rotating magnet pairs during transient torque loads, could that not be considered as slip. There would be a speed differential between the two halves of the coupling.

In the context of the original question, where slip is evident. It is safe to assume that the application features a coupling that does have an element of slip. Other applications that have massive permanent magnets and low torque would have no apparent slip.   

RE: DO MAG-COUPLINGS SLIP?

(OP)
Please refer to this link. It is a youtube video, it does show how mag-couplings (all with an air gap)
are being used. No pipe strain to worry about, no precision alignments to worry about, less energy to run. But always carry a good strobe light...haha as I have found out.

http://www.youtube.com/watch?v=TtIkYbReMbk

Yes, we are talking about slip and how to accurately work with it.

Thanks all!!!....Scott

RE: DO MAG-COUPLINGS SLIP?

In the generator world there is a measurement which we know as the load angle. This is a load-dependant angular displacement between driver and driven shaft. Once it reaches 90° the shafts will break out of sync. This angular displacement is not slip. When a synchronous coupling breaks out of lock that condition is what we know as 'pole slipping'. When it occurs on a synchronous machine it is normally very bad news for the machine because of the torque pulsation which occurs.

BigInch,

I'm an analogue guy. smile
  

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If we learn from our mistakes I'm getting a great education!
 

RE: DO MAG-COUPLINGS SLIP?

Difference in rotating speeds.  I'll remember that.  

Scotty, I fear that there's few of you left out there.

Only put off until tomorrow what you are willing to die having left undone. - Pablo Picasso

RE: DO MAG-COUPLINGS SLIP?

Permanent mag drives don't slip, They decouple when the torque is exceeded. check if there is any partculate build up in the  bottom of the can.Remember the shaft od where the magnet are run a close to inner dimeter of the can.

Reducing the air gap will increase the torque handling of the coupling but.... there is a compromise as the wall of the can sit in the air gap and that has to hold the pressure. so pressure and wall thickness of the can will ditatate the minium air gap.

Is it decoupling at start up use a soft start. If it is decoupling when loaded check the can and bearings for  particulate build up. Or use a bigger coupling size.
 

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