Lift Capacity @ 24" for SubCompacts

[LD1]

Can you show me where it says the 24" point needs to be straight and level on any manufacturers spec sheet? Can you show me where they specify a toplink configuration? What do they care how or why you install your implement?

How's this example.... I make a ball of cement for a ballast box. It weighs 550lbs. I hang it 24" behind the ball eyes on a bar that lets the cement ball spin freely. On the ball is a toplink mount. Will any of the three toplink configurations change the center of mass point? No, they will not. There will be 550# at 24" no matter how you spin the ball and no matter how high you lift it. And thats what the manufacturer is talking about.

All they're doing is saying you shouldn't buy a rotary cutter that weighs more than X pounds. You guys are talking about something else entirely.

Changing the toplink doesn't change the center of mass. It is still at 24". What it does do is change the amount that it is lifted.

If it were a true parallelogram, and the ball ends lifted 16", then the ball would lift 16" and would not require any more force to be exerted and the ratings would be the same.

But it a lower toplink position is chosen, and that deviates away from the true parallelogram, the ball will be raised higher. If the ball ends are raised that same 16", the concrete ball (still @ 24") will be raised MORE tan 16". The lower the toplink, the MORE it raises in relation to the ball ends. Thus a reduced capacity. It takes more force to raise the cement ball at a faster rate than if it were a true //ogram lifting at the same rate as the ball ends.

 

[arrabil]

But it a lower toplink position is chosen, and that deviates away from the true parallelogram, the ball will be raised higher. If the ball ends are raised that same 16", the concrete ball (still @ 24") will be raised MORE tan 16". The lower the toplink, the MORE it raises in relation to the ball ends. Thus a reduced capacity. It takes more force to raise the cement ball at a faster rate than if it were a true //ogram lifting at the same rate as the ball ends.

Raising it HIGHER does NOT require more force. In fact, the very geometry you guys are going on about makes it EASIER to raise the ball as it goes higher than the ball eyes. Why? Because as the ball goes higher than the pins, the 3pt arms bottom attachment point starts to take some of the weight of the object. So there is LESS mass for the hydraulics to lift at the higher points. Its the LOWEST point that requires the most lift force because its the only position in which the hydraulics are lifting the entire mass of the object. And again, the toplink has nothing to do with it other than changing the angles of attack.

 

[tsteahr]

Raising it HIGHER does NOT require more force. In fact, the very geometry you guys are going on about makes it EASIER to raise the ball as it goes higher than the ball eyes. Why? Because as the ball goes higher than the pins, the 3pt arms bottom attachment point starts to take some of the weight of the object. So there is LESS mass for the hydraulics to lift at the higher points. Its the LOWEST point that requires the most lift force because its the only position in which the hydraulics are lifting the entire mass of the object. And again, the toplink has nothing to do with it other than changing the angles of attack.

We need a picture. See attached. Two horizontal lines, one above the other. Points AB is the top link, CD is the lower link, AC is the implement attachment point, BD is the tractor attachment for the top link and lower link pivot point. B' is the top link attached at a lower point on the tractor.

With the horizontal top link configuration, lifting lower link CD about fixed point D results in a AC remaining vertical. You lift point C by x inches, the CG of implement attached to AC lifts by x inches.

With the top link lower attachment point B', raising point C x inches causes AC to rotate clockwise. The implement tips forward. If the CG of the implement is behind point C, the implement lifts higher than C (x+delta).

This extra lift does not come "free". Resolve the vector AB' into horizontal and vertical components and you see a force component in the downward direction (toward C). This is the extra force needed to be supplied by the 3ph to carry the same load with the top link lowered.

The converse is true if the top link were to be raised above vertical. If B' were above A then lift would be reduced but required force to support load would be reduced as well. You would see the top link taking some vertical load (the implement would be somewhat "hanging" from the top link),

Hopefully this makes a little sense. It really does come down to geometry.

 

[arrabil]

As soon as point C rises above point D, point D starts taking some of the weight of the implement. You guys are not taking that into account in your estimations.

 

[tsteahr]

As soon as point C rises above point D, point D starts taking some of the weight of the implement. You guys are not taking that into account in your estimations.

Yes, and the vertical component of AB' increases at a faster rate than the vertical component of CD. Because of the geometery. The required lift force (moment about D) increases the higher you lift C.

 

[arrabil]

D isn't the point at which the tractor lifts the implement. So any mass held by D is mass that doesn't need to be lifted at the lift point.

 

[tsteahr]

D isn't the point at which the tractor lifts the implement. So any mass held by D is mass that doesn't need to be lifted at the lift point.

D is a reference point for summing moments. The lift links create the moment about D, which is translated to the lift force at C. For this to be clear for you. I think you really need to resolve the forces and sum moments, about any point you wish. If you do this you will see how the relation of the top link impacts the force at C. This question is a basic 3rd year undergrad engineering statics class.

The above approach is known as the method of statics. I can try a different a approach known as method of virtual work. We first have to agree that in the B' configuration, any point behind CD will lift higher than point C. In the B configuration, any point behind CD will lift the same as point C.

Work = force x distance. No linkage system can create work. It can only transmit work. Lets say we want to lift a implement with a center of gravity located behind AC a foot off the ground. The work required to lift is the weight times vertical distance. (any horizontal movement does not count) With AB link configuration, point C needs to move 1 foot. With link AB' point C needs to move something less than 1 foot. How much less depends on the exact geometery. Since point C moves less, and the work done is the same, the required force delivered by the 3ph must be greater to satisfy the principle of virtual work. Regardless of where or how you choose to apply the force from the 3ph lift arms.

 

[Don87]

So, after all that (which totally lost me), Have I purchased a tractor that is being outperformed by every other model

 

[LD1]

So, after all that (which totally lost me), Have I purchased a tractor that is being outperformed by every other model

If it does everything you need it to, then no.

Which is one thing not accounted for in the OP's question. Sometimes numbers only look good on paper. But in the real world, it is what you can actually accomplish that matters.

For example, what is the point of owning a car that can go 200+ MPH when in the real world, it will most likely never see anything close to that. It just looks good on paper.

my kubota doesn't have the highest specs on paper. But it can lift ~2000 on the 3PH and ~1500 @ 24". I don't have a need to lift anythign even close to that heavy. The bushhog is the heaviest thing I have @ ~600lbs. So kubota could beef up the lift to 6000lbs and I'd never see a bit of difference.

 

[SPYDERLK]

Your example is not a viable 3ph mounting configuration and it will not lift the ball. We have to work within the physical geomatery of the 3ph configuration. In you example the ball will just sit on the ground, regardless of the position of the 3ph lower arms.

In order to get that ball 24" behind the ball eyes, you need to use a bar attached to the ball eyes with a pin. Now you need to have a top link attached to a fixed point to lift it. If you want to have the top link attached to the ball, the ball can not spin on its own axis. If it spins it will not lift. So either the ball remains fixed or the top link is attached to another point on the framework supporting the ball. Either way, as you shorten the top link the ball travels in a arc around the ball eyes. The shorter the top link the closer the ball CG is to the ball eyes (relative to direction of gravity). So Yes, the top link configuration does change the center of mass point.

:thumbsup:Truly I thank you for lucidly taking care of this tedious task of explanation. It really needed to be done and I sure didnt want to.
larry