Mowing Blades running backwards

[diesel85]

Good luck getting the gear box rebuilt. It cracks me up when people don’t read the entire thread and keep giving advice that won’t help because you already know what’s wrong.

I did predict correctly -- I knew this was going to be interesting. Indeed.

I was a mower deck engineer a million years ago. This thread conjured up a memory I forgot all about!

I was at the test site in the cut quality plots making cuts with a prototype mower i was working on (proto'd in fiberglass before ordering $600k stamping dies).

An engineer one plot over was running his latest build commercial 7-iron deck prototype on a front mount mower. He'd make a cut then get off and swear.... alot. I could hear he was having trouble so I walked over there. "Look at this cut!" he said. He fired up and took a pass. I'd never seen anything like it... it was 'combing' the grass not cutting! WTH? A moment later I saw the drive sheaves spinning down as he shut off the PTO... CCW! Experimental assembled the deck using the wrong gearbox! Hilarious!

Back in the office I quietly 'borrowed' one of his drawings and stuck it back. He was less than thrilled years later at his retirement party when the drawing came back out! Engineering gold!

I always wondered what would happen if it spun backwards. Now I know.

 

[dodge man]

Kind of like putting a chain saw chain on backwards I admit to putting lawn mower blades on backwards once. It’s obvious immediately.

 

[DarkBlack]

Kind of like putting a chain saw chain on backwards

Did that once in my younger days, when I was always in a rush.

 

[diesel85]

Kind of like putting a chain saw chain on backwards I admit to putting lawn mower blades on backwards once. It’s obvious immediately.

When I was younger, I tried mowing snow. It does absolutely nothing but make a brown mess.

Does make me wonder what would happen if one were to throw ice chunks in a chipper shredder? Would it make shaved ice for snow cones? LOL

 

[dadohead]

At the remote test sites... the operators do all kinds of stupid things. I've seen them mow over giant puddles (bend blades every time) and chase gators. Sometimes the critter will attack a tire. The logs show pics of a destroyed tire and a pinhead loses his job.

 

[diesel85]

At the remote test sites... the operators do all kinds of stupid things. I've seen them mow over giant puddles (bend blades every time) and chase gators. Sometimes the critter will attack a tire. The logs show pics of a destroyed tire and a pinhead loses his job.

I never knew water could bend blades.

Out of curiosity, is it true if you have a high HP machine connected to smaller deck and mow wet grass it causes cavitation pitting to blades?

Like say one attaches a 75 HP tractor to a 60" finish mower and mows wet grasses.

Someone told me this a long, long time ago.

 

[Fuddyduddy1952]

To me the hp is irrelevant.
The motor spins blades at an rpm determined by pulley diameter. Too low hp would just bog engine down, to much would just be a hp waste.

 

[dadohead]

Out of curiosity, is it true if you have a high HP machine connected to smaller deck and mow wet grass it causes cavitation pitting to blades?

Idk... I've never heard of 'cavitation pitting' on a mower blade before. That's a new one.

The test sites in FL and TX always sandblasts decks/blades in their sandy soils. Can also play havoc on electrical systems. Mowing in the Northern states brings more impacts. You want to test in both conditions.

 

[Richard001]

When I re-sealed my gear box on my MMM a few years ago, it was probably the easiest thing I worked on once I got it off the deck and up on to the bench. I took it all apart and inspected everything for lash wear. All looked great, and I re-assembled it and put new seals on it.

What ? You have a Minneapolis Moline Manufacturer Tractor?

Minneapolis-Moline tractors were iconic American agricultural machines known for their rugged construction and distinctive "Prairie-Gold" paint. Formed in 1929 by a massive merger, the company produced a wide variety of legendary models. Today, these workhorses remain highly collectible and popular in the antique tractor community

TractorData.com - Minneapolis-Moline farm tractors sorted by model

www.tractordata.com

Richard

 

[diesel85]

To me the hp is irrelevant.
The motor spins blades at an rpm determined by pulley diameter. Too low hp would just bog engine down, to much would just be a hp waste.

What they were getting at was when you have that much HP going to the blades, that essentially the tractor has so much force that it doesn't slow down and the clumping and water that is being impacted against the blades is what causes cavitation.

Right, normally a machine will slow down, but if you have some high HP/Torque going to it and the machine doesn't notice, that is when this is supposed to happen.

No clue, figured I'd ask.

 

[diesel85]

Idk... I've never heard of 'cavitation pitting' before. That's a new one.

The test sites in FL and TX always sandblasts decks/blades in their sandy soils. Can also play havoc on electrical systems. Mowing in the Northern states brings more impacts. You want to test in both conditions.

Yeah, I was told this many years ago. Who knows, it could all be snake oil.

 

[Shawn T. W.]

What ? You have a Minneapolis Moline Manufacturer Tractor?

MMM = Mid Mount Mower ...

---------------------------------------

Most all finish mowers are actually belt driven . . . So high HP doesn't really come into play ... The belts would slip at some point ...

