Putting a vac in overdrive...

[Putter]

If you didn't get the right size pulleys, that would really suck! ...or ... wouldn't suck!?!? Sorry Kent I couldn't resist.

[tbar1.1]

BE CAREFUL bout going to a smaller pulley, don't want to pull the grass out of the ground. :)

[MPH]

I recall Dutch posting that he suckin the green outta his grass. Could be he is near the limit

[Guest]

Another formula that applies is that on an air blower or fan the power required goes up as the cube of the rpm. Thus if you increase rpm 33 percent the hp required goes up 235 percent. (1.33x1.33x1.33)=2.35 I think you engine might be running out of power before it really gets up to peak speed. This is the same reason that it is tough to double the speed on an airplane at sea level. to go twice the speed you need 8 times the power

[dlcentral]

I just cut grass REAL LOW and wind blows everything over to neighbor's!

[Kent]

And here is a picture of the smaller 3" impeller drive pulley.... http://www.simpletractors.com/transfer/images/vac_clutch_impeller.jpg

[Kent]

OK. Here's the actual measurements instead of going from memory -- I had a couple of them wrong. I removed a 4" (not 3.5") pulley from the vac impeller and replaced it with a 3" (not 2.5") one. Plus, the rear PTO pulley is 1" larger than I thought. BGB 3" pulley to PTO 10" pulley PTO 6" pulley to jackshaft 6" pulley Jackshaft 6" pulley to impeller 3" pulley What speed would this generate at 3600 engine RPM on the driveshaft? In comparison, the pulley that goes on the right side of the BGB to drive it directly would be: 4" BGB pulley over the idler shown below, to the 4" impeller shaft pulley (before I changed it). This gave the direct drive for 3600 RPM. The picture below shows the 3" pulley added to the left side of the BGB for the rear PTO (on the left), the bracket and idler removed from the vac that allowed right hand drive (in the center), and the 4" pulley (on the right) that is added to the BGB right side to drive the vac. http://www.simpletractors.com/transfer/images/vac_drive_pulleys.jpg

[Kent]

Dave, Is that your neighbor's 725 you have For Sale..? It looks like his yard. http://simpletractors.com/transfer/images/dlc_dsc00083.jpg

[Kent]

I used a variable pitch pulley. It's set to about 2.5" like Kents.

[Woodydel]

After seeing that picture I think I'll hook up the snowblade for my leaves. Looks like it might work. And if I'm lucky I won't need my mower deck in the spring!

[Woodydel]

I was just doing some math and the pulley changes from 3.5 to 2.5 inches works out to a 33% increase in the impeller speed. I think this is a bit more speed than the engineers figured for. The forces outward I think are exponential and could be a disaster waiting to happen. Maybe not right away but eventually so. Just a thought. I wanted to add that the impacts with objects sucked into the impeller at greater velocities could trigger an unhappy incident. I forgot too much from physics classes and am looking up formulas now. Only because I'm curious. Woody

[John_RI]

I hope someone will correct me if I'm wrong here - but here's what I think: the 3" to 10" reduces the 3600 bbox output to 1080 rpm. Assuming your 1st 6" pulley is on the same shaft as the 10" pulley, the 6" to 6" makes 1080 >=> 1080 rpm. From there the 6" to 3" increases the 1080 rpm up to 2160 rpm for the final impeller speed.

[Woodydel]

I just looked up the relevant formula and found this: KE = 1/2 m v2, When the velocity increases the energy of impact is four times as great. Speed increases on the vacuum, I think, can kill if something impacts the blades. The blades may not take the strain. I'm not an engineer but I really trust them. If they could have gotten more speed in there they probably would have. I think the engine may not speed the vacuum up to full speed anyway and your mower deck will probably slow down as well. The vac is sucking power to go faster and the engine may not keep up. Anyway better safe than sorry. Woody

[Woodydel]

Well I just got back from a wedding, it's 12:53(am) and I've had quite a few and why am I looking at these postings? I quess it's fun. It looks a lot more complicated right now to figure out all these pulleys and ratios and such. I really think that the blower will not ever spin too fast. The motor will run out of push and maybe go a little faster at best. I had a good time with this discussion. Everything matters but I think nothing bad will happen. Before I left for the wedding I remembered long ago replacing my countershaft sprocket on my Honda motorcycle. I wanted to make the vibrations subside while I was cruising. What I remember was the top end actually dropped because of this tall gear. It didn't vibrate as much but ran slower at the top end. I think that the engine on the tractor will strain its guts out turning the blower maybe a little faster at best. Poor old tractor. Again thanks for the fun I have on this site. I realize I look forward to coming here. Woody Del Mar

[Woodydel]

Just foolin around with your pulley sizes Kent. I came up with the engine at 3600 rpm the 3.5 inch pulley had the impeller spinning at 3082 rpm approximately and the 2.5 inch boosted it to 4320 rpm. I just used the approximate sizes you came up with did the calculations and by coincidence if you multiply 3082 times 1.333 (33% quesstimate increase in speed from previous post) you come up with 4108 which is close to the 4320 rpm number. There is just no way to say whether it would break or not. I have a Marine Corps flak jacket you could borrow. haha.

[Kent]

Woody -- that assumes that there was 3600 RPM on the rear PTO -- but when you go from a 3" drive pulley to a 8" or 9" driven pulley, you reduce that 3600 RPM to a fraction of that speed... Now if I were driving off the left side of the BGB to the same size driven pulley on the vac, your calculation would be more correct.... My vac has either 5" or 6" driven pulleys on the jackshaft, so driving it with the 4.5" (I think that's the size) pulley off the left BGB would still slow that 3600 RPM down before it was transferred to the impeller.... I'll try to get some pics this weekend to show what I mean. My vac isn't turning 4000 RPM, I'm sure....

[Woodydel]

Kent, I started with the 3" pulley at the bevel box which should be turning at the same speed as the engine then to the 9" pto pulley which drops down to 1200 or so rpm then to the 5" which brings it up to 2160, back down to the 2.5 which brings it up to 4320. If the 3.5 where there it would end up at 3082. These other size pulleys I didn't know about. But I thought you stated they were the same size approximately so there should be no change at those pulleys as far as rpm's go( the jack shaft and small rear pto pulley). All I did was figure all the diameters of the pulleys in inches. Then depending if it was a step up or down in pulley size determined the ratio by simple multiplication or division. I still think the impeller is turning that fast or that the engine can't keep the speed up because of the extra power needed to push the impeller around with the smaller pulley. If the engine can't keep up to the original speed with the larger pulley then maybe there is no danger of over revving the vac. Most vacs I have ever seen spin pretty fast or sound like it and 4000 rpm seems reasonable.When the blowers are direct coupled to an engine they will spin at engine speed so 3600 seems achievable in most cases. Woody

[Woodydel]

The impeller speed is low as you suspected Kent. There's a lot less pulley action from what was first stated. I think the more direct driving of the impeller will get you your improved results with the vacuum action without any problems as you thought. At least we all seem to know some math. Thought I had an opportunity to get one of the vacuums a couple of weeks ago in the for sale section but it didn't work out. Still hopeful he'll get back to me so I can get it. Woody

[Kent]

Woody, That was my thought -- that the vac is still turning far below max speed. I had been thinking of actually trying a smaller, 5" pulley on the left side of the jackshaft to see what that would do.... Guess I need to try your math and calculate what'll get me close to 3600 RPM impeller speed. On my B-210 that I mow with, I can't drive the vac from the right side because of the Varidrive setup (the Varidrive works good for mowing and snowblowing -- not a huge fan of it for anything else). I'm going to try configuring my Big Ten for using the vac also, but I have a lot of work to do first. My rebuilt mowing deck has the weird double-V pulleys on it, as does my B-210 center PTO (actually 3 Vs there, including the small one for the sickle bar mower). So, to interchange the mower decks and vac, I need to swap out center PTO pulleys, mower deck pulleys, add the right drive pulley to the Big Ten, and then put this bracket/idler back on the vac so I can drive it from the right.... See why I haven't gotten around to it? The alternative is to rebuild all the arbors in my Big Ten's deck, also no fun! I actually paid to have a shop rebuild the arbors on my good deck, rather than me trying to do it without a press or the right pullers... (a bearing splitter is on my Chrismas wish list!) I guess I'm going to have to do some math and get to work. Rather than do all this work to my B-210 with wornout BGB and varidrive, I'm actually thinking to trying to swap in a Sundstand hydro, i.e. replace the BGB and everything from there back.... If I'm going to do all this work, I'd like to have something more when I'm done! Kent

[Kent]

I probably should add that I typically mow/vac with the throttle at about 7/8 or so, simply because the old throttle cable on my B-210 seems to always vibrate back to that setting. The "lock" on the original cable wore out long ago, and I haven't spent the $30 plus to replace it with an original one... All I know is that before I put the smaller pulley on it, the black walnut stems would frequently clog the tube between the deck and the vacuum and that seems to have been solved. Now it only clogs if I've disengaged it and forget to start the vac up again before starting to mow.... I'm also using the rear PTO/tiller drive combo, which seems to significantly slow down the RPMs. The drive belt goes from a 3" pulley on the bevel gear box to about a 9" on the rear PTO. The small pulley on the rear PTO is about the same size as the driven pulley on the jackshaft so, there's not much of a ratio change there, if at all. The impeller drive pulley is about 5" (maybe 6") driving the now 2.5" pulley on the impeller... Talk about some fun calculations, when the ratios could change on each of three belts! For example, the tiller specs show 175 RPM with the engine at 3000 RPM, but of course it has a different gear ratio than the vac. Yet the vacuum specs show 3600 RPM max, implying a 1:1 ratio off the engine's driveshaft... NOTE ADDED: The pulley sizes above are incorrect in some cases -- see the discussion further down...[A href='http://simpletractors.com/attachments/vacuum_specs.htm']http://simpletractors.com/attachments/vacuum_specs.htm[/a]

[Kent]

Both Dutch and I have "overdriven" our vacs, by changing pulley sizes and finding an appropriate new length belt. I didn't make huge changes, moving from about a 3.5" pulley on the impeller shaft to a 2.5" pulley. You could similarly put a larger drive pulley on the tractor or jackshaft to get the same effect. They certainly increase the vacuum generated.... Larger drive pulleys or smaller driven pulleys will cause the impeller to spin faster and vacuum better, if needed. Please use common sense and caution though, and don't make too large of a change from what was originally on there. I'd hate to think what would happen if a heavy impeller blade broke or came off... and these things are made of heavy steel, if you've never checked it out. NOTE ADDED: The pulley sizes stated above are incorrect (I was going from memory -- or lack of one). I actually removed a 4" impeller pulley and replaced it with a 3" pulley. See the discussion farther below.