Gear box ratio.

[EMB5530]

So I've put to pasture an old Cheetah gear box due to a broken input shaft and the fact that the parts to repair it will be about 3/4 of the cost of a new higher HP rated box. In order to get the larger Omni to fit I will have to drill out the mounting holes for the new box but there is plenty of clearance for that so no worries.

The real question is about the ratio of the new box. My old box was a 1.47:1 and it seemed as though when trying to mow in real thick brush I had to go obnoxiously slow(even with sharp blades and by sharp I mean brush cutter sharp) due to the fact it would just lay the grass over more than cutting it. The tractor has plenty of HP to run the original gear ratio. Would there be any down side to going w/ a 1.93:1 ratio instead of the 1.47:1 gear box in order to get a faster blade speed. (About 4 more rotations/sec)The boxes are available in both ratios.

What does the majority think? Please ask questions as needed to get any picture of the situation that I have not included.

And yes, I do run the mower over thicker brush and saplings.

I am also going to a slip clutch instead of just a sheer pin.

 

[Xfaxman]

What size mower is/was the gear box on?

 

[bcp]

I would use the same ratio as used by other makes of the same size. There is the possibility that the broken gearbox you have is a replacement of the wrong ratio.

Bruce

 

[Fallon]

Running a higher ratio will mean faster blade speed at the tips. Generally I've seen them top out at a speed no matter the size. 1,400 fpm or something? Its been a while. But if that tip speed gets to fast the blades will be entering self destruct territory with or without hitting anything. Make sure your blades & running gear are rated for the maximum speed your gearbox will produce.

 

[EMB5530]

Its a 6' brush mower

 

[EMB5530]

Its the original box to the mower.

 

[EMB5530]

Running a higher ratio will mean faster blade speed at the tips. Generally I've seen them top out at a speed no matter the size. 1,400 fpm or something? Its been a while. But if that tip speed gets to fast the blades will be entering self destruct territory with or without hitting anything. Make sure your blades & running gear are rated for the maximum speed your gearbox will produce.

What would be the best way to go about this?

The stump jumper and blades have no ID on them.

 

[EMB5530]

Thanks for the input thus far. As fallon suggested "check the rating for the blades and drive train." That is what I wondered about. W/ the extra speed and the chance of finding some underbrush hidden treasure w/ the brush mower what happens to the stump jumper at that speed if a blade contacts it?

As far as all the reading I've done goes the stump jumper is the same size (thickness of steel) as similar ones on the brush mowers rated to run over 4" trees but the deck and original gearbox not so much.

I just cannot find any articles or other information on gearbox ratio vs. best uses/what to watch out for so I wondered if anyone here had a solid knowledge on such things.

:confused3:

 

[EMB5530]

Even at a 1.93 ratio at 540 rpm on a 6' mower the blade tip speed would be under 350 fps or at least that is what I came up w/.

1.93gear ratio * 540rpm@pto = 1042 rpm on blades
6' mower so blade distance is aprox. 19 feet for a full rotation

So

1042 rotation per minute = aprox. 18 rotations per second

18 rotations * 19 feet for length of an entire rotation = 342 feet per second

Am I thinking this out right?

 

[Leejohn]

Boy to me the 1.9 would be slower blade speed then the 1.4.

 

[bcp]

Here are some of the first spec sheets I found. Blade tip speeds are 13,000+ to 17,000+ feet per minute.

Light
http://www.landpride.com/ari/attach/lp/public/lit/lit_rcr18.pdf


Heavy
http://www.landpride.com/ari/attach/lp/public/lit/lit_rcf36.pdf

 

[EMB5530]

Well the fastest of these is under 300 f/s. But I still am not sure if there is a reason for this besides the gearbox not requiring as much power to cut or if it is a safety issue w/ the components involved.

Thank you for the links and info bcp!

 

[EMB5530]

Hopefully the sales rep's know a bit about their products and can give me some more info on this subject. It usually seems as though I get the sales but not the rep b/c anymore finding someone whom knows about what they are selling comes few and far between.

Thanks to all who gave info.

:thumbsup:

 

[RickB]

Even at a 1.93 ratio at 540 rpm on a 6' mower the blade tip speed would be under 350 fps or at least that is what I came up w/.

1.93gear ratio * 540rpm@pto = 1042 rpm on blades
6' mower so blade distance is aprox. 19 feet for a full rotation

So

1042 rotation per minute = aprox. 18 rotations per second

18 rotations * 19 feet for length of an entire rotation = 342 feet per second

Am I thinking this out right?

Seems so.

 

[Texasmark]

HP is (torque x rpm)/5252. Changing the gear ratio changes the ratio of torque and rpm. I am going to say that in this application, since the blades pivot at the stump jumper (assumed) hub, you are working more with kinetic energy in the spinning blade than raw torque like you would put on a shaft to get it to turn.

With that thought KE is essentially E = MC exp2 which you have heard many times. So with a given mass in the blade, the faster you can spin it the more the KE and the better the whallop when you hit something.

So get the blade up to speed then get into the crop and keep the rpms up.

Go with the higher ratio.

My 2c,
Mark