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magnetic clutch

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Taris Jewell24/11/2020 19:58:13
16 forum posts

so i will try and keep this as simple as posible but i have a small ish milling machine that i am putting a bigger motor on going from a 1500rmp 1.5 hp 3 phase to a 3000rpm 3hp 3 phase both being driven from an inverter. this isn't too inportnat but it is a facter, basicly as i am making the new shaft for the motor and running the change wheels for difrent speeds i thorght i would put a magnetic clutchon the motor so that i didn't have to shut down the motor when i am checking somthing or changing speeds seemd easy to do but i can't find a good clutch that doesn't cost £100+ i was thiking more like £50. so the question is can anyone point me in the direction of a cheper magnetic clutch or have a good alternertive?

many thanks for your imput

Robert Atkinson 224/11/2020 20:07:20
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887 forum posts
17 photos

How about car airconditioning compressor clutch?

Either used one or usaed scrap one normally comes complete with pulley.
I've been looking at fitting one to the ML2 along with VFD. First I need a round tuit.

Robert G8RPI

Nigel Graham 224/11/2020 21:58:57
921 forum posts
16 photos

The nearest I have been to any sort of clutch on a machine-tool was the fast-and-loose pulleys on the overhead drive to a large IXL lathe; but the one thing that worries me about the description here is that the clutch is to allow testing the work or changing gears. If so, this is one component that must be either very reliable or fail-safe. No corner-cutting.....

Steviegtr24/11/2020 22:48:03
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1801 forum posts
244 photos

The clutch on an air con unit is energise to engage so should be fail safe. However would a car one take 3 hp of jip.

Steve.

peak424/11/2020 23:46:10
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1328 forum posts
148 photos
Posted by Steviegtr on 24/11/2020 22:48:03:

The clutch on an air con unit is energise to engage so should be fail safe. However would a car one take 3 hp of jip.

Steve.

The various sources I've seen suggest 4 to 5 HP of the engine's power
Wikipedia, if you believe that suggests about 4

Bill

Clive Foster24/11/2020 23:51:17
2540 forum posts
83 photos

Taris

Like Robert I've been considering the same sort of thing. Have a compressor & clutch under the bench but the round tuit isn't yet mature enough to harvest.

Stumbling block is trying to work out the torque rating of the clutch so as to figure out what speed to drive it at.

I've yet to find any sensible data as to what the design power transmission actually is. Plenty of speculation that generally a horse power or so is the average power draw but it can be up to 3 or 4 hp. Found a link in an article on EV servicing stating that belt driven car AC systems are rated at 4 to 5 kW but EV systems are generally in the 2 to 3 kW range. However EV systems are driven by VFD controlled motors so compressor speed is independent of road / main drive motor speed whilst belt drive systems run at roughly engine speed. That article says up to 7,000 rpm.

Another source implies that pump efficiency is in the region of 65% implying 1.5 kW of drive power for every kW of cooling.

Assuming that the 4 to 5 kW rating is cooling power at 7,000 rpm we can do some handwaving analysis of input power and therefore clutch transmission capability.:-

4 to 5 kW cooling power = 6 to 7 kW drive power = 4.5 to 5.6 hp @ 7000 rpm = 0.65 to 0.8 hp per thousand rpm

So, if driven at motor speed, this suggests a car air con pump clutch would handle roughly 1 hp from a 1,400 pm rpm motor and 2 hp from a 2,400 rpm motor.

In practice belt drive air con pump clutches are toggled on and off as required so a good deal of overload capacity is needed to avoid premature wear out from slipping at each cycle. I reckon at least 50% overload capability would be built into the old style units as clutch failure used to be pretty rare. Some of the more modern ones seem to be less durable. I'd be suspicious of using anything where replacement clutch units are readily available at modest price. That said a fair variety of new clutch units seem to be available for around £30 to £60.

All the ones I've seen appear to have the driven unit bolting onto the end of of the pump drive shaft with the coils bolted to the pump itself so you cannot have a through bore system. Unless you plan to drill and tap the end of the motor shaft for the driven plate it will have to go on an intermediate shaft.

Clive

 

 

Edited By Clive Foster on 24/11/2020 23:51:57

Jeff Dayman25/11/2020 00:12:33
1990 forum posts
45 photos

For years I had a portable air compressor rig made from an Eaton car AC compressor driven by a 3.5 HP gasoline / petrol engine. The compressor had a 12 VDC electric clutch which I operated from the battery of the truck or tractor used to haul the compressor around. Worked for about 25 years and the engine wore out, but the compressor and clutch survive in a friend's shop. Clutch on that compressor never had any problem with 3.5 HP just FYI - real life experience

Clive Foster25/11/2020 10:12:55
2540 forum posts
83 photos

Jeff

Thats an interesting application but its not a great deal of help with this question as, for all practical purposes, you were running the compressor pump unit as it was designed to run. So the clutch would clearly be sized to handle the torque and power demanded by the compressor at any speed within its operating range.

