Conversion Factor for 3PH load capacity

[SPYDERLK]

Spyder, I'd reconsider thinking along those lines. The lift capacity is determined by a function of the weight (force) and the distance from the lift assembly, or moment (torque). You are completely discounting the moment part.

I honestly don't know the point you're arguing. My only contention with what you said, or didn't say (originally), is you didn't mention anything about the induced moments on the 3PH. Even with a //, the moment exist and will cause a failure if the component forces in the links are allowed to grow too large. I see now (post 13?) where you mention the moments of the 3PH linkage; I missed that @ 3:30 this morning. ---I also dealt with that several other times with replys regarding your cantilever comment, general description to separate it from the hydraulics, and when I addressed a couple of your points in post 25.--- That was my original point. The lift will be limited by either the forces of the moment or the forces on the lifting links, with a // being a special case which is not realized here. If it were realized, the moment consideration still holds.

After the moment issue, I think we are on the same page.

OK, so when you said "lift capacity" you meant catastrophic failure of link parts or front lift rather than pushing upward as much as the hydraulics will do at the eyes and not being able to lift something weighing that much. I had thot that was the subject. It changes when you want to talk about the required tensile and compressive strength of link elements to transfer the moment/torque into the tractor body and the required body mass distribution to resist. It certainly wasnt clear to me what you meant when you used the term in so cryptic a statement. I consider it almost a change of subject ocurring without warning on the heels of similar terms thrown about previously but referring to lift arm torque. And particularly since these links are way stronger than needed to tip a tractor back at whatever backset the required force is applied. Of course you can contrive to make them buckle or stretch if you prevent backtip by adding weight.

Yes, the moment consideration as developed does hold. The misunderstanding came from me thinking you had put it into a part of the system where it was not an issue.

 

[matt_isserstedt]

You guys are making me chuckle...

Two things must be true

1. sum of forces on lower arms must be zero.
2. sum of moments on lower arms must be zero.

Whoever said that it doesn't matter how long the lower lift arms are, that the hydraulics can still lift the same amount of weight needs to go back to engineering school

Think about how you could or couldn't lift a bag of concrete placed on the end of an 8 foot wooden 2x6 board, depending on where you chose to grip the board !!

There were also some arguments related to the tractor tipping backwards...what I didn't see was that the sum of moments are taken about the center of the rear axle (IOW the contact point between the dirt and the rear tires). The force holding the tractor down can be resolved into a point mass located where the tractor's center of mass is...probably about where the clutch/flywheel is.

Analyzing the bending stresses in the lower arms has practical merit, but for analyzing what the OP was asking about it means nothing.

 

[SPYDERLK]

Here's a thought;
If the perfect parallelogram is worthwhile it might just be worth doing a project.
Any interest ?
No, I do NOT propose hacking up the rockshaft housing and everything in it.
I propose very little "hacking up", in fact I would expect to adhere to the "easily returnable to factory standard" goal.

Is it worth doing ?
How much work, hassle, trouble, etc would it be worth to have this ?
Would it be worth giving up something for ? some feature or function of the tractor.
Assuming that the feature/function given up would be returned when the perfect parallelogram were removed.

Iv thot along the same line and I have a couple tractors it would not take much on. The 3 tractor fixed points are in a vertical plane. Id just have to extend the top point up about 6". [It has 3 choices now but the highest one is ~6" low compared to std implement spacing.]

Is it worth it is a good question. +/-You lose some lift range but dont lose force as you reach. Might be worth doing to have on hand for the job where you really need it. Tractor has a darn strong lift for its size anyway... If I only had time to play constantly.
larry

 

[SPYDERLK]

You guys are making me chuckle...

Two things must be true

1. sum of forces on lower arms must be zero.
2. sum of moments on lower arms must be zero.

Whoever said that it doesn't matter how long the lower lift arms are, that the hydraulics can still lift the same amount of weight needs to go back to engineering school

Nobody did that I noticed or Id have jumped on it immediately. What post is that in??
larry

 

[datanull]

Hi,

Thanks everyone. I didn't know this was such in interesting subject.

I was not able to follow this thread completly because it had grow so large while I was buying and driving my new tractor. My comparisions are done.

If a reasonable conversion factor exists it may be useful to others.

 

[Reg]

I think PART of the problem here may be analogous to trying to perform a detailed analysis of exactly WHY a perpetual motion machine of a particular design will not work.
Yes, this part seems to work here, yes that part works there, in fact every individual part appears to work, so why doesn't the whole thing work ?

As a practical matter the lift capacity 24 inches (or 24 ft) behind the lift arm eyes is much more limited by the tractor's mass and the distance of it's center of mass from the rear axle center line.
Clearly hanging a loader on the front would change the arithmetic, as would suitcase weights or (just for experimental fun) another front mounted 24ft boom pole.
I am guessing that the manufacturers worry about steering stability, i.e. their de-rating for 24 inches behind the lift eyes is much more about stability and liability than it is about 3PH geometry.
There are 2 examples of manufacturers' figures posted in earlier replies, one of them is from an 85 HP tractor (Class 2) and the other from a Class 1.
They almost certainly have different length lift arms, I doubt that much can be done with these figures.

I am content to accept that a) the 3PH works well enough as is b) Steering stability matters at least as much as absolute lifting capacity. c) Manufacturers are more concerned with liability than capacity even when it means that an item or two in their spec sheet has to be less than competitive.

I'm still a little bit interested in the development of a true parallelogram mechanism.
It has appeal, at least on paper. I'm not (yet) sure that it has practical advantage.

