Ihab Saad – Scrapers
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The speaker explains how productivity is calculated using various factors and factors like job conditions and operational efficiency. The loading and loading methods for various types of loads, including scraper engines, are discussed, including the importance of performance data for determining optimal loading time. The importance of maneuvering and performance data is also emphasized. The speaker explains the details of a construction project, including the scraper and its characteristics, and discusses the weight and resistance of the device, including the weight distribution and resistance. The importance of practicing and learning to improve speed is emphasized in upcoming exams.
AI: Summary ©
So on and so forth. So the productivity is equal to the rated
capacity times the operational efficiency divided by the cycle
time. So let's see how we're going to calculate that.
Here we have
different tables that give us different factors that affect
whether it's the job conditions and
that are going to primarily affect the fixed time and so on. How are
you going to load it? Is it? Push, loaded single engine, push, loaded
twin engine, push, pull, or self loading elevator. In our case, we
have an elevator self loading
scraper. And then we also have, if you remember when we discussed the
trucks and the hauling and so on, if you are going to be operating
on a very short stretch of road, then by the time you reach your
maximum speed, you have to start braking to stop at the end of that
stretch. So we're going to have a factor that's going to affect that
maximum speed. Because once you calculate the maximum speed from
the performance curves, you have to multiply it by that factor. And
in this case, the factor for the short speed is going to be point
four, five is going to affect the maximum speed. It's going to be
only 45% of the maximum speed. And if you have a long stretch of
road, like 5000 feet, then the factor in this case, going to be
96% we are already familiar with this table because we have used it
before,
and what we have here on this slide is again, something that we
have seen before, and we already know how to use it, which is a
performance chart. In this case, it's for a scraper. We have the
weight both loaded and empty. We have the effective grade
represented by these parallel lines. And then we have the
different gears. So we are going to use the weight to intersect
with the effective grade. Go horizontally and check which gear
is that going to be intersecting with, and then that's going to
give us the speed and which gear are we going to be operating
within.
Here's another example of very similar one. Again, here's the
loaded weight, and here's the empty weight,
the push loading scrapers, again, as we have seen before, sometimes
the engine of the tractor in front of the board is not enough to move
that scraper, so scraper sometimes require assistance in loading to
fill the rated capacity. Push, pull. Scrapers can work in tandem
to help each other, as we have seen in the video clip that we saw
a few minutes ago, non elevating scrapers need pushers to load
because they cannot lift the soil on their own. Crawler tractors are
usually used since they have better traction than wheeled
tractors,
especially on when you have a high
resistance, then in this case, the crawlers are going to be better.
We have seen with the wheeled tractor how there was some
slipping, and then it overcame that slipping and started moving
forward. The loading method can be either backtrack loading, chain
loading or shuttle loading. We're going to see some pictures
representing each one of these different types of loading.
So when scrapers are push loaded, the material in the bowl gets
compacted due to the pressure of forcing the material into the
bowl. Now imagine the bowl is moving forward, the blade is down,
so it's being filled as the ball is getting filled, the new soil
compresses the old soil inside the bone already. So the density of
the material in the bone is determined by the equation. The
density is equal to 100%
110%
times the bank density divided by 100% plus swell, which means it's
110%
of the loose
density of that sort.
Now here we have this picture representing the the push
phase. You have here the tractor, and then you have another tractor
here in the back. It pushes it forward. And now, once the scraper
is fully loaded, that pusher goes back to push another scraper. So
they work. They are in parallel. Here you push the first one, and
then only the pusher goes back. Now the tractor is going to move
that ball forward, and the pusher here in the back is going to move
back and it's going to push another tractor. So this is called
the backtrack loading backtrack, because only the pusher goes back
and it pushes another so.
Scrape, whereas in the second one, called Chain loading, basically
the second scraper is standing little bit ahead of the first one,
so you push the first one until it's able to move. The ball is
filled. It moves away, and then the pusher just positions itself
in behind the next scraper, and then keeps pushing it and so on.
So it's called, this is called Chain loading, because it's not
backtracking to go back to the same original position.