The problem here is figuring out how much torque the clutch can safely pass when run at a different, lower, speed.

With a machine tool application the clutch is unlikely to be engaged under serious loads so its safe to exploit the inherent overload capacity built into the design to cope with a hard engagement of a compressor at engine speed.

As I suggested in the previous post something approaching 2 hp at 1400 rpm and 4 hp at 2,400 should be quite doable.

Which begs the question of how much power folk like us are going to be putting into the cutting tool tip anyway? By my maths fully utilising even 1 hp is getting quite as far into into the scary hot chip shower and supersonic birds nest build up regions as I care to go!

Clive

clogs25/11/2020 10:34:22
587 forum posts
12 photos

Taris,

my Toro Time Cutter ride on mower has a 18HP pertol engine and the electro clutch is def ok with that HP.....

it is a modified version of a Car AC unit.....

but I'd prefer something easier....how about the motor on a rocking mounting plate and just slacking off the drive belt will be enough to disengage the drive.....

I did the same thing on my Colchester Student lathe...works a treat.....

the belt has to be just loose enogh to slip/diengage the drive but not loose enough to fall off.....

mine has been faultless for the last few years of constant use....

Dave Halford25/11/2020 11:28:29
1156 forum posts
11 photos
Posted by Jeff Dayman on 25/11/2020 00:12:33:

For years I had a portable air compressor rig made from an Eaton car AC compressor driven by a 3.5 HP gasoline / petrol engine. The compressor had a 12 VDC electric clutch which I operated from the battery of the truck or tractor used to haul the compressor around. Worked for about 25 years and the engine wore out, but the compressor and clutch survive in a friend's shop. Clutch on that compressor never had any problem with 3.5 HP just FYI - real life experience

An air compressor and a mill have comparable start up conditions as they both start off load.

Clive Foster25/11/2020 11:35:20
2540 forum posts
83 photos

clogs

I've used, and built, several slack belt clutch systems of both rocking motor and jockey pulley types.

I entirely agree that they are effective but can't consider them a particularly safe method of disengaging the drive whilst you do other things such as measure or adjust. Especially if the device being driven is free running.

I used mine to give a more gentle spindle engagement than simply starting the motor under load and to get a slow running "slipping clutch" drive when I needed to see how things turned under power. When I wanted things stopped I switched the motor off. Theoretically slipping belt clutches increase pulley and belt wear but I never noticed any such effects.

Great thing about an electromagnetic clutch off a car aircon pump is that its designed to spin freely when driven whilst disengaged.

I want to use a pair to select speeds in a two speed belt drive system.

Clive

Circlip25/11/2020 12:07:02
1224 forum posts

Nobody do "Powder" couplings anymore?

Regards Ian.

duncan webster25/11/2020 13:28:36
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2963 forum posts
34 photos

I know this isn't answering the original question, but as the main motor is 3 phase driven from a VFD what's that advantage of a clutch? Just seems like adding complexity to me

Brian Sweeting25/11/2020 14:07:09
452 forum posts
1 photos
Posted by Circlip on 25/11/2020 12:07:02:

Nobody do "Powder" couplings anymore?

Regards Ian.

Had one in my Subaru Justy years ago, great device.

Clive Foster25/11/2020 18:14:05
2540 forum posts
83 photos

Taris

I raised the question in another place and one correspondent said he had run a hydropump of approximately 5 hp power demand at 1,800 rpm for many years using a belt drive via a car air conditioning pump magnetic clutch.

Which, I think, pretty much answers your question and confirms that such a clutch will be up to the job.

It was also mentioned that a seized air con pump will shred the belt rather than slip the clutch indicating that the clutch can transmit more power than the belt.

I wonder if there is scope to reduce the clutch coil current, and thus power needed, to more closely match the transmitted power to the demands of the cutter.

Duncan

From a safety perspective a clutch that can disengage pretty much instantly if things go wrong is a little more comforting than waiting the 10 seconds or so for a VFD to ramp down and stop. Even the E-Stop routine takes finite time.

The extra step of having to engage the clutch is also more protection from being rush headed and starting things up before you should.

Clive

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