Lift Capacity @ 24" for SubCompacts

[SPYDERLK]

You know, I'll assume I don't know squat. So educate me..... After reading your linked thread I have the exact same concern as I did beforehand:

As the object is lifted and its weight begins to be transferred to the lower 3pt mount at the tractor (D), doesn't the effective/visible weight seen by the hydraulics go down?

Without having fully perused the questions and answers I would say yes to this, if you were levering the lift directly from the bottom [lift] links and the hyd force was applied such that it was always tangential on the arc it is driving. This is not normally the case because the hyd force is applied linearly to a rocker that swings an arc. The line of action swings from tangential toward a less torque effective [but faster rotating] radial.If the sweet spots of the hyd driven [rocker shaft] and the lift arm arcs coincide throughout it will yield constant lift capability thruout the arc. If not you could very well have a case where it gets better with height ... or worse. If the rocker shaft, instead, had a sprocket driven via the linearmotion of a chain imparted hydraulically - then driving force is always tangential and available output torque [and therefore force] to the lift links would remain constant with arc --- and hyd load would go down as the lift arms raised due to the situation you brought up.
Were always going to have to keep an eye on the whole system end to end tho to be sure that individual true statements come together seamlessly in predicting the outcome. Seeing the interrelationships is difficult. Everything tends to be conditional. .... Im giving up for now. Gotta go to work.
larry

 

[LD1]

I'll try to take a stab at explaining it in a not so complicated manner.


Short answer is YES, the lower arms begin to take some of the load. But the top link in tension adds to the load.

If this were a perfec //ogram, the toplink and lower links will have exactally the sam angle through out the range of travel. So If you get to a point of say 30 degrees for the top and lower links, the lower links where they mount to the tractor is carrying given load x.

But with the toplink in that same 30 degree downward angle, under tension, it is resisting that same load x so they cancel each other out.

Imagine this, put a harness on and use a chain and anchor yourself to the ground with the chain on a 30 degree angle. You are the implement and the anchor point is the tractor and the chain is the toplink. Now try to stand up. You are putting the chain in tension and are being pulled in that 30* angle. But that 30* angle can be broken into two components. A vertical force and a horizontal force. there is resistance pulling you down as well as back toward the anchor point/tractor. These horizontal and vertical components are exactally the sae but opposite forces seen by the lower arms and thus cancel out.

In a semi//ogram like most tractors, where the toplink is shorter or mounted lower, the toplink angle will increase at a faster rate than the lower links. Thus the downward(vertical) component of the toplink will be greater than the vertical weight held by the lower link @ the tractor mount. Thus a reuced capacity. And the further out of parallel the 3PH system becomes, the less it can lift.

And to try to further complicate everyone, If you could mount the toplink on a tractor in a way that it was going uphill to the tractor in relation to the lower arms instead of the conventinal down hill, it would cause any point behind the ball ends to be raise slower than the ball ends themselves. This would result in an INCREASED capacity @ 24". The same as deviating away form true // in the other direction causes a reduction in capacity.

You could use the force x distance, or whatever formula you like, but if you coudl raise the toplink like this, and you had a blade on for example, and you raised your hitch and the ball ends raised 24", the blade edge may only raise 16", where as a true //ogram it would raise exacally 24" like the ball ends, and like a conventional tractor with a shorter/downward top angle, it may lift 30". This is why if your tractor has multiple mounting holes, the top has the greatest capacity and least amount of travel, and the bottom will give you the most lift height, but the least capacity.

 

[tsteahr]

I cannot imagine any physics professor accepting your explanation of Newton's Third Law. In fact you've got it backwards.... if the link is in tension, it is applying a force away from the object as its the object that is applying a downward force on the toplink to put it in tension in the first place. Gravity is what my feet are resisting and I'm pulling on the toplink as gravity works on me. The toplink then resists this in the direction of the tractor to maintain equilibrium.

Either way, I understand your point on Newton's 3rd. But I'd like you to address my question on the effective weight seen by the hydraulics as the weight is transferred to points on the tractor and away from points lifted by the hitch.

This is true

This is false Consider what you said. "The object applying a downward force on the top link to put it in tension in the first place". If the object applied a downward force on the top link it would be in compression. A pinned link can only support a force in line with the link. The top link resists the implement (load) from pivoting around the lower link eyes. To resist the tendency of the load to rotate around the ball eyes, there needs to be force applied orthogonal to the line AC. When the top link is not perpendicular to to AC, the tension in the top link increases. With the B configuration, when the load raises this increase in top link tension is offset exactly by the increase in compression in the lower link. In the B' configuration the increase in top link tension is greater, and it is more than the compression load transferred into the lower link (which is the same load transfer as above). This extra tension in the top link has to be carried. It travels down AC to the ball eye. It now has to be supported by the 3ph lift links. (When lifting a dead load, the lower links have both a compressive stress and a torsional stress present, with the torsional stress centered about the lift link attachment point).


