Front-End Loader Forces

[WpgKarl]

I am wondering how one goes about calculating the impact load of a front end loader wheel/tire against a bumper wall? (See attached sketch). We have a weigh scale, so I could verify the weight of the loader but I need help to come up with the impact force "F" in order to design the bumper wall.

 

[SlideRuleEra]

IMHO, there is no "right" answer for the question. However, if you make reasonable assumptions you can come up with an answer that makes sense. For example:

1. If the bucket hits the wall at a significant velocity, something on the machine will break... absorbing quite a bit of energy - ignore this case.

2. If a tire hits the wall at a significant velocity, the tire will flex (somewhat)... absorbing energy - ignore this case.

3. So what is left... Assume a conservative coefficient of friction between the tires and and surface here are a few examples:

http://ffden-2.phys.uaf.edu/211_fall2002.web.dir/Ben_Townsend/StaticandKineticFriction.htm

4. Since there is no need try to "pinpoint" the forces, I would take the worse case and say the COF = 1.0

5. Then assume and "Impact Factor". Just to have a reference, I like to use 2.0 from the AISC 9th Edition Steel Manual. Of course you are not designing with steel, but impact is impact.

6. With these assumptions, the impact force = weight of machine (assume that the bucket is fully loaded) x 2

7. This approach can be disputed, but the affect on your design will probably be minimal. With industrial design / construction, pays to be conservative.

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[JStephen]

If the bucket hits the wall at significant velocity, the wall will break, is my guess. Not to say this should be the design condition one way or the other, but those things don't just fall apart when you bump something, either. Come to think of it, they have a counterweight on the back, don't they? So you can back into stuff, too.

 

[SlideRuleEra]

JStephen brings up a good point. I suggest that the minimum thickness be 12", regardless of what the assumptions and calculations show. This is the thickness (more or less) that is needed to have two rebar mats, with proper cover. and still have a reasonable distance between the mats to resist moment.

Oh, yes... no matter what you do, over time expect some damage to the wall.

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[COEngineeer]

I would do another bumper that is not connected to the retaining wall. Make the retaining wall go up only 6" and put some kind of precast barrier that is easy to replace 2-3 ft away from the wall. Maybe use rebars down the soil.

Never, but never question engineer's judgement

 

[JaredS]

First, unless the guy in the loader is being a real tool then the bucket should never be close enough to the ground to hit the wall. Second a half foot rubber bumper would probably be a good idea to prevent damage should this in fact happen. For the load itself I would want experimental data, in all likelihood the loader operator will at some point use the wall to come to a stop.

 

[hokie66]

COE,

If you look at his sketch, that is what he is doing. He is just trying to figure out how robust to make the bumper.

 

[WpgKarl]

Thanks all for your responses. Was thinking a bit more about it and since F = m a, and we are taking a front end loader moving at say 5 to 10 mph and bringing it to a stop over a period of say 1 second, could one not work out the acceleration from the initial assumed velocity?(acceleration, a, has units of ft/sec^2, which can be obtained from velocity (ft/s) divided by time (s))

This is really no different than a car applying the brakes and going from a constant velocity to a standstill over a few seconds. Does anyone have a physics sample type of calculation that I could reference, to work out "a" and subsequently the force "F"?

By the way, I agree that it should be min 12" thick for two mats of steel and I would expect some damage as well in the long-term. Perhaps I will protect the top corner of the bumper wall with a contin. 6"x6" angle c/w studs or rebar anchors....

 

[hokie66]

I don't know about the angle. That would just make it harder to repair. But I might look at bolting a big lump of hardwood on to protect the top corner of the concrete.

 

[COEngineeer]

F = m * v/t

Multiply both sides by time you get

F t = m v (momentum)

m = mass of the loader which should be in slug (divide lbs with 32.2 ft/s^2)

v= speed (ft/s)

Now t is a little tricky. t is the time the loader hitting the wall. The more elastic it is, the smaller the t. I would use less than 1 second... maybe 1/20 of a second.

This is why automaker make the front of the car easily crush to make t longer. I hope i am making sense. It has been a while.

Never, but never question engineer's judgement

 

[swearingen]

I strongly suggest armoring the wall as you mentioned. The concrete corner will quickly come apart with heavy use. In materials handling, there are many cases where FEL's are dumping into pits to load a conveyor and we always armored the corners.

I think a deflection of 1 foot is very unconservative. Look at where the deflection must come from: the deflection is a combination of the deflection of the wall and the tire or bucket mechanism. If the tire hits the wall, the deflection would be maybe a few inches. If the bucket hits the wall, it will be less than that, with more force generated.



