Vintage Double Hose

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?

Yes, over-tightening a belt can definitely damage bearings and even the shaft, but I can’t really give you any further guidance.

Rotating machinery has to be specifically designed for that type side load. Keep in mind that every revolution with a side load adds-up into its fatigue live. Therefore rotating machinery has to be carefully designed for that side load.

Obviously that is part of the expected loads and it is designed for it, but if the expected load is exceeded then it will affect its designed life expectancy.

The number of cycles is additive and at the revolutions this type of equipment runs, the cycle add up fairly fast. Therefore deviating from the designed load can reduce the life expectancy fairly quickly.

So you should follow the manufacturer’s recommendation for each individual rotating machinery type. They are not all the same.

In general a belt needs to be tight enough so it will not slip, but not too much tighter. But that “rule of thumb” is kind of broad and is not a substitute to the manufacturers recommendation.

When I built a new frame for my compressor I used a hinged mount for the motor. The weight of the motor puts enough tension on the belt

Thank you Luis and Tom. 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!

Tom thanks for the tip on the hinged motor mount. I've been thinking of rigging up a hinged mount for the RIX but I didn't realize just the motor weight might keep it tight enough. 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 Luis and Tom for your help.

Update - Aerotecnica Belt

- Well the A28 30" belt definitely WAS too tight... It failed in a cloud of fibers and black rubber dust. The next size up was 31". But I put that one on and it was too loose. I looked under the machine for the bolts to loosen and slide the motor (to tighten the belt) and they are set up not to adjust! So, what I realized that I needed is 1/2 size between 30 and 31 inches.
- Thankfully I found a source for A28.5 - 30-1/2" v-belts. So, we'll try that one in a couple days when it arrives. I don't know if anybody is interested in this stuff but I thought I'd post it anyway. Some places give the impression there are no "1/2" sizes in V-belts. The "V-Belt Guys" have them.

https://www.vbeltguys.com/products/a28_ ... 4a1c&_ss=r

Sometimes we overthink things... Figure out what size belt you need and go to the local Tru Value hardware store. READ THE PACKAGE THAT THE NEW BELT COMES IN.... It will give you the horse power rating that the belt is designed for. If your compressor has a four horse power motor get a belt that meets that specification. I set up my belts so they have about a quarter inch deflection. This has worked well on my floor sanding machines for almost thirty years and has worked on my scuba compressor for 203 hours as of this afternoon.... Don't use the cow method and buy the cheapest belt you can find off of Ebait. LOL

Well, you got me there. I have bought belts from 3 sources online and the Farm Equipment supplier locally and none of the packaging gives a horse power rating. I have seen a spec sheet that lists one type of cross section "4L" as being for fractional horsepower and another cross section "A" as being for Classic V-Belt... Dimensional drawings and individual belt specs indicate these are identical cross sections.

For my 30.5" length, I found a "Classic" in Kevlar that's labeled "A28.5". Since I saw your post I checked again and found another 30.5" belt that's labeled "4L305"... Which is the "Fractional Horsepower" belt. Cross section spec is A/4L. After reading your post on horsepower, that one may be what I should have ordered but I already have the A28.5 on order. It looks identical as far as the fit goes so I'll give it a try... If that one fails I'll order the 4L305.

Thanks for your help.

Exact Belt Size Required
- The discussion on adjusting the belt tension is my reason for posting this time. This compressor does not have the ability to adjust the belt tension. I've taken a couple of photos to show this. See on the bottom how the engine mounting bolts are maxed out in opposite directions...


- I'm thinking Coltri has designed this intentionally and its a really good idea. To heck with screwing around trying to get the belt tension adjusted just right. If you can put a belt on it that fits perfectly without adjusting, that would be easier, more consistent, and more predictable.
- Anyway, their system seems to work. I finally got the half size between 30" and 31"... I got the 30.5" belt and the compressor is now running great. NO vibration and self destruction from being too tight. And no slipping and smoking from being too loose. I tested it by filling a 3442 psi tank from 0-3100 psi. The compressor actually has an over pressure valve that releases at 3000... So it can't pump much higher than that. I pumped the tank up to 3100 and let it run for awhile at that level to see if the belt slipped from the load. NO slippage. Works great.
- A happy ending for this story.

SurfLung wrote: ↑

Mon Jun 22, 2020 10:50 am

Exact Belt Size Required

- I'm thinking Coltri has designed this intentionally and its a really good idea. To heck with screwing around trying to get the belt tension adjusted just right. If you can put a belt on it that fits perfectly without adjusting, that would be easier, more consistent, and more predictable.

- A happy ending for this story.

