Rotary Cutter Rotary cutter blade balance

[rims421]

Any ideas on the best way to balance a set blades for a rotary cutter. I am familiar with balancing blades with a center bolt but it is not as straight forward with a free swinging brush cutter blade.

 

[BB_TX]

I think the only thing you could do is to weigh each blade on a fairly sensitive scale to ensure each weighed the same. That is not truly "balancing" the blades after they are installed. But until they come up with a "blade spin balancer", I think it is the best you will do.

 

[SPIKER]

probably the only REAL wat to do it would be to use TWO scales, set up like how they mesure piston connection rods. measuring each end, then each blade would be close enough to consider the same weight... basically though if the two weight about the same then it probalby is close enough.

maybe if you are concerned that one blade has a big chunk out of it then take the other blade and wack it hard too! this leads back to that 50 HR service test question posted back in the owning operation forum! /forums/images/graemlins/tongue.gif

anyhow good luck
Mark M /forums/images/graemlins/ooo.gif

 

[AndyMA]

Balance a brush hog blade. Something I have never done or see anyone do. I run my blades ususlly until a piece brakes off and then you can tell they are out of balance /forums/images/graemlins/frown.gif Even then I usually replace only the broken half. Just remember, unlike finish mowers, these blades are getting the c#@p beat out of them each time you use them. And the loads definately aren't equal.

My thoughts after "hogging" for over 30 years.

Andy

 

[AndyMA]

By the way, if you really feel a need to balance them, why not take a piece of metal lets say 12" long, 4" wide and 3/8 thick and drill 3 holes in it. 2 near each end to bolt the blade to. and one in the center. You could then do a static balance on any pointed object like you do with a conventional blade. The weighing method talked about above won't work since balancing weight has nothing to do with statically or dynamically balancing around a central pin. To illustrate this imagine your mower running with a 5 lb weight mounted 6" out on a zero weight arm on one side and a 5 lb weight mounted 2 feet on a zero weight arm on the other side. Even though the arms would weight the same I wouldn't want to spin it very fast. Just looking at one dynamic, Angular Momentum which is equal to mass times velocity times distance from the point of rotation, at only 100 rpm shaft speed the short 6" would be going 100*6*3.14 or 1884 inches per minute and have angular momentum of 1884 times it's mass and the long 24" arm would be going 100*24*3.14 or 7776 inches per minute nad have angular momentum of 7776 times its mass which is the same as the other arm and drops out of the equation. All in all not a very balanced affair. /forums/images/graemlins/shocked.gif Both sides would balance using the "scale method" /forums/images/graemlins/confused.gif The 2 scale method is somewhat better.

Andy

 

[AndyMA]

By the way, if you really feel a need to balance them, why not take a piece of metal lets say 12" long, 4" wide and 3/8 thick and drill 3 holes in it. 2 near each end to bolt the blade to. and one in the center. You could then do a static balance on any pointed object like you do with a conventional blade. The weighing method talked about above won't work since balancing weight has nothing to do with statically or dynamically balancing around a central pin. To illustrate this imagine your mower running with a 5 lb weight mounted 6" out on a zero weight arm on one side and a 5 lb weight mounted 2 feet on a zero weight arm on the other side. Even though the arms would weight the same I wouldn't want to spin it very fast. Just looking at one dynamic, Angular Momentum which is equal to mass times velocity times distance from the point of rotation, at only 100 rpm shaft speed the short 6" would be going 100*6*3.14 or 1884 inches per minute and have angular momentum of 1884 times it's mass and the long 24" arm would be going 100*24*3.14 or 7776 inches per minute nad have angular momentum of 7776 times its mass which is the same as the other arm and drops out of the equation. All in all not a very balanced affair. /forums/images/graemlins/shocked.gif Both sides would balance using the "scale method" /forums/images/graemlins/confused.gif

Andy

 

[Bird]

While I don't think balancing those blades is very critical, I did just what BB TX said. When I removed my blades and sharpened them a bit on the bench grinder, then I weighed each of them on the same 20# scales I had; ground them to a point that the scales showed they weighed the same.

 

[John_Mc]

</font><font color="blue" class="small">( While I don't think balancing those blades is very critical, I did just what BB TX said. When I removed my blades and sharpened them a bit on the bench grinder, then I weighed each of them on the same 20# scales I had; ground them to a point that the scales showed they weighed the same. )</font>

If you want to take Bird's method one step further (and correct for the angular momentum concept mentioned in another post):

In addition to making the two blades weigh the same, you need to make sure that each blade balances at the same point... that is, the balance point of each blade should be at the same distance from that blade's bolt hole (rather than trying to balance it at the midpoint as you do with a single, double-ended lawnmower blade). Grind a bit off one end or the other of the blad to move the balance point, then recheck that blade's weight & balance against the other blade.

As Bird said, however, it's probably not that critical. The bearings in a rotary mower are built to take a lot more abuse than in a finish mower (I believe a rotary mower spins more slowly than a belly-mount finish mower as well).

John Mc