? on gearboxes on rotary cutters

[whitearrow]

why are gearboxes on rotary cutters so high?example:i have a jd 5203 tractor,48 hp pto,and a bushhog 296,130 hp gearbox.why is the gearbox hp on the rotary cutter so much higher than the pto hp on the tractor?
thanks,
whitearrow->>>------------------->

 

[whitearrow]

why are gearboxes on rotary cutters so high?example:i have a jd 5203 tractor,48 hp pto,and a bushhog 296,130 hp gearbox.why is the gearbox hp on the rotary cutter so much higher than the pto hp on the tractor?
thanks,
whitearrow->>>------------------->

 

[browns40]

Hi whitearrow,

Those gearboxes are (hopefully) built to be able to withstand some tremendous forces put on them via the blades during normal operation & especially when impacts occur with rocks, stumps, etc.... when the blades slow down nearly instantly.

Also to withstand forces from the PTO shaft from bigger Ag type tractors... 90 PTO horsepower could theoretically (assuming inadequate slip clutch or shear pin protection) grind up a 40 hp rated gearbox as it continued turning after blades stop.

If not "overbuilt" gears would strip or shatter when those forces are put on it.

 

[browns40]

Hi whitearrow,

Those gearboxes are (hopefully) built to be able to withstand some tremendous forces put on them via the blades during normal operation & especially when impacts occur with rocks, stumps, etc.... when the blades slow down nearly instantly.

Also to withstand forces from the PTO shaft from bigger Ag type tractors... 90 PTO horsepower could theoretically (assuming inadequate slip clutch or shear pin protection) grind up a 40 hp rated gearbox as it continued turning after blades stop.

If not "overbuilt" gears would strip or shatter when those forces are put on it.

 

[Farmwithjunk]

Interestingly enough, most slip clutches won't slip when they hit stumps and rocks either. There's a good reason why.

People don't SERVICE those slip clutches like they should. They freeze up while sitting. Rust is the usual culprit. A slip clutch should be serviced a couple times a year. Loosen all the bolts. Use the tractors PTO to turn the drive shaft. If it doesn't release without help, use a long pry bar or something simular to hold the blades and carrier from turning, then apply LIGHT power (with PTO) to the drive shaft. Let it slip for a minute or so to "polish" the friction disc(s). Re-tighten slip clutch bolts to spec.

Eventually, hitting "immovable objects" with a bush hog, and having no WORKING shear point, or slip clutch, and you're bound to do destruction to tractor and mower gears, shafts, and the operators nerves too.

BTDT, got the broken PTO shaft hanging on the shop wall to prove it.

 

[Farmwithjunk]

Interestingly enough, most slip clutches won't slip when they hit stumps and rocks either. There's a good reason why.

People don't SERVICE those slip clutches like they should. They freeze up while sitting. Rust is the usual culprit. A slip clutch should be serviced a couple times a year. Loosen all the bolts. Use the tractors PTO to turn the drive shaft. If it doesn't release without help, use a long pry bar or something simular to hold the blades and carrier from turning, then apply LIGHT power (with PTO) to the drive shaft. Let it slip for a minute or so to "polish" the friction disc(s). Re-tighten slip clutch bolts to spec.

Eventually, hitting "immovable objects" with a bush hog, and having no WORKING shear point, or slip clutch, and you're bound to do destruction to tractor and mower gears, shafts, and the operators nerves too.

BTDT, got the broken PTO shaft hanging on the shop wall to prove it.

 

[BB_TX]

The HP rating of the cutter gearbox should always be somewhat higher than the PTO HP of the tractor so that the tractor output power will not destroy the gearbox. Then the shear pin or slip clutch on the PTO shaft protects the tractor PTO drive from damage if the cutter should hit something solid. Hopefully the result is that nothing is damaged, except maybe a sheared pin.

 

[BB_TX]

The HP rating of the cutter gearbox should always be somewhat higher than the PTO HP of the tractor so that the tractor output power will not destroy the gearbox. Then the shear pin or slip clutch on the PTO shaft protects the tractor PTO drive from damage if the cutter should hit something solid. Hopefully the result is that nothing is damaged, except maybe a sheared pin.

 

[N80]

I understand overbuilding the gear box for the rated horsepower but I'm not sure I understand the assigned number needing to be higher than the actual PTO hp. It seems like a gearbox made to handle everything a 90hp PTO can give, should be a labeled a 90 hp gearbox. Seems like it would avoid confusion. Calling that same gearbox a 150hp gearbox just seems to be inviting disaster if someone actually hooks it to a 150hp PTO.

 

[N80]

I understand overbuilding the gear box for the rated horsepower but I'm not sure I understand the assigned number needing to be higher than the actual PTO hp. It seems like a gearbox made to handle everything a 90hp PTO can give, should be a labeled a 90 hp gearbox. Seems like it would avoid confusion. Calling that same gearbox a 150hp gearbox just seems to be inviting disaster if someone actually hooks it to a 150hp PTO.

 

[GaryM]

A 150HP gearbox should be able to handle 150HP tractor power. I don't think it's over built for the power it handles, but rather that the rating is it's upper limit.

Tractors caome in all sizes from maybe 15HP to 150HP or more. I could use a BH with a 150HP gearbox on my 20HP tractor, but I'd be wasting my money. I have a 40 HP gearbox and it will probably never break.