We arn't talking about your body weight, gravity on your body mass is outside of this discussion. Pretend you don't weigh anything.

I realize this is not intuitive. I have tried to explain it in a intuitive fashion. I feel I have failed. A semester of engineering statics would provide the tools and skills for the understanding you are looking for. The answer lies in the distrbution of forces throughout the linkage system. The basis has already been provided. There is not much more I can say. I would suggest you let the concepts digest for a little while and then return.

Funny you mention professors. My father was a university professor. It takes a special person and a special skill to be a good teacher. In taking classes from him and growing up watching him work I came to appreciate how difficult good teaching is. Unfortunately I don't have that skill and that's why I'm not a teacher. I guess that is obvious.

 

[tsteahr]

LD1 and I are saying EXACTLY the same thing. :thumbsup:

 

[arrabil]

Short answer is YES, the lower arms begin to take some of the load. But the top link in tension adds to the load.

Explain how this is possible. How can the toplink add to the load being lifted outside its own mass?

This is why if your tractor has multiple mounting holes, the top has the greatest capacity and least amount of travel, and the bottom will give you the most lift height, but the least capacity.

I don't see why the lowest hole has the least capacity. The hydraulics determine the capacity in their entirety anyway. In all cases, the hydraulics stop lifting before any toplink configuration will.

We arn't talking about your body weight, gravity on your body mass is outside of this discussion. Pretend you don't weigh anything.

I was responding to what you brought up. Post #37 since you have no memory of it.

This is false Consider what you said. "The object applying a downward force on the top link to put it in tension in the first place". If the object applied a downward force on the top link it would be in compression.

The object is being acted on by gravity. Since the object is attached to the tractor via the toplink, the link is in tension because it is resisting gravity. The only way for the toplink to ever go into compression in a static situation is when the point B/B' is supporting weight from above.

LD1 and I are saying EXACTLY the same thing.

That cannot be true as in the same exact situation you are saying the link is compression while LD1 is saying its in tension. In fact you have compression and tension reversed in every one of your examples.

 

[arrabil]

Were always going to have to keep an eye on the whole system end to end tho to be sure that individual true statements come together seamlessly in predicting the outcome. Seeing the interrelationships is difficult. Everything tends to be conditional. .... Im giving up for now.

As far as I'm concerned, this is the whole problem with your guy's explanations. Just like the pictures in the other thread and your //ogram explanations in this one, you are treating the 3pt hitch as a closed system and hence the 3pt hitch mounts are acting as infinitely static points (points that don't move or change). In the real world of the tractor these things are not true. LD1 says the toplink determines lift capacity. In your closed system it may. In the real world that would result in thousands of broken links.

And so I will say again, in the real world the reason the MF cannot lift as great a percentage of weight as the JD is not because the hitch geometry is improperly designed but because some physical part of the tractor is a limiting factor.

And seriously, just think about what you guys are arguing.... that Massey Ferguson didn't get the geometry right on their tractor. They've been designing these things since before most of us were born. I bet they even have simple plug and play formulas for designing a hitch at this point. No way they made an error in geometry that costs them sales due to mismatched specifications. No way.

 

[LD1]

Explain how this is possible. How can the toplink add to the load being lifted outside its own mass?For the same reasons you are saying the lower link can take away from a load being lifted outside its mass


I don't see why the lowest hole has the least capacity. The hydraulics determine the capacity in their entirety anyway. In all cases, the hydraulics stop lifting before any toplink configuration will.
Because the lowest hole cause points behind the ball ends to raise at a faster rate than the ball ends themselves. It is the geometry of the setup. But it the most the hydraulics are capable of lifting is xxx pounds at the ball ends, it cannot lift that same amount @24" if it is trying to raise it at a faster rate/farther than the ball ends themselves. This would be "creating" work. Remember work =force x distance. If the max range of travel of the ball ends is 20", when the lowest point on the toplink is chosen, a poind 24" behind the ball ends is going to be raised some distance MORE than the 20" that the ball ends moved. The 3PH cannot do anymore than said work, so for the distance to be increased, force(the load) has to be decreased.






That cannot be true as in the same exact situation you are saying the link is compression while LD1 is saying its in tension. In fact you have compression and tension reversed in every one of your examples. The toplink is most deffinatally under tension. Put a chain in place of the toplink) if it were under compression, the chain would go slack. The lower arms are the ones under compression. The toplink is never under compression with a load in the air. The only time it is under compression is with a ground engaging implement on the back engaged in the ground.

I am not sure of an easy way to explain it to you. First, we agree that in a perfect //ogram, it can lifted it's rated amound at any point behind the lift arms with enough counter weight right???

If we agree there, if the parallelogram is NOT perfect, and the point behind the ballends is raised a greater distance than the ball ends themselves, how is it possible to NOT have a reduction in lift capacity? You are asking it to do more work. Again, if the tractors MAX capebillities are 1000lbs at the ball ends and they have a range of travel of 20", that is a work done of 20,000. Now if you move that 1000lbs back to 24" behind and have the toplink in the lowest position, it is going to move more than the ball ends. Something like 30". (NOT ACTUAL #'s, BUT JUST EXAMPLES). This would require a work done of 30,000, when we already established that the hitch is only capable of 20,000 units of work. So the most it could actuall lift in this example would be 2000=F x 30". So 20000/30=667lbs.

 

[LD1]

And seriously, just think about what you guys are arguing.... that Massey Ferguson didn't get the geometry right on their tractor. They've been designing these things since before most of us were born. I bet they even have simple plug and play formulas for designing a hitch at this point. No way they made an error in geometry that costs them sales due to mismatched specifications. No way.

No one is saying they got the geometry wrong. We are just saying that other MFG's may have it a little closer to a true //ogram. But that doesn't mean they are right either. There are pro's and cons to both. I have already established that you can increase the height you can raise an implement by giving up some lift capacity. So maybe that is what they were considering. Weighing the balance between lift capacity and lift height.

And as far as costing them sales, regardless of what the reasons are, on paper it IS inferior to the others listed. So that may cost them the sale anyway, even though there is probabally no noticable real world differences. But everyone likes superior numbers, and numbers are what sells.

 

[arrabil]

Explain how this is possible. How can the toplink add to the load being lifted outside its own mass?For the same reasons you are saying the lower link can take away from a load being lifted outside its mass

I said nothing about a lower link. I said that link point D is holding some of the weight when the object is above it. This is an obvious fact and I'm not sure why you don't see it. By taking some of the weight, there is less weight to lift. Kind of like using the top of your stomach when you lift something big. It takes less force to lift and hold it after you've put some of the weight on your body. So the bottom link point can TAKE away load.

But how does the toplink, which has no power source, ADD load?

That cannot be true as in the same exact situation you are saying the link is compression while LD1 is saying its in tension. In fact you have compression and tension reversed in every one of your examples. The toplink is most deffinatally under tension. Put a chain in place of the toplink) if it were under compression, the chain would go slack. The lower arms are the ones under compression. The toplink is never under compression with a load in the air. The only time it is under compression is with a ground engaging implement on the back engaged in the ground.

Really? I agree with you and you still decide to be argumentative with me?

Because the lowest hole cause points behind the ball ends to raise at a faster rate than the ball ends themselves. It is the geometry of the setup.

This is the exact problem between what I'm saying and what you're saying. Your theoretical capacities, while true, are never reached in the real world. If you have infinite hydraulic force from the rockshaft, then you could reach a point where the toplink will determine the maximum lift capacity. But in the real world the rockshaft runs out of force first. And so it doesn't matter where you put the toplink.... at the point the rockshaft can't lift the object, it can't lift it from any toplink position. Why? Because the rockshaft runs out of force to lift the object when it is still on the ground and the toplink is doing nothing!

 

[LD1]

I said nothing about a lower link. I said that link point D is holding some of the weight when the object is above it. This is an obvious fact and I'm not sure why you don't see it. By taking some of the weight, there is less weight to lift. Kind of like using the top of your stomach when you lift something big. It takes less force to lift and hold it after you've put some of the weight on your body. So the bottom link point can TAKE away load.

But how does the toplink, which has no power source, ADD load?It adds to the load because it is a physical connection point in which makes the lift arm NOT lift the load straight. The toplink is FORCING the implement to rotate about the lower pins, thus causing the tail wheel of a bushhog for example, to be lifted proportionally higher than the lower links. Thus a proportionally lower rating. It will require more force the higher the tailwheel is lifted in relation to the lower arms. Which is exactally what changing the toplink position does.



Really? I agree with you and you still decide to be argumentative with me?
I was in no whay trying to be argumentative with that statement. I was mearly stating the obvious facts and in a way, agreeing with you on that one.

This is the exact problem between what I'm saying and what you're saying. Your theoretical capacities, while true, are never reached in the real world. If you have infinite hydraulic force from the rockshaft, then you could reach a point where the toplink will determine the maximum lift capacity. But in the real world the rockshaft runs out of force first. And so it doesn't matter where you put the toplink.... at the point the rockshaft can't lift the object, it can't lift it from any toplink position. Why? Because the rockshaft runs out of force to lift the object when it is still on the ground and the toplink is doing nothing!

I have tried explaining it everyway I can with the given examples. I guess the only way to really solve this is to have someone with a carryall hook it up to their tractor with the toplink in the highest position, ballast the front, and start stacking on blocks or whatever for weight until the 3PH can no longer lift the load. And then, do nothing else but change the toplink to the lowest position and see how much weight has to be removed until the 3PH can lift the load again.