PTO lawn vac

I'll second the V-belt suggestion.

First of all, going that level of multiplication will result in the shaft speed on driven end to be very high, so the shock (from chain drive) will be very high at such a short reversal arc... it'll beat the chain, sprocket, and high-speed shaft bearings.  Belt won't do that.

Next... belt pulleys WILL be available in that size...

and if something goes wrong, the belt will slip.

NOW...

With respect to horsepower, the previous note about belts being used in 17hp applications... this is true, but the power transmission capacity of any belt or chain has some serious dependancies.  You can't just say it's a 'xx horsepower' and expect to get that out of it.  There's drivetrain engineering to consider... there's two sides, and the belt or chain in between.  I won't go into thorough detail, but let's look at just a few:

Consider the track on a tank...  it goes across the top of the bogeys, changes direction at the front, then along the bottom, up, and changes direction at the top.  When rolling down the road, the TOP of the track (belt) is travelling forward at TWICE the vehicle's speed, then the opposite direction at the same speed.  At the back end of the vehicle, each segment is forced to go from moving in one direction, to the other direction, in a very short period of time.

Even with no physical load on a chain or belt, the act of changing direction, imposes a very high load on the chain or belt. It's centrifugal force.  If you spin a chain or belt fast enough, it will break, strictly by virtue of centrifugal force.  The LIGHTER the chain or belt is, the lower the tension due to centrifugal force will be.

Next...  NO belt sheave, or sprocket is perfect... nor is the shaft or bearing that it runs on.  This means that the RUNNING outside diameter of a sprocket is not consistent, and when on a fixed-position shaft, varies slightly as it spins.  A chain turning on that sprocket will experience 'tight' and 'loose' spots.  Add that to a sprocket on the opposite side that has a different diameter, and you have TWO things that are moving towards, and away from eachother, in different frequencies... and as a result, your driveline combination will have TWO sequences of tight/loose that change, and cause a third sequence (like neap tide, and high tide)... so you got vibration.  You also have loose spots and tight spots...  the vibration from loose to tight... even without load... can beat hell out of a chain or belt, and cause it to fail on it's own.  While they both experience runout, belts are generally more forgiving than chains... and when they fail, they tend to be a little less dramatic... than a chain.

Driveline power is limited by chain/belt strength, and tractive effort.  chain/belt strength is pretty obvious... tractive effort is a little different, but the idea is same:

Let's say a chain pulling on a tooth... the tooth is only strong enough to withstand 100lbs of pull before it rips off the sprocket.

Let's say BOTH sprockets have 20 teeth... so the chain WRAPS around 50% of BOTH sprockets.  For practical purposes, you can figure that HALF of those teeth-in-wrap are actually in a contact position (it's actually closer to 73.2%, but 50% is easier math with safety factor included)  You can figure that 10 teeth on each sprocket have chain-in-contact.  That's 10x100=1000lbs of tension before the teeth rip off, right?

Let's say the chain is only good for 400lbs of tension.

Your power limit, will be determined by the chain's break tension.

Now let's do a 10tooth sprocket, driven by a 50 tooth sprocket.  Obviously, the large sprocket will have lots of wrap... but due to the angle and distance, the small sprocket will only have 3 teeth in working contact...  your tension limit is 300lbs now.

Oh, and once you get the arrangement spinning... the centrifugal force acting on chain at that turnaround point... comes out to some astronomical figure... like   500lbs...

You'll find the limits really fast.

With belts, all same applies, but it's about CONTACT SURFACE on the sheaves... and don't think you can 'make up' for surface by increasing belt tension-  you can't snug up the belt and expect it to survive long amidst runout of both ends... ;-)

and because of the distance between the two sprockets, and sizes of the sprockets

 

So, I've been doing some Leinie's contemplation. I rummaged through the pulley section at Fleet. Found a 3.5" pulley and a 12" pulley type A or B pulley.

 

That would yield a 3.43 to 1 ratio. So wide open, the fan should be turning 1852 rpm, and should be OK at a little below wide open.

 

I'm a little worried about if the pulleys will fit with my current mounting configuration, so I ran up to where the tractor is stored and measured. Its approximately 9.5" center to center on the shafts.

 

The 3.5" pulley is not meant to go on the 1 3/8" hub. But, a few minutes on the lathe should remedy that.

 

 

 

So, I laid it all out on a piece of cardboard to mock it up. My main concern was the amount of belt contact on the small pulley, being in such close quarters. Need to get a tensioner in there too, if possible.

 

I drew the one on the left at 9.5" center to center, it looks like a little less than 180 degrees of contact.

 

The one on the right I drew up at 13.5" center to center, looks like 180 degrees or maybe a little more, if I just raise the whole vac 4".

 

 

THEN, I googled "how much wrap do you need on a belt" and found a general belt page, and it says 60 degrees or more is acceptable.

 

Well, it seems like it should work with the current 9.5" center to center then. I don't think it will take much power to maintain the fan once it is running, but the load at startup will be significant. Going to have to ease the clutch out to get things turning, that's for sure.

 

Its just a fan, moving air and leaves, not like an auger full of grain, or an elevator full of bales, you know?

 

OK belt and power transmission experts, how is my aim?

 

Loosened everything up, swapped out the chain and sprockets, installed the pulleys so I can measure up for a new belt. Used the old fan belt off the engine and a tape measure to get close.

 

 Need to drill the small pulley for the roll pin and get a 44" belt and see where we come out.

 

Looks like all my existing setup will work, I was wondering if that big pulley would hit my brace or not.

 

 

 

I'm thinking a simple bracket and spring will suffice here for a little tension on the slack side of the belt.

 

 

More to come...

 

 

 

 

 

Fabricated some solid mounts out of 2" square tubing.

 

 

Bolted them on under the rockshaft mounts

 

 

Tacked some cross bars into place. Remember my failed attempt at making new cylinder bars for my all crop? Yeah, me too. Still using all the 1 1/4" angle iron from that,

 

 

Set the fan down into place. It still leans back a little, so I had to shim it up just a touch to try and get the pulleys aligned as close as possible.

 

Flipped over my upper support brace and it bolted right up.

 

Fabricated a tensioner. I wanted a spring loaded tensioner, as I expect the belt to be moving a bit. I tried straightening out the big drive pulley in a press, and I got it better, but its still going to have runout.

 

 

Put just a bit of preload on the spring. Fired the old girl up, and was not impressed. It didn't seem to be doing any better. Only about 1000rpm! What the hell?

 

Then I tightened up the tensioner. I could hear the fan starting to roar as I tightened it.  More tension=louder fan. Belt must have been slipping.

 

At wide open throttle, got about 1550 rpm.

 

 

More gooder. The airflow from the fan seems to have doubled. I'd still like to see it at 1800 rpm, but this will have to do for now.

 

One other idea I had was to mount the vac on the trailer, build a jackshaft and use two belts to gear it 3:1, and drive it with a pto shaft like any other implement, but that would be a project for the future.

 

Got the new discharge elbow mounted up.

 

 

Now, just need to build a guard around the belt drive, and put something under the seat to hold it up because when I plunk my carcass in the seat, it will hit that top pulley. Maybe I can just built that into the guard.

 

Other than that, need to take it all back apart, do some final welding, and wait for the leaves to fall! I'm sure that will be soon enough.

 

Thanks for looking.

 

 

 

 

 

 

 

 

 

 

 

Good luck, you might be chasing the wild goose!

Brower used to make vacs, I think its called brower-kesmec, now, but it might be a search term you can use.