Brush hog gearbox ratio

[geneP]

Is there a "standard" ratio for cutter gearboxes? I saw an ad for one (40hp) that stated that the output shaft turned 1.47 times for each revolution of the input shaft. I checked mine, and it takes a little more than one input turn for one turn of the output shaft. It appears to me that varying ratios would affect the blade tip speed, therefore the quality of the cut. Maybe this is why some mowers cut better than others?

 

[Fredex]

Don't know for sure, but I doubt there is a standard ratio. Cutting smoothness would seem to be more a function of blade tip speed. Further, for example, a set of blades cutting 48" would have to be turned at higher rpm than a set of blades cutting 60" Tip speed in feet-per-minute = blade rpm X circumference of cutting circle. You can do the math to convert diameter to circumference and to convert pto rpm to spindle/blade rpm through the gear box ratio. Tip speeds for the Bush Hog SQ Series (10 yrs ago) were 11,000 to nearly 15,000 fpm at 540 pto rpm depending upon specific models in the series. Of course for large machines with multiple gear boxes the above applies to each set of blades individually and each gear box must have the same ratio.

 

[geneP]

Thanks for the formula. Using it verifies the advertised ~15000 fpm tip speed of the typical 6' cutter. Using the same ratio for the smaller diameter cutters would reduce the tip speed accordingly. Maybe 1.47 speedup is typical. I'm going to have to check the ratio on my 6' cutter again, a little closer this time. Thanks again
Gene

 

[john_bud]

Gene,

I have not seen a standard that they all use. Faster gives a better cut, but it also throws stuff farther and harder. I think there is a law on max speed, at least there used to be in the mower biz.

Some mfg's will use the same gear box on 72" and 60" cutters. The tip speed will be ~12,000 for the 60" and 15,000 for the 72". Others will use different ones for each size to get the tip speed in the 15,000 range. Basically, it is what it is. I don't usually cut at full PTO speed anyway unless the stuff is extra thick.

jb

 

[geneP]

Well, I checked my gearbox ratio today by marking the starting points and turning the input shaft and counting the revolutions of the input and output shafts. At just a few degrees under three full turns of the drive shaft the output shaft turned five times. I would estimate that the ratio is about 1.70 to 1.75 upspeed. Those numbers convert to about a 17,700 fpm tip speed on this 6' cutter. It seems reasonable because at 540 PTO rpm, the blades are whistling. The mower has "Andy 600" stickers on it, never heard of one before. It's new to me, so I'm going to try reducing engine speed and going to one higher gear when cutting lighter grasses.