Direct vs. Belt Drive...

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SurfLung

Contributor
Messages
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Location
Central Minnesota
# of dives
500 - 999
Direct vs Belt Drive
- I blew out the belt on my little Aerotecnica MCH-6 and had to fit a new belt to match the specs of the original. Same length, width, angle, etc. But I am surprised with how tight it is. Reviewing the video tutorial from Coltri Sub, the new belt seems to have gone on exactly like the video. It's kind of different (to me) how they just lever the belt onto the pulleys without loosening the motor mount.
- On my RIX SA3, I loosen the motor mount to change to a new belt. Then, I get a pry bar and lever the motor to slide and tighten the belt. And while holding it tight, I snug up the motor mount bolts to hold it in place. The belt is as tight as I can get it and conforms to the tightness test in the manual. But it still is floppy. Some of you have commented on the videos I've posted of the RIX where you can see the belt getting floppy while running. Also, the RIX manual recommends caution not to get it too tight as it might damage the compressor bearings if there's too much side tension.
- The little Aerotecnica belt is tighter and has zero floppy-ness. And I'm wondering if there is any danger to the bearings from side tension.

On the Other Hand...
- My Tornado/Kidde compressor is direct drive (no belt). It even has a coiled spring in the fan to take up the shock at start-up. And of course there is no side tension. I think its pretty common to take the odd voltage direct drive motors off of surplus Kidde compressors and replace them with a side tension belt drive.

- I'd like to know how tight a belt should be and how serious is the danger of over tightening?
 
there should always be deflection on belts for side loading on bearings which they don't really like. Sufficiently snug so there is no slippage is all that is needed. IIRC the Rix manual has pretty clear deflection instructions for the belts and they will stretch as they warm up.

Direct drive is always better than belt drive. You know how loud your kidde is... Anyway, the reason direct drive is impractical is because inexpensive electric motors are 3600rpm which is too fast for most compressors and designing them for that would be irritating due to heat. 1800rpm motors are available, but are typically twice as expensive and are twice as large. 900rpm motors are also available, but again are twice as expensive as 1800rpm and twice as large. Since hp is a function of torque and rpm, direct drive gasoline motor options at the lower rpm would be highly impractical due to size of the motor

I wouldn't call overtightening of belts to be dangerous, but it could get expensive depending on how pricey the main bearings are. The manuals should call for deflection specs and the procedures on how to measure it. Typically a 5-8lb weight at midpoint between the pulleys with deflection generally being around 1/4". This is an industrial standard, nothing special about this happening to be a scuba compressor
 
Set mine up with a kevlar belt. The mount had adjustment to move the diesel engine. Had a frame made previously for another compressor to have a gas engine slide for belt adjustment. Cost about $1000 US.
 
additional useless info....

So one of the advantages of the belts is you can do some fine tuning of speed without a VFD. Direct drive on an AC motor is full out unless you put a VFD on there. VFD's are good, you should use them, but that's a different thread.
If you have concerns about belt slippage, there are cogged belts that are used in many industrial applications but also in DPV's and various other high torque applications. You can swap the pulleys over and use that kind of belt if you want.
 
Thank you T-Bone and Solo Diver. After putting the new belt on the Aerotecnica MCH-6, I was getting a lot of vibration in the belt guard screen. Yesterday after posting this, I noticed that the V-belt packaging had instructions on the back for how to install and how to check for the correct tension. Interestingly, it advised NOT to change belts the way Coltri does it in the video!

So last night after work, I took off the belt guard screen to check the tension and it was almost perfectly as advised on the packaging. But then I noticed a bright spot on the inside of the screen... The belt guard screen had been vibrating because when I reinstalled it a little too close, the center of a pulley was rubbing on it! I re-installed with it backed off and the vibration disappeared... Duh on me!

I've been thinking of rigging up a hinged mount for the motor on my RIX SA3. I was thinking a hinge and a threaded rod tightening system with a spring to give it a little flex might stop flopping?

We're going on the dive trip next weekend and I'm planning to bring this little Aerotecnica MCH-6 compressor... She's got a new belt, changed the oil and filter, filled up the Briggs and Stratton with aviation gas, and test filled a scuba tank... Ready to go! Thanks T-bone and Solodiver for your help.

 
@SurfLung the hinged motor mount should be enough. There should not be much of any bouncing though since it should be a stable system. Worth a try, can always stiffen it up if it proves problematic
 
https://www.shearwater.com/products/perdix-ai/

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