Posted by dgoddard on February 11, 2016 at 18:34:35 from (104.245.136.182):
I do not know how others do it but When making attaching an implement with a 3 point hitch, but getting the location of the
Lower Link point (where the lower link attaches to the tractor)
and
Upper Link Point (where the top link attaches to the tractor)
can be a real pain, especially the upper one).
My technique is as follows:
Tools required:
-- Concrete slab to park the tractor on.
-- Plum bob
-- Yard stick
-- Chalk
-- Level
-- Square (Carpenters square usually but a smaller one may be useful
Using the plumb bob to drop a vertical from the lower link point and mark it on the concrete Do both sides and draw a line between them
1,
Drop a vertical from the upper llink point. (some times you can actually do this but if necessary use the level to hold the yard stick parallel to the floor and drop a vertical a known distance behind the upper hitch point and measure back under the upper link point and mark that on the concrete.
2.
Get the lower links level and Using the plum bob drop a vertical from the hitch point at the ends on both sides and draw a line on the concrete between them. This must be done because The true effective length of the lower links is the distance between the line of the lower link points and the line of the lower hitch points. (Do not just measure the length of the lower links )
3
Measure the vertical height of the lower link points.
Measure the vertical height of the upper link point.
And subtract the lower from the upper heights. This is dimension A
Measure the distance between the line of the lower link points back to the mark for the Upper Link Point This dimension B
--------------Keeping an implement level or at a chosen angle -------------------
You might want a "carry-all" level but you might want a tiller at a bit of an angle to keep its input shaft level and parallel to the tractor's PTO shaft.
Now level up your implement (or tilt it to what ever other angle you would like). Measure from the hitch pins vertically by the amount of dimension A
Then measure back from that line by dimension B horizontally. (I do mean vertical and horizontal! not just perpendicular to some surface on the implement !) The square and the level may be of some help on this.
Measuring from the hitch pins, A up and B back is where the top pin hole needs to be in order to lift without the implement changing its angle. But to make this work you have to adjust the top link to the same as the "true effective lower link length".
Many manufactured implements will not have the top link hole at this location as the mast may be vertically in line with the hitch pins.
Depending on how for instance a subsoiler plow is built moving it with its bar vertical and its foot at the manufactured angle may be what you want, but you can make it more agressive at pulling down into the soil if you shorten the top link. whereas you can make a grader blade more agressive at pulling down by lengthening thetop link . For a PTO driven implement keeping the input shaft parallel to the tractor's output shafe parallel at all degrees of lift is necessary unless your shaft joints are CV joints. This would be important for a rotary mower or a rotary tiller. However with a carry all you might want it to sit level on the ground but tilt slightly forward once it has a load so as to not dump stuff off the back.
For you old timers with plenty of experience you probably have a good handle on this. And in that case, at best maybe all I have done is shared a way to get accurate measurement. Because to this point all I have described is how to make a parallelogram linkage out of the lift links.
But What I can add is that it is possible and fairly easy to use a computer spreadsheet Like excel or Libre office calc That will let you see what happens to the position and tilt of the implement at every angle or height of lift of the hitch.
Since the lower links and linkpoints on the tractor are pretty much immovable the only effective options are changint the upper link length or moving the hitch pins or the top link pin on the implement But it is possible to experiment with those. At the same time for those who are not all that much into computers, the same thing can be done using push pins and a cork board to make a scale model using cardboard links. For the pins on the implement you can use thumb tacks with the points sticking away from the cork and just stick small corks over the points if you don't want to get stuck. And if you put a piece of graph paper on the cork board You can trace the position of the cardboard cut out of the implement on the paper for different experiments.
BUT with a scale model you have to take very accurate measurements.
Then again if you are going to have some expense tied up in the construction of the implement you might want to do the cardboard model thing full scale With a sheet of plywood or sheet rock etc. and do it full scale, just to be sure that there are no surprises. With models you may want to draw the tractor tires or other parts on the sheets where the links will be moving to chec for clearances.
Just for the record it is also possible to use that spreadsheet to calculate the forces on all the links to see if the hydraulics can handle it or how strong the links have to be, but in that area I have the advantage of being a retired college professor of mechanical engineering with a speciality in machine design.
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