Hi Eben,

I am glad you got your compressor working, but that is not correct for a number of reasons. The list is long, but the basic reasons have to do with design tolerances and excessive cost of manufacturing with very high precision. It would just be very bad practice to try to design high precision with a flexible belt.

Belts are not only flexible, but they have manufacturing tolerances, therefore if you had a fixed shaft to shaft dimension, the tension would definitely be less consistent and much less predictable.

Belt drive designs all over the world (including Italy) always have to deal with real world loose tolerances from the available belts and their inherent tolerances.

In any case, you can see the adjustment slot. The slots were just intended for a motor with a different footprint. Someone got a different motor (either at the factory or later) that had a slightly different bolt pattern. It is pretty obvious to me that there was a change and someone didn’t follow with all the needed changes.

Why would they have machined or stamped four slots if they intended to use them as four fixed hole?


I could add a lot more about the relationship between the dimensional tolerances and friction, but the explanations would be too lengthy for here. You are just going to have to trust me with some basic machine design.

Yep. You sent me the top picture, but not the bottom. I agree with Luis. That is not the original motor to the compressor. Bet you a six pack. I would take an angle grinder and lengthen the slot some so I could tighten the belt when required. Belts do loosen with wear, so you will need that option.

swimjim wrote: ↑

Mon Jun 22, 2020 5:43 pm

Yep. You sent me the top picture, but not the bottom. I agree with Luis. That is not the original motor to the compressor. Bet you a six pack. I would take an angle grinder and lengthen the slot some so I could tighten the belt when required. Belts do loosen with wear, so you will need that option.

You owe me a six pack.

SurfLung wrote: ↑

Tue Jun 23, 2020 4:46 pm

swimjim wrote: ↑

Mon Jun 22, 2020 5:43 pm

Yep. You sent me the top picture, but not the bottom. I agree with Luis. That is not the original motor to the compressor. Bet you a six pack. I would take an angle grinder and lengthen the slot some so I could tighten the belt when required. Belts do loosen with wear, so you will need that option.

You owe me a six pack.

I don't see a way to get beer out of this for me, but I do agree I would probably modify the slots for a little adjustment room.

antique diver wrote: ↑

Tue Jun 23, 2020 5:11 pm

SurfLung wrote: ↑

Tue Jun 23, 2020 4:46 pm

swimjim wrote: ↑

Mon Jun 22, 2020 5:43 pm

Yep. You sent me the top picture, but not the bottom. I agree with Luis. That is not the original motor to the compressor. Bet you a six pack. I would take an angle grinder and lengthen the slot some so I could tighten the belt when required. Belts do loosen with wear, so you will need that option.

You owe me a six pack.

I don't see a way to get beer out of this for me, but I do agree I would probably modify the slots for a little adjustment room.

All the pic's on the interweb, except for yours show your compressor being powered by either Honda or Yamaha engines. Miller High life would be good......

This is a 1980s model... From back in an era when everybody used Briggs and Stratton. Here are a few photos I collected when I was researching this compressor last year:



And here's a video


You guys have to realize I'm kind of proud of the "vintage" aspect of my little Aerotecnica compressor. I want to keep it as original equipment as possible, so it kind of puts me off that someone would suggest it hasn't got the original engine. And as much as longer adjustment slots may make good sense, I'm not about to take a die grinder to it.

I am proud to have corresponded with Claudio Coltri and further established some of the history of this machine. He said he was assembling these very machines when he was in college and working for his Dad... Now he's the President and CEO and probably the same age as most of us. He recently sent me this note along with some complimentary Air Station signs one might find in a dive shop window.

Confirmation from Claudio Coltri...

- I wrote an email to Claudio Coltri and I got back an answer that should satisfy all of us. I sent him one of the original photos of my Aerotecnica MCH-6 and asked him to confirm that the Briggs and Stratton was the original engine and whether or no the belt is adjustable. Here's his response:

"Good morning Eben,

what a lot of memories with this picture, I remember any single piece of this unit made and assembled with my father Carlo in 1982 or an year later, this is a time capsule, everything it’s original as made in the 80’s, intake filter, filling hose, stickers and all the rest.

I suggest you refrain to go into the compressor forum when you can talk to me directly. The frame I think was made with slots, you have the possibility to adjust engine distance. We use A31 but if too loose go to the smaller size and push motor forward. The all made in China belts allow some extra tension initially to adjust itself without breaking.

About your 100% ORIGINAL Briggs & Stratton made in Milwaukee it was the only available at that time, I use to buy from the Italian Distributor that now his son is the Subaru distributor. Honda come’s much later and was not allowed in Europe at that time... Best Regards, Claudio Coltri"