Why are loader arms tapered?

[rhamer]

In designing my home built loader for my tractor, I noticed that most loader arms are tapered.

Does anybody know why?

The only thing I can come up with is it makes the connection points easier to manufacture as the swivel point is narrower.

I intend to do the same thing, if only to make it look right, but I would like to know why they do it.

Cheers

Rohan

 

[bucmeister]

More likely is engineering for handling the weight versus distance from the point of movement kind of like the way a fishing rod is designed.

 

[SPYDERLK]

In designing my home built loader for my tractor, I noticed that most loader arms are tapered.

Does anybody know why?

The only thing I can come up with is it makes the connection points easier to manufacture as the swivel point is narrower.

I intend to do the same thing, if only to make it look right, but I would like to know why they do it.

Cheers

Rohan

They are stronger where they need to be. They have to be bigger or thicker near the center in the length of the arm, where force is applied off the axis of the arm. Large bending moments occur in these areas. Push forces have no leverage on the ends, but max leverage in the middle. Etc. Bigger adds strength with less weight than if you just used thicker walls. [Leverage again.]
larry

 

[rdbrumfield]

the farther from the tractor the less it will pick, so it is a waste of metal and weight to be that beefy out there. I suppose the pivot is able to rotate farther also as said above.

 

[rhamer]

Interesting, I would have thought it would have been cheaper to use standard retangular tube right through, rather than manufacture tapered arms.

Although, as you say it would weigh more.

Anyway, for me only doing a one off, I will taper them appropriately.

Cheers

Rohan

 

[browns40]

Hi Rohan,

Like said earlier: wider at the base is used for strength & stability on all straight applications: fishing rods, sky scrapers, trees... you name it. Strength (width) increase along the length until you reach the point of attachment discourages collapse and rotation.

In the case of what you are doing, use of gussets & bracing to accomplish the strengthening & prevention of twisting ought to work fine using square stock.

 

[Brad_Blazer]

As was said, bending moment is the big player and it accumulates as force multiplied by distance. 200kg (1961 Newtons) cantilevered at 2 meters results in 3922 N-m (or 2894 ft-lb) of bending moment.

Some good light reading for junior engineers.
Beams
Topic 4.1 - Beams Shear Forces/Bending Moments
The whole shebang of Statics
Index
Beam cross-section design. e.g. when you make up your tapered arms, would it be better to make the top and bottom plates thicker than the tapered sides?
Second moment of area - Wikipedia, the free encyclopedia

In the end, you do not want to approach the yield stress of your material.

I don't calculate the combined stress on everything I make. Once you understand the basics and take a good look at what is already out there, it's not too risky to copy the basic designs of others. Just don't do something like use bigger diameter hydraulic cylinders than anybody else. Err on the conservative side.

If you are making 10,000 or it has to go fast or fly, do the calculations.

Brad

 

[joes_427_vette]

I guess i'm not 100% with this thinking. The loader frame is 6' from the mounting point where it attaches to the bucket. When the bucket contacts an immovable object the stress is at the end of the tube near the bucket. this is where the bend is most likely to take place. My bobcat has very narrow tubes near the end and that is where they moved back ( bent ). Now the only solution I can come up with is to cut, reinforce, and reweld. I believe the fancy designs for the loader frame is to save steel and astetics. I'm not cutting and slicing when I build mine. I will take the slight extra weight and have it strong where it needs to be. And.............. I'm not a junior engineer, just practical with some common sense who hates to redo screw ups.

 

[SPYDERLK]

I guess i'm not 100% with this thinking. The loader frame is 6' from the mounting point where it attaches to the bucket.
[[[[[[[[[When the bucket contacts an immovable object the stress is at the end of the tube near the bucket. this is where the bend is most likely to take place. ]]]]]]]]]
My bobcat has very narrow tubes near the end and that is where they moved back ( bent ). Now the only solution I can come up with is to cut, reinforce, and reweld. I believe the fancy designs for the loader frame is to save steel and astetics. I'm not cutting and slicing when I build mine. I will take the slight extra weight and have it strong where it needs to be. And.............. I'm not a junior engineer, just practical with some common sense who hates to redo screw ups.

NO! Something is missing here. Please post pictures.
larry

 

[Bill_C]

I guess i'm not 100% with this thinking. The loader frame is 6' from the mounting point where it attaches to the bucket. When the bucket contacts an immovable object the stress is at the end of the tube near the bucket. this is where the bend is most likely to take place.....

No, the most bending stress is at the point where the cylinders are resisting the load, not at the bucket (the point where the load is). The arms are acting as a lever. You're putting a force on the end of the arm with the bucket, but this force is multiplied by the distance (back to the cylinders). This is the bending moment, it's a torque.

Imagine using a wrench to loosen a bolt. With just the wrench, you can't get the bolt to break free. So you grab a piece of pipe to put on the wrench as a "cheater". Presto, you've multiplied the torque on the bolt. The force you are exerting is the same, but by increasing the distance, the torque (force multiplied by distance) has been increased.