Blade Tip Speeds...educate me!

[Tejas]

Folks, I need help understanding and calculating blade tip speeds for rotary cutters/shredders. I assume there my be an actual formula of sorts floating around??

I have converted a Bradco SSL 72" cutter that originally had a hi-flow hydraulic motor attached to a RC-71 gearbox. Some of you helped me to figure out what I needed to get that job done via another thread. I ended up using a 9.59 CID motor. This, powered by a 19GPM SSL comes out to 458RPM....seems a bit slow but I wanted the higher torque. I know there is a missing number here, the gear ratio of the gear box...The deal is, I contacted Bradco and they sent me the specs on two of their gear boxes, a RC-51 and RC-61, both of these have a 1:1.93 ratio. I Will send another email to Bradco in attempts to clarify what the RC-71 has for a ratio.

Some questions:
- Are all shredders running the same Blade Tip Speed, or even close? As in, does a 5', 6', 7' shredder all run the same speeds? Understanding the longer or shorter the radius will effect the final speeds...does this get compensated for with a different gear box ratio or by controlling the speed of the motor/PTO turning it?
- What is the real limiting factor on maximum blade speeds?
- What is an average gear box ratio for a normal 3-point/PTO driven gear box?

I am preparing to build a 5' brush cutter for a neighbors 15GPM Bobcat. I really want to ensure I get everything correct. I appreciate all the help!

 

[GreenYellow]

OK Here goes...

You said your shaft into your gear box was 458 RPM. The gearbox has a 1:1.93 ratio. That means your gearbox output (actual blade RPM) should be 883 RPM. Keep that number for later.

You have to know the circumference of the blade exactly. If it's exactly 6' diameter, then the circumference is 17.39'. (circumference = diameter x pi (3.14))

The blade tip speed (in ft per minute) is = to the RPM x the circumference.

883 x 17.39 = 15,355

Blade tip speed is 15,355 feet per minute.

This is assuming all the original figures are accurate.

And for useless trivia -- that is roughly 175 mph.

 

[GreenYellow]

When you verify the ratio of the RC-71 gearbox, just figure the final RPM. If it's the same, use 883 RPM x the circumference of your 5' cutter. (15.75)

This would give you a blade tip speed of 13,907 ft per min.

To achieve a 15,000 ft per min tip speed on a 5' cutter, the blade RPM needs to be about 950-960. Which means your gearbox ratio needs to be about 1:2. Roughly...

 

[bcp]

Go here. Click on the different models. Specifications will give input rpm and blade tip speed.

Rotary Cutters

Bruce

 

[brown40]

There's quite a bit of variability among cutters, with higher speed leaving a better cut on fine stuff. The faster you gear it up, the more harsh impacts you potentially set up for the life of the machine, but I'd think build it heavy and make it fast. You can always slow down the output. I've seen some in the 11000 range, and others up near 18000 at 540/1000--PTO. What gauge of metal are the sides and deck/do you plan on reinforcing them? Would make sense to try and match the built speed of the unit if you aren't adding material, at least to the leading side and deck corner.

 

[Tejas]

AWESOME!!! THANK YOU THANK YOU!!! I am stoked to now have the formula to work this out. I very much appreciate you taking the time to type this out.

Bradco responded and told me they never used the OMNI- RC71 ...so, either he is wrong (it does "look" original) or it is aftermarket...it was used when I bought it. Is there a simple way of figuring out the gear ratio on the box...if there is no tag? Is this like a truck differential...turn input shaft twice and count how many times the blades go around (to a marked spot of course).

OK Here goes...

You said your shaft into your gear box was 458 RPM. The gearbox has a 1:1.93 ratio. That means your gearbox output (actual blade RPM) should be 883 RPM. Keep that number for later.

You have to know the circumference of the blade exactly. If it's exactly 6' diameter, then the circumference is 17.39'. (circumference = diameter x pi (3.14))

The blade tip speed (in ft per minute) is = to the RPM x the circumference.

883 x 17.39 = 15,355

Blade tip speed is 15,355 feet per minute.

This is assuming all the original figures are accurate.

And for useless trivia -- that is roughly 175 mph.

 

[Tejas]

Thanks for the link....man, they have all sorts of BTS I wasn't expecting such a wide range, especially from one manufacturer.

 

[Tejas]

There's quite a bit of variability among cutters, with higher speed leaving a better cut on fine stuff. The faster you gear it up, the more harsh impacts you potentially set up for the life of the machine, but I'd think build it heavy and make it fast. You can always slow down the output. I've seen some in the 11000 range, and others up near 18000 at 540/1000--PTO. What gauge of metal are the sides and deck/do you plan on reinforcing them? Would make sense to try and match the built speed of the unit if you aren't adding material, at least to the leading side and deck corner.

These are open front designs, exposed blades for getting to the bigger brush. Both decks are 1/4" plate, sides/skirts. The Bradco I cut 14" off the front so the blades are exposed about 5" at the dead center/front. I added a large push bar/brush guard to it for pushing over the larger stuff.

Both of these units will likely be running 1/2" thick steel blade carriers for more "heavy flywheel" momentum.

 

[brown40]

If you can see both sets of teeth you divide one by the other to get the ratio; turning would give an idea.

 

[Jim Timber]

It's easier than that guys - put a mark on the input shaft, and a mark on the stump jumper. Turn the input shaft and count the revolutions of the stump jumper. Too easy!

I have a RC71 for my stump grinder I'm building. They have several ratios available and none of them should be particularly tricky to figure out with the basic "hands on" approach I just described. You'll likely either have the 1:1.93 or the 1:1.46.

1.21:1, 1:1:, 1:1.21, 1:1.46, 1:1.93 are the available options.