Help with 3 point fork design please

[beppington]

I replied to your other post, but I didn't see this one.
How do you intend to keep your lift arms parallel? They lift by rotating, so you'll never keep them level. If it could be done, all carry-alls would be built that way. There will be a point that you have the top link attachment so low that it will bind when you try to lift, but I can't say exactly where that is.

I didn't see where he asked about keeping his lift arms level. Maybe I missed it.

Should be no need for it, though: As long as his lift arms & top link are the same length & parallel to each other, his carry all should stay level ... i.e. a parallelogram.

 

[MMagis]

Okay, now I see what you guys are saying. But for the carry all to remain level, wouldn't that require the top link to compress as it lifts? Something has to change, though maybe that's not it.

 

[bcp]

Three point hitch geometry is so confusing even the inventor got it wrong!

"It is extraordinary in that it has two upper links and one lower link whereas ever since the 3 point linkage has consisted of two lower links and one upper link. Harry Ferguson quickly recognised the problem of the linkage geometry and turned the invention upside down!"

From:
3 point tractor linkage

 

[beppington]

Okay, now I see what you guys are saying. But for the carry all to remain level, wouldn't that require the top link to compress as it lifts? Something has to change, though maybe that's not it.

If the lift arms & the top link are the same length & parallel to each other, then no. They would remain parallel to each other, & therefore the carry all would remain level, no matter how high or low you raise or lower the 3PH.

 

[MMagis]

If the lift arms & the top link are the same length & parallel to each other, then no. They would remain parallel to each other, & therefore the carry all would remain level, no matter how high or low you raise or lower the 3PH.

So, what causes the forks to remain level? Like I said, if it worked it would already be standard practice. It doesn't work.

 

[LD1]

This is actually really simple to simulate in real life (other than the program listed at the beginning of this thread).

Go to a post in your polebarn and put two nails in it somewhere abouts 10" appart.

Now take a short peice of scrap 2x4 or something and put two nails in it the same distance appart as the ones on the post.

Now imagine the nails on the post are the tractor side TL pin, and lower pin holes that attach near the rear axle.

The two nails on your scrap board are teh implement TL pin, and lower pins.

Now take two peices of string, equal in length, and about 20" long. These will simulate the TL and lower arm. Attach them to the nails accordingly and pull the strings tight. Now raise and lower the board(implement) while keeping the strings tight (remember the strings simulate NON-flexible steel arms). You will notice that the board in your hand(implement) does NOT change angles as it goes up and down. It only goes closer to the wall as it raises. Same as on a tractor, the implement actually moves a little closer to the tractor @ full raise.

I have actuall used this method when designing my own trailer hitch. (before I stumbled onto that program). The nails In the post were exact simulations of measurments on the tractor. I use them whenever I am designing or changing something else on other implements. Because some things I do want the angle to change, causing the back of the implement to be raised higher.

The method is simple yet effective. Especially if you have a hard time with a spreadsheet program like I keep refering to. And it actually gives you the ability to see it IN PERSON rather than trusting your eyes to the computer:confused2:

 

[LD1]

So, what causes the forks to remain level? Like I said, if it worked it would already be standard practice. It doesn't work.

Do what I suggested in my previous post. IT DOES WORK. You cannot deny the physics, plain and simple.

It is not standard practice because most implements are designed to raise AND angle. Making the back of the implement raise more than the front. This is for greater clearence. Most implements, like blades, bushhogs, etc are long. Imagine if your blade only raised perfectally parallel. The rear of the blade would only be ~ 12" off the ground. That isnt much clearence for way back there. It would dig in if you lived in hilly terrain, or trying to load on a trailer.

 

[bcp]

Maybe someone could make a flash animation of a 3 point hitch, like these. There is one animation on top of the page and seven near the bottom. Then, if we could input our dimensions and see the motion...

Simulation of four-bar linkages – Display with SVG and Flash

Bruce

 

[beppington]

In this pic, the lift arms & top link are parallel & the same length.

Does this help?

 

[LD1]

In this pic, the lift arms & top link are parallel & the same length.

Does this help?

Nice:thumbsup: