 |
 |
Braking Determination
Name: Buzz
Status: other
Grade: 9-12
Location: OH
Country: N/A
Date: 1/24/2006
Question:
I'm a truck driver that hauls loads of steel coils and
other types of sheet steel. My question is this: How can I
determine the G forces applied to my loads under severe braking
onditions?
I know that makers of the chains we use to secure the loads provide
information on how much of a load (weight) the chains can handle
before breaking, but they do not carry this computation further by
exploring how the weight of steel loads is amplified by truck speed
and what happens under extreme braking conditions.
I'm sure someone in the scientific community has an answer to this
question and I would appreciate knowing that answer.
Replies:
Hi, Buzz. If I understand correctly, you want to determine what sort
of G forces your load experiences in the course of a trip.
The easiest way to determine this accurately is to purchase a
"G-meter". It's an accelerometer (a device that produces a change in
electrical signal with a change in acceleration) integrated with a
display and various electronics into a single package that mounts to
the chassis. It's usually powered by a 12V cigarette lighter plug.
It's used by racers to help determine performance, so it may be
expensive (I've never looked at the prices). If it is too costly, then
there may be something developed by the packaging companies just for
shipping cargo.
Anyway, it tracks the magnitude of the G forces. Some track it in just
one direction, but some track it in all the various directions.
Usually they will have some sort of memory so you can recall the
highest acceleration experienced on a certain run. In a semitrailer,
the forces can be a little different on the trailer when you are
turning/cornering, so to get a good measurement, you could mount one
under the trailer bed to determine what G forces your load experienced.
For just forward-backward direction, though, it'll generally be the
same on the trailer and in the cab.
If the model you get doesn't do several directions at once, then you
could change the direction of the meter every few runs.
Once you know the G-force your load is experiencing, multiply the
weight by that, and that is the force the load experiences in the
particular direction. Be sure to take combinations of directions into
account. For example, if your load experiences 1/2 G forward at the
same time as it experiences 1/2 G to the side, the total force is then
the square root of ((1/2)^2 + (1/2)^2). You also may need to take into
account how the chains are arranged.
I'm glad you're concerned. I drove next to a trailer with a
chained-down load of 6" schedule 40 pipe that broke loose. That was
scary! It punched a hole in the overpass and shut it down for a few
days. I imagine a coil of steel would be quite a bit worse!
Hope this helps!
David Brandt, P.E.
Buzz,
There is a fairly simple calculation to provide an estimate for this value.
IF the rate at which a truck's speed is fairly constant, we can use the
kinematic equation v^2-v0^2=2ad. "v" is the final velocity. In the truck's
case, this is zero when braking. "v0" is the initial speed, how fast the
truck is moving before braking. For a truck, I expect this to be about
55mph. "d" is the distance required to stop, how far the truck travels from
when the brakes are hit to when the vehicle stops. "a" is this
acceleration. Since "a" is opposite the motion, it will be a negative
value. The magnitude of acceleration is therefore "a=(v0^2)/d".
Note something important. Units matter. Convert v0 to meters per second.
Convert d to meters. This will yield acceleration in m/s^2. Gravitational
acceleration is 9.8m/s^2. Divide your acceleration by 9.8m/s^2 to yield the
number of G's felt by the truck, by the load, and by the driver as well.
Dr. Ken Mellendorf
Physics Instructor
Illinois Central College
Buzz -
Your question is practical and variable, rather than ideally definable.
However a few things may make you feel better oriented.
While you are driving smoothly, the bed supports the 100% weight of the load,
and the chains take a fairly small percentage of the load, maybe 10%.
That ~10% comes from braking gently at ~1/10 of a Gee, and from
bouncing an jiggling that, due to friction,
accidentally conspire to push towards one side more than the other.
That last could happen depending on loading details,
and no physicist is going to be able to predict it's magnitude confidently.
The friction coefficient of your rubber on the road is only about 1.0, so
in a panic all-out) stop, the braking Gee's will be limited to 1.0 by skidding.
(or may 0.6 or 1.1, and less on sand an ice.)
Do you have a Gee-meter in your cabin ?
Most of the time, the load will be experiencing a rather similar force.
Not much amplification there.
The force on the whole set of chains will be the load weight times the number of gee's,
plus some angle corrections, plus some brief amplifications due to jerking and vibration.
So these chains have to be able to pull the full weight of the load
for a few seconds, plus at least 40% more if they are sloped at 45 degrees.
Exactly how big the chains have to be to do that reliably
after years of jouncing and weathering in relatively low-force conditions,
only the manufacturer will have known by testing & experience.
So there's a fudge-factor of 2:1 or more you can't really calculate,
unless somebody has told you the right fudge-factor and you adopt that.
The actual strength of steel varies with grade, condition, and the shape of the chain links.
But it might help you out to measure the thickness of a chin link,
figure it's cross-section in square-inches, and compare the load weight
with a typical steel strength of maybe 40,000 pounds per square inch.
Many steels will start higher, and many will end life lower.
Maybe the total strength of the chains you use will tend to be some simple factor
(1? 4? 10?) times the load weight.
For bouncing and vibration to put additional forces greater than the load weight on the chains
would require the chains to be tight the whole trip.
I imagine this would put a bending force on the bed; is this something you'd avoid in practice?
Then the forces could be only about 1 to 3 times the weight, I think...
Load-forces while the truck accelerating upwards in a bounce are taken by the bed, not the chains.
The truck cannot accelerate downwards after a flying bounce faster than 1.0G,
nor in the front-back direction due to traction limits.
I suppose slamming into a perpendicular ditch would be worse.
If chains are tight and vibrating, or a loose load swings forwards
and is stopped _abruptly_ by the chains in a time 10x shorter than the time which pushed it that way,
then a magification factor occurs, which could be brief jerks of any size, maybe 10 Gee's,
resulting in momentary 10 load-weights on the chains.
This kind of factor is also not predictable by a letter-writing stranger.
You'd have to measure it during your trip, or take measures to minimize somehow.
In tough jobs, having springs, shocks, or dampers of some kind in line with each chain could be important,
especially if you are carrying hard heavy cargo, loaded your own way, on unpredictable roads.
A similar magnification might occur if a springy resonant jiggling tendency in the load or chains
happens to match frequency with the vibrations from driving.
Added forces due to resonance of the chains themselves vibrating laterally
should generally be far less than their rated load-weight,
but perhaps they could fatigue the metal, making it weaker with time and long use.
Or perhaps they don't really happen in your practice.
Having side-pull ropes significantly deflecting each chain in the center of it's length,
might be a useful safe-guard against many such things.
Ropes are more damping than chains, so there'd be less rattle as you drove.
When huge peak loads occur the ropes might notify you by stretching or breaking
or by letting the chain suddenly reach high tension when it reaches a straight&taught state.
Normally this wouldn't happen. It would save wear on the chains and the rig would feel quieter, ride softer.
With the right kind of rope it would be the easiest kind of shock to improvise.
Nylon rope is known for stretchiness. Dacron a little less so.
I don't really know what chain-deflection angle to choose.
It might depend on how much load movement you could tolerate in various cases.
Having shoes or spikes on the floor to take most of the forwards forces might help all this.
Any instrumental way of reporting the peak chain tension to the truck's driver, even roughly,
would be helpfull to accumulated experience about this.
It could be anything from oversized-load-cells in line with a chain,
to a microphone taped to a chain or to the frame near a fastening-point.
A chain jerked to straightness and held in tension long enough
can produce a low-frequency twanging sort of sound you might someday recognize.
If you hear a higher pitch twanging, it must have been produced during
a moment of higher tension in the chain.
I once had a bathroom scale that worked on this principle, so it's a plausible idea.
Some of the solution would be human attention to properly damped chains
to prevent most twanging, snapping or jerking events,
rather than chain specs and computations.
Jim Swenson
Click here to return to the Physics Archives
| |
Update: June 2012
|
|