If you "heard" it on the internet, it's guilty until proven innocent. - DCS

 

[SlideRuleEra]

I agree with hokie66 about the armor angle - leave it off. At a coal-fired electric power plant we built in the early 1980's all of the foundations had armor angles. Very quickly, two problems appeared:

1. During construction, it is very difficult (read that as virtually impossible) to consolidate the concrete under the angle. The result is that the angle is not anchored very well, no matter how many studs or how long they are.

2. There is a lot of heavy equipment used all over a power plant. Within five years, many of the angles had been knocked loose and twisted like pretzels - tearing out chunks of concrete with them, but still attached (more or less) to the foundations. Had to send a crew around with a cutting torch and take off ALL of the loose angles. As other come loose, they are cut off, too.

At the next unit built on the same site in the early 1990's, we went out of our way to specify that no armor angle were allowed. Got a few "squawks" from the design engineer, but no armor angles. Now, 15 years later there is damage to the 1990's foundations, but a lot less than the 1980's foundation have (even if you take into account the difference in age of the structures).

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[WpgKarl]

JAE you're awesome! No comments?

 

[msucog]

having played on a loader like that in my contracting days, you'd better armor the wall with a steel plate if you think he's going to be scooping up material against the wall. that loader could easily take the top of that wall off.

maybe slap a steel H-section (laid down horizontal of course) to the front of the wall and add a bumper in front of that. or maybe a steel plate that runs vertical up the wall and turns to cover the top of the wall (simply attach with a few bolts epoxied in to the top of the wall so that it can be removed in needed). at least the steel would protect the wall from a point load from the bucket's corner and would also help protect the wall corners too. i guess the bumper would need to be centered on the wheel axle. if there's a gap below the steel column/I-beam/bumper and the ground, slop some 2-3" stone under there as armoring to keep the operator from accidentally lifting his bucket up and catching the edge of the steel section/bumper.

 

[EngineerTex]

In my opinion, the issue is going to be dealing with the bucket being rammed into the concrete. In thinking about that, how is that any different from a jackhammer in slow-motion? Isn't that exactly what you would do to demolish this wall after you're done with it? I don't think you need a force balance equation for it. The bucket is going to take it apart.

Look back in your physics book for impulse forces. As COEngineer says, determining "t" is where it gets tricky. In fact, it's pretty much impossible. The concept of the impulse force in physics is generally considered to be something that is unknown where "I" = the area under the curve of a function on an acceleration vs. time plot. The problem is that without close scrutiny of moduli of elasticity of everything in the system (loader bucket, linkage, cylinders, pins as well as the concrete, the rebar, cushion sand and everything else), you don't know if the area under the curve looks like the capital building or the Washington Monument.

However, you did ask how to determine the force from the tire. So, how do we solve it?

If you absolutely HAVE TO KNOW how much acceleration you're going to get, weight the front end of the loader on your scale. Then raise the front end and measure how high you have to lift it to get the front wheels off of the ground. Now you know force and deflection on the spring of the system (the tires) from force and deflection, you use that as your K in the system. Then, the energy as determined by velocity and total mass is applied to the spring in an energy balance equation, you can determine the maximum force required to deflect the spring (tires) enough to equal the energy of the moving loader.

But I wouldn't bother with all of that.

My (non-civil-engineering-background) opinion would be to look at what everyone else does in similar situations. Off the top of my head, I can think of a couple of different scenarios where enormous loads are slammed into concrete structures:

1) Truck loading docks
2) Harbors

On the loading docks, they put rubber bumpers on the dock and back the (80,000+ lb) truck into it. In a shipping port, they use wood piles to accomplish the same effect on a larger scale.

So, with that in mind, you might put a row or two of wooden rail ties offset from the wall about 6 - 8 inches to allow some springiness and put some rubber bumpers at the point of connection to the wall so that in the non-cushioned points, you still have protection.

And there's my two cents.

-T


Engineering is not the science behind building things. It is the science behind not building things.

 

[btrueblood]

Why is the bumper there in the first place? To keep a loader from falling in the pit? How about just training the operators not to drive into pits? Most surviving construction operators have learned to do this (Darwin's theory at work).

A slightly more user-friendly idea would be to paint a stripe "stop line" at the point where adequate clearance for the bucket to dump exists.

If you MUST have a bumper, let the bumper just be a length of concrete block (ecology block or jersey barrier, or a shorter kerb if you must) that is at most retained by a chain/cable to embedded posts to either side of the drive ramp. Then, the blocks could be removed (slid out of the way) occasionally to scrape/sweep debris behind it into the pit. If an operator is stupid enough to rely on the bumper to stop, he is likely to knock the bumpers into the pit, and then waste time retrieving them (better yet, install impact-triggered cameras on the retaining posts, to photograph the idiots that bump the barrier, and then dock them pay for every impact, or just fire the highest violator as an example to the rest).