 

[JWR]

Yeah, I was told this many years ago. Who knows, it could all be snake oil.

I never heard mower blade cavitation suggested from any source, being driven by more power than necessary or anything else. I am an engineer but NOT one with fluid dynamics credentials. In my opinion mower blade "cavitation" is very far-fetched and cannot happen. Cavitation involves bubble formation due to extreme sudden negative pressures in a liquid. When the bubbles collapse (essentially out of control) it can be violent enough to cause erosion of the container material or vessel involved. That is common with things like boat engine props, turbines, etc. Mower blades running in wet grass are going to slow down regardless of other factors. There is nothing in the vicinity to create THAT much pressure. High HP input doesn't really matter because that high HP is not going to create high enough rpm and blade tip speed to even think about some effect like cavitation. The high HP would certainly drive the blades through heavier load (like deep wet grass, etc.) but that isn't going to produce high pressure bubbles collapsing hard enough to eat away metal... in my opinion. A fluid dynamics expert in some pump design group could put this to rest once and for all but I say it is very far fetched.

 

[JWR]

To me the hp is irrelevant.
The motor spins blades at an rpm determined by pulley diameter. Too low hp would just bog engine down, to much would just be a hp waste.

Also the rpm is not going to exceed the zero load rpm regardless of the engine horsepower. Nowhere near enough to be thinking about cavitation.

 

[DarkBlack]

You could have a million hp engine.
Cutter blades only experience whatever amount of force is needed to push through the grass

 

[Fuddyduddy1952]

Also the rpm is not going to exceed the zero load rpm regardless of the engine horsepower. Nowhere near enough to be thinking about cavitation.

Yes. Some mowers run higher rpm blades and some people change pulley diameter so they can mow faster.

 

[ning]

What they were getting at was when you have that much HP going to the blades, that essentially the tractor has so much force that it doesn't slow down and the clumping and water that is being impacted against the blades is what causes cavitation.

Right, normally a machine will slow down, but if you have some high HP/Torque going to it and the machine doesn't notice, that is when this is supposed to happen.

No clue, figured I'd ask.

Ideally you have enough power to drive the blades at the right RPM for cutting. I've been doing some rotary mowing here (different beast I know) and it's definitely slowing the blades down (very heavy grasses, probably not the right cutter but it's what we've got)... so I've increased throttle, and keep the RPMs at the right place.

We're not going to get "cavitation" from blades running the right RPM or from not slowing from 540 to 500, you get cavitation from a wing (usually propeller) slicing through water and the "lift" causing enough of a reduction in water pressure that bits of water turns to steam (forming a cavity) which then collapse, with the collapse inducing very localized temperatures of several thousand degrees and intense shock waves, both of which can erode metal -- but this happens in a liquid, and not just going through a zero psig water droplet. The water drop could well turn to steam, but that's not the part that hurts - there's just no reason for the steam to return to liquid state after the mower blade smacks it and even if it did, it probably wouldn't be on the blade.

 

[Fuddyduddy1952]

The best lawn cut in my opinion is a reel mower since it slices the grass like scissors instead of a spinning blade. I believe that's the mower golf courses use.
A funny mower story (I have plenty ) is as a kid my Grandfather next door was forever making something (machinist/mechanical engineer/M.D./D.O./surgeon). He buys six new reel mowers from a hardware store and fabricates this "triangle"...so one, two behind either side, three in back after removing handles of course.
This he pulls behind his Farmall to mow his lawn and it was fantastic since they independedly 'floated'. Except they didn't have traction especially with any ground moisture. So he cut grooves sideways in all the tires and it helped. Except he had two very large oak trees in his yard as well as a few other trees and bushes which were forever dropping dead limbs, sticks, leaves, acorns, etc.
Grandad never cursed, almost never because that was an exception because just a twig would stop one reel leaving a streak.
Finally all that stuff would end up in the garage which was like Disneyland for us kids to make into all kinds of weird contraptions.

 

[dadohead]

Right... shear cutting is fabulous. It is precise and uses very little power compared to impact cutting. It's downfall is high maintenance.

The industry is still waiting for the next level technology to cut grass... one that cuts like a reel, simple like a rotary, quiet, and low power. The most important breakthroughs come from the fringe... not industry. C'mon people... think ;<)

 

[4570Man]

Right... shear cutting is fabulous. It is precise and uses very little power compared to impact cutting. It's downfall is high maintenance.

The industry is still waiting for the next level technology to cut grass... one that cuts like a reel, simple like a rotary, quiet, and low power. The most important breakthroughs come from the fringe... not industry. C'mon people... think ;QUOTE]

A rotary motor with flat blades pretty much checks all of those boxes. That’s pretty much a hay disc mower. But a flat blade mower doesn’t do very good with regular height yard grass. A shear blade mower like a reel mower or a sickle bar mower doesn’t do very well with regular height grass either.