Kubota B7510 HST vs Bolens G174 on a 48″ bush hog in hay (updated)

June 17, 2014

Normally I keep the black walnut fields carefully trimmed, but this year I have a lot of younger seedlings and so I decided to let the two four-acre fields fend for themselves. Some are now bearing, so they’re hardly tender shoots any more.

But my wife complained that her favourite dog walk had become “spooky” with the dense hay growing between the trees. This new worry might have something to do with the large black bear we saw on the property earlier in the spring.

Anyway, I made good progress with the TAFE on the 5′ Rhino, but only down the centre of the aisles. With its ROPS, sun cover and loader, the 35 hp field tractor is now too cumbersome to work around the rapidly growing and brittle branches.

I had already put a couple of tanks through the Kubota trimming around 15,000 younger seedlings with the 48″ Braber rotary cutter. As long as the grass wasn’t too long it did a good job. When I tackled a one-acre plot of hay running rich to wild parsnip, however, it did a lousy job. The wide turf tires flattened about a quarter of the hay and the blades tore up the rest. At the time it didn’t look too bad, but five days later I couldn’t stand the mess any more and re-did the job with the Rhino on the TAFE.

Up until now I have blamed the crappy, off-brand 48″ bush hog. The blades seem shaped improperly on it, pushing the grass (and rocks, I suppose) down, rather than drawing up.  (See update below).

With its ROPS the Kubota is too high for work under trees, so today was the turn of the Bolens. Normally I use 1st gear HI for mowing, but the hay was too heavy. We crawled around under the trees in 3rd LO with the PTO in 2nd gear. The “crummy” mower did a surprisingly good job.

If the increase in pto revolutions from 540 to 750 produces a major improvement in cut, could it be that 48″ mowers turn too slowly with a standard, 540 rpm shaft? Simple geometry would suggest that the same gearbox used on a 4′ and on a 5′ will only deliver 80% of the tip speed on the smaller chassis.

The 5′ mower produces an effortless cut. Until now I have attributed it to the high-priced components in the Rhino. But if I were to buy a 48″ Rhino, would it still produce only 80% of the tip speed of the 60″? (Again, see update below).

UPDATE: 29 July, 2014

I looked up gearboxes for Rural King rotary mowers. They list two 40 hp gearboxes, one geared to produce a final drive speed of about 750 rpm for 5 and 6′ mowers, and the one for the 48″ mower geared to spin at 1000 rpm. So a good 48″ rotary mower won’t have the same gear ratio as a good 60″.  (Turned out the Braber mower has the correct 1:1.9 gear ratio.  The label on the gearbox makes that plain.)

UPDATE: 14 August, 2014

I found that TSC in its American stores stocks replacement blades for the Chinese rotary mower under their proprietary brand name, so I replaced my broken blades.  The contour of the new blades looked great, though they still looked as if they had been shaped and sharpened by a blacksmith.  Initially they did a very good job, causing me to rethink my complaints about the mower.  It was even somewhat quieter with the new blades.  Perhaps that grinding sound from the gearbox was blades out of balance?

Today I discovered a 3/4″ nut and washer in the chaff I cleared off the deck of the mower.  The shear pin was about to give up.  A new one installed, I checked the blades.  They’d had a couple of hard days mowing the worst three acres on the farm, and I discovered the problem with these blacksmith-sharpened blades:

A normal rotary cutter blade is very dull.  It has a steep angle and blunt edge.  The Chinese blade has quite a sharp angle.  An impact on the cutting edge of the blade seems to bend it upwards.  Repeated impacts produce a blade that looks like a snaggle-toothed propeller designed to push the grass downward.  Perhaps they use soft steel so that the blades don’t shatter when they hit a rock.

That was why this morning I had noticed the cut on short grass was no longer very good.  The other revelation was that my theory about tip speed was out to lunch.  The gearbox is clearly labelled a 1:1.9 ratio.  The 48″ mower has the same tip speed as a 60″ mower with a 1:1.4 gear ratio.

So I pulled the back end of the cutter up and went at the blades with an angle grinder.  There’s no way to restore the contour of the blades by grinding.  The best solution will  be to replace the $54.00 US blades with a new set.  That’s not such a bad idea.


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