From a Marketing POV; Golly Gee if it is THAT good why isn't there market pull for it ?
Why hasn't somebody cobbled something together (Hewlett Packard garage style) shown it around at Ag fairs and become very rich very quickly ?
SOME time in the last 80 or so years... though VHS did beat out Betamax (-:

 

[gpflepsen]

You guys are making me chuckle...



There were also some arguments related to the tractor tipping backwards...what I didn't see was that the sum of moments are taken about the center of the rear axle (IOW the contact point between the dirt and the rear tires).

Matt, it's like a barrel of monkeys around here.

The sum of the moments can be taken from any point you wish and doesn't have to be the rear axle. If the moments don't sum to zero, things are rotating. But choosing the rear axle can eliminate one of the unknown moments.

The parralellogram mechanism that spyder brought in will and does work. He sited several examples in the real world. The lifting forces would remain the same but the tension and compression forces in the members increase with distance.

Here's one real world example of a parrallelogram linkage; a "Tommy Gate" lift that can be found on the back of pickups and trucks. It takes the same effort to lift the load anywhere on the platform. The lift effort does vary as you go through the lift distance though.

Why is this arrangement not used in 3PH? One reason could be because the current arrangement gets the implement further off the ground, such as for transport reasons.

 

[mwb]

Hope you all had a good weekend! I spent mine welding tractor parts (new stuff, not fixing).

Parallelogram has no significant effect on the lift capacity of the 3ph; its only purpose is to maintain the orientation of the implement. The best way to see this is to simplify the mechanism and eliminate the extra parts. There might be a little difference in capacity at lower, mid and highest position but that is not part of the equation here. The OP wanted to know how to calculate the projected capacity using published specs. The only way to practical way to do this is to make the hitch static as already stated, the sum of all forces including torque must be zero.

The capacity of the hitch is limited by the amount of torque from the rockshaft. It can稚 matter what you add to the front of the tractor, tire pressure, or the weather. Everything you add to the hitch itself is still limited to the torque output of the rockshaft.

Also - with the hitch in a horizontal position and static - the horizontal forces in the top link (tension) and the lift arms (compression) are equal and they are at 90 degrees to the moment. Knowing this you can eliminate the whole parallelogram thing and really simplify the problem.

 

[SPYDERLK]

Parallelogram has no significant effect on the lift capacity of the 3ph; [[[its only purpose is to maintain the orientation of the implement.]]] The best way to see this is to simplify the mechanism and eliminate the extra parts.

[ ...Ya think?...Try accomplishing this in a fundamentally different way and see what you get.] A tractor with 2k# at the eyes would be hard pressed to get a 1k# implement off the ground.
larry

 

[LD1]

Wow I don't know how I have managed to miss this thread for the last few days

All I can say is that I do agree with Spyderlk. His fundamentals and math seem to be right on with everything I know. However their is an easier way of measuring lift capacities of loaders and 3ph's.

It has to do with ratios and percentages. The force of a cylinder/length of travel is directally proportional to the lift force/distance of the lift. This is a whole lot easier than trying to figure angles and forces.

Example: Let's say lower lift arm is 32" long with the lift link exactally half way. In order to achieve 2000lbs lift at the ball ends, you need to apply 4000lbs force at the lift link. This part is a simple lever. Likewise, for every inch of travel at the lift link results in 2" travel at the ball ends. Thus the ratio of lift force to lift distance that I mentioned.

Lets say your tractorhas 2000lbs lift at the ball ends, and you want to know how much it can lift @24" back, first is to find out how much it can lift at the lift links, as described above, we'll use the 4000lbs. Measure the hight of the lift link, and measure the height of a point on your implement @24". Raise it up some, and take the measurements again. If the lift link moved say 4" and the point on the implement moved say 10". 4/10 =40% of lift at the lift links. Which would be 1600lbs @24" and 2000lbs at ball ends. You could also do this equation using the lift cap at the ball ends to start with. Which in this example would have moved 8". So 8/10 = 80% of 2000 still = 1600lbs.

As mentioned, the distance lifted, and the force required are directally proportional. In A perfect parallelogram, it doesn't matter how far back you go, it raises the same as the ball ends, thus what SPYDER said, would have no effect on lift capacity. But in the real world, none of us have perfect parallelograms, and even one tractor can be changed to lessen or increase the lift @24" just by adjusting the length or position of the toplink to make the implement raise slower or faster.

Same method can be applied to loaders, if you know the spec at the pins, pressure, and cyl diameter. Use the pressure(spec and not actual for this part) and cyl diameter to figure the force of the cylinder, times 2 cylinders and you have your total force. Now you need to find the ratio of force lost due to the angle and levers effect.

With the loader on the ground, measure to the pivot pin, and take a measurement on the cylinder. Raise the loader some and take the measurements again. If the pin raised 40" and the cylinder extended 8" that is 20%. Take the 20% and multiply by the cylinder force.
Now you got the total lift force at the pins, which may seem way higher than your lift spec, and it is. You have to take the lift spec @pins out of your book, and subtract it from this. So lets say total lift force is 2500lbs and your lift spec is 1300lbs, 2500-1300=1200lbs. It take 1200lbs just to lift the loader frame and empty bucket.

If you want to know the capacity at the bucket edge, put it flat on the ground, (so you can start with 0" for the measurement) and measure the cylinder. Raise the loader (but do not curl) and measuer again. Using the same method as above, find the % and use the total cylinder force # we used above. Then subtract the force required to raise the loader frame and bucket and you have your answer.

Sorry for the extremely long post, but I was bored.