Oh, and they don't build gearboxes for every power level, but there are steps inbetween. Maybe 30 HP, 50HP, 90HP, 150HP.

 

[GaryM]

A 150HP gearbox should be able to handle 150HP tractor power. I don't think it's over built for the power it handles, but rather that the rating is it's upper limit.

Tractors caome in all sizes from maybe 15HP to 150HP or more. I could use a BH with a 150HP gearbox on my 20HP tractor, but I'd be wasting my money. I have a 40 HP gearbox and it will probably never break.

Oh, and they don't build gearboxes for every power level, but there are steps inbetween. Maybe 30 HP, 50HP, 90HP, 150HP.

 

[Soundguy]

</font><font color="blue" class="small">( It seems like a gearbox made to handle everything a 90hp PTO can give, should be a labeled a 90 hp gearbox. )</font>

Applying that logic to the real world, and manufacturers would have to stock 'every' size gear box from about 8hp and up to over 200 hp... to coincide with manufacturers pto hp ratings...

Can you imagine walking in and needing a 93 hp box? They make a few different levels and call it good.

Soundguy

 

[Soundguy]

</font><font color="blue" class="small">( It seems like a gearbox made to handle everything a 90hp PTO can give, should be a labeled a 90 hp gearbox. )</font>

Applying that logic to the real world, and manufacturers would have to stock 'every' size gear box from about 8hp and up to over 200 hp... to coincide with manufacturers pto hp ratings...

Can you imagine walking in and needing a 93 hp box? They make a few different levels and call it good.

Soundguy

 

[GaryM]

Kind of like light bulbs. They make 40W, 60W, 75W, 100W etc. They don't make 27, 29,31,35. If 40's too dim go to the next larger size. Too bright, too bad.

 

[GaryM]

Kind of like light bulbs. They make 40W, 60W, 75W, 100W etc. They don't make 27, 29,31,35. If 40's too dim go to the next larger size. Too bright, too bad.

 

[John_Mc]

One other thought here: You are not just subjecting the gearbox to loads from the tractor. You are also getting some pretty hefty loads every time you hit a rock. These are instantaneous, and could conceivably subject the gearbox to much higher loads than your tractor engine would (think of all that spinning mass coming to an abrupt stop).

The gearboxes are probably used in other applications besides brush hogs. In something not subjec to the impact loads that a brush hog sees, you could probably use a gearbox rated fairly close to the rating of the driving horsepower.

John Mc

 

[John_Mc]

One other thought here: You are not just subjecting the gearbox to loads from the tractor. You are also getting some pretty hefty loads every time you hit a rock. These are instantaneous, and could conceivably subject the gearbox to much higher loads than your tractor engine would (think of all that spinning mass coming to an abrupt stop).

The gearboxes are probably used in other applications besides brush hogs. In something not subjec to the impact loads that a brush hog sees, you could probably use a gearbox rated fairly close to the rating of the driving horsepower.

John Mc

 

[N80]

</font><font color="blue" class="small">( </font><font color="blueclass=small">( It seems like a gearbox made to handle everything a 90hp PTO can give, should be a labeled a 90 hp gearbox. )</font>

Applying that logic to the real world, and manufacturers would have to stock 'every' size gear box from about 8hp and up to over 200 hp... to coincide with manufacturers pto hp ratings...

Can you imagine walking in and needing a 93 hp box? They make a few different levels and call it good.

Soundguy )</font>


I think you've missed my point. It seems that the general wisdom says that if you have 90 pto hp than you really need a 150 hp gearbox just to be on the safe side (or something higher than 90). My point is that a gearbox rated to 90 hp should be capable of handling anything up to 90 pto hp and anything 90 pto hp can dish out within the realm of reasonable use. You shouldn't have to second guess and buy up just in case.

In your example of 93 hp, you'd get the next size up 90. If the pto hp is 83.25 then you get a 90 hp gearbox. If the pto hp is 93 you get a 100 hp gearbox. I didn't intend to suggest that there needed to be a gearbox rating to match every possible pto hp. That would be silly. /forums/images/graemlins/wink.gif

 

[N80]

</font><font color="blue" class="small">( </font><font color="blueclass=small">( It seems like a gearbox made to handle everything a 90hp PTO can give, should be a labeled a 90 hp gearbox. )</font>

Applying that logic to the real world, and manufacturers would have to stock 'every' size gear box from about 8hp and up to over 200 hp... to coincide with manufacturers pto hp ratings...

Can you imagine walking in and needing a 93 hp box? They make a few different levels and call it good.

Soundguy )</font>


I think you've missed my point. It seems that the general wisdom says that if you have 90 pto hp than you really need a 150 hp gearbox just to be on the safe side (or something higher than 90). My point is that a gearbox rated to 90 hp should be capable of handling anything up to 90 pto hp and anything 90 pto hp can dish out within the realm of reasonable use. You shouldn't have to second guess and buy up just in case.

In your example of 93 hp, you'd get the next size up 90. If the pto hp is 83.25 then you get a 90 hp gearbox. If the pto hp is 93 you get a 100 hp gearbox. I didn't intend to suggest that there needed to be a gearbox rating to match every possible pto hp. That would be silly. /forums/images/graemlins/wink.gif