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.

 

[Brad_Blazer]

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.

I think in the case of ramming a skidsteer bucket into an immovable object, it becomes a combined stress issue where the short downturned ends of the skidsteer arms experience a lot of shear in addition to the bending force both which were really due to compression of the curved arms with very little lifting involved. That's why dozers have straight arms designed to act in compression. If the skid-steer arms were constant cross-section they would have bent in the middle. Since they are tapered the ends were the weakest point for the load condition you applied. Of course it is impossible to optimize the design for every load condition. It sounds like something had to give. You should be thankful the damage was confined to the ends.

The arms are designed to save steel. It saves money and fuel and increases lifting capacity. Aren't you glad your limbs are tapered? If your calf muscles were the size of your glutes how fast could you run?

 

[czechsonofagun]

Spyder got it right. It is the strongest where the torgue/bending force is highest. Same reason tall electrical posts are tapered from bottom up, the top can be ligter and save some weight and material.

 

[joes_427_vette]

My Bobcat is probably 40 years old and I bet has met alot of immovable objects in its life. That is why it was 1200 when I bought it. I can understand the telephone pole analogy but loaders get shock loads and twist with the added forces. The old saying is something has to give and at some point it does. To my way of thinking the taper down to 2 1/2" at the end of a torsion rod so to speak doesn't make much sense, then again I'm not the engineer or parts salesperson. Just my 2cents.

 

[SPYDERLK]

[[[My Bobcat is probably 40 years old]]] and I bet has met alot of immovable objects in its life. That is why it was 1200 when I bought it. I can understand the telephone pole analogy but loaders get shock loads and twist with the added forces. The old saying is something has to give and at some point it does. To my way of thinking the taper down to 2 1/2" at the end of a torsion rod so to speak doesn't make much sense, then again I'm not the engineer or parts salesperson. Just my 2cents.

And I bet the tips are badly rusted internally due condensation running down there. Time has selectively weakened them there so they cant withstand their proportion of the load anymore.
larry

 

[CCWKen]

I think it's to save weight and money. Not to mention looking better. The stress is distributed between two arms and the cross bar acts as an equalizer. The cross bar also keeps the arms from twisting. The roll cylinders take a lot of stress off the cantilever calcs. It depends on what what the FEL is going to be mounted on and the design.

I built this small grader then added the FEL later. (I call it the "Kenbota".) The bucket is a little over 4cu.ft. I've had it heaped with #2 base and bounced it. I've mounted forks on the bucket and pulled cactus out of the ground whole. The arms are made from 2x4x3/16" square tubing and there's no sign of "bending moment". It does what I need it to do but I wouldn't try to lift a car with it.

 

[3RRL]

What a great little build, Ken.
So your loader works and does everything you want and is adequate for the tractor then? What about the grader ... that looks really cool. Does it come down to the Earth via hydraulics too? I see the hydraulic cylinders for angling, but not for lowering. Are there two cylinders for movement or just one?

 

[CCWKen]

Don't want to hijack the thread but Thanks Rob. Yes, the grader blade angles as well as lowers and/or tilts. There's a cylinder on each side that's independantly controled for ground contact. The cylinders are just in front of the rear up-right for the FEL.

This is a shot of one of the cylinders with the FEL removed.

 

[blacksmith]

I think it looks better and I knew I could do it. Here is a picture as I was installing the loader arms. The frame is made of 3"square tubing.

Here is the finished project.

 

[Iplayfarmer]

I think it looks better and I knew I could do it. Here is a picture as I was installing the loader arms. The frame is made of 3"square tubing.

So how did you taper the arms?

 

[joes_427_vette]

CCWKen; 2x4x3/16 material is what I plan to use for my loader. Why waste time and material hacking the tubes to a taper when the 4 inch profile is what I am after. I know they will be strong enough for my purpose.
Your "kenbota" turned out great. Alot of thought and time went into the grader blade. To do some serious cutting in granite or hard clay do you have to add weights front and back ?