The third method is called shuttle loading. So basically, again, you
push the first scraper until it moves out of the way, and then you
go back the other scraper is facing the other way around, so
the other way the other scraper, the second scraper, is moving in
the opposite direction. So you go behind the second scraper and
start pushing it. So this is called shuttle loading because it
goes sort of in a circle or in a closed circuit
to determine the number of scrapers and pushers,
to determine the number of scrapers a pusher can load, we
must determine the pusher cycle time. So the cycle time for the
pusher is the time to contact the scraper, to touch it, and then
time to push it while it loads, and then time to boost it out of
the cut once the ball is filled, and then time to maneuver to
contact the next scraper. And that depends on the method of loading,
if it's going to be shuttle or is going to be backtracking and so
on. If no performance data exists, the cycle time for the pusher can
be estimated as
1.4
LS, which is the loading time for the scraper in minutes plus point
two, five minutes, which is the time to make that contact and to
start pushing that scraper. So it's point it's 1.4 times the
loading cycle time for the scraper, plus point two five
minutes or 15 seconds, basically
the boost time, which is time assisting the scraper out of cut
point one minute return time is estimated to be 40% of load time,
because now the pusher goes back empty. It does not have the same
resistance, so it can come back at higher speed. The maneuver time is
point one, five minutes again to maneuver and to position itself
behind the next scraper. Therefore, the number of scrapers
that a loader can push is determined by the equation n. The
number of scrapers is equal to the cycle time of the scraper divided
by the cycle time of the pusher. CTS is the cycle time for the
scraper, and CTP is the cycle time of the pusher. Let's
look at an example again. It's going to make things easier.
A CAD six, 630, 1e single engine. Scraper will be used to excavate
the side, the side of a large fill to level a construction site. The
soil is sandy clay, weighing 2700 pounds per bank cubic yard, with a
swell of 18%
the scraper has a 450 horsepower turbocharged diesel engine.
The haul road is 4000 feet. The distance is 4000 feet, with an
uphill grade of 3% from the cut area to the dump area. The running
resistance is 8585
pounds per ton, and the coefficient of traction is point
four,
A, D 8n crawler will be used to push the scrapers to load them to
heat capacity. The project site has an elevation of 3000 feet. Now
look at the problem, and each word has a certain meaning. Now here it
gives us an elevation of 3000 feet. In the older problems that
we have solved with other pieces of equipment, we had the derating
factor due to the elevation. But notice here that it told us that
this is turbocharged diesel engine. And we have mentioned
before that turbocharged diesel engines are not affected by the
elevation. So this is sort of a trick, if you do understand the
nature of the engine, and it's not going to be affected by any
derating factor, then this number is totally redundant, and we're
not going to be using
it. The scraper has the following characteristics, the rated
capacity, 31 cubic yards, loose cubic yards, of course, the empty
weight, 96,880
pounds, the maximum load, 75,000 pounds, the weight distribution
when empty on the drive axle is 67% rear axle, 33%
and when loaded again, is sort of balanced. The drive axis, 53% and
the rear axle, 47%
what's the estimated production of the scraper in.
Cubic yards, if the operation efficiency is 50 minutes per hour,
and the scraper does not wait in the cut for a pusher and how many
scrapers can a pusher load? So we're going to need to calculate
the cycle time for a pusher and the cycle time for a scraper to
determine how many pushers are we going to need. This problem might
look familiar, because we have solved something similar to that
when we're talking about haulers, when we're talking about trucks
and the different types of resistance that they're going to
face, and so on and so forth. So we're going to follow exactly the
same steps.
First of all, we're going to check if we're going to be weight
controlled or volume controlled. The density of the material the
scraper will carry is determined by the equation density. Now here
we're going to apply this is going to be unique to the scrapers,
because of the compression factor and the compaction factor that's
going to take place. So the density is 110%
the bank density divided by 100% plus the swell factor, which gives
us basically 110%
of the loose cubic density. Loose density, which is 2517 pounds per
cubic yard. Per loose cubic yard, the weight of the load when filled
to heap capacity, the capacity is 31 loose cubic yards. So the total
weight is going to be 31 times 2517
and that gives a total weight of 78,027
pounds, which is beyond the load capacity of that scraper. Because
the load capacity the maximum weight was 75,000 pounds, which
means we are not going to be able to fill it to the heat capacity of
31
loose cubic yards. So what would be the volume that's going to be
applicable? In this case, it's going to be 75,000 pounds divided
by the density, which gives 29.8 loose cubic yards, which can be
translated into bank cubic yards by dividing by the bank capacity,
75,000 pounds divided by the bank capacity, the bank density, which
gives 27.8
bank cubic yards. So that was the first check. So we learned here
that it's going to be weight controlled, not volume controlled.
The next step is to estimate the cycle time of the scraper. With
average job conditions, we get a loading time that's from the
tables that we have seen a couple of slides before. We get a loading
time of point seven minutes, spotting a delay time of point
three minutes, and a dump time of point five minutes. All of these
from the table, which makes the fixed time point seven plus point
three plus point five. That's 1.5 minutes,
the maximum rim rim pull generated by the scraper is going to be the
coefficient of traction times the weight on the moving axles. So
it's going to be when it's empty, the coefficient of traction is
point four times point six seven, which is the weight on the moving
axle, times the total load, the total weight, one empty, which is
96,880
which gives a rim pull of 25,964
pounds. Now, when it's full, we're going to add the load of the soil,
in addition to the weight of the scraper itself, the coefficient of
traction is still point four. The weight distribution has changed
because of the heavier load on the other axle. So it's times point
five, three times 96,008 80 plus, the weight inside the bowl, which
is 75,000 pounds, which gives 36,439
pounds. So that's the generated rim pole.
Now we have, we can convert that into tons, the total weight into
tons, by dividing by 2000 which is 85.9 tons. That's to calculate the
resistance, because the resistance is. The factor here that we use is
in tons. So the resistance is going to be
the force of the resistance going to be the rolling resistance, plus
the grade resistance, which is 80 pounds per ton, the factor times
the total weight, 85.9 tons, plus the grade 3% times 20 pounds per
ton, per percent slope times the weight in tons, which gives a
total of total resistance of 12,456
pounds. Now the required drain pull to overcome the resistance is
less than the generated drain pole. So basically, the scraper
can move forward. The resistance is not going to be enough to stop
the scraper from moving. We have seen here while it was empty,
25,900
almost 26,000 and when it's full, 36.4
and the required the resistance is 12.4 so we are quite safe now to
catch.
The effective grade, because we're gonna need to plug that into the
performance chart. We divide the running resistance by 20 pounds
per ton, that's a constant. So 85 divided by 20, that's 4.25
plus the already existing grade, 3% that gives us a total grade of
7.25%
when it's loading and when it's moving forward on the way back,
we're going to subtract the grade. So we have 85 divided by 20, which
is 4.25
minus three. So we have still an effective grade of 1.25%
using the effective grade on the performance chart, we get a speed
of 10 miles per hour. To get the average speed, we incorporate the
speed factor from the tables. We mentioned that
for starting from standstill for half of the road and coming to a
stop for the other half. So basically, if the road distance is
4000 we divide it by two. So the factor from the table is going to
be point nine. Two. Therefore the average speed when loaded is going
to be point nine, two times 11. That's 10.12 miles per hour. And
the average speed this should be 11, by the way, not 10. The
average speed one empty on the way back is going to be point nine,
two times 33 miles per hour, which gives 30.3 mile, miles per hour.
And this is basically how we obtain these numbers. We plugged
in the weight one loaded with a 7.25
effective grade, and that gave us this line horizontally. We go
here. We're gonna hit the fifth gear, and that's going to give us
a speed notice for by the way, that the first numbers are in
kilometers per hour, and the lower scale is in miles per hour. So
it's going to be about 11 miles per hour, and that's what what we
use in the equation.
Now, when empty, here's the weight one empty, and we had an effective
grade of 1.25
so basically, if we keep just going, it's not going to hit even
the 1.25 so we're going to use that speed, which is
basically we're going to keep going down until we reach the
maximum speed, which is going to be about 33 miles per hour.
So in the 4000 feet, which we're going to divide by two, as we had
just discussed,
the time is going to be 4000 divided by the 88 to convert the
feet into miles per hour
divided by the speed. And that gives us a time of 4.5 minutes,
moving forward, moving backward, the speed is 30.3 so we have 1.5
minutes. Therefore the variable time is the sum of these two, 4.5
and 1.5 that's six minutes. We had already calculated the fixed time
to be a minute and a half. So the total cycle time is fixed plus
variable. That's a total of 7.5 minutes. The Productivity is going
to be the load per cycle
times the number of cycles.
So
the load per cycle is 27.8 bank cubic yards
times operation efficiency, which is 50 minutes per hour
divided by the cycle time, which gives
the total capacity, or total production, of 185.33
band cubic yards per hour.
The pusher cycle time is equal to 1.4 because we are not given any
information about the pusher. So we're gonna use the equation 1.4
the scraper load time plus point two, five minutes, which is 1.4
times point seven, which is the loading time for the scraper,
plus point two, five minutes. And that gives us and the point seven
minutes is from the table that we have used before. So it gives us a
cycle time of 1.2 minutes. Now if the cycle time for the scraper is
7.5 minutes, and the cycle time for the pusher is 1.2 minutes, so
how many scrapers can one pusher serve? The number of scrapers is
going to be scraper cycle time divided by pusher cycle time. So
that gives 6.25 scrapers, which is going to be rounded down to six
scrapers. So one pusher can serve six scrapers. That's basically how
we calculate the production for a scraper and the number of pushers
and the number of scrapers, and so on and so forth.
I hope that has been
well understood, and we we have seen the solved example. Please
make sure that you try to change the numbers and resolve the
problem, to get more practice and to gain
speed in solving the problems which are going to help you
finishing the problems on the exam.
Well, good luck, and I'll see you in another lecture. Bye.