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Mower ratio


alanscott

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not real sure what you are after, I know you're 1:1 at the bgb,I think you're 1:1 from the bgb to the pto assembly too,but the pulley sizes are about 5 or 6 inches,and out side of the pto drops down to about 3 inches as does the center deck pulley. According to my math it might be 2 or 2 1/2 : 1. It can get confusing when your talking about multiple pulley arrangements. For all I know I may not be correct,I'm sure somebody out there has the answer. Seems to me the max blade speed is around 9000 rpm's.
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The correct mower blade tip speed is not to exceed 19,000 feet per minute and that is how blade RPM is determined. Hence your earlier 21" 3-1/2HP WB mowers with a 3600 RPM engine had a 19,792 FPM tip speed. They have since reduced engine speeds somewhat to bring tip speeds down and also had to increase horsepower ratings to compensate for slower running engines. If you look at a 6 HP curve engine performance curve running at 2400 RPM today versus the old 3-1/2 HP engine of years past you might see a 1/2 HP increase in performance. A 48" 3 blade mower deck should be able to handle an arbor speed of 3800 RPM and keep the tip speed under 19,00FPM. It comes down to what you have for a drive speed diameter versus the driven speed diameter and ratioing it out. You then have to know what maximum engine RPM you will be mowing at and the mower blade diameter. This is how you calculate blade speed: Length of Blade measuered diagonally times 3.1416 divided by twelve. That tells you how many feet the balde turns in one revolution. Take that sum and divide it into 19,000 and this will give you maximum RPM blade speed. Anything under that value should give a fuzzy feeling that the blade won't chop off any toes on the walk behinds and the blade won't make a bad mark on anyones mower deck. All these assumptions are made on the fact that the mower blade/s are not BENT and if they are, all assumptions are null and void. I also hear a lot of WB mowers running at speeds never dreamed of to get thru the thick grass and it amazes me that most of these people still have ten toes.
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Thanks for the info. I have been thinking of building a detachable bracket for mounting a hydraulic pump under the front and using the mower's PTO to drive it. I think the pump I have is a bit big, off the top of my head, it draws 30HP at 780 rpms so I guess I need to find a smaller pump before I go much farther.
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Alan, I don’t know what you’re trying to build, but don’t be discouraged by the pump’s maximum rating. If you have the specs, check them out. As long as the pump can produce the PSI & GPM you need, using your available HP and your RPM, and in the direction your belt is running, you can use it. Your first concern should be…….. Is the pump capable of being driven by a belt? Pumps designed to be “direct driven” won’t last long with the side forces of a belt or chain.
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