ByronBob said:
How do you figure out how much weight the rear 3 point can lift? I went onto the Kubota web site for a BX2230 to see but I'm confused by the numbers listed there.
You're typically given two numbers: (1) the capacity at the lift point (ball ends of the lift arms) and (2) the lift capacity 24" behind the LP, because this is a standardized representation of the location of the implement's CoG.
Why two numbers and why does it vary w/length? The 3PH hydraulics work by applying a torque to the lift arms - but the weight of the implement acts as a counteracting torque. Since Torque = Force (weight) x Distance, the further away the CoG of the implement is, the more torque it exerts on the lift arms, consequently the harder it is for the 3PH to lift. The max lift capacity is where the two torques cancel each other out.
Calculating the lift capacity of a given implement
exactly is complicated by a few things:
(a) Actual CoG of the implement? 24" behind LP might be pretty close for a rear blade, but the CoG of a RFM for example is much further away.
(b) We don't know the exact distance between the LP and the point on the tractor about which the torque rotates
(c) Height of the implement. Lift capacity will actually vary a bit with the angle of the hitch due to the geometry involved.
Fortunately you can get a good approximation by making a few assumptions:
(a) make a best guess at the distance between CoG of the implement and LP
(b) extrapolate from the two figures given
(c) assume lift capacity is linear
The unknown distance (b) from pivot to LP is (I'll simplify the math; plug in values from any tractor):
b = (24")*(680) / (1210-680) = 30.8"
The lift capacity at any distance x from the LP is then
approximated by:
1210*b / (x + b) = 37268 / (x + 30.8)
x = 24": 680 lbs
x = 48": 472 lbs
x = 72": 362 lbs
Final thing to remember is
lifting (static) and
carrying (dynamic) are two different things. Typically you can safely carry only about 1/3 to 2/3 of what you can lift, depending on